Colour, the art and science behind sight

COLOUR WHAT WE SEE AND THE SCIENCE BEHIND SIGHT Margaret Franklin Tom Kennedy Ian Elliott Stefan Hutzler Tim Durham & other contributors

Science Spin DISCOVERY 1

All About

MAYO

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Tom Kennedy

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Cover: Rainbow over Monaghan cathedral. Photograph Tom Kennedy

COLOUR

What we see and the science behind sight Margaret Franklin Tom Kennedy Albertine Kennedy Publishing Cloonlara, Swinford, Co Mayo www.albertinekennedy.com

Print edition ISBN 0 90600210 9

First edition, November 2005 eBook September 2021

Contributions from Ian Elliott Stefan Hutzler Tim Durham and others

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Acknowledgements Margaret Franklin is a Senior Lecturer in the School of Science at Athlone Institute of Technology, where she has been sharing her passion for chemistry with generations of students over the past thirty years.

After majoring in chemistry for her B Sc from UCD, Margaret obtained an M Sc in Organometallic Chemistry. Following two years teaching chemistry at an international school in Züirch Margaret joined Athlone Technical College. Margaret is a fellow of the Institute of Chemistry of Ireland and she currently serves on the Council of that body.

Ian Elliott, author of notes on red shift and colour in astrophysics is a scientist with the Dublin Institute of Advanced Studies; Stephan Hutzler is a physicist at Trinity College Dublin; and Tim Durant is a photographer who recently held an exhibition, Soap Opera, at the Draíocht Gallery in Dublin.

Our thanks to Richard Mills for his wildlife photography, Margarita Voronova, and Vorsin Dmitri, two of the Russian photographers who exhibited at the Source Foxford Gallery, Co Mayo, Florence Gruben for her colourful painting, and Gerard Wheelan and the ever helpful RDS library.

Marianne Jonker for the plant woodcuts, Appie Kennedy and Seán and Maria Duke for weeding out some errors, Oliver Heffernon and Sharon Murphy for drawing our attention to some amazing pictures, and thanks also to the countless number of experts who shared their knowledge through scores of books and scientific papers.

Tom Kennedy is an editor of Science Spin, founder of the Source Archive photo collection, and IBM/Forfás print Science Journalist of 2002. His family has a strong association with the decorative and fine arts, and Tom, who has given third level courses in creative photography has exhibited in Dublin, London, Amsterdam, and most recently at the Linenhall, Castlebar.

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Painting a ship, Dublin. The greater the area, the higher the intensity of colour. Photograph Tom Kennedy

COLOUR

The science of colour

What is colour? •

6 The wonder of colour •

8 Paints and dyes •

10

Selective absorption•

12

Electromagnetic spectrum•

13

Compounds •

14

Organic dyes •

15

Azo dyes •

17

Inorganic pigments •

17 Green energy •

19 Adding and subtracting •

20 Defining colours •

22 Why is the sky blue? •

27 John Tyndall •

28 Cosmological redshift, Ian Elliott 29 Raleigh scattering, Ian Elliott 30 Northern Lights •

31 Soap Opera. Stephan Hutzler with Tim Durham 32 Refraction •

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False colours •

Natural colours •

Animal colours •

Animal sight •

Looking at heat •

Subjective sight •

Mixing colours •

Defining line •

Inks •

Colourful art •

Saffron•

Indigo •

Colour and the eyes • Colour blindness •

Making sense of our world • Colour in print •

Inks for print •

Photo colour •

Colour film •

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38 42 46 50 52 54 57 59 63 65 70 73 75 80 82 84 86 88 89

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John Joly •

90 Colour television •

90 Atomic structures • 92 Fluorescence •

96 Spectroscopy •

98 Fireworks •

99 Lasers •

100

Chromatography •

101 Testing sight, Ishihara charts 102 Index 110

• Margaret Franklin — • Tom Kennedy

Coloured pebbles by the shore, Howth, Dublin. Photograph Tom Kennedy 5

COLOUR

What is colour?

T

he reds, blues, greens and all those other colours are so real to us that they hardly seem to need description, yet, like smell, sound, and even pain, colour is all in the mind. The colours we see are our own personal responses to certain wavelengths of light, Light, which has no colour, is made up from a bundle of wavelengths, and it is only when these are separated, as when the Sun’s rays strike water droplets that we see all the colours of the rainbow. The shorter waves give us violet and blue, and the longer rays are red, Our eyes have millions of receptors that respond to these waves, and the signals from them are processed in the brain to create a vivid impression of colour. Thus, what we see is highly subjective.

When light strikes an object some of the wavelengths are reflected back to us, and these give us the colours we see. If most of the wavelengths are reflected equally we see bright white like snow or quartz, but if only the shorter waves are reflected we see blue, and if all the light is absorbed we see nothing, so the object we are looking at is as black as soot.

Because of white light we live in a colourful world. Relatively few objects are totally reflective across the spectrum, so most of our surroundings are coloured.

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COLOUR

Our eyes are sensitive to a narrow band of wavelengths in the electromagnetic scale. This scale ranges from the long waves used for radio and television up to and beyond the penetrating X rays and highly energetic cosmic radiation. With the exception of this narrow window, 780 nm to 380 nm, all other rays are invisible to us. Between 780 and 380 nm we see white, visible, light. Apart from detection of low intensity light, which we see in monochrome, our eyes have special sensors for colour. These tiny sensors, the cones, are of three types, each responding to a different segment of the visible spectrum. There is some overlap, but overall the sensitivity peaks at 445 nm for blue, at 535 nm for green, and at 570 for red. We see a full range of colours by recombining these responses in our mind.

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The wonder of colour

S

ince the dawn of human history, the sight of a rainbow in the sky has been a source of wonder and inspiration for people everywhere. The rainbow is mentioned in the Book of Genesis, as the sign of a covenant between God and humankind, yet, though observed by the ancients, it was not explained scientifically until the seventeenth century, when Sir Isaac Newton showed that a glass prism could split white light into its constituent colours, while Descartes showed how a single raindrop can produce a similar

effect. We have become so well accustomed to such everyday phenomena, that we take them for granted.

We have all seen the beautiful colours in soap bubbles and oil films, we observe the fact that the sky is blue on a fine day, but changes to various shades of red and orange at sunset. But, it takes a rare event, such as the recent sighting of the aurora borealis in these latitudes, to make us wonder what causes these spectacular colour effects.

We enjoy being surrounded by colour. Photo: Margarita Voronova 8

Primrose, Primula vulgaris, bringing colour into spring. Cloonlara, Mayo. Photograph Tom Kennedy

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Paints and dyes

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rom an early age, as children, we enjoy colouring with crayons and paints and soon learn to experiment with mixing various primary pigments to produce new colours. Maybe we learned how to make a colour wheel and spin it to produce a white effect, the reverse of what happens when a rainbow is formed.

As adults, we appreciate great works of art and find delight in such examples as the glorious red of Monet’s poppies, the use of blue in the famous ‘Blue Boy’ by Gainsborough and the brilliant yellows so often exploited by van Gogh.

Colour is of primary importance in the world of fashion, where the availability of synthetic as well as natural dyes have provided designers and their customers with an almost unlimited choice. Consider the paints industry and the multitude of hues that interior and exterior decorators have at their disposal.

Think of the printing industry and the brilliant colours that modern technology can reproduce in a glossy magazine such as Science Spin or this book, or the beautiful colour photographs we can take using a relatively inexpensive camera. But where does all this colour come from?

Take, for example the dyes, which are used to give

Painting by Grand Canal, Dublin. Photograph Tom Kennedy

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COLOUR colour to the clothes we wear. These significance of his discovery, decided are usually organic substances, which to give up his studies and with some were originally extracted from plants financial assistance from his father, such as indigo and madder. However, opened a factory in 1857 to for over a hundred years, chemists manufacture the dye. When Queen have been making synthetic dyestuffs, Victoria took a fancy to the colour, his some of which are exact copies of reputation was made. Over the next plant extracts, while others are two decades, he became a wealthy colourful compounds that never man and was able to sell his factory at occurred in nature. the age of 36 and retire from business The synthetic dyestuffs industry in order to devote his time to chemical was started in 1856, quite by accident research. (as has happened with many scientific In the meantime, a major dyestuffs discoveries) by an 18-year old student, industry had developed in Germany William Henry Perkins. He was where azo-dyes, made from substances attempting to synthesise the drug extracted from coal tar, had been quinine, widely used in the treatment Becoming wealthy from his discovery discovered and were being of malaria, by the oxidation of aniline. manufactured. These proved to be of William Henry Perkins was able to At the time, the only known source of retire from business and devote his time major commercial importance and to research. quinine was from the bark of the were used, not only to dye fabrics, but cinchona tree, which was grown in also as food colourings and in south-east Asia. However, as it happened, the aniline cosmetics. Their manufacture proved very lucrative he was using was somewhat impure, being and it is significant that when the major German contaminated with traces of nitrobenzene, with the chemical companies merged in 1925, the new result that, instead of preparing a sample of quinine, company was named ‘IG Farben’, from the German his reaction mixture became a dark sticky mass, word for paint. which, on suitable treatment, eventually yielded a But, what do all these substances have in common, beautiful mauve-coloured dye. William, realising the that makes them appeared coloured to our eyes? To 11

COLOUR answer this question, we have to realise that we cannot see any colours at all in the dark, so colour results from the interaction of light with various objects. When light strikes a surface, various things can happen. It may be reflected, absorbed, scattered or transmitted, or a combination of these.

Consider first reflection. We are familiar with the phenomenon of specular reflection, when we see our image in a mirror. This happens when the surface is so smooth that light rays are reflected in a regular way, so that they appear to emanate from points behind the mirror and what we perceive is a virtual image of any object in front of the mirror. (It is called a virtual image because the rays do not actually pass through the image, but seem to come from it.) On the other hand, if the surface is somewhat rough, like a piece of white paper, then the reflected rays are scattered in random directions. Provided all wavelengths are reflected to the same extent, they mix to form white light, as in a child’s colour wheel. Conversely, if all of the wavelengths are absorbed to the same extent, then the surface appears black. However, if some wavelengths are absorbed more than others, then the reflected light contains more of the colours that are NOT absorbed.

Thus we see the complementary colour to that being absorbed. Grass appears green to our eyes because there is a substance in it, called chlorophyll, which selectively absorbs red light. Which brings us to the

question, why do some substances selectively absorb certain frequencies (colours) of light?

Selective absorption

To understand the mechanism of the selective absorption of certain colours of the light spectrum by certain materials, we need to know a little about the structure of matter itself.

As most people are aware, matter is composed of tiny particles called atoms.

Atoms contain even tinier particles called protons, which are positively charged, and electrons, which are negatively charged. An atom is mainly empty space and the protons are confined to a small dense central portion of the atom, known as the nucleus. The electrons revolve in the space round the nucleus rather like the way the planets of our solar system revolve around the sun. But the path of a particle as small as an electron cannot be tracked exactly, so rather than describing them as moving in fixed orbits, the electrons can be envisaged as moving in regions of space, around the nucleus of the atom, called orbitals.

These orbitals can have various shapes, but the way in which they accommodate electrons follow very strict rules.

Elements are the simplest form of substance, as they consist of one type of atom only. Elements are 12

COLOUR distinguished from the others by the number of protons in the atom’s nucleus, the atomic number.

In general the number of electrons in the orbitals is equal to the number of protons in the nucleus, so atoms are usually electrically neutral.

The orbitals are arranged in layers called ‘shells’ and each orbital can accommodate one or two electrons, but no more.

Electrons in the outer shells possess more energy than those in the inner shells. These differences in energy level are not continuous, but are distinct steps.

The levels are ‘quantised’ so that electrons can only have a certain number of ‘allowed’ energy states.

Normally, an atom is in its ‘ground state’, the lowest energy state available to it and therefore the most stable state. This means that all of the available orbitals are filled from the bottom up. Although higher-energy orbitals are available, they remain unoccupied in the ground state. For electrons to jump up into a higher orbital, energy has to be added.

This can happen when the atoms are exposed to light. The photons, ‘light particles’, deliver energy in proportion to their frequency. On absorbing the photon’s energy, the electron may make a ‘quantum leap’ into a higher orbit. In doing so, the energy absorbed by the electron must match exactly the difference between the ground and excited state.

On exposing materials to light, the electrons in

many millions of atoms absorb energies from a broad spectrum of frequencies. Those that absorb photons of the right frequency will make the transition to a more excited state. With total absorption all we would see is black, because no light would be reflected. With selective absorption a portion of the light is absorbed, so the balance is reflected back to us as colour. Because grass is so good at absorbing the energy in red light we see it as green.

Out of sight

Our eyes are only sensitive to wavelengths of about 800 to 400 nanometres in length. This is just a small portion of the electromagnetic spectrum, and photons at shorter wavelengths have higher energy.

Wavelengths within the visible range are only capable of exciting low-level energy transitions. Therefore an atom or molecule needs to have very closely-spaced energy levels in order for its electronic transitions to be excited by visible light. If the energy levels are spaced farther apart, the atoms would need to absorb the more intense ultraviolet radiation, which is invisible to our eyes. This is the reason why not all substances are coloured.

Electromagnetic spectrum

Ultraviolet waves are of higher energy than visible 13

COLOUR light, while infra red rays have lower energy. Along with visible light, X-rays, gamma rays and radio waves, all types of radiation which are part of the electromagnetic spectrum.

The waves all consist of vibrating electric and magnetic fields, hence the description ‘electromagnetic’. All travel with the same enormous speed through a vacuum.

This speed, the speed of light, denoted by the symbol c, is the maximum speed that can be attained and is effectively the speed limit of the universe. It is thus a universal constant and has the value of approximately 3 x 108 metre per second, or almost three hundred thousand kilometres a second. The different regions of the electromagnetic spectrum may be distinguished by their different wavelengths and therefore different frequencies, since they are related by the equation:

c = l x n (speed = wavelength x frequency) The energy of a photon is directly proportional to its frequency, so that ultraviolet photons have more energy than visible light photons.

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COLOUR between the molecular orbitals can occur, in the same way as happens with atoms.

If a molecule has very closely-spaced energy levels, it may be possible to excite an electron in that molecule to a higher energy level, using red light. If white light is shone on to the surface of a substance made up of such molecules, then the red light will be absorbed as the electrons take up the energy and the material appears bluish to our eyes. On the other hand, another material, with its energy levels a little further apart, may require the more energetic blue photons to cause electron transitions. It will thus absorb selectively blue light and appear reddish in colour to our eyes. However, as has been noted, many substances, such as water, salt or sugar, are white or colourless. This is because they do not absorb visible light, as their energy levels are too far apart and visible light photons do not have sufficient energy to excite the electrons to make the transition. Electronic transitions can be induced in these substances only by ultra-violet light, which is not visible to the human eye, so that such materials look white to us.

Colouring compounds

When we consider coloured substances such as dyes and pigments, we are usually dealing with compounds rather than elements. Compounds are substances formed when two or more elements combine in definite proportions. Refined sugar, for example, a compound of carbon, hydrogen and oxygen, is white because it reflects all colours of the visible spectrum. But ripe tomatoes are red and carrots are orange, and this is because they contain substances which selectively absorb green and blue light.

When elements come together to form compounds, their atoms interact in such a way that the atomic orbitals combine to form new molecular orbitals. The electrons are accommodated in these molecular orbitals and their energies are quantised in the same was as has been described in atoms. This process can be explained theoretically using quantum mechanics, which treats electrons in atoms as if they were waves, so it is sometimes called wave mechanics. The resulting molecular orbitals have definite allowable energies associated with them ß carotene and electron transitions 15

Organic dyes

COLOUR Organic dyes consist of molecules which have sufficiently closely-spaced energy levels for visible light photons to be absorbed. This is because the molecules possess a group of atoms known as a ‘chromophore’. This may consist for example, of a chain of carbon atoms, linked by alternating single and double bonds, as in the carotenoids, which are found in red and orange coloured vegetables such as carrots and tomatoes. The structure of Beta-carotene, shown on the previous page, is a typical example of a carotenoid. In this type of structural formula, each single straight line represents a single chemical bond and contains a pair of electrons, shared between two adjacent carbon atoms, while the double lines represent two shared pairs. There is a carbon atom located at each of the angles or branch points in the chain and at each end of the chain there is a sixmembered ring structure. There are also hydrogen atoms present, though they are not specifically shown.

Because of the double bonds, the molecule is ‘unsaturated’, meaning that it has not reached it full potential for binding (valency) with hydrogen atoms.

There are only two elements present in this molecule, carbon and hydrogen, so that beta-carotene is a highly unsaturated hydrocarbon.

The alternating double and single bonds are said to be ‘conjugated’, which means the electrons, which bind the atoms together, are not restricted to the same

location, but are highly ‘delocalised’ and are free to move along the whole length of the conjugated chain, by means of a system of linked molecular orbitals known as a π system. These π bonding orbitals through which the electrons move, have resulted from the quantum mechanical interaction between certain atomic orbitals (known as π orbitals) possessed by the carbon atoms in the chain. But quantum mechanics, also known as ‘wave mechanics’ allows for destructive as well as constructive interference of the electron waves. While constructive interference produces the bonding orbitals, destructive interference at the same time produces corresponding antibonding orbitals, higher in energy than the π bonding orbitals and denoted by an asterisk, as π* orbitals. Normally, the π bonding orbitals in the carotenoids are the highestenergy levels in the molecule containing any electrons.

They are referred to as the HOMO (highest occupied molecular orbitals). But not far above them in energy are the antibonding π* orbitals, which are known as the LUMO (lowest unoccupied molecular orbitals).

Now it so happens that it does not take very much energy to cause electrons to jump the gap between the π and π* energy levels; all that is required to excite an electronic transition in a single molecule is a photon of visible light, with a frequency corresponding to the energy difference between the π and π* orbitals.

So if sunlight, which contains all the colours of the 16

COLOUR spectrum, is shone onto a bunch of carrots, there will be plenty of photons of the right frequency available. The beta-carotene molecules in the carrots will selectively absorb these frequencies, which will then be absent from the reflected light, which looks orange.

Inorganic pigments

Another mechanism for generating colour in chemical compounds involves certain metals, from the central block of the Periodic Table, known as transition elements. Reference has already been made on page 12 to different types of Azo dye structure atomic orbitals, denoted by the letters s, p, d and f. What distinguishes transition metals is the fact that the energy levels being filled Azo dyes involve d orbitals. In the case of atoms of lower atomic A different type of chromophore occurs in the azo number, such as hydrogen (atomic no.1) carbon (atomic dyes, which contain the element nitrogen in addition no. 6), and nitrogen (atomic no. 7), the d orbitals are not to carbon and hydrogen. The molecular structure is involved, because there are not enough electrons in again highly unsaturated; containing so-called these atoms. But, for atoms with more than 20 electrons, ‘aromatic’ rings (derived from benzene) conjugated the d orbitals come into play. In fact, d orbitals occur in with double bonds between the two nitrogen atoms, sets of five, known as a d sub-shell, which can which form a bridge between the two rings. Such accommodate up to ten electrons. Some of the familiar molecules therefore contain π and π* orbitals. But, in metals such as chromium (no. 24), iron (no. 26), nickel addition, each nitrogen atom possesses a non-bonded (no. 28) and copper (no. 29), are transition metals. They electron pair (referred to as a ‘lone pair’ because it is are characterised by their ability to form coloured not involved in bonding the nitrogen to another atom). compounds, such as the reddish colour of rust and The lone pair electrons are actually higher in energy certain iron ores and the green colour developed by than the electrons involved in π bonding, so it is these copper when it corrodes on exposure to the which are very easily excited, by visible light, which atmosphere. Artists will be familiar with pigments such provides sufficient energy for them to be lifted to the as cobalt blue and cadmium yellow, which contain π* antibonding molecular orbitals. compounds of less common metals. The use of cobalt to produce blue glass has been common for centuries and 17

COLOUR transitions within the d subshell. However, this cannot occur in the case of isolated atoms, where the five d orbitals in the same subshell all have the same energy, so that no energy input is required for an electron to move from one d orbital to another. However, if a transition metal becomes chemically bonded to certain molecules possessing lone (non-bonding) electron pairs, then the d orbitals are separated into different, distinct but closely spaced, energy levels. Transitions between these energy levels can now occur, excited by absorption of visible light and colour is observed. The characteristic blue colour of copper salts, such as copper sulphate (commonly known as ‘bluestone’) is due to the fact that in the crystalline state, and in solution, the copper is bonded to several water molecules, which has the effect of splitting the d subshell of the copper atoms into two separate, but closely spaced energy levels. If copper sulphate is ground up and heated in a crucible or test tube, the water of crystallization is driven off and a white powder, anhydrous copper sulphate, is formed.

Addition of a few drops of water will instantly restore the blue colour.

The red colour of blood is due to another transition metal complex, the haem structure in haemoglobin.

This contains an iron atom in the centre of a square formed by four nitrogen atoms, which are linked by carbon chains and rings to form a closed structure,

Haem structure

the very name given to the element chromium, from the Greek word for colour, is because it forms a variety of coloured compounds.

The reason for the colours found in transition metal compounds is due to the absorption of visible light photons of the correct energy to excite electronic 18

COLOUR known as a porphyrin, as shown in the structural formula to left.

Porphyrins are very important biological molecules.

Other examples are vitamin B12 (cyanocobalamin), which contains a cobalt atom in the centre of a porphyrin ring and the green pigment chlorophyll,

found in green plants, which is responsible for photosynthesis and which contains a magnesium atom at the centre.

But, of course, we couldn’t appreciate any of this if we didn’t have the sensory apparatus to distinguish different colours.

Green energy

This, however, is just one part of the process, and during what is known as the dark phase of photosynthesis, energy is trapped for use in a whole range of chemical reactions. One chemical, ADP, uses the energy to bond with an inorganic phosphate, becoming ATP. The bonding is reversible, so ATP acts as a very efficient energy carrier. The ADP-ATP-ADP transfer of energy is, in fact, one of the basic driving processes in life, and without it we could not, for example, flex our muscles.

Chlorophyll, extracted from plants such as nettles and spinach, is classified as Natural Green, and this dye is used extensively in inks, soaps, waxes, cosmetics, perfumes and mouthwashes.

P

lants are green because they have chlorophyll, the pigment that enables them to absorb light in the red region of the spectrum.

Chlorophyll absorbs light in two peaks, 650 microns in the red, and about 430 between blue and violet, so the green we so enjoy, lying in between, is rejected.

Many materials absorb light only to release the energy immediately, but the energy absorbed by chlorophyll is trapped and channelled into a series of chemical reactions. Using the energy of photosynthesis carbon dioxide from the atmosphere and water are converted into oxygen and carbohydrates, such as starch and sugars.This reaction can be represented in the chemical equation CO2 + 2H2O CH2O + O2 + H2O

Photo: Nettle leaf

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COLOUR see how this works in projection. Next time you go to a live show in the theatre watch how the coloured spotlights on stage merge. In the same way, if two slide projectors are set up, one with a green filter, and one with a red filter, superimposing the beams produces yellow on the screen by addition. Instead of taking from each other, as in paint, transmitted colours add up. Mixing all the paints would eventually, in theory, make black, but all the colours in transmission would eventually give us back the white light we started off with.

The old idea that a full range of colours could be produced by mixing a limited range of pigments was first seriously questioned by Isaac Newton. His book, Opticks, published in 1704, annoyed artists because he pointed out that paints do not behave as true primaries.

Furthermore, he revealed that yellow, traditionally held to be a primary, is in fact a mix of blue and green.

Artists were not impressed because in practice bright yellows often mixed in well with other colours to produce satisfactory results. At the time most people simply could not accept that there was a significant difference between transmitted colour, which adds up, and the colour we see by reflection, which, by its nature, is subtractive. So Newton was dismissed as a dreamer, his colours, unlike the real paints of the artist, were regarded as a clever optical illusion.

Adding and subtracting

W

hen we see objects around us we are looking at colour by reflection. Light itself has no colour, but when it falls on objects some wavelengths are absorbed, and the other wavelengths are reflected.

Thus, the reflected light we see as colour is the result of subtraction. If all the light was reflected we would see pure white. Plants are green because they take the red out of light and soot is black because all the colours are absorbed — there is in fact nothing left for us to see.

Because the colours we see reflected from objects are the result of subtraction, mixing dyes or paints can lead to some very odd results. With every mix there is subtraction, so the total amount of light reflected decreases, and for a long time this caused a lot of confusion because everyone assumed that paints and dyes could be used and mixed as if they behave like pure colours. As anyone who has ever dipped into a box of water colours knows, mixing paints usually produces dull and disappointing results.

The problem is with the paint, and not with the light itself. When light is transmitted, rather than reflected, absorption is no longer an issue, and we can 20

COLOUR Painters stuck with yellow as a ‘primary’ for the simple reason that it is bright, so in practice it works.

Printers also follow this tradition, and they make up their colours from magenta, cyan, and yellow, and while the first two hues look chemically raw on their own, magenta with yellow gives a vivid red, and cyan with yellow gives a saturated green. These reds and greens are common in tabloid headlines because they can be created directly from the inks without the necessity of screening. Painters and dyers, likewise, have found clever ways to compensate for loss of brilliance. Artists often work in some extra blues and contrast to make paintings more vivid, and likewise a

hint of blue was traditionally added to whitewash, making it brighter because our eyes are more sensitive to this end of the spectrum.

An object appears red because the green and blue are absorbed. Likewise, if red and green are absorbed we see blue. By mixing paints we reduce the amount of light reflected. With transmitted colours, shown above, we get addition, so red, green and blue combine to give us white.

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COLOUR made up from cyan, magenta, yellow, and black, Le Blon was reproducing works of art by printing from four plates, red, yellow, blue, and black to give depth.

Although the colour wheel was a practical aid, it was of no use in classifying hues by saturation and depth. The colours we see can be darker, lighter, or more saturated than those shown on the wheel. A German astronomer, Tobias Mayer made up for that deficiency by arranging the three colours, red, yellow, and blue into a triangle and introducing another dimension, representing depth. Mayer went to a lot of trouble, first by arranging the grading of colours from one to the other into a series of steps, each of which could be defined as a mixing formula, such as ten of red, two of yellow and one of blue. To this two dimensional triangle, he then added depth, darkening each colour of the triangle in series of steps. In 1775, when Mayer published his Trichromatic Mixing Triangle, he was able to define 819 different colour mixtures.

One of the problems with Mayer’s system was that it could not be represented in the same way as a flat colour wheel, and a German astronomer, Johann Heinrich Lambert attempted to get around this difficulty by painstakingly constructing a wax pyramid incorporating all these colours.

Defining colours

T

he experience of seeing is highly subjective, and defining colours with a high degree of accuracy only became possible when a system of classification based on the additive primaries, red, green and blue, was developed. Various attempts to code colours have been made, and while some of the names invented by paint makers are highly descriptive, such as burnt opera house for an ochre red, or elephant’s breath for a grey, these terms could hardly be called objective.

In the early 18th century Isaac Newton observed that transmitted colours could be mixed to produce others, and although he explained that paints can not be combined in the same way, his arrangement of hues into a circle had an immediate and lasting appeal. The arrangement is very useful because it gives a good idea of how colours are related to each other.

The notion that primaries can be combined to produce other colours inspired a German printer, Jakob Christoffel Le Blon, to devise a variation on the colour wheel of just three hues, red, yellow, and blue, which he employed to good effect in his work. Long before printers adopted what we now know as process colour, 22

The colour wheel gives a good idea of how colours relate to each other. By subtracting the colour from one side of the wheel we get the opposite hue.

Colour has hue, saturation, and brightness, and three dimensional modelling, although harder to visualise, led to more accurate systems of classification.

COLOUR Back to basics

Although the concept of describing colours had been worked out by Meyer in the 18th century, the Scottish scientist, James Clerk Maxwell improved the concept with his trichromatic mixing triangle, based on a choice of three primaries; scarlet, emerald green and ultramarine. Like Meyer, Maxwell made a triangle mixing his primaries in steps, but he discovered that some of the more intense hues we see in life could not actually be contained within the triangle. For example, no possible mixing of his scarlet and emerald green could match a particular intense yellow, gamboge, commonly used by artists. It was just not possible to produce the required intensity with existing paints. So, Maxwell compensated for this by calculating how much grey would have to be added to make these intense colours come back inside this triangle. It was an unsatisfactory compromise, and Maxwell’s trichromatic mixing triangle remained flawed until the physically ‘real’ primaries were simply discarded and replaced by mathematically defined alternatives. The mathematical model immediately had an enormous advantage in that it was no longer limited to less than perfect primaries.

In 1931 the Commission Internationale d’Eclairage, a body more often simply referred to as CIE, developed a standard model, the XYZ space, based on Maxwell’s trichromatic triangle. The X and Y are

The standard XYZ model developed by the Commission Internationale d ’Eclairage is based on mathematically defined colours, and so is not limited by less than perfect primaries.

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COLOUR simply vertical and horizontally defined positions, light, so the setting on the meter can be changed to and the Y represents overall brightness. In this the compensate. For example, the white can be nudged primaries were treated as equals in intensity, so the towards the red if colours are being viewed indoors. white is close to the centre of the triangle. Later, in 1976, the model was changed so that representation of Hands on primaries comes closer to the average human While the three dimensional model is scientifically sensitivity. In the standard CIE model the top edge of sound, and provided the foundation for the current the triangle spans wavelengths from 750 to 520 nm, CIE standards, artists and designers have always been and the left edge spans from 520 to 400 more at home with charts and swatches. nm. As a result of all these changes the While scientists were perfecting the original straight edges of the triangle mathematically defined models, become skewed, and in the 1976 LAB painters continued to look for colour space, the white area has moved something they could get their hands nearer the top edge.The L in the name on, and in 1905 an American artist, A H denotes lightness, while a and b are the Munsell, devised a colour classification co-ordinates defining hue. system using card arranged by hue, The big advantage of the CIE models lightness, which he called value, and is that they enable us to analyse colour saturation, which he referred to as objectively and unambigiously by chroma. The cards became popular as machine. This can be a colour meter the Munsell Renotation system. Cards with just three colour sensors, or a more and swatches have always remained advanced spectrophotometer which can popular, and many graphic designers, measure a whole range of parameters when they need to have a very specific including hue and reflectance. colour in a print job, choose from the In addition, the model itself can be extensive Pantone range. This is a bit Colour cards used by printers to adjusted to take different light sources like the colour cards in hardware shops, pick out inks for spot colours into account. Compared to daylight, but far more extensive, with hundreds household tungsten bulbs give yellow 25

COLOUR of colours and shades. Picture processing programs always show the equivalent on screen, and these automatically translate into RGB or CMYK versions ready for printing. The colour match although quite good, is limited to the range possible from CYMK separations. In big printing jobs where the precise rendering of something like a company logo might be important, graphic designers often specify a particular Pantone reference for that particular colour, and this is made up as a special ink. Thus, in addition to the process cyan, magenta, yellow, black, there would be an additional spot colour. Spot colours are often used with black, and this two colour printing is often more attractive and elegant than full process. Many of the classic designs, particularly in book work, are two colour, and part of their attraction is that lines and type, rendered in solid, unscreened inks, are sharper.

From space we see a colourful world and as light travels through the upper atmosphere it becomes visible as blue.

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COLOUR true solution. In a true solution a beam of light passing through will not be visible, but if colloidal particles are present, although invisible to us, the beam of scattered light can be seen. The same effect allows us to admire the sunbeams in a darkened church.

Tyndall correctly concluded that extremely small particles in the upper atmosphere must, in a similar way, be responsible for scattering the shorter, blue waves. In the atmosphere the scattering is caused by molecules, which are much smaller than dust or water droplets, The Tyndall effect, as it became known, occurs widely in nature. The intense blue in some birds is due not to pigments, but to scattering by the fine structure of translucent feathers. Likewise the iridescence of many highly colourful insects is due to scattering and interference as intersecting rays interact. In birds scattering is often combined with pigmentation, and brilliant greens, for example, can be the result of the Tyndall effect over a yellow colouring, and in other birds surface texture can diffuse and mute the tones.

The whiteness of clouds is also due to scattering from the millions of suspended water droplets, which, because of their relatively larger size scatter all wavelengths and so we see white.

Rooftops and blue sky in Brussels by Vorsin Dmitri

Why is the sky blue?

I

n space the sky is black because there are no particles to reflect rays of light. If outer space is dark, why is our sky blue by day?

It was a Carlow born scientist, John Tyndall, whose observations led to an explanation of why rays of light entering our atmosphere become visible. During his experiments he noticed that finely divided particles suspended in solution scatter light, making it visible to us as colour. In size these colloidal particles can be extremely small, somewhere between those of a suspension, which tends to settle out, and a 27

COLOUR He experimented with light, and, on finding that light cannot be seen passing through clear water, concluded that the rays only became visible when they bounced off particles.

Different particles, he found, scattered light in different ways, a discovery that led to the invention of the spectrometer, an instrument used in the analysis of materials.

Tyndall also found that light could be sent through a pipe, a discovery that ultimately led to the development of fibre optics.

Always busy, he finally found time aged 56 to marry Louisa Hamilton, but plagued by insomnia his health deteriorated and he died in 1893

John Tyndall

J

ohn Tyndall, son of a constable, was born at Leighlinsbridge, Co Carlow in 1820. Encouraged by his father and a good tutor, John became a surveyor, working for a while in England where he got into trouble for protesting at Tyndall drawn by G Richmond how Irish labourers were being treated during in 1864 construction of the railways. In 1847 he took up teaching at Queenswood College in Hampshire, where again his independent attitude led him to argue that scientific thought is of more value to science than belief. His lecture on the subject, given in Belfast, received a cool reception by the clergy. He completed his doctorate at Marburg, Germany, in just two years, and at the same time found time to become well known as a mountaineer. Tyndall was a well rounded person, and like a number of scientists of that time, he was an excellent communicator, and his public lectures were popular.

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Like many fellow countrymen Tyndall was no republican, and after remarking that he would ‘without hesitation, shoulder my rifle with the Orangeman” the humourous magazine, Punch, ran this cartoon, titled The scientific volunteer.

COLOUR largest redshifts. This effect is know as the cosmological redshift and was discovered by Edwin Hubble in 1929. At first the redshifts were assumed to be Doppler shifts caused by the galaxies moving away from us through space. A better interpretation is that space itself is expanding and clusters of galaxies are swept along by the general expansion which is known as the Hubble flow. Within each cluster, gravitational interactions lead to local deviations from the Hubble flow; this effect is very obvious in our local group of galaxies some of which have blue-shifted spectra.

We now understand that the cosmological redshift is due to the general expansion of space. When light waves were emitted long ago from a distant galaxy, their wavelengths were as they would be in the laboratory. But as space expanded, the waves lengthened with it so they now appear red-shifted.

The more time the waves have travelled, the longer they have become. The cosmological redshift allows us to estimate the distance of very remote galaxies and to test various cosmological models.

The cosmological redshift

O

bservations of the optical spectra of galaxies led to one of the most important discoveries of the 20th century – the expansion of the Universe – and to our current theory of its origin, the Big Bang.

The spectrum of a star is crossed by dark lines due to the presence of chemical elements in the star’s outer layers. The wavelength or precise colour of each line can also be measured in the laboratory and compared with the dark lines in the star’s spectrum. If the star is moving towards us, its spectral lines are shifted bluewards and if away from us then red-wards; this can be understood in terms of the Doppler effect familiar to us from listening to moving sound sources and hearing the resultant squashing or stretching of the sound waves as a change of pitch.

When astronomers examined the optical spectra of galaxies each containing many billions of stars, they found that the spectral lines were nearly always shifted to the red or longer wavelengths, with the faintest galaxies (i.e. the most distant) having the

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COLOUR

Raleigh scattering

A

quantitative explanation for the blue sky phenomenon was given in 1871 when J W Strutt (later Lord Raleigh) constructed a mathematical model which showed that the intensity of the scattered light was inversely proportional to the fourth power of the wavelength. Since red light at 7200Å has a wavelength 1.8 times greater than violet light at 4000Å, the relation predicts that violet light will be scattered 1.84, or about ten times more strongly than red light. Thus colourless air molecules cause preferential scattering of blue light and account for the blue colour of the sky. At sunrise or sunset the path length in air is so great that virtually all the blue and green light is scattered and we see the Sun as a red disk.

Raleigh scattering can also help us to study the distribution of dust in the spiral arms of our galaxy, the Milky Way. In 1931 the American astronomer Robert Trumpler noticed that the stars in distant clusters appear redder than similar stars in nearby clusters. The reddening is due to small dust particles which scattered blue light much more than red light. We deduce from this that the interstellar dust grains are smaller than the wavelength of visible light. The reddening gives a measure of the amount of dust in the line-ofsight.

Reddening caused by scattering should not be confused with the red-shift of spectral lines of distant galaxies. The cosmological red-shift is due to the expansion of the Universe.

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Milky Way, the Cygnus region with dust, and ionized gas. From Canadian Galectic Plane Survey.

COLOUR on the type of gases, the amount of energy they absorb, and magnetic forces, so the Northern Lights change, flicker and vary to create spectacular patterns of the Aurora Borealis.

The Aurora Borealis is best seen in northern latitudes, where the charged particles are drawn down towards the magnetic pole. Both poles attract particles, so in the Southern Hemisphere we have the Aurora Australis.

The colourful skies at night only become visible from Ireland during peaks in solar activity, which follow an eleven year cycle. The next solar upheaval is due in 2011. The Aurora Borealis, like the shimmering stars, is actually there all the time, but in the bright light of day the colours are just too faint for us to see.

Northern lights

A

part from light the Sun radiates a solar wind of charged particles. These clouds of plasma take about three days to reach the Earth, where most of the charged particles are repelled by the magnetic field.

The more energetic particles, however, penetrate the Earth’s magnetic shield and on meeting atmospheric gases molecules are excited and on dropping back to their ground state light is emitted. The colour depends 31

COLOUR into a Froth, that is into an Aggregate of Numerous small Bubbles, whose convex Superficies fits them to Reflect the Light every way Outwards.”

This whiteness contrasts with the colours of individual soap films observed by Isaac Newton and Robert Hooke. They also noticed the appearance of small black spots, now called Newton black films, when the soap film is allowed to drain for some time.

Their experiments can be easily repeated by dipping a loop of metal wire vertically into a solution of ordinary washing-up liquid and water. Slowly withdraw the loop and gaze in wonder at the soap film as it changes its appearance. The washing-up liquid contains surfactants, that is molecules which prefer to locate themselves at the surface of the film, and enable it to survive intact for a little while.

As liquid drains out of the soap film, its shape becomes that of a thin wedge. This is revealed by a pattern of slowly shifting coloured stripes similar to those seen in an oil slick on water. Eventually small black dots are visible moving upwards, but one needs a strong and quite viscous soap solution to observe this. Generally the film thins very quickly and bursts, ejecting many tiny droplets.

Those beautiful colours deserve some more thought. Light that falls on the front surface of the soap film is partly reflected, but some of it also enters

Soap films Stefan Hutzler explains the physics behind Tim Durham’s brilliant photographs.

T

hese are not computer-generated images, but are real photographs of real soap bubbles, at home as much in the art gallery as at a scientific conference. A scientist might well be asked how they can be explained? The answer brings us from the realm of art into physics.

The first known display of soap bubbles in art appears to be that on an Etruscan vase. The theme of children playing with soap bubbles used to be quite common in painting, and can be seen for example in the work of Jean Simeon Chardin (1699-1789) or Edouard Manet (1832-1883). Often this theme was chosen as an allusion for the transience of life, and it recurs in poetry as well.

Irish-born scientist Robert Boyle noted in 1663 “That the White of an Egg though in part Transparent [...] being long agitated with a Whisk or Spoon, loses its Transparency, and becomes very White, by being turn'd 32

COLOUR

the film and some of this in turn is reflected at the back surface, and so on. The effect of all this to-and-fro motion of light waves is to cancel out some colours, while leaving others, the phenomenon we know of as constructive or destructive interference. The colours selected depend on the thickness of the film. Any part of the film with a width much smaller than a quarter of a wave length of visible light is black in reflection: all colours are cancelled. Such films have a thickness less than 30 nanometres, about 2000 times thinner than a human hair. Evaporation of liquid can decrease this

further to about 5 nanometres. Since nanoscience is a current buzz-word, it might be surprising that the nano-world can already be probed with simple soap films.

At the same time as the black film grows at the top, where the film is thinnest, the observed colour patterns grow in complexity and black spots also appear in the lower sections of the film and move upwards. The detailed explanation of all of this was first given by Karl Mysels and others in the 1950s.

The wire frame itself has an important role.

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COLOUR Drainage is faster at the edges where the film is attached to the frame in so-called Plateau borders. As liquid is drawn into the Plateau borders thinner regions are created which, being lighter, move upwards in the film.

Thermal convection can also play a role. In preparing his photographs, Durham at times

deliberately introduces a heated surfactant solution.

This also leads to increased evaporation of the liquid and changing the surfactant also affects the moving patterns.

Soap bubbles, like a film of oil on water, have extremely thin transparent walls, and the colours we see are caused by interference between light waves reflected from the outer and inner surfaces.

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Blowing bubbles Bubbles do not have to be small, and Sue McGrath, well known for her lively demonstrations of science in action, explains that giants can be produced from common wash up liquids.

I

t is impossible to blow bubbles with just water because the surface tension is just too strong. By adding detergent surface tension is reduced, typically to a third of that of pure water. Detergent molecules are composed of long chains of carbon and hydrogen atoms. At one end of the chain is a group of atoms that like to be in water, and at the other end, a group that are hydrophobic, but attached easily to grease — hence their usefulness in washing dirty dishes.

In a detergent solution, the hydrophobic ends try to escape from the water, and those that successfully manage to squeeze their way to the surface do so by pushing water molecules away from each other, so the surface tension, caused by mutual attraction of water molecules, decreases. Because the hydrophobic ends face out, the bubble is protected from evaporation, one of the main reasons for bursting. Adding glycerine

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helps, and it is also important to avoid dry surfaces. To catch a bubble you must first dip your wand or hand in the solution. A wet nail will not puncture a bubble!

In areas where water is soft, tap water is fine, but otherwise it is best to use distilled or rain water.

One of my favourite recipes, ‘the Sploose mixture’ is one litre of Fairy Liquid, three quarters of a bucket of rain water, and half a cup of glycerine or glucose syrup. With this mix, really big bubbles can be produced, and a handy trick is to seal everything in with a quick twist of the wrist.

COLOUR

780

nm

VISIBLE LIGHT

10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14

hen light passes through a transparent body, such as water or glass, it slows down, so it is bent, or refracted. The different wavelenghs pass through the medium at different rates so they separate out into different colours. We perceive these colours as red, orange, yellow, green, blue, indigo, and violet.

When light passes through a convex lens the rays are refracted in much the same way as in a prism.

Thus the shorter waves of violet are refracted more than the longer waves of red. Convex lens, as in a magnifying glass, or our eye, brings the rays from an object into focus to make an image. However, because reds are refracted more than violet, the colour image has no clearly defined plane of focus. In a camera such a simple lens produces images with a halo or fringe of colours, known as chromatic aberration, so a second lens element, concave on one side, is cemented onto the first to provide correction.

The eye does not have this correction, so it is not possible to focus sharply on a violet and a red object at the same time. We are never aware of this halo because in perception chromatic aberration is simply ignored.

Visible light is just one tiny band within the electromagnetic spectrum which ranges from waves longer than a metre used for broadcasting to the extremely intense X rays and high energy Cosmic rays

SHORT WAVES

1 metre

W

Refracting LONG WAVES

Wavelength 102

380

nm

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COLOUR

FALSE COLOURS

B

y looking through instruments such as telescopes, cameras, and microscopes we can extend our sight. Our eyes are sensitive to the primary colours, but cameras and electronic sensors are not limited to these narrow bands, and indeed in astronomy wonderful images, revealing structures in deep space, are produced from long wave data gathered by radio telescopes. Thus waves we cannot otherwise see are rendered visible.

Large radio telescopes provide us with another way of looking into space. In this image we see a pattern of radio emissions near the centre of our own galaxy which are believed to follow magnetic fields.

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Two high energy jets emerging from opposite sides of the nucleus in a large elliptical galaxy. The galaxy is 350 million light years away from Earth, and what we see here is an image of the radio emmissions around the galaxy itself. Optically, the galaxy would be one tenth the the size of the radio image. Data to generate this image came from a large array of 27 linked radio telescopes. (Image: NRAO/ AUI)

In this galaxy the radio emission forms a complex double-lobed structure with narrow jets. In the lighter areas, extremely high energy streams are thought to be colliding with cooler surrounding gases.

By combining the data from different radio telescopes around the world resolution of detail becomes greater. Promoters of the ARTI ( A Radio Telescope for Ireland) project believe that Birr in County Offaly would be an obvious choice for another location.

In the mid 19th century Birr had the largest reflecting telescope in the world. William Parsons, the Earl of Rosse, built the telescope and discovered the first spiral nebula.

A radio telescope at Birr would continue a great astronomical tradition, and it would enable astronomers to generate and analyse more images like these from the depths of space.

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COLOUR Data processing makes information visible, such as the contours and depth of the continental shelf.

Geological Survey of Ireland.

Opposite page: Looking down on Kilmore Quay, Co Wexford. a false colour view in infrared highlighting details that would not be as clear in true colours. From the GSI/Marine Institute coastal survey.

A Siemens computer tomograph scan revealing the innards of a living body. The scanner rotates around the body in just 0.33 seconds, capturing details of less than 4 mm.

Image: Siemens 40

COLOUR

NATURAL colours

P

lants have a reason to be showy. Flowers attract pollinators by advertising with with colour Above, Ragweed, Cloonlara. and scent. Colourful blooms Photo: Tom Kennedy attract the insects, and in much the same way, bright Mistle thrush attracted to the red berries berries draw the birds so on rowan. Photo: Richard Mills they can eat their fill, fly away, and deposit the seeds elsewhere.

The predominant colour in plants is of course green, and this too is for a reason. The green is from light trapping chlorophyll. Plants take energy from the longer end of the spectrum, so red is absorbed and green reflected.

The flower colours are there for display, and sometimes petals have coloured stripes which act as guides for visiting insects. pointing the way towards the nectar.

Leaves, stems and other plant parts are often coloured, and 42

COLOUR chemists have classified a number of woollens red. A crimson was produced these pigments. The oranges, yellows and from the pulp of common privet berries, reds are often from the widely distributed Ligustrum vulgare, and when alum was carotenoids. Carotenoids are synthesised added, this became a green for dying silks. not just in higher plants, but in bacteria, The flowers from the all too common algae, and fungi. We also need ragwort, Senecio jacobea, were once used to carotenoids, for example for rhodopsin, make a yellow dye for wool previously the visual pigment in our eyes, but unlike treated with alum. This associated plants we cannot synthesise them, so we treatment was often necessary to fix the depend on eating greens. colouring and prevent rapid fading.

The reds of young shoots, autumn Two of the most widely used plants leaves and beets usually come from were Indigofera tinctoria, and Isatis another group of chemicals, called the tinctoria, These plants were a source of a anthocyanins. Other common groups are glucoside, indican, which produces the the anthroxylanins, which occur in onion deep hue of indigo or, as in ancient skins, and the flavenoids which can occur Britain, woad. Elizabeth I was among in a range of colours. those who objected to the stench of Before synthetics, these colours were preparation, and apart from refusing to widely used in dyeing materials, and visit any towns where woad was being some plants were grown for the colouring. prepared, she banned processing within Rubia tinctorum was the source of five miles of any royal estate.

Rubia tinctorum madder, a red dye.

Natural indigo is thought to have Source of madder At one time local lore about plant been in use for at least 5000 years, and it dyes was strong, and common plants, was popular with the Sumerians. such as Lady’s Bedstraw, Galium verum However, this came to an end in the 19th were gathered for this purpose. In Ireland there are century, when Adolf von Baeyer worked out the folklore records of the stems and leaves being used to chemical structure, and the synthetic substitute produce a yellow dye, and the roots were used to dye completely displaced the natural dye, cutting trade 43

COLOUR annatto, extracted from Bixus ocellana seeds and sold commercially as a tasteless powder classified as the food additive E160. Annatto is used extensively in margarine, Bixus ocellana is known as the lipstick plant because annatto is so popular with the manufacturers of cosmetics.

Natural colourings such as henna and chamomile, traditionally used to colour or brighten hair, are now a big selling point in modern shampoos.

links which had endured for a thousand or more years. Synthetic indigo was itself in danger of being overshadowed by the more colourful chemicals, but deep blue is one of those colours that never seems to go out of fashion, and with the popularity of jeans keeps demand for the dye high.

The dark, purple black berries of elderberry, Sambucus nigra, have been used traditionally to deepen the colour of wines. In some areas, such as Kent in the 16th century there were plantations of elder grown for this purpose. The practice meant that the appearance of indifferent wines could be improved, and in Portugal adulteration of port became such a problem that at one stage the growing of elderberry trees was banned.

Elderberry trees are still grown extensively in Denmark and Germany and the berries are harvested to prepare a red food colouring dye.

Another natural colouring is

Elder berries, common in Autumn, and yielding a deep colouring for food and wine.

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Isatis tinctora Source of indigo

Ragweed, Senecio jacobea, was once used for dyeing wool yellow.

Lady’s Bedstraw, Galium verum, close relation to the madder plant, Rubia tinctorum. Widespread by paths, Lady’s Bedstraw was also used as a source of red dye.

Photographs Tom Kennedy

COLOUR

Animal colours

A

nimals use colour to lure, identify, hide or warn.

The peacock’s colourful feathers are spread out in a display to impress the mate. The black and yellow stripes of a wasp are so effective that other insects sometimes mimic the colouring for protection. Stripy tigers stand out in the zoo, but in the jungle merge into the background. Birds know that the orange and black Monarch butterfly is not good to eat, and a different species, the Viceroy butterfly has copied the pattern to avoid becoming a meal.

The colour of an animal can come from pigments, or they can be the result of iridescence. The brilliant, metallic greens and blues of some insects is due to the scattering of light, and colourful snakes sometimes mix a bit of iridescence with a splash of pigment.

The pigmented chromatophore cells often have a reflective backing layer which serves to enhance the brilliance of colours. A granular substance, guanin, closely related to uric acid, is abundant in fishes where it accounts for the silvery appearance. Whether the guanin is granular or crystalline determines how much and how colours are displayed.

Pigments in animals are classed into a number of

Researchers in industry have been looking at insects to find out how to mimic the dazzling colours in consumer products. Photo: BASF. chemical categories, of which carotenoids are among the most common. Carotenoids are seen in the yellows, oranges, and reds of many invertebrates, but they can also be blue or purple. Other places where carotenoids occur are in the skin of goldfish, and in the pink flesh of salmon.

Some animal pigments are chemically similar to haemoglobin, C758H1202N195S3FeO218. Haemocyanin is a copper containing pigment, the blue blood occurring in mollusks and arthropods where it acts as an oxygen 46

COLOUR carrier in respiration.

Coloration can change, and it can be under hormonal control. Some animals, such as the octopus, can change colour within seconds. The change can be due to contraction or expansion of pigment containing cells, chromatophores.

Diet can be a big influence, as fish farmers who have to supplement feed know. Without a supplement salmon raised in captivity would be too pale for the market. Without a special diet flamingoes in the zoo would not remain pink.

Aquatic animals are often highly coloured because they have a plentiful supply of flavinoids from algae.

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Some natural colourings Melanin. Dark pigmentation, the main colouring agent of human skin and hair.

Sclerotins. Dark pigmentation, often found in eggshells and crustaceans. They are generally associated with hardening of materials.

Porphyrins. Mostly occur as pinks and reds in eggshells and sea shells.

Bilichromes. Usually green, and occurring in insects and egg shells.

Bilirubin. Reds when concentrated, and yellow if dilute. Occur as the reds in skin and eyes.

Biliverdin. Occur as the blue and blue-green of egg shells and some sea animals.

Pterins. Occur as whites, reds or oranges in insect wings.

Quinones. Occur as pinks to deep red in sea urchins.

Carotenoids. Widely distributed, yellows, reds or oranges. Can also be blue or purple.

Carotenoids occur in the skin of goldfish, and in the pink flesh of salmon.

COLOUR luciferase, that catalyses the oxidation of a substrate, luciferin. The chemical composition of the luciferin varies, so the term is used to include a number of substances that act in a similar way, and of course, some are more reactive than others. As the luciferin is oxidised protons are released, which we see as red, blue, green, yellow or orange light. Different species emit their own distinctive light, and in one type of firefly, Photinus, the yellow-green colour goes red when conditions become acidic.In another marine species, the hydromedusa, Aequorea, the process has been further refined to produce green and blue lights.

Calcium in the luciferin makes the light blue, and this in turn acts on a fluorescent protein, which glows green.

Bioluminescence is thought to have evolved separately in different organisms. One of the biological oddities is a deep sea fish, Malacosteus niger, which emits a red light that no other fishes, except itself can see. Thus the prey never knows it is being observed until Malacosteus makes the kill.

Fireflies, which are beetles, not flies, signal to each other in synchronised flashes. At least 2000 different species of fireflies are known, and each has a distinctive code in which males flash and the females respond.

Bioluminesence

R

ather than rely on reflection or even refraction, some animals and fungi actually glow. Fireflies and many deep sea creatures emit light, a process known as bioluminescence. In a world flooded with artificial light, we do not always realise that bioluminescence is fairly common in nature. In some regions the sea can be seen to glimmer, as millions of tiny Odontosylis fireworms organisms swim to the surface in huge numbers signalling in rhythmic flashes. When the rock-boring Piddock mollusc, is disturbed a substance named lucifern, in the presence of a catalyst, luciferase, reacts with oxygen to generate light. Plankton, in tropical regions often glow when disturbed, so when bays shimmer it is not an optical illusion. In deeper waters fishes often glow and some, like the angler fish, Anomalops katgoptron, are specially equipped with a long appendage from which dangles luminescent bait. In this case the glow is produced by symbiotic bacterial passengers inside the tip of the fish’s appendage.

The light is usually produced as an enzyme, 48

Top left: A new species of jellyfish collected in the Canadian Arctic region.

Photograph: Kevin Raskoff, CoML

Main picture, Aeginura grimaldii, a jellyfish species living in depths from 500 to 1000 metres.

Photo: David Shale

The ocean depths are dark but the inhabitants are surprisingly colourful and many are bioluminescent

COLOUR

Animal sight

W

hen animals look, do they see what we see? The simple answer is no; birds, mammals, fishes, and insects have their own distinctive view of the world, and information from other senses, such as smell, or heat sensors can be more important to them than sight. In dogs and cats detection of movement is more important than colour, and in high flying hawks the eye can resolve extremely fine detail, such as a mouse creeping through the grass a hundred metres below. A blind cat can still catch mice. The mosquito is guided to its target by body heat.

In mammals and birds the overall structure of the eye is similar to our own, but the proportion of rods and cones varies according to what the animal needs to see. In nocturnal animals, such as the owl, rods predominate, while the retina of the hawk is made up entirely of cones.

Many insects, such as bees, are drawn to flowers reflecting ultra-violet light. When out foraging bees have no need to pause in contemplation of the landscape, but instead head for the pollen. When we

look at the white petals on a daisy we see the distinctive ring of white, but to a flying bee this stands out as a vivid halo of ultra-violet.

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Are bats blind?

A common myth because bats use echo location instead of sight to catch flying prey. Bats have sight like other mammals, but they hunt at dawn and dusk when light is dim. They emit high pitched sounds that bounce back from objects. The echo enables bats to locate flying insects at high speed and with great precision.

Drawing by Gregory O’Corry Crowe Colour is obviously important in birds, like this cock who asserts himself with a brilliant show of red.

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COLOUR Infra red cameras do not depend on visible light, so they can record heat emitting images in the dark. We radiate heat, so even in the dark people show up in ghostly outline. Police in helicopters use infra-red cameras to track their quarry from above.

Thermal image of an engine

Colour temperature

Pouring white hot metal into moulds is a familiar image, and from it we know that hot objects can glow like the tungsten filament in a lamp. Heating any object will cause it glow, first dull red, then orange, yellow, and then white.

One way of measuring light is by refererence to the temperature of the source. Instead of using degrees centigrade, the absolute Kelvin scale is used. In photography this is referred to as the colour temperature, and and normal daylight films are made to give best results at 5500 degrees Kelvin, this being the colour temperature of the mid-day Sun. The colour temperature of a household bulb is much lower; for example, a 100 watt bulb is about 2900 °K. To compensate for this, and remove the yellow cast, photographers use a blue filter. Sometimes sources of light have too high a colour temperature, so an amber filter is used to mellow down the image. Modern films are a lot more forgiving than they were in the past, but in film making it is usual to use different stocks for indoor or outside shooting. Of course electronics have made this distinction less important, but, even so, recording of colours is strongly influenced by the colour temperature of the light source.

Looking at heat

A

ll objects above absolute zero, -273ºC, emit heat in the form of infra-red radiation. Although infra-red radiation, like light, is part of the electromagnetic spectrum, we feel rather than see these rays. However. with special films and sensors the infra-red rays of heat can be made visible. Satellite images are often in infra-red because some details are clearer and the longer wavelengths are better at penetrating cloud cover.

Infra-red cameras are also used to photograph buildings and machinery to see where heat is being lost. Specialised heat sensitive cameras are used to check the efficiency of insulation in industrial plants, and the same approach is often used to check wiring in electric circuits.

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What do you expect to see? Look at the white faced lady on the right, then look at her again on the left and see her blush. Our mind is very good at filling in the missing gaps, even if there is nothing at all there.

Based on a drawing by Aubrey Beardsley 53

COLOUR white often appears to be tinted by the complementary colour.

Complementary colours, when projected one over the other, produce white light. Every colour has its complementary, and like positive and negative, the two together always give white, yet colour contrast is highest when complementaries are paired.

After looking at a bright colour, its complementary will appear more vivid. After looking for a while at red for example, greens will appear stronger.

After looking at a bright coloured object, an afterimage can persist when eyes are closed or lights are put out. However, if instead of darkness, the eye is directed to a white surface, a negative complementary colour afterimage may be perceived.

Artists and home decorators, often without understanding why, make use of how we perceive colours. Blues added to shadows, for example, make painted scenes more vivid. Artists simply come to know through trial and error and experience that certain colours work together to accentuate atmosphere. While hobby painters might try to reproduce colours as in a photograph, real artists use colour as a creative tool.

J M W Turner’s brilliant skies are dramatic, and likewise the great strength of Paul Henry’s western landscapes lies in his distinctive use of colour.

Subjective sight

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hat we see depends on the lighting. Of course we need light to see, but not all light is the same.

Home lights are yellow compared to daylight. Cold overcast days are indeed quite blue, and it is only in bright sunlight that we see colours at their best. In normal life we hardly notice these differences because our perception compensates for all but extremes. Colour films cannot adjust in the same way, so photographs taken indoors without flash usually look like they were taken through an amber filter. Without our ability to normalise, such photographs give a good idea of what we should actually see.

Wise shoppers realise that the goods on display do not always match up to expectations. Lighting inside a shop can be quite different from the lighting out on the street, and for this reason its always a smart idea to look at colourful materials in daylight.

The colours we see are influenced not just by light, but by where we see them. A small sample of wallpaper, for example, can look pale and insipid, but when spread over a wall the colour can become surprisingly intense. The same applies to paint.

Our perception of colour is also influenced by the surroundings. When a colour is placed beside white, the 54

Artist Florence Gruben combines complementary colours to produce her characteristically vibrant paintings.

Whether with oils or watercolours, each mix results in a reduction in brightness.

An artist at work by Notre Dame, Photograph Tom Kennedy 56

COLOUR

Making colours

MIXING COLOURS

While transmitted red, green, and blue light can be mixed to produce the full spectrum of colours, artists often favour red, yellow, and blue, because, in their experience a wide range of hues can be produced from paints in these colours. However, paints, which absorb light and reflect the colours we see, do not behave like pure light. Thus mixing blue ultramarine and chrome yellow paints usually gives a green, but this cannot be done by mixing projected colours. With pure transmitted colours yellow is actually a mix of green and red.

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hese days artists can go into a shop and buy ready made paints, but this was not always the case. In the past preparing paints required a great deal of knowledge and skill. Until the 15th century pigments mixed with egg, tempera, were usual, but then Dutch painters led a change over to oils. One of the big advantages in using oil based colourings was that they could withstand the damper northern conditions.

Water paints became popular in the 19th century, but coloured inks have been used since antiquity in China and Japan. The modern, brightly coloured cartoon is an extension of this great tradition.

Artists have always used whatever materials are to hand, and at one stage turf ash painting was popular.

Until recently every house had open fires, and turf was the common fuel. As the fire died down ash of different tints, from white to dark brown, was left, and among those to take advantage of this was the celebrated song writer, Percy French. Mixed with water and a little gum, the ash makes a perfectly good paint. Percy French’s turf ash landscapes have become collectors items.

Portrait artist at work in Valikavkaz, Nth Ossetia.

Photograph: Tom Kennedy 57

The Irish chiefs submitting to the Crown, one of Derrick’s Images of Ireland, from 1581. Elaborate woodcuts like this were painstakingly carved in relief, and on inking the image could be printed out onto paper. Woodcuts had a special character making them distinct from drawings, and like many other examples of outstanding line art there is no need for colour.

COLOUR

Defining line

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ong before colour there was recognition of shape and pattern. Some of the earliest animals that we know to have sight were the remarkable Trilobites with their multiple crystalline calcite eyes. Because light was channelled down in from hundreds of separate crystals Trilobites would have been remarkably sensitive to the slightest movement, and no doubt this was an early warning system to detect predators. Detection of movement has been with us for millions of years, and edge definition is also a long established part of sight. Like hunting dogs we are instantly alerted by movement, and we are very good at separating objects from their surroundings.

Because of this sense of edge artists can draw, and it could be argued that the pen, or its equivalent, has been more important than the paintbrush in the development of art. Artists such as Rembrandt and daVinci are celebrated for their paintings, but their pen and ink drawings are also remarkable, as are the wood engravings of Dürer. These line images need no colour, in fact Dürer’s woodcuts are so complete that colour would ruin them.

Some of the most outstanding line art originated

A naturalistic style was perfected in Japan under the influence of Buddhism and Chinese artists. Artists drew on nature for their inspiration, and the elegant drawings are full of observation and sometimes humour. This is a detail from an irreverant sketch by an 11th century artist, Joba Sojo depicting priests as frogs, monkeys and other animals.

over two thousand years ago in China, and on reaching Japan the style became established as a tradition that has continued with minor variations to this day.

In Japan print making reached the peak of perfection during the 17th and 18th centuries. The print makers were quite selective in their use of colour, 59

and the sharply defined edge always remained the essential, dominating feature. The popular pulp comics of the 1940s and 1950s, often dismissed by concerned parents as decadant and crude, drew heavily on this tradition, and about the same time Walt Disney realised that animation badly needed to get back to the eastern roots of cartooning.

Many of the images produced by his team of artists in the 50s and 60s could have come straight out of the 12th century.

In Japan where the tradition remains as strong as ever, hard edge comics are enjoying a boom, and not surprisingly, given the history, the standard is extremely high.

Black and white

Just as we can accept images in line, colour is not always essential in the creation of works of art. Less colour in a painting can make it special and the same is true in photography.

One of the problems facing early photographers was that it was impossible to work in colour. Far from being a drawback, this limitation became a great strength. The impact of old press photographs has little to do with their age. Black and white prints have a quality that colour is never going to match. As many press and art photographers discovered, colour is a different medium. Until digital swept away the difference, many of the best press photographers insisted on loading up with fast black and white film. Colour was for the glamour, the fashion, the food, but actuality was always stronger in black and white. In powerful, iconic images, colour just gets in the way.

With all the improvements in colour stock, film makers faced a similar dilemma. For years audiences had been enthralled by adventures in black and white, and quite a few directors argued that a change over to colour would detract rather than enhance the experience.

Which of these two sets of running birds is lighter? Both are exactly However the market, and the same, but a little bit of outlining makes a lot of difference. studio bosses, made sure 60

COLOUR that directors and their arty camera operators moved with the times. Some of the more independent directors, such as Bergman continued to work in black and white, and more recently Stephen Spielberg, who gave us such colourful productions as ET finally won approval from his peers as a serious film maker by shooting Schindler’s List in black and white.

We are not likely to see many new films in black and white but a lot of the old classics are going through a revival in DVD. Charlie Chaplin and the Keystone Cops are still as enjoyable as ever, and no one complains about the lack of colour.

Watching all the colour of the passing parade in Camden Street, Dublin, a moment captured in monochrome by Tom Kennedy

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Beautiful executed script upon which some modern typefaces have been based. Part of the Gospel according to Mark in the illuminated Book of Kells manuscript.

Sourcing and compounding inks to create the interlacing figures, such as the one above, required a great deal of knowledge and skill.

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anuscripts can often be traced back to a particular monastery through an analysis of the inks used by the scribes.

For writing a wide variety of substances have been found to meet the basic requirements; freedom of flow, clarity, and a high degree of permanency. Boiled tree bark, the black mush produced by ink-cap mushrooms, blue from cornflowers, powdered root of the yellow flag iris, and even strong coffee have been used. A black ink was made from the winter blackened bark of blackthorn twigs mixed with milk or glue. One common type of ink was made from oak galls, the round balls formed by insects on oak trees. One formula for preparation was five pounds of iron sulphate, five pounds of gum, 12 gallons of water, and measuring by volume, 12 gallon of oak galls.

Collecting enough oak galls for 12 gallons must have been difficult but it just shows how big the demand for ink was. On an even more extensive scale Indian ink was made from lampblack and gum, and producing fine grained soot became a big, although very dirty industry in parts of south eastern Europe. The soot, lampblack, was mixed

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Photogr

INK

aph: RI A

COLOUR

The quality of medieval inks had to be high for manuscripts such as this to survive.

This is a page from a medical manuscript, the Book of the O’Lees, preserved at the Royal Irish Academy.

The amazing quality of ancient art, thousands of years old, yet vivid and fresh. Using the most basic material the artist, with great perception and skill, created this image of a wild boar in a standing and a galloping position. Culture and appreciation of form and colour was already old before the emergence of city states.

Facsimile of one of the cave paintings at La Caverne d’Altamira, Altamira, Spain.

COLOUR tribes of North Africa with horses and purple dye. dried and ground up bodies of the female, which is However, during the dark ages most of that considerably larger than the much less plentiful male. knowledge was lost to Europe. Venetian trade links to Males are half the size and for every one there are the east and south are believed to have sparked off a about 150 to 200 females. Cochineal, chemically revival and guilds were formed in important cities. carminic acid, C17H18O10, is also used in colouring The introduction of dyes from the New World was a foods. In 1630 a Dutch chemist, Drebbel discovered further stimulus, and in Europe cultivation of woad, that wool could be dyed scarlet with cochneal and a madder and other dye plants began in the early 16th tin solution, but even so, secrecy in trade was century. important, and most people had no idea where the In 1518 the Spanish, colouring came from. Until invading Mexico, were the early 18th century it was struck by the vivid colours commonly held that in native materials, and in cochineal came from a plant, particular the reds produced and then in 1703 Anthony from the cochineal insect, van Leeuwenhoek Coccus cacti. This parasitic determined its real nature insect infests a cactus, with the aid of his Opuntia coccinellifers, but can microscope. also thrive on other species.

Immediately that knowledge was brought back to Europe and raising cochineal insects became an important activity in the Canary Islands and around the Mediterranean. The Prehistoric cave painting of a bison from Altamira, Spain. colouring is made from the 65

COLOUR As we know from the earliest remains of buildings and decorations, colour was often an important feature. Glazed tiles, gold, and precious stones were common, and sometimes climatic conditions helped in preservation of painted murals, as in Egypt, where tourists now come in their thousands to see the vividly coloured images adorning the walls of ancient tombs.

Colourful art

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t is hard to believe that the cave paintings at Lascaux in France and Altamira in Spain were already thousands of years old before any of the recognisable civilisations emerged. Vivid and confident in style, these coloured drawings are among the most amazing works of art ever produced, yet The female cochineal infests cacti, forming they must have been executed colonies coated with a protective cover.

Colour and rank under difficult circumstances Photograph by Dres.

Colours were not so easy to and the artists could only have produce as they are now, and in had access to the most basic materials. Animal fats, ancient societies colours were often a sign of rank. In soot, red ochre, yet the command of colour is strong, Rome purple belonged to Caesar, and as a mark of and obviously colour was an important element. distinction senators were allowed a purple trim on We know little of how our ancient ancestors their togas, a tradition that survives as a symbol lived, and as these works of art suggest, our of high status in the Roman Catholic Church. distant ancestors were a lot more sophisticated The Celts in Europe seem to have been a than we are often led to believe. From the little colourful sight. and observers from Roman we know, red seems to have had great times described them as fond of bold check significance, perhaps as a symbol of life, and patterns and stripes with cloaks over coloured some of the most ancient burials had bones marked tunics. with red ochre, a mineral rich in iron.

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COLOUR In Gaelic Ireland colours were also associated with rank, and the Annals of the Four Masters, a compilation of history and legends, an Irish king from about 1538 BC , Tighearmas, is given the credit of being the first to dye clothes purple, green and blue. His successor, Eochaidh Eadghadhach, which translates as Eochaidh the clothes designer, is recorded as imposing a rule limiting the number of colours people could wear.

While kings and queens were entitled to wear seven colours, slaves were expected to lead a drab one colour life. Descriptions of kings and heroes often stressed the finery and colour of clothes, Cú Chulainn appears in the sagas wearing crimson secured with a brooch of gold. Even allowing for poetic licence, colour was obviously a mark of distinction.

Producing colours was a specialised trade, and one of the ancient sources of purple was Inishkea, a now abandoned island off the coast of Mayo. The purple dye was extracted from periwinkles, small sea shore snails. A large accumulation of periwinkle shells was found by archaeologists, who concluded that Inishkea had a substantial export trade in purple dye. Although we think of islands off the western coast as isolated now, in the ancient past the sea off Kerry, Clare, Galway, Mayo and up around Donegal was like a highway for trade with the rest of Europe.

The vivid colours in manuscripts such as the Book of Kells were produced with difficulty, and indeed some

undoubtedly had to be sourced abroad. One of the dyes, known as folium, used widely as a colouring in medieval manuscripts, is now known to have come from a plant, Chrozophora tinctoria, formerly grown in the south of France, and now a threatened species because of herbicides.

In the ancient world trade in dyes was important, and the Romans, for example, associated the Berber

ROUGE

One of the colours used widely in cosmetics came from Safflower, Carthamus tinctorius. Originally from the East Indies, Carthamus tinctorius, named from the Arabic, safra, yellow, was cultivated in Egypt and southern Europe for production of safflower dye.

The dye material, carthamin, C14H14O7 was obtained by digesting the washed, dried and powdered flowers in an alkaline solution into which a cloth clean cotton is dipped. The alkaline solution is then neutralised with weak acetic acid, the cotton removed, and immersed in another alkaline solution. On neutralising this second solution with acetic acid, carthamin is precipitated as a dark red powder. The colouring although strong, is not permanent, and so it was used extensively for preparing toilet rouge.

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I

n Japan printing colours from separate wood blocks has been practiced since at least the 17th century, and one artist, Suzuki Harunobu, who was active from about the 1760s, is sometimes credited with inventing the process. However he almost certainly drew from a long tradition, and the prints produced at this time are amazing for their beauty and observation of nature. The colour was applied from wood blocks, cut from fine grained cherry wood to leave the areas for inking in relief. After making the first impression, the next block was placed down, using small notches as a guide for perfect register. During the 18th and 19th centuries prints were popular, and work from artists, such as Harunobu, Katsukawa Shunsho, and Hokusai, were in great demand.

Coloured woodblock by Japanese printmaker Ando Hiroshiga, 1797 to 1858.

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Uncluttered, balanced, and perfectly in tune with the natural world. Monkey trying to catch the Moon, a beautiful print by Shosan from the early 20th century in a classical, ageless style.

Over a thousand years old and still sharp and amusing as ever, detail from a sketch by 11th century artist, Joba Sojo. With a bare minimum of strokes these masters of the brush and pen, laid the foundations for the modern cartoon.

COLOUR Sicily, and the usual method of harvesting was to gather the flowers in the morning before they were fully open. Then the stigmas and part of the styles were picked out and stacked on paper up to a depth of 2 to 3 inches. A cloth was placed over this layer and the saffron was pressed and heated to make a cake.

Saffron was extensively used in medicines and in foods. Just one grain was enough to give ten gallons of water a distinct yellow tint. It was also used for its perfume, the streets of Rome being sprinkled with saffron when the megalomaniac Nero entered the city.

In medieval manuscripts saffron was sometimes used as a substitute for gold.

However, the main use for saffron was as a dye. In ancient Greece it was a royal colour, and in Ireland it was also a mark of high status. In ancient Ireland a king’s mantle was dyed with saffron, and up to the 17th century a saffron dyed shirt, the lein-croich, was a sign of rank.

Saffron was a luxury, and as with rare medicinal herbs even now, dealers were often tempted to stretch their precious supplies Adulteration has always been a problem, and in 15th century Germany the penalty for cheating was severe. In one case the dealer was burned with his products in the market-place, and two others are recorded as being buried alive.

In Ireland saffron was so popular that it came to be seen as an Irish colour, and Henry VIII passed laws

SAFFRON

O

ne of the most popular and enduring colours throughout history has been saffron, the strong yellow obtained from the Autumn crocus. The purple flower is similar to the spring flower of gardens, but it blooms in Autumn. The colour comes from the flower stigmas and part of the style, which have been harvested and dried since ancient times. Homer, Hippocrates and other Greek writers referred to saffron which was cultivated in Persia and Kashmir, and is said to have been introduced to China by Mongol invaders.

The original centre of cultivation is thought to have been Corycus, now known as Korghoz, in Turkey, and at one stage the Arabs brought it to Spain.

Like many other things, saffron disappeared from Europe during the Dark Ages, only to be re-introduced by the Crusaders. One story is that a pilgrim returning to England from Tripoli hid a stolen corm, the underground stem, in a hollow staff. Given that many plants of economic importance were jealously guarded this story is probably true, and cultivation of autumn crocuses began at Hinton in Cambridgeshire, and at Saffron Essex.

Saffron was widely cultivated in Spain, France, 70

COLOUR banning it. Mairead Dunleavy from the National Museum and an expert on Irish costumes believes that saffron might have been introduced from Spain in the late 10th century, or the twelfth to thirteenth century from the Middle East. The dye became popular also in England, but the fashion persisted longer in Ireland. One of the reasons saffron remained popular is that it was believed to protect the body from disease.

In 1466 attempts to ban saffron began in Dublin. Women were banned from wearing saffron smocks or kerchiefs. In 1536 Henry VIII banned the wearing of saffron in Galway and a year later extended the ban to cover all Ireland.

However the ban was largely ignored, and in 1578 complaints were made about the English continuing to wear it in Dublin. In

1577 imports of saffron were taxed at 2 pence a pound in Galway to raise money for paving and town walls.

At the same time a toll of one penny a pound of ginger was being charged.

Mairead Dunleavy records that about 1566 the Rev Fr Good described that the dye was prepared by heating poplar leaves, bark and boughs together with the bark of the arbutus tree, salt, and saffron.

Clothes were soaked for days and urine was used as a mordant, fixing the colour to make it more permanent.

17th century view of a Wild Irish Man. His tasselled cloak is likely to have been bright yellow. One of the figures on John Speed’s map of Ireland, c1612.

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COLOUR Cultural colours

Southern red

The sober navy blues and pastel shades of female dress in northern Europe contrasts strongly with the vibrant oranges and greens of their sisters in Nigeria, and likewise women in India stand apart by their distinctively coloured saris. The colours worn are as diversified as language, but some have strong cultural associations. For example brides in Europe wear white, but in Japan white is the colour of death, and while fridges here are clinically white, in Arab countries green is preferred because it signifies coolness.

In the ancient world various attempts were made to arrange colours into some form of order, and China by tradition had a spectrum of five. Red represented the south, fowl, and summer; green was the east, dragons, spring, and wood; black was the north, winter, water and the tortoise; white was the west, tigers, and autumn. Yellow was earth and source of nutrition, fire, metals and water, it was sweet and perfumed, and the inhabitants of the world were the yellow race.

Volendam in Holland, where traditional dress is now a tourist attraction.

Photo: M Jonker

Weaving in the colour at the Foxford Woollen Mills in Mayo Vivid colours in Thailand dress 72

Photo: Eamonn Boyle

COLOUR were being imported from Java, Guatemala and other places. In the Bengal Indigofera sumatrana was sown for harvesting. The indigo was extracted from the leaf as a colourless glucoside, indoxyl. In air leaf enzymes convert this into indigotine. Draining off the liquid left a muddy residue, known as mal, which after drying, was cut into blocks.

When a synthetic substitute was discovered by Adolf von Baeyer in 1883 the trade in natural indigo rapidly declined, a huge blow to producers and suppliers.

Not only did the trade in natural indigo drop dramatically, but interest in dark blues declined as well because now dyers had a much wider range of colours to choose from.

In the 1960s, when jeans became popular the market for blue dye picked up. Blue jeans had their origin in cotton work clothes made by Levi Strauss, a Bavarian merchant who went to San Francisco in 1850. The word ‘Denim’ comes from the textile centre of Nîmes in France, where in earlier times seamen’s clothing was known as “Bleu de Genes”.

Indigo

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he glucoside dye indican occurs in Indigofera tinctoria, a mallow, and Isatis tinctoria, woad. Natural indigo is thought to have been used first by the Sumerians 5,000 years ago in what is now modern Iraq, and it was also used in Iron Age Europe.

The Sanscrit name was niti, from which came the Arabic al-nil, in turn becoming aniline, the term we now associate with synthetic dyes.

It was principally extracted from plants of the genus Indigofera, which belongs to the Leguminosae. To a lesser extent it was obtained from Lonchocarpus eyanescens, from West coast of Africa; Polygonum tintorium, from China, and from the woad plant, Isatis tinctoria.

Until the 17th century Isatis tinctoria was the only source of the dye for English dyers. In the early years of the 20th century woad was still being cultivated in Europe for use as a starter in dying wool.

Most of the indigo coming into Britain in the early 20th century came from India, while smaller quantities 73

Optic nerve

Carrying signals to the brain

Retina

Thin light sensitive layer of rods and cones

Iris

Regulating the light by adjusting the aperture

Cornea

Blind spot

Transparent outer layer

Where nerves lead out of eye

Fovea

Lens

Most sensitive area

Focuses the image on retina

Nerve fibres

BACK OF EYE

Rods

Cones

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Occipital lobe,

the area involved in processing sight

COLOUR

were reluctant to give Dobelle permission to conduct risky medical procedures, but wealthy from a string of patents, the inventor arranged to have the surgery performed in Portugal. Since then, several implant operations have been carried out. No deaths or complications have been reported, but the restored sight is extremely poor, and lacking in colour.

In time we can expect artificial vision to become a lot better, and possibly cheaper. In spite of Dobelle’s spectacular success, retina implants may turn out to be more useful. In most cases of acquired blindness, the fault lies only in a failure of rods and cones, and as many researchers argue, if something is not broken, why try to fix it? When all the nerve network supporting the retina is intact and potentially functional, chips in the eye would give better results than wires to the brain.

Colour and the eye

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hen rays of light are brought into focus by the eye’s lens, the pupil, an inverted image is formed on the retina. We, however, interpret this image as being the right way up.

The retina, a thin layer of light sensitive rod and cone cells could be compared to the recording plate inside the back of a digital camera. In the camera thousands of tiny sensors record the image as dots, so the more sensors there are, the higher the resolution.

In the retina there are millions of light sensitive cells, each about 1µm in diameter, so we are well equipped to read small print. In addition the cells lining the base of the retina are dark with melanin, and this helps keep our vision sharp by preventing backscattering of light.

The rod cells, so called because of their long and slender outer ends, respond to low level light, but they do not give us colour. Each eye has about 100 million rods, and they are extremely sensitive. Just one photon of light is enough to trigger a response.

Dobelle’s patient wired up for artificial sight. Electrodes placed in close proximity to the brain pass data on from a camera which can then be seen by a blind person as images.

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COLOUR For colour and detail we rely on the cone cells, this process the electrical charge on the cell membrane named, like rods, from their distinctive shape. Each changes. This change is immediately picked up as a eye has about 7 million cone cells, so they are a lot less signal by the neural network overlying the light in number than rods. However, they make up for this sensors, and transmitted via the optic nerve to the in performance. Cone cells are concentrated in an area brain. known as the macula, and this is where sight is In cones the process is similar, but instead of keenest. The resolution has to be high because the scotopsin, there are chemicals known as photopsins. image formed on the retina is quite small, smaller in The photopsins are of three types, with peak fact than the frame of a 35 mm film. The smallest sensitivities of 445 nanometres, 535 nanometres, and visible object might only become .0025mm on our 570 nanometres, corresponding to blue, green, and red retina. light.

Rods and cones are specialised cells Rods and cones are capable of containing a light sensitive chemical, responding to ultra violet, but this high EYE COLOUR rhodopsin. Rhodopsin is also known The coloured part of the eye is energy light is harmful, and after some as visual purple. and it undergoes a called the Iris. This is a ring of absorption by the cornea and lens a reversible reaction on exposure to very fine muscles which control yellow pigment in the area of the fovea light. The reaction is extremely fast, the size of the pupil. The colour acts as a protective filter. virtually instantaneous. is determined by the connective Even at this stage the eye has begun to In rods, rhodopsin is composed of tissue and pigmented cells. Less organise the messages. Before passing the proteins, scotopsin, and 11-cispigment makes eyes blue, more on to the long nerve fibres, the retinal, the latter a derivative of pigment makes the eyes brown. impulses pass through a layer of vitamin A. These muscles dilate or ganglia, nerve cells, with many cross On being energised by light the reduce the aperture of the pupil, connections. angled molecule of 11-cis-retinal from about 2 mm up to 8 mm, Although the fine nerve fibres lie over instantly straightens out to becomes giving a total difference in light the rods and cones, they are so fine all-trans retinal. However the straight that they do not impede the rays of intensity of 30 times. shaped molecule is unstable, so it light. However, at one point they all turns into activated rhodopsin, and in converge to exit the eye as the optic 76

To find your blind spot close your right eye and look at this black bar.

You see the circle and the cross. Now, keeping watch on the circle, move closer and closer. The cross disappears, and that is the point at which the image has fallen on the nerve exit from your eye. The blind spot can cause overtaking drivers to miss seeing a car close behind.

nerve. Where the optic nerve, made up of millions of fine fibres, exits the eyeball, we have a blind spot. We are not aware of this, yet, as a simple test can show, the blind spot is quite real. Close your right eye and look at the picture of the cross and circle. Keep looking at the circle, and as you move closer, the cross disappears. At that point the image of the cross has fallen on your blind spot.

The optic nerve leads to an area towards the back of the brain known as the occipital lobe. In effect this lobe, a thin layer of outer cortex cells, is our image processor, converting the constant stream of signals into sight. At birth we cannot make sense of these signals, babies must learn how to see, and at first all we can perceive is light and shade. However, we learn fast, and remarkably the ability to make sense of what we see remains intact even if our eyes are lost or damaged.

Stimulating the optic nerve, or the occipital lobe makes us ‘see’ spots of light so the brain is well equipped to create visual images provided there is some way of by-passing the eye.

Researchers have had some remarkable success in feeding visual data directly into the brain. Fine electrodes have been used as the interface between living tissue and electronic cameras. About four years ago, in an extremely delicate operation, a tiny electronic chip was inserted into the retina of a blind patient at the Chicago Medical Centre. One researcher described this process as like placing a heavy brick on a moving piece of wet tissue paper. The electrode, fed with power and information, stimulated the underlying nerve endings, and amazingly, the implant actually worked. With this implant some low resolution monochrome vision, equivalent to about 10 by 10 pixels, was restored.

The idea that bionic vision does not have to be just a dream sparked off research around the world, and electronic engineers at the Tyndall Institute in Cork are aiming to create smaller, more flexible chips with much higher resolution. Dr John Alderman at the Tyndall Institute has been working with opthalmic surgeons, Philip Cleary and Hossein Ameri at Cork University Hospital and their aim is to reach 250 by 77

COLOUR 250 pixel resolution, enough to read a newspaper.

Numerous platinum electrodes come close to nerve endings in the retina. The closer these get, the better the contact, but, because the chip is powered, this increases the risk of inducing minute burns. The Cork researchers believe that this problem can be solved by more efficient use of power, and with better connections, the quality of vision would increase.

Another approach involved

by-passing both the eye and the retina, to tap directly into the visual cortex of the brain.

Following long years of experimental work, an independent researcher in the USA, William Dobelle, astounded the world by successfully restoring limited vision to a 38 year old man who had been blind for most of his life. After fine tuning the electronics and improving the software, Dobelle offered to do the same for anyone willing to pay. The American authorities

in rooms illuminated by red light before taking off on night missions. In red light crew could still read, but being at the lower energy end of the spectrum, the red light did not deplete their rhodopsin.

In the absence of light rhodopsin is resynthesised from the pigment and vitamin A. A diet deficient on vitamin A can cause night blindness.

TWILIGHT

In dim light sight depends on the more sensitive rods, so we see in monochrome. As the level of illumination goes up the cones start responding so in bright light we see in colour and in detail. In going from bright light into the shade our eyes take time to adjust because the rods have been over-stimulated and need a few seconds to recover. For the same reason a brief exposure to bright light, such as the headlights of a car, can leave us blind for a brief period.

During the Second World War aircraft crew stayed

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A

Response to primaries In a narrow band of the electromagnetic spectrum our eyes respond to light between about 380 to 780 nm

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lthough we have separate red green and blue sensors our response to light is not just like a simple on/off switch. The sensitivity of cones overlap to a considerable degree, and while each has a peak for each particular colour, there is a tailing off of response into other wavelengths. Rods also respond to light, and it is likely that their input has a strong influence on how we interpret colours. In addition we perceive some colours as brighter than others, and if the blue end of the spectrum was less intense than it actually is, we would see everything with a yellowish tint.

It is convenient to refer to red, green, and blue as primaries because this is the closest we can get to our experience.

However without additional stimulation outside the narrow RGB range our experience of colour would be limited, as it is on computer or television screens.

COLOUR John Dalton, 1766-1844, was not alone one of the pioneers to investigate the nature of light, he was also the first to describe colour blindness. In 1794 he described how he, and his brother, could not see one of the colours, red, observed by others in an experiment involving different types of illumination.

Partial colour blindness is the most common defect in colour perception. The perception of red or green is below normal. However, while such people may not see green or red, their perception of brightness in these colours is more acute.

Colour blindness

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he most common type is red-green. 8 per cent of males and 0.4 per cent females cannot distinguish the two as separate colours. The condition occurs when either the red or the green cones are not present, or not functioning properly. Colour vision is located on the X chromosome, so the condition is inherited, and as women have two X chromosomes, the condition is less frequent in females.

Perceiving all colours in shades of grey is a much rarer condition and it can be caused by absence or failure of the cones.

• See colour test at end of book.

primates of South America have just one gene for red or green, while the primates of Africa, from which we descended, have two genes, one for red and one for green.It is thought that the two genes began as a doubling up, perhaps a mutation, and over time they began to accumulate small differences, which in turn led to separate receptors for red and green.

The blue recptors gene is on chromosome 7, and as males and females both have a pair of 7s, instead of XY and YY, blindness to blue is equally rare in men and women.

Extra gene

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he arrangement of genes for red and green on the X chromosome is thought to have originated about 40 million years ago. It is possible that a doubling up of genes occurred then, and the evidence in support of this comes from our closely related primates. At that stage the primates of the old and the new world were separated from each other, thus South American monkeys are quite different from those of Africa. The

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Late evening at Athlone. As the light begins to fade colours also start to disappear.

Photograph Tom Kennedy.

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COLOUR taste or sight are routed through a structure called the thalamus deep within the brain, and from there they are sent on to the cerebral cortex. A lot of signal processing occurs along the lines of communication, and in the case of sight, this begins with the neurons in the eye. The interconnected ganglia sort out and concentrate the messages, so that only the essential signals get passed on to the next stage of interpretation. This greatly reduces the volume of signalling traffic, and efficiency is also enhanced by parallel processing. The high degree of efficiency is actually necessary, because nerve impulses do not travel fast; they are much slower than data transfer in a computer, but we make up for this by having lots of nerves and plenty of processing stages.

Different areas of the brain’s outer covering cortex have evolved to process this flow of information; a region called the occipital lobe is associated with sight, while other regions processes signals from the ears, the nose, the tongue and so on so that the information is perceived as sight, sound, smell, or taste.

Stimulation of the nerves leading into any of these regions will produce the appropriate sensory experiences, such as sound or taste, and to some extent these distinctions, although very real to us, are imaginary. Sometimes the barriers between our different senses come down, and there is a good reason why people often think of red as warm, or a

Making sense of our world

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ithout any obvious effort on our part we gather a lot of information from the world around us.

Scents, sounds, shapes, colours, all adding up to an awareness of our environment. We think in terms of sight as senses, but in fact these are mental impressions formed on the basis of available information. We don’t just have eyes and ears, our body is equipped with all kinds of sensors, many of which we are not even aware of, such as those triggered by chemical changes in body fluids, or temperature. The sensors we are aware of, such as ears and eyes, are more obvious because we are conscious of sounds and images. Those images, like sounds, seem very real and coherent to us, yet our brain does not store physical pictures, the images are literally in our mind. The images which we think of as indivisible, are not really so. When we look at a coloured cube, for example, our eyes are also detecting edges, shape, and depth, and if the cube falls over, we register movement, so the picture we see is in reality a seamless composite. Without a sense of perspective we misjudge sizes, a common cause of traffic accidents. We often sense movement before seeing an object, and shadows let us know when objects are the right way up.

Most of the incoming messages, whether from sound, 82

COLOUR sound might evoke a scent, a crossing over of senses called synthesia. In some people synthesia is a normal part of awareness, they might simply assume that everyone sees smells, but in most of us the breakdown of barriers only occurs under the influence of the so-called mind bending drugs. Everyone is influenced by crossing over of the senses, even if only by habit and association. For example numerous trials by food manufacturers have shown that colour affects taste. A strawberry flavoured food will not taste of

strawberries if it is coloured green, and the deeper the colour, the stronger the reported taste. A simple test of tasting foods without looking, or in the dark, shows how surprisingly hard it is to be sure of the difference between flavours such as rasberry and orange. The expected colour enhances the taste and aroma.

Taking your pick has a lot to do with colour. Blues and purples make us expect a sour taste, and greys hardly excite the palate.

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COLOUR

primaries. The original is copied through red, green, and blue filters to give screens corresponding to the three complementary colours, cyan, magenta, and yellow. Up to quite recently these screens were made onto separate sheets of black and white film, and from them the printing plates were produced, each being inked with one of the complementary colours. Now these screens are made electronically, but the basic process is much the same. Fine dots in the complementary colours lie in close proximity, and at normal viewing distance they merge and blend to give us a close approximation to full colour.

In theory the three complementaries should in combination produce full colour, but inks are not perfect, and in practice a fourth colour, black, is added to increase the contrast and give deeper tones. Without the black, colour reproductions would look pale and washed out.

Cyan, yellow, magenta, and black, usually referred to as CYMK process colours have become the standard, but in the earlier days of colour printing red, green and blue or other combinations were often used, sometimes with great success.

Colour in print

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single colour, black, gives us clear legible text and line images. In drawing, cross hatching, dots and fine lines in black are often used to create the impression of greys from this single colour. The same technique is used to reproduce continuous tone photographs. The discovery that a photograph could be rendered into thousands of tiny dots made it possible to reproduce photographs in books and newspapers. Look closely at the photographs in a newspaper and you can see that every dot is actually a solid. In areas of darkness the dots are so big that they almost merge, while in lighter parts of the picture, the dots are tiny. The dots, or screen, is usually too fine for us to see without a magnifying glass, so they merge giving us the impression of continuous tones. To convert the continuous tones into dots, the image is copied through a very fine screen.

From one colour, black, we can make greys, but of course we get no colour. To produce colour the same process of screening is repeated for each of the 84

COLOUR

Process colour

Jakob Christoffel Le Blon, 1667-1741, developed a system of colour printing with four separate plates, each inked with a different colour, red, yellow, and blue, and black, to produce coloured mezzotints. Unfortunately his colour printing enterprise was a commercial failure. However he inspired imitators. The cyan, magenta, yellow, black, which has become universal, was devised by Alexander Murray in 1934.

To show how each colour adds to the image yellow has been gradually added to cyan in this example.

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COLOUR influence the inks in a variety of ways. To take two extremes, blotting paper would absorb everything and have zero gloss. Metal foil is the exact opposite, and in practice printers, who refer to this as surface efficiency, work with papers that fall somewhere in between.

Humidity also has to be taken into account, so on big jobs where quality is an important issue, printers and inkmakers need to work closely together as collaborators.

Compounding the ink begins with taking small amounts of the basic ingredients and preparing a 10 gramme sample. The colours are ground up with varnishes or other ingredients, and when this mix is satisfactory the formula is scaled up for production in the works. At one stage all this was done on cards, and scaling up a formula could take hours. Now all this is done on a computer so getting results takes less time, all the formulae are stored, and revision and changes are easier to implement.

The ink is mixed, and a sample is taken back for another test to ensure that the colour is as it should be.

Small stripes of colour are made on strips of paper with a hand roller, and these are examined on a light box. A spectrophotometer is used for this checking, but as the ink makers observe, the naked eye still remains the final arbiter.

Inks for print

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he raw materials for printing inks are produced in bulk in large chemical plants, and the compounding is carried out by specialised firms. One of these firms, Graphocolor in Dublin, has been compounding inks for printers since 1975, and as director Bob Christie explains, the company makes up just about every colour except black. Black comes directly from big suppliers by the tanker load, but colours are usually blended in small volumes, anything from one kilo to two or three hundred.

The colour demanded by a customer can be very specific, and the ink that produces the right hue on one job might not work on a different paper. Paper itself has a colour, and often incorporates brightening materials to give higher contrast and saturation.

Many companies take a keen interest in their image, and that includes using specific shades and colours in promotional and packaging materials. Matching a specific hue, like Microsoft grey, from process colours can be difficult, so it is often better and more consistent to use a special ink. At one stage Graphocolor had about 100 different ink formulations made up for one particular grey so that printers could still meet tight specifications although using a range of papers.

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COLOUR printers there to get the typical glossy Natural colourants are The amount of ink used in printing is magazine look. On solvents, Bob Christie popular in the food quite small, and modern machines are remarked that environ-mentally friendly industry where they are efficient. In the earlier days of colour alternatives can be three times as dear and a seen as safer. Some of printing and up to the 1980s hundreds of third less efficient. the synthetics have sheets might have to go through before the

Print finishing can greatly enhance the given problems and cyan, magenta, yellow, and black screens colouring of inks, and varnishing is often colourants have been were in perfect register. Electronics changed withdrawn because they used to improve the quality. Varnishing is all that, and apart from higher resolution applied just like another ink, and as many were associated with screens, register is now spot on from the health risks. In printing, printing machines already have the capacity start. Because of this, full colour can be used where almost all natural for extra colours, the whole process can be colourings were on relatively short print runs, and inking is completed in one pass. For most jobs the displaced by synthetics, four process colours would be all that’s kept to a minimum. Even when a print job natural dyes are coming required, but because cyan, magenta, is big, the volume of inks required is back into favour in surprisingly small, and in cost terms, inks yellow, and black cannot produce certain packaging where direct tints to a high enough standard, extra, account for about two per cent of printing contact with food is cost. In printing 50,000 milk or drug cartons single colour inks, are sometimes included. involved. with a stripe of one colour, the amount of On big packaging jobs, for example, specific ink used might only come to less than one colours are often run for logos and areas kilo. where an exact match to a standard is critical.

Many of the older inks were opaque solid blocks of The use of a finishing varnish can give a different colour, but modern litho inks are transparent. Volatile look to a job and it can protect the surface from solvents are also becoming a thing of the past, mainly scuffing. The extra coating does not even have to be a because of environmental concerns, and while making gloss, but it can make colours change in unexpected the printing work safer, the change-over is not always ways. With some varnishes the vivid pinks in print easy to achieve. In the US tighter regulations, which simply fade away. banned optical whiteners, make it more difficult for The paper is another critical factor because it can 87

COLOUR could be produced.

In 1893 John Joly patented a method of photographing in colour using a glass plate ruled with fine red, green, and blue lines.The plates were manufactured at the Grubb optical works by the canal at Ranelagh bridge, at the time one of the leading optical workshops in the world. The lines were about 200 to an inch, and the exposure was made through this onto a black and white film.

Auguste and Louis Lumiere improved the process by changing the lines into a random pattern, which they produced from orange, green, and purple starch grains. This became known as the Autochrome process, the first to become available commercially. In contrast with the aggressively vivid colours of modern films, the Autochrome process produced softer, and often more attractive, pastel shades.

Colour films improved rapidly, and in 1935 Kodachrome went on the market, and although it had to be sent off to Kodak labs for processing, it remained popular for many years because of its warm colours.

Shortly after the launch of Kodachrome the German manufacturer, Agfa produced a better product, and at the close of World War II this technology was taken over by the Allies and applied to the development of modern colour films.

Photo colour

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n May 1861 Sir James Maxwell, the Scottish physicist, demonstrated that a wide range of colours can be produced by mixing the three primaries; red, green, and blue. From this he deduced that colour could be recorded photographically by making three exposures, one for each primary, and superimposing the images.

He photographed a tartan on black and white film using in turn a red, green, and a blue filter. From these three exposures he made black and white lantern slides. With three projectors, fitted with colour filters, he superimposed the images, and a colour picture was created — the first colour photography.

A short while later, Charles Cros, and Louis Ducas de Hauron, both working independently, began producing colour images using a similar approach. In 1869 de Hauron published his method of producing colour photographs. He took three negatives of the subject in turn through a green, orange, and a violet filter. From these he printed three separate positives on red pigmented, blue pigmented, and yellow pigmented bichromated gelatine. By superimposing the three images in making prints from the bichromated masters, a colour print was produced. By printing on glass rather than paper, a transparency 88

COLOUR The film is developed up to the point where a negative image is produced, and then, before fixing, the film, which is still sensitive, is exposed to a brief controlled flash of light. The light must pass through the negative, and thus a secondary exposure is made.

Developing continues, and after fixing to make the film permanent, we are left with a positive transparency slide. Until recently slides were very popular, notably in press and magazine work, because images could be seen immediately after processing, and the quality is high. However, the rapid rise in digital printing, and digital cameras which use no film at all, is making slides redundant.

Colour film

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n black and white film there is a single light sensitive layer of silver bromide. On exposure to light a latent image is produced which can be rendered visible and fixed in the developing process. Development causes the exposed silver bromide to reduce as tiny grains of metal, and where no light has fallen the bromide can be dissolved away to leave clear areas. Thus the developed film is a negative image, which is convenient because on projection to silver bromide coated printing paper the process is reversed to give a positive photograph.

In colour films there are three sensitive layers with silver bromide coupled to blue, green, and red colour dyes. As in our eye there is also a yellow mask to prevent over-exposure to higher energy light. In colour films the yellow mask is below the blue layer, to protect the green and red layers.

In negatives the image is recorded in complementary colours, so exposure to blue produces yellow on the negative, exposure to green produces purple, and red produces blue-green.

Projection onto colour paper gives us positive prints.

Colour transparencies are the product of a combining the negative and positive stages into the one process.

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Although his colour process had great promise, it was not a commercial success, there were patenting issues and a dispute with Kodak in the United States which could not be resolved, and John Joly moved on to other interests, especially in geology, where his work in calorimetry led to the first scientifically valid estimate of the earth’s age.

One of Ireland’s distinguished scientists, John Joly died aged 76, in 1933.

John Joly

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ohn Joly, born at Bracknagh, Co Offaly, in 1857, began his career as assistant to the Professor of Civil Engineering at Trinity College Dublin, where he eventually became Professor of Geology and Mineralogy. John Joly was a prolific inventor, and his colour process is just one of the projects he worked on. Among his patented inventions was a telegraphic barometer, a photometer, and a solar sextant.

also three sets of phosphorescent dots. The glowing dots are tiny, and as in printing, the primaries merge to produce an image of continuous colour.

Apart from merging dots which are too small for us to see at normal viewing distance, experiments in which colours are rapidly projected also show that we can perceive two separate colours as a combination. If a red light is projected for up to 70 milliseconds, followed immediately by a green light for the same duration, the observer will see yellow.

COLOUR TELEVISION

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he image on a television is produced by a stream of electrons striking thousands of phosphorescent dots on the inside of the screen. The electrons are beamed out from the back of the tube and the phosphorescent dots glow when struck. Some of the lesser known ‘rare earth’ minerals are used to create the colours, such as salts of europium for reds, and terbium for green.

With black and white there is one stream of electrons, but for colour there are three electron guns, one for red, one for green, and one for blue. There are 90

COLOUR right angles to each other, so without the crystals in between, no light would pass through. In a typical laptop screen, about two million transistors are able to regulate the twisting with great precision, thus controlling the amount of light passing through, and by having three sub-pixels, filtered for red, green, and blue, we get full colour.

FLAT SCREENS The change over to liquid crystal displays, LCDs, and plasma technology has made screens a lot slimmer.

Compact laptops could not have been developed without LCD, and plasma screens are bringing cinema quality television into the home.

How does plasma work?

The screen, enclosed between two flat plates, is made up of tiny gas filled cells, each of which contains three chemical phosphors, similar to those used in a normal television. An inert gas, such as xenon, is used, and in the resting stage it has no overall charge. On applying an electrical charge the gas becomes excited, the atoms become positive ions, and, as in a fluorescent tube, ultra violet light, which we cannot see, is emitted. This UV light, in turn, excites the phosphors, and on dropping back to their ground state visible light in red, green, or blue is emitted. Millions of these pixels lighting up give us a very clear, distortion free picture.

The process is triggered from fine electrodes placed behind and in front of the pixels. The electrodes are arranged in rows, and those in front, which are transparent, are set at right angles to those at the back, so together they can act as a controlling grid.

How does LCD work?

Liquid crystals are peculiar in that they share some of the characteristics of solids and liquids. Like solids they have a regular crystalline shape, but under certain circumstances the crystals can move around as if they were liquid. For long regarded as a chemical curiosity, liquid crystals have become very useful in electronics. One particular type of crystal has a twisted shape, and the degree of twisting turns the plane of light passing through. Because the twisting can be controlled by electrical current, these liquid crystals can be used to make flat screen LCDs. The crystals, all facing the same way, are laid down in rows to create one layer. Another, matching set of rows is laid down at right angles to make a second row, and these are sandwiched between two polarizing filters. The polarizing filters are also at 91

COLOUR constant, represented by the letter E, on the right hand side of the equation. The value obtained for E is the energy possessed by an electron in an orbital represented by y. . Knowing the form of the appropriate wavefunctions allows the behaviour of electrons in atoms to be predicted. The square of the amplitude of the wave at a given point in a molecule, gives the probability of finding an electron at that point in the molecule. This allows electron density maps to be constructed for the molecule. Obtaining the value of the energy associated with each orbital shows the energy levels available to the electrons in a molecule. Applying certain quantum mechanical rules, known as ‘selection rules’ shows what electronic transitions are the most probable and enables interpretation of the way in which the molecule interacts with light. The more probable the transition, the more molecules in a given sample will absorb light and the more intense the colour will appear. The transitions that occur in organic chromophores, which involve n -> π*or π– > π* are fully allowed by the selection rules, so that it only takes a very small quantity of dye to impart a deep colour to a large quantity of fabric. On the other hand,the absorption of visible light by transition metals complexes, involving the d orbitals, is often ‘forbidden’ to a greater or lesser extent by the selection rules, so that absorption of photons occurs with a lower probability and the

resulting colours of inorganic pigments are relatively pale by comparison with organic dyes.

The spectrum of a coloured compound can be measured using an instrument called a spectrometer.

Most modern instruments can display the spectrum on a monitor screen and it can also be printed in the form of a chart. This appears as a plot of absorbance against wavelength, as shown on previous page.

The figure shows absorbance spectra for various transition metal ions in aqueous solution. Notice that copper (Cu) has its maximum absorption at 800 nanometers. This is at the red end of the visible spectrum and explains why copper salts, in solution, appear blue to our eyes. Titanium (III) has a broad peak with its maximum absorption close to 500 nm.

This is in the middle of the visible spectrum, so it is mainly yellow light which is absorbed. Thus solutions of Ti(III) appear violet in colour.

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COLOUR discovered in 1911 by Rutherford, who also showed that the atom is mainly empty space. It was then assumed that the electrons revolved in orbits in the space around the nucleus, much in the same way that planets revolve around the sun in our solar system.

Two years later, the Danish Physicist, Neils Bohr, came up with a model of atomic structure which incorporated this idea, but which also took account of the quantum theory, which had been proposed by Max Planck. However, it was not until the 1920s that the full quantum mechanical model of atomic structure was developed. The notion of electrons moving in fixed orbits was replaced by the concept of ‘orbitals’. This was necessary because of the impossibility, recognised by Heisenberg, of tracking an electron in its path (the uncertainty principle). A similar concept was developed by Louis deBroglie, who suggested treating electrons as waves, rather than particles. Since we cannot pinpoint the exact location of an electron, the best we can hope to achieve is to calculate the probability of finding it at a given point in space. The probability is found by solving the Schrödinger wave equation, which is a differential equation, involving calculus. To solve this equation was a very tedious process in the 1920s, but with modern computer technology, becomes somewhat less daunting. The solution of the wave equation for the atoms of any given element, at least in theory, would

Atomic structure

Quantum Mechanics and Spectra

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he word ‘atom’ is actually a misnomer, because it comes from the Greek, meaning ‘indivisible’ and was originally introduced over 2,000 years ago by Democritus, who gave the name ‘atom’ to the smallest particles of matter, which couldn’t be split into anything smaller. However, there was another school of thought, led by Aristotle, which believed that matter was continuous, rather than discrete. In other words, one could try to go on cutting and grinding ad infinitum but never reach the smallest possible particle.

This view was accepted by the majority of philosophers of that time, so the atomic theory fell out of favour and was forgotten for almost two thousand years.

At the start of the nineteenth century, Higgins in Ireland and Dalton in England re-introduced the atomic theory of matter and recognised that each of the chemical elements is made up of identical atoms, unique to that element. They believed that atoms were the smallest particles of matter and could not be split into anything smaller. However, the discovery of electrons, by J.J. Thompson in the 1890s, revealed the existence of sub-atomic particles. The nucleus was 93

COLOUR provide us with all the information we need to know two electrons. in order to predict the properties of that element. But Both the Bohr model and the Schrödinger model of in practice, it becomes more complicated the more atomic structure involve the concept of ‘quantised’ electrons there are. Now the number of electrons in an energy levels. This means that an electron in an atom atom is determined by the atomic number of that cannot possess any arbitrary energy, but only the element, which is also the number of positively energy associated with the particular orbital it charged protons in the nucleus, so that for atoms with happens to be in. To understand the concept of a high atomic number, approximations have to be discrete, or quantised, energy levels, a simple analogy made. In fact, it turns out that the wave equation can may be helpful. Consider two men working on a be solved exactly only for the hydrogen atom, which building site. Both wish to climb to the top of a high has just one electron. Nevertheless, approximations wall, but one uses a ladder, while the other walks up yield reasonably good results. From a mathematical along a wooden plank, which is conveniently leaning point of view, the wave equation would yield an against the wall. The higher the men climb, the greater infinite number of solutions, but in order to make is their gravitational potential energy, which is a physical sense in the real world, we have to impose measure of the amount of work they have to do in certain ‘boundary conditions’, which order to reach a given height. But the limit the number of solutions to those man on the ladder is, in effect, DISCHARGE specified by certain quantum numbers. restricted to certain ‘allowed’ Passing electricity through a These are denoted by letters s, p, d and gravitational potential energies. He can gas causes it to glow, and if f and they specify the shape of the possess only energies corresponding to mercury is present ultra violet orbital. It is found that s orbitals are the height of each rung above the light is emitted. In fluorescent highly symmetrical, having a spherical ground; it is not possible for him to lamps electricity is passed shape, while p, d and f orbitals are less stop at any level in between the rungs through mercury vapour, and symmetrical, but can be distinguished of the ladder. The man on the plank, the inner surface of the tube is by characteristic symmetry properties. however, can stop to rest at any point coated with phosphorescent An important rule, known as the Pauli along the plank, so that there is a chemicals which glow on exclusion principle, states that each continuum of energies available to him exposure to UV. orbital can accommodate no more than Now, an electron in an atom is like the 94

COLOUR man on the ladder. There are certain energies available to it, each corresponding to the energy of a particular atomic orbital. As long as an electron remains in a given orbital, its energy remains constant. However, it is possible for an electron to be ‘excited’ to a higher energy level or orbital, for example by heat, electricity or light of a specific wavelength. Once excited, the electron finds itself in an unstable, high-energy state.

It cannot remain in an excited state indefinitely, so it will eventually drop down to the ground state again.

This is exploited in the familiar sodium street lights, which contain sodium vapour, whose atoms are being excited by a continuous electric discharge, which causes so many of the outer electrons to be raised to higher energy levels that at any given time, there are more atoms in the excited state than in the ground state. This situation is known as ‘population inversion’.

The excited electrons, being in an unstable state, don’t remain in that state for long, but soon drop down to the ground state, emitting light of a characteristic wavelength as they do so. It so happens that photons of yellow-amber light correspond in energy to the difference between the excited and ground states of the sodium atom. This is the reason for the peculiar colour emitted by this type of street lamp. If an electric discharge tube is filled with a gas or vapour of a different element,

light of a different colour is emitted. Illuminated advertising signs use neon gas to produce the characteristic red colour of neon lights. Mercury street lights emit light containing a mixture of wavelengths, corresponding to several electronic transitions, which can occur between the available energy levels of the mercury atoms. Because there are a number of wavelengths present, the light is almost white, but has a slight bluish-green appearance. This is in contrast to the sodium light, which is almost monochromatic.

But, these examples involve emission of light, which is what occurs when there is a population inversion.

This occurs in glowing gases and vapours, which have been excited by heat or electricity. The examples given above involve elements, rather than compounds. Net

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absorption occurs when there are more atoms in the ground state than in the excited state.

Most of the coloured materials we encounter in everyday life are compounds rather than elements, but the quantum mechanical theories described above can also be applied to compounds. When elements, which are the simplest form of matter, combine to form compounds, their atoms interact. The atomic orbitals overlap to form molecular orbitals and the Schrödinger wave equation can be applied to these molecular orbitals too. In its simplest form, this equation may be written as follows:

y represents the wavefunction; H stands for an operator, known as the Hamiltonian; (named after the famous Irish mathematician Rowan Hamilton) E stands for the energy associated with the orbital.

So the wave equation states that if we operate on the wavefunction, with the Hamiltonian operator (which contains a number of terms representing the kinetic and potential energy of the electron) the result is the same wavefunction, now multiplied by a

A number of natural substances absorb ultraviolet light causing them to emit visible light.

UV fluorescence is often used in microscopic examination of specimens, because it brings up details in vivid colours. With many samples no special treatment is needed, but UV sensitive dyes can also be used to stain specimens.

Fluroscence does not always require UV light, the electron beams in a TV tube cause chemicals, such as zinc sulphide to emit visible light, and similar compounds are used to make safety strips along stairwells which continue to glow faintly when lights fail.

Fluorescence

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ome substances, dull in ordinary light, glow strongly when illuminated by ultraviolet. By shining an ultraviolet lamp vets can see if pets have ringworm, a fungal infection of the skin, and in much the same way blouses and shirts become ultra-bright in the UV lights of a nightclub. Because the UV itself is invisible to us, the glow is strange and intense against the dark background.

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Fluorescence A green fluorescent protein from a species of jellyfish has become widely used in microscopic examination of tissues. The fluorescent protein binds to its target acting as a label. At a specific wavelength the labelled targets glow, so they stand out from an otherwise confusing background. In this case the mitochondrial intracellular network in canine kidney epithelial cells was picked out by the yellow fluorescent tag To create this image a number of greyscale pictures were made for different wavelengths, and these were then colour coded and recombined.

Image: Olympus

COLOUR

Spectroscopy

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pectroscopy is the name given to that branch of science, which studies the interaction of electromagnetic radiation with matter. In the late nineteenth and early twentieth centuries, spectroscopic studies enabled Bohr and others to come to a greater understanding of the nature of matter. It has provided scientists with much information about the structure of atoms and molecules. Now that we have a greater understanding of chemical bonding and electronic energy levels in atoms and molecules, spectroscopy has become an important tool for chemists. Today, most modern laboratories are equipped with a variety of spectrometers, which are used for chemical analysis. These instruments are capable of detecting and measuring the quantity of coloured substance present in samples at extremely low concentration.

Such measurements are important in the quality control of products such as printing inks, food colourings or dyes used in cosmetics or textile fabrics. The method may be extended to the analysis of substances which are not themselves coloured, but which can be treated with a specific reagent to produce a colour, before using a spectrophotometric measurement. The fact that each substance has its own unique absorbance spectrum makes these measurements highly selective. They are also extremely sensitive, as extremely low levels can be detected and measured. Today, spectroscopic methods of analysis have become routine in most laboratories. However, none of this would have been possible without a knowledge and understanding of the quantum mechanical model of the atom.

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Magnesium produces a brilliant white, calcium and barium give different shades of green, and potassium gives a pale lilac.

The reason each of these elements produces a different colour is because the atomic structure of each is unique. The atoms of each element possess a set of 'allowed' energy levels which the electrons can occupy, with a unique spacing between the levels. It is the energy difference between these allowed energy levels that gives rise to the unique pattern of wavelengths in the photons emitted when the atoms relax to their ground state, following excitation by heat, light or electricity. So each element is characterised by its emission spectrum. In the case of certain elements emissions occurs in the visible region of the spectrum and so can be detected by our eyes.

FIREWORKS

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n sodium street lamps light is emitted when the electrons, excited by electricity, drop back to their ground state. In fireworks the energy comes from the heat of the explosion, and the brightness and colour is produced as the atoms return to their ground state. As in the discharge lamps, sodium produces a strong orange yellow, and other metals also produce distinctive colours.

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COLOUR more photons in the same wavelength, so light in a well defined range is amplified for release as a single controllable beam. With powerful lasers the beam can be intense enough to cut through metal.

Lasers development originated with research by Charles H Townes into spectroscopy in the 1940s.

Townes had been with Bell Labs, and then he went to work at Columbia University where he was joined by Arthur L Schawlow, a young phycicist and his son-inlaw. The researchers wanted to produce shorter wavelengths to achieve a stronger reaction from molecules, and in 1953 Townes, with two others, Gordon and Zeiger, were able to demonstrate a device producing intense microwaves, the Microwave Amplification Stimulated Emission of Radiation, MASER. At Bell Labs, Townes and Schawlow continued to collaborate on shorter wavelengths, and in 1958, two years before a working device was built, they took out a patent on the laser. At the time no one anticipated the significance of the invention, and the researchers themselves thought that their work might be useful in communications or, as originally planned, analysis.

Later, after both scientists had been awarded Nobel prizes, Schawlow commented that the discovery showed the value of pure research. No one, he observed, could have deliberately set out to invent lasers.

Laser research at the Optronics Ireland centre at TCD

LASERS

Light Amplification by Stimulated Emmission of Radiation

I

n short, lasers work by generating and concentrating light of a single or very narrow band of wavelengths and ‘pumping’ it up to a higher intensity. The light is produced by exciting a suitable material by chemical, electrical or optical means, and the first laser, developed in the 1960s used a ruby to generate a narrow band of red. Since then a variety of materials have come into use including carbon dioxide, argon, metals and their vapours. As photons are produced they are reflected back into the material and this triggers the release of 100

COLOUR action, the substances in the spot separate out.

In column chromatography the stationary material, enclosed in a tube, might be silica gel and the moving phase a solvent such as acetone. The solution is fed in from the top of the column, and over time the different constituents emerge from the bottom. In high speed liquid chromatograpy, HSLC, the same process is automated and the moving phase is forced through the column by pressure. In HSLC the emerging liquid is analysed and charted continuously, and the method is widely used in examination of small, 1µg to 100 mg samples.

In gas chromatography, GC, or GLC, the stationary material consists of crushed minerals coated with oil, and the moving phase is a gas such as nitrogen. The sample is introduced hot, 50 to 200 degrees C, and as in HSLC the emerging compounds are continuously recorded. This type of chromatography can be used for difficult separations, such as between chemically similar isomers.

The basic concept involved in separation was further developed in ion exchange chromatography, where particles, such as proteins, can be selected out by differences in their charge. Ion exchange chromatography is extensively used in the pharmaceutical industry for purification of proteins.

Chromatography

I

n 1903 a Russian botanist, Michael Tswett, developed an ingenious method of separating plant pigments, and this is generally thought to have marked the beginning of chromatography. The principle of the technique is extremely simple: simply dipping the end of a paper into a coloured solution makes it act like the wick in an oil lamp, and the dissolved pigments will slowly creep upwards. If different substances are dissolved in the solution, they will creep upwards at different rates, so the further they go the more they become separated.

With paper dipped into a solution there is no need to force the process, but the same sort of approach can be applied by pushing dissolved compounds through a variety of fixed media rather than paper. Chromatography has developed into a powerful tool both for analysis and for separation out of substances which may be hard to purify by any other methods.

Various types of chromatography have been developed, such as thin layer chromatography, TLC, where the stationary absorbent is coated onto glass, constituting the thin layer, and the sample is deposited near one edge as a spot. One end of the plate is dipped into the solvent. In rising up the plate by capillary 101

COLOUR While the following charts are presented here in their original order, testers were always advised to mix them up to confound the fakers.

The first colour test charts are thought to have been the ones devised by a professor in Strasbourg, J Stilling, in 1876. These presented numbers and shapes against a confusing background of colours. This approach was further refined and indeed perfected by Ishihara Shinobu, who began his medical career in 1905 as an army doctor. In 1908 he began opthalmic research at Tokyo University, where he was asked to develop a colour vision test for screening army recruits. Ishihara began composing hand painted charts, aided by his colour blind assistant. The quality of these charts was so much admired that in 1917 he prepared a set using Arabic numerals as the ‘international’ edition, and eventually these became the standard for army and air force tests. With the defeat of Japan at the close of World War II rights for the Ishihara charts were taken over by the Allied forces, and new editions went into circulation. Since then Ishihara charts have been widely used in assessing colour vision throughout the world.

Testing colour sight

W

e seldom pause to consider the fact that we might have our own unique view of the world.

There is nothing to tell us if our neighbours see the same reds and greens for perception is highly subjective. However we can be more certain about differences in ability to see, or distinguish between particular colours. Some of our light receptor cones, for example, may be defective, or missing, so it is possible to be insensitive to certain colours. Many people go through life perfectly unaware of an inability to distinguish between certain colours, and there is strong evidence to suggest that the mind compensates by enriching other hues, but in some jobs this can become a problem. For example. a train driver who cannot readily see the difference between a red stop sign and green for proceed is more likely to make a serious error.

Several forms of colour blindness exist and once the condition became known various tests were developed for doctors and recruiters. For some reason armies around the world became obsessed with colour vision, and apart from disguising flat feet, many aspiring warriors became highly proficient at faking eye tests.

On the following pages all 32 of the Ishihara plates are reproduced in process (CMYK) colours.

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COLOUR

Reading the chart

evidence to suggest that a a failure to distinguish between hues is compensated for by enhanced perception of other qualities.

The charts can be viewed from a normal reading distance, and ideally they should be looked at in isolation, so if concentrating on one is difficult, simply cover the others. Some of the charts are much easier to read than others, but others require closer attention, and of course response is going to vary — it is a test after all.

W

hile the following colour charts are of diagnostic value it should be remembered that vision and perception of colour is not just a simple response to red, green, and blue light. Except in extreme cases the term colour blindness is not really accurate. Our response to light is actually spread over a range of wavelengths, and what we see is not physically in the eye, but in the mind. Our mind constructs colour from the information available to it and there is strong

1 Normal and colour blind read this the same way.

2 Normal reads the figure as 8, red-green blind read it as 3.

3 Normal read the figure as 6, the red-green blind read it as 5.

4 Normal read the figures as 29. The red green read them as 70.

5 Normal read figures as 57, the red-green blind read them as 35, and totally colour blind read can hardly read the figures at all.

6 Normal read the figure as 5, the red green blind read it as 2, and totally colour blind can hardly read the figures at all.

7 Normal read the figure as 3, the red green blind read it as 5, the totally colour blind can hardly read the figure at all.

8 Normal read figure as 15, red-green blind read them as 17, and totally colour blind can hardly read the figures at all.

9 Normal read the figures as 74, red-green blind read them as 21, the totally colour blind can hardly read the figures at all.

10 Normal read the figure as 2, and the colour blind can hardly read the figure.

11 Normal read the figure as 6, but the colour blind can hardly read it.

103

COLOUR 12 Normal read the figure as 97, but the colour blind can hardly read them.

13 Normal read the figure as 45, but the colour blind can hardly read them.

14 Normal read the figure as 5 but the colour blind can hardly read this.

15 Normal read the figure as 7 but the colour blind can hardly see this.

16 Normal read the figures as 16, but the colour blind hardly see them.

17 Normal read the figures as 73, but colour blind can hardly see them.

18 Most red-green blind read the figure as 5. The majority of normal, and the totally colour blind can barely read the figure.

19 Most of the red-green blind read the figure as 2.

The majority of normal, and the totally colour-blind, can barely read the figure.

20 Most red-green blind read the figure as 45. Most normal and the totally colour-blind can barely read the figure.

21 Most red-green blind read the figure as 73. Most of the normal and the totally colour-blind can hardly read the figures.

22 The completely colour blind read the figure as 6.

The completely green blind read it as 2. The normal

and the incomplete red-green blind read the figures as 26.

23 The completely red-blind read the figure as 2. The completely green-blind read the figure as 4. The normal and the incompletely red-green blind read the figures as 42.

24 The complete red-blind read the figure as 5. The completely green blind read the figure as 3. The normal and the incomplete red-green blind read the figures as 35.

25 The completely red-blind read the figure as 6. The completely green-blind read the figure as 9. The normal and the incomplete red-green blind read the figures as 96.

26, 27, 28 and 29. These are read by following a colour from one X to the other. Most can follow the line, but it is difficult for the colour blind.

30 and 31 Read by following the winding line of colour from one X to the other. Most normal and the totally colour blind find this difficult or impossible, but the majority of red-green blind can follow the line.

32 Both colour-blind and normal can follow the winding line.

104

COLOUR

1

2

3

4

5

6

105

COLOUR

7

8

9

10

11

12

106

COLOUR

13

14

15

16

17

18

107

COLOUR

19

20

21

22

23

24

108

COLOUR

25

26

27

28

29

30

109

COLOUR

INDEX

31

32

Absorption of colours 12 Additive colour 20 Agfa 88 Altimira 65 Animal colouring 46 Animal sight 50 Annals of Four Masters 66 Annatto 44 Anthocyanins 43 Arisatole 92 Atomic numbers 93 Atomic structure 11, 92 Aurora Australis 31 Aurora Borealis 31 Autochrome 88 Azo dyes 17 Baeyer, Adolf von 43, 73 Bell Labs 100 Berbers 66 Bergman, Igmar 61 Bilichromes 47 Bilirubin 47 Bioluminescence 48 Bixus orcellana 44 Black and white art 59 Blind spot 76 Bluestone 18 110

Bohr. Niels 92 Book of Kells 66 Bubbles 32 Canary Islands 67 Carminic acid 67 Carotene 15 Carotenoids 43, 46, 47 Cartamus tinctorius 67 Carthamin 67 Cartooning 59 Cave art 65 Celts 66 China 72 Chlorophyll 19 Chromatic aberration 37 Chromatography 101 Chromatophores 47 Chrozophora tinctoria 66 CIE 25 CIE, Commission Internationale d’Eclairage 24 Cinchona 11 Classifying colour 22 Clothes 72 Coccus cacti 67 Cochineal 67 Colour blindness 80

COLOUR Colour blindness test 102 Colour classification 23 Colour film 89 Colour in print 84 Colour space LAB 25 Colour temperature 52 Colour wheel 22, 23 Colours defined 6 Comics 60 Commission Internationale d’Eclairage 24 Cones 75 Cones sensitivity 79 Conjugated bonds 16 Copper sulphate 18 Crocus 70 Crusaders 70 Cú Chulainn 66 CYMK 84 Dalton, John 80 deHauron, Louis Ducas 88 Democritus 92 Disney, Walt 60 Drebbel 67 Durham, Tim 32 E160 colouring 44 Edge definition 59 Elderberry 44 Electromagnetic spectrum 13, 14, 37 Eochaidh Eadghadhach 66

Eye colour 76 Eye structure 74 Farben I G company 11 Film 60 Fireworks 99 Fireworms 48 Flavenoids 43, 47 Fluorescence 96 Fovea 76 Gaelic Ireland 70 Galium verum 43 Gene influence 80 Graphocolor 86 Grubb 88 Guanin 46 Haem structure 18 Haemoglobin 18, 46 Haemoglocyanin 46 Heat 52 Henry VIII 71 Hinton 70 Hubble, Erwin 29 Indican 43 Indigo 43, 73 Indigofera tinctoria 43, 73 Indigofers sumetrana 73 Inishkea 66 Inks 63 Inks printing 86 Interference 34 Iridescence 27

Iris 76 Isatis tinctoria 43 Ishihara, Shinobu 102 Joly, John 88, 90 Kodachrome 88 Korghoz Lady’s Bedstraw 43 Lambert, Johann Heinrich 22 Lascaux 65 Lasers 100 Le Blon, Jakob 22 Leeuwenhoek A van 67 Lein-croich 70 Light speed 14 Ligustrum vulgare 43 Line art 59 Lonchocarpus eyenescens 73 Luciferase 48 Luciferin 48 Macula 75 Madder 43 Maser 100 Maxwell James Clerk 24, 88 Mayer, Tobias 22 Melanin 47 Molecular orbits 15 Monkeys 80 Natural colourings 47 Natural colours 42 Neon signs 94 Nero 70 111

Newton Isaac 20, 22 Night sight 78 Northern lights 31 Oak galls 63 Occipital lobe 76, 82 Orbitals, atomic 92 Organic dyes 15 Pantone colours 25 Pauli exclusion 93 Perception 82 Periwinkles 66 Perkins, William Henry 11 Photographs colour 88 Photons 13 Photopsins 76 Physiology of sight 75 Plank, Max 92 Polygonu tinctorium 73 Population inversion 94 Porphyrins 18 Primaries 79 Primary colours 7, 20 Process colour 22, 84 Pterins 47 Pupils 75 Quantum mechanics 15 Quinones 47 Radio telescopes 38 Raleigh scattering 30 Raleigh, Lord 30 Redshift cosmological 29

COLOUR Reflected light 12 Refraction 37 Retina 75 Retina implant 78 Retinal 75 Rhodopsin 75 Rods 75 Safflower 67 Saffron 70 Sambucus nigra 44 Schawlow 100 Scherotins 47 Schrödinger 95 Scotopsin 75 Senecio jacobea 43 Senses 82 Shampoo 44 Sight artificial 76 Sky colour 27 Slides 89 Sodium street lamps 94 Spectroscopy 98 Spielberg, Stephen 61 Status social 65 Stilling J 102 Strauss, Levi 73 Strutt W 28 Subtractive colour 20 Sumerians 73 Synthesia 83 Synthetic dyes 43

Taste 83 Thalamus 82 Thermal images 52 Thompson J J 92 Tighearmas 66 Townes, Charles H 100 Transition elements 17 Transparencies 89 Trichromatic Mixing Triangle 22 Tswett, Michael 101 Tyndall effect 27 Tyndall Institute 78 Tyndall, John 27, 28 Vegetable dyes 43 Visual purple 75 Vitamin A 75 Wave equations 95 Woad 43 X chromosome 80

112

Back cover Evening light over the Liffey.

Photograph: Tom Kennedy

COLOUR

WHAT WE SEE AND THE SCIENCE BEHIND SIGHT

Albertine Kennedy Publishing