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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

AIC Meeting – Mar del Plata, Argentina – October 2010

Introduction to Colour Measurement Lindsay MacDonald Professor of Digital Media

London College of Communication

Overview Part I Colorimetry – Spectral nature of light, object, observer

Part II Measurement and colour differences

Three Elements of Colour

Newton’s Experiment

Radiant or Emitted (direct)

Observer

Source Illumination

Reflected

Absorbed

Newton’s birthplace at Woolsthorpe Manor, Lincs.

Object Transmitted

Prof. Lindsay MacDonald, London College of Communication

Using a prism to produce a spectrum, Trinity College, 1665

1


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Newton’s Apparatus

Demonstration Dispersion of white light into the spectrum.

Visible Light in the Electromagnetic Spectrum Gamma Rays Cosmic Rays

Ultra-Violet X-Rays

Infra-Red Radar Microwaves

VHF Radio

Radio

Television

Division of the Spectrum

UV violet

SHORT

MEDIUM

LONG

Blue, cyan

Green, yellow

Red, orange

indigo g

blue

green g

yyellow

Orange g

IR red

Visible Light violet indigo

blue

green

yellow

Orange

red 400 nm

400 nm

500 nm

600 nm

600 nm

700 nm

The colour of a light depends on which part of the spectrum contains the greatest power.

700 nm

Barely one octave out of 40.

Spectral Power Distribution 

500 nm

Incandescent Tungsten Light Continuous smooth spectrum, rising toward longer wavelengths.

Abbreviation = SPD Power (watts) = energy (joules) x time (seconds)

Fundamental property of any light source. source

Shows how power varies as a function of wavelength throughout visible spectrum. Two main types: continuous and spiky.

Wavelength (nm)

Graph as power (Y axis) at each wavelength (X axis).

Prof. Lindsay MacDonald, London College of Communication

Relative Power per un nit Wavelength

Powe er (Watts)

Spectral Power Distribution (SPD) 250

200 150

100

50 0 380 405 430 455 480 505 530 555 580 605 630 655 680 705 730

Wavelength (nm)

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Fluorescent Light

Exercise

Relative Power per un nit Wavelength

Spiky spectrum, with power concentrated at a few wavelengths.

Use a spectroscope to view various light sources in room.

80 70

Observe whether each spectrum is smooth or spiky.

60 50 40 30

violet

indigo

blue

green

yellow

Orange

red

20 10 0 380 405 430 455 480 505 530 555 580 605 630 655 680 705 730

Wavelength (nm)

400 nm

500 nm

600 nm

700 nm

High Pressure Sodium Light

Smooth and Spiky Spectra

Berns, p.5

Observer

Relative Power per unit W Wavelength

Light source

http://www.iupac.org/didac/Diidac%20Eng/Didac03

Spiky spectrum, with power concentrated at a few wavelengths.

Wavelength (nm)

ďƒ˜

Sources with smooth spectra appear continuous. ďƒ˜ Sources with spiky spectra appear as a series of lines.

High pressure sodium lamps are widely used for lighting urban streets. They are preferred for their high efficacy rather than colour rendering.

Daylight

Relative Power per unit Wavelength

Cooler

Warmer

140 120 100 80 60 40 20 0 380 405 430 455 480 505 530 555 580 605 630 655 680 705 730

Wavelength (nm)

Prof. Lindsay MacDonald, London College of Communication

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Outdoor Illumination

Colour Temperature

Reflected skylight – causes the shadows to be bluish.

Directt Di sunlight – causes the lit side of objects to be yellowish.

Colour temperature is expressed in Kelvin (K). As the temperature increases blackbody increases, (Planckian) radiators emit more power and the spectrum moves toward shorter wavelengths.

Hunt, p.85

Every outdoor scene is lit by two sources: direct sunlight indirect skylight

http://www.itchy-animation.co.uk/tutorials/light03.htm

Colour Temperature

Correlated Colour Temperature

Relative SPD of blackbody (or Planckian) radiators at temperatures of 2850K, 5000K and 10,000K.

12000

Clear blue sky at noon

11000 10000 Typical desktop computer CRT display 9000

Temperature, in Kelvin, of a blackbody radiator that most closely resembles the colour of a stimulus of equal brightness.

Normalised at 560 nm a

E E560

Berns, p.4

S 

8000 North facing sky light 7000 Overcast sky at noon 6000

Sunlight at noon

5000

Graphic arts viewing standard

4000

Cool white fluorescent

3000

Photoflood tungsten Domestic tungsten

2000

Sunlight at sunset Candle light

CIE Standard Illuminants

Sources and Illuminants 

Illuminant A - Incandescent & tungsten light sources

Illuminant D - CIE Daylight Series

– Used in homes & as accent lighting in stores. Correlated colour temperature ~2850K.

A light source is a physical originator of light, such as a lamp, laser or the sun.

An illuminant is a numerical representation defined by a SPD. It may or may not be possible to make a real light source to produce the SPD.

– D50 - Noon Sky Daylight at 5000K (yellowish shade) • Used for colour quality evaluation in the graphic arts industry, as specified in ANSI standard PH 2.32 and ISO standard 3664. – D65 - Average North Sky Daylight at 6500K (neutral shade) • Conforms to international standards in Europe, the Far East, and South America. • Used to correlate with instrumental measurements in textiles and television. – D75 - North Sky Daylight at 7500K (blue-ish shade) • Used for design and visual evaluation of opaque materials as recommended by ASTM (American Society for Testing and Materials). 

Illuminant E - Equi-Energy

Illuminant F - Fluorescent Light Series

– Hypothetical source having equal power at all wavelengths across the visible spectrum. – F2 Cool white fluorescent (CWF) a common wide band fluorescent used in the USA. – F7 Broad band white fluorescent. Common in the USA. – F11 TL83, TL84 are narrow tri-band fluorescent common in Europe & Asia.

Prof. Lindsay MacDonald, London College of Communication

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

D50 simulator used as standard source for viewing of prints in the graphic arts.

Simulation of D50

Field, p.5

Fluorescent tube approximates theoretical SPD of D50 defined by CIE.

Three Elements of Colour

Reflected Colour

Radiant or Emitted (direct)

The surface reflects wavelengths of the incident light that are not absorbed. Observer

Source

Incident ray colour of light

Illumination

Reflected ray colour of surface

Reflected

Absorbed

Light source

Reflective surface

Object Transmitted

Interaction of Light with Material Incident light

Transmitted Colour

Specular reflection

The material transmits wavelengths of the incident light that are not absorbed.

Diffuse reflection SURFACE

Hunter & Harold, p.30

Transmitted ray colour of material

Reflection is a mixture of specular and diffuse

Yellow pigment particles

Incident ray colour of light Light source Transmissive material (filter)

Transmission

Prof. Lindsay MacDonald, London College of Communication

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Three Elements of Colour

Demonstration How R,G,B filters absorb different wavelengths of white light out of the spectrum.

Radiant or Emitted (direct)

Observer

Source Illumination

Reflected

Absorbed

Object Transmitted

Structure of the Human Eye

Cross-Section through Retina

Cross-section of the right eye, looking from above. Signals

EAR SIDE

Light NOSE SIDE

red green blue Demonstration that colour vision is trichromatic.

Prof. Lindsay MacDonald, London College of Communication

Colour after-images – a retinal illusion

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Spectral Sensitivity of the Cones

Color Deficiency

Three broad overlapping curves give different responses in different regions of the spectrum.

Relative Senssitivity

Short Medium

Lacking one of the cone photopigments

Red-green Red green confusion is most common form

Affects 1 in 12 adult males (8% population)

Long

Ishihara isochromatic test figure 400 nm

500 nm

600 nm

700 nm

Wavelength (nm)

Spectral Luminous Efficiency

Colour Matching Experiment

Perceived relative luminance of a monochromatic stimulus at each wavelength throughout the visible spectrum.

Relative respo onse

1.2

V'( ) V( )

1

Scotopic (Rod vision)

08 0.8

Photopic (Cone vision)

0.6

Bi-partite Field

Monochromatic test light generated by successive wavelengths throughout the visible spectrum.

Screen

R Projected Test Colour

0.4 0.2

G

B Primary Lamps with Controls for Intensity

Observer

0 -0.2

The observer adjusts an additive combination of three primary lights to make a visual match with a test light.

Wavelength (nm)

Transformation of Primaries

The CIE Standard Observer Standardised in 1931 by CIE. The functions were defined as an average of only 17 observers in two separate experiments.

2 1.8

Tristimulus Values

Spectral Sensitivity Curves (or Colour Matching Functions) 4

Guild’s matching functions (1931) ( (Primaries 700nm,, 564nm,, 436nm))

3

1931 CIE 2-deg Tristimulus Functions

1.4 1.2

y

1

x 

0.8 0.6 0.4 02 0.2

Negative

Intensity of light required to achieve match

z 

1.6

0 380 405 430 455 480 505 530 555 580 605 630 655 680 705

2

Wavelength (nm)

x-bar

y-bar

z-bar

1

Matrix transformation removes the negative lobes of the three functions. 0

-1 405 430 455 480 505 530 555 580 605 630 655 680 705

0.49 0.31 0.20  x( )   r(  )   y( )  0.17697 0.81240 0.01063   g ( )      0.00 0.01 0.99  z( )    b ( )

Wavelength (nm)

Prof. Lindsay MacDonald, London College of Communication

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Three Elements of Colour

Computation of XYZ Tristimulus value depends on the illumination, object and observer!

Radiant or Emitted (direct)

Power (Watts)

Reflected

Absorbed

X

Reflectance (R) %

Illumination

X

Wavelength (nm)

Standard Observer

X

Object

X

Tristimulus functions

Source

Observer

Source

Wavelength (nm)

=

X,Y,Z Colour

=

X = 13.54 Y = 14.36 Z = 47.89

Wavelength (nm)

Tristimulus Value

Object Transmitted

Example

Computation of XYZ Relative Spectral Power

380

780

 S ( ) R ( ) 

z ( )

X Wavelength (nm)

Tristimulus functions

X Wavelength (nm)

Reflectance (R) %

 S ( ) R( ) y( )

 380

Z k

Standard Observer

X

=

X,Y,Z Colour

=

X = 13.54 Y = 14.36 Z = 47.89

780

Power (Watts)

Y k

Object

X

Lighter Object = Blue

D65

X Wavelength (nm)

Wavelength (nm)

Darker

Observer = CIE 2 Degree

X

Tristimulus functions

Source

Reflectance (R) %

780

 S ( ) R( ) x( ) 

Relative Power S (watts)

X k

Wavelength (nm)

Wavelength (nm)

380

Where :

S   Spectral power; normally a CIE standard illuminant R    Spectral reflectance or transmittance

x ( ), y ( ), z ( )  Tristimulus functions of the CIE standard observer

  wavelength in nm k  normalising factor, usually determined when Y  100 (for a perfect white diffuser)

Metamerism

Light Blue X=30.05; Y=37.36; Z=82.57 Dark Blue X=9.46; Y=8.13; Z=31.5

Illuminant Metamerism

If two objects have different spectral power distributions but the same tristimulus values, they are considered to be a metameric pair.  Metamerism makes trichromatic colour imaging practical!  It is only necessary to produce a stimulus that is visually equivalent q to the original. g  Metamerism is the matching of stimuli -- not the matching of objects!

X 1 Y 1 Z1 = X 2 Y 2 Z2

Prof. Lindsay MacDonald, London College of Communication

Two colours may appear to match under one source, but appear completely different under another source.

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Metameric Spectra

Demonstration

0.7

Metamerism of fabric and paint samples under different light sources.

Reflectance e factor

0.6 0.5 0.4 0.3 0.2 0.1 0.0 380

480

580

680

Wavelength (nm)

Colour Communication The reflectance spectrum is a means of precise colour specification.

The Observer ďƒ˜

Response depends on the spectral sensitivity of photoreceptors.

1

Jenoptik eyelike

Relative sensitivity

0.8

0.6

0.4

0.2

0 380

480

580

680

780

Wavelength (nm)

Human Eye

Digital Camera

51

Observer Metamerism

Colour Rendering Effect of a light source on the colour appearance of an object or scene in comparison with its colour appearance under a reference white light.

Two colours that appear to match for one observer may appear completely different to another observer.

Prof. Lindsay MacDonald, London College of Communication

9


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Summary 

Three elements of colour specification: the light source, object and observer.

Any spectral colour can be matched with three monochromatic primaries.

Interval

Metamerism occurs when two stimuli are visually equivalent – this makes trichromatic colour image reproduction practical!

10 minutes

Light Source

Colour

Object Observer

Spectral Power Distribution Spectral Reflectance / Transmittance Colour Matching Functions

Remember this Colour!

Colour Measuring Instruments Spectroradiometer – measures spectra of radiant energy, either emitted by a source or reflected from a surface. Spectrophotometer – measures reflectance spectra of surface colours (light source included). Tristimulus colorimeter – measures surface colours by passing reflected light through three filters to derive visual stimulus.

Spectroradiometer

Sampling the Spectrum Sharma, p.113

Monochromator

Light source

Lens Scanning device A t Aperture

Slit

Prism or diffraction grating

The spectrum of the light reflected from the sample is sampled differently by the three instruments: – Densitometer

Status-T filters

– Colorimeter

CIE standard observer filters

– Spectrophotometer

Regular intervals of 10nm or 20nm

Photo-detector SPD

Prof. Lindsay MacDonald, London College of Communication

10


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Demonstration – Radiance Measurement Measurement of spectral power distribution of a light source.

Spectroradiometers

Photo Research PR-650

Minolta CS1000

JETI spectroradiometer

Spectroradiometers

Standard Calibration Lamps Lamps of known radiance (for sensitivity) and line spectra (for wavelength) are used as transfer standards from national standards labs (NIST, NPL) for calibration.

Advantages

Disadvantages

May be used for both radiance and colour measurement

May be slow and expensive

Ideal for display colour measurement Can derive many different measurements

Two Modes of Colour Generation Additive

Separate wavelengths of light emitted by one or more sources are added together.

Need to be recalibrated periodically Must have illumination reference for colour Many sources of possible inaccuracy

Demonstration Young’s experiment, showing how colours are produced by overlapping RGB lights.

Subtractive Separate wavelengths of light are subtracted from white light when reflected from, or transmitted through, a material.

Prof. Lindsay MacDonald, London College of Communication

11


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

RGB Spectra of CRT Display

Addition of Emissive Spectra

Spectral power distribution of RGB channels of a desktop CRT display, measured with the X-Rite eye-one Pro spectrophotometer.

Spectral power distribution measured from white equals the sum of the spectra at each wavelength of the individual RGB channels.

4

4

3.5

3.5 3 Red

2

Green Blue

1.5

Power

Power

3 2.5

White

2.5

Red

2

Green Blue

1.5

1

1

0.5

0.5

0

0 350

400

450

500

550

600

650

700

750

350

400

450

500

550

600

650

700

750

Wavelength (nm)

Wavelength (nm)

Demonstration

CMY Filter Spectral Absorption Cyan - absence of red violet

indigo

blue

green

yellow

orange

red

White Light B G R

Cyan

Colours obtained by overlapping C,M,Y filters. 400 nm

500 nm

600 nm

700 nm

Magenta - absence of green violet

indigo

blue

400 nm

green

yellow

500 nm

orange

red

600 nm

indigo

blue

400 nm

The surface reflects wavelengths of the incident light that are not absorbed.

500 nm

orange

red

600 nm

R

White Light B G R

700 nm

Yellow

G R

Reflected ray colour of surface

Blue Object

100 Lighter

Reflectance ffactor (%)

Light source

yellow

B

Magenta

Spectral Reflectance Distributions

Reflected Colour

Incident ray colour of light

green

White Light g B G R

700 nm

Yellow - absence of blue violet

B G

Reflective surface

Curves characterise the colour of an object as a function of the percentage reflection of each wavelength l th across th the visible spectrum.

80 60 40 20 0 380

Darker - more colourant - more absorption - lower reflectance

430

480

530

580

630

680

Wavelength (nm)

Prof. Lindsay MacDonald, London College of Communication

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Spectrophotometer Prism or diffraction grating

Demonstration – Reflectance

Optics Aperture Sample

Photodetector array

Illuminate at 45°

Electronic controller/ digitiser

Measurement of the reflectance spectrum of a surface.

Internal light source R%

Signal processing and data recording

Display

X-Rite eye-one Pro Spectro photometer

Wavelength 

Ideal White and Black

White Paper Reflectancce factor (%)

Refflectance (R)

* A black surface absorbs almost all the incident light energy. Its reflectance curve is ideally a straight horizontal line at 0% reflectance.

%

Spectral Reflectance Distribution

* A white surface reflects all light energy across visible spectrum. Its reflectance curve is ideally a straight horizontal line at 100% reflectance.

White Blue

Black Wavelength (nm)

1.2 1.1 1.0 0.9 0.8

0.1 0 350

For real white surfaces the reflectance is typically in the range 90-95%, while for real black surfaces it is typically in the range 2-3%.

Heavyweight 220g

0.7 0.6 0.5 0.4 0.3 0.2

Acrylic 230g Artshop A3 140g Card 300g Inkjet matte 170g Artshop A2 210 g Laser printer 80g

400

450

500

550

600

650

700

750

Wavelength (nm) The Artshop A3 art paper has the flattest reflectance spectrum, because it is free from optical brighteners that cause the peak at 450nm.

Subtractive Colour – Printing Inks

Spectra of secondary colours are formed as the product at each wavelength of the contributing primaries (e.g. green = cyan x yellow).

1

1

0.9

0.9

0.8

0.8

0.7 0.6

Cyan Magenta Yellow

0.5 0.4 0.3

Reflectance fa actor

Reflectance fa actor

Spectral reflectance factor of CMY prints of a desktop inkjet printer, measured with the GretagMacbeth eye-one spectrophotometer.

Multiplication of Reflective Spectra

0.6 0.5 0.4 0.3

0.2

0.2

0.1

0.1

0

Red Green Blue Cyan Magenta Yellow Black

0.7

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

Prof. Lindsay MacDonald, London College of Communication

350

400

450

500

550

600

650

700

750

Wavelength (nm)

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Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Spectrophotometers

CIE measurement geometries Bidirectional, at specified angles, avoiding specular component – most common for graphic arts

High quality instruments with integrating sphere, to measure total reflectance from surface (all angles).

Diffuse, integrates reflections at all angles

Minolta portable 8/Diffuse

Datacolor

Standard Calibration Tiles

Spectrophotometers

Ceramic tiles of known reflectance are used as transfer standards for calibration of instruments.

Advantages

Disadvantages

Accurate colour

May be slow

Illumination source is built in

Expensive

Ideal for reflective colour measurement (ink on paper)

Need to be recalibrated periodically

Can derive many different measurements

Newton’s Insight “And if at any time I speak of Light and Rays as coloured or endued with Colours, I would be understood to speak not philosophically and properly, but grossly,

Demonstration

and according to such Conceptions as vulgar People in seeing all these Experiments would be apt to frame. For

Newton using a prism to produce a spectrum, Trinity College, 1665

the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour...” Newton, I. Opticks, Dover Edition (1979), Book One, Part II, Prop. II, pp.124-125.

Prof. Lindsay MacDonald, London College of Communication

14


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Computation of XYZ X k

780

 S ( ) R( ) x( )

Source

 380

Z k

Standard Observer

X

=

X,Y,Z Colour

=

X = 13.54 Y = 14.36 Z = 47.89

780

 S ( ) R ( )

z ( )

X Wavelength (nm)

Tristimulus functions

X Wavelength (nm)

Reflectance (R) %

780

 S ( ) R( ) y ( )

 380

Power (Watts)

Y k

Object

X

Wavelength (nm)

 380

Where :

S   Spectral power; normally a CIE standard illuminant R    Spectral reflectance or transmittance

x ( ), y ( ), z ( )  Tristimulus functions of the CIE standard observer

  wavelength in nm k  normalising factor, usually determined when Y  100 (for a perfect white diffuser)

Visual Colorimetry

Tristimulus Colorimeter Filters

 Observer

makes measurement based on visual match between test colour and composite through variable RGB filters.

x

Photo-detector

Sample

y

 Measurements

recorded in terms of the density of each filter required (RGB).

 Still

widely used in industrial applications for liquids, e.g. Tintometer.

z Light source

X = 25 Y = 45 Z = 55 D65/2°

Tristimulus Colorimeters

1931 CIE Standard Observer

Colorimeters

Contact Minolta CA-100 Color Analyser

Advantages

Disadvantages

Relatively cheap

May not give correspond to colour vision, unless great care is taken to match filters to CIE standard observer.

Quick measurement Easy for process control

Non-contact LMT 1200C

Prof. Lindsay MacDonald, London College of Communication

Cannot derive other measurements

15


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Chromaticity Co-ordinates Spectrum locus

X,Y,Z may be normalised to calculate the ratios called chromaticity coordinates.

The 2-D chromaticity diagram is used to specify small colour differences between colour stimuli.

Monochromatic colours lie on a horseshoe-shaped boundary – referred to as the spectrum locus.

The ends of the spectrum locus are connected by straight line known as the purple boundary.

x=

X X +Y + Z

y=

Y X +Y + Z

Chromaticity Co--ordinates Co

Macbeth Color Checker Chart

X X Y  Z Y y X Y  Z

x

Purple boundary

Perceptual Colour Models Chromaticity x,y plot of pixels in Macbeth Color Checker chart.

Opponent primaries

Three dimensions: lightness, colourfulness and hue (L,C,H)

Related to processes of human visual perception

A meaningful way of describing colour

Triangle shows sRGB colour gamut.

Perceptual Dimensions

Conversion XYZ  CIE L*a*b* Normal formulation

           

L*  116

Hue

Value

Hue is the attribute of a visual sensation according to which the area appears to be similar to one or two

Chroma

Y

YW

1

3

 16

L*

Alternative formulation ha b

1 1   X Y 3 3  a*  500   XW  YW   1 1   Y Z 3 3 b*  200    YW ZW  

of the primaries red, yellow, green and blue. for :

Y

YW

b* a*

 0.008856

         

 Y 16   L*  116f  116   Y W  X Y  a*  500f f  Y W   X W  Y Z  f b*  200 f  ZW   Y W where : f  x    x 

1/ 3

else

Value is the perceived lightness/darkness of a colour. The value scale ranges from black to white.

where : X , Y , Z are tristimulu s w w w values of the white reference.

Chroma is the attribute of a visual sensation according to which the area appears to exhibit more or less of its hue, relative to white. Black, greys and white are achromatic.

f

C*ab

    X

XW

for x  0.008856 ;

f  x   903.3 x 

&f

Z

ZW

are similarly defined.

CIELAB is generally preferred for subtractive devices such as printers, film and textiles because of its superior chromatic adaptation to white.

Prof. Lindsay MacDonald, London College of Communication

16


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

CIELAB a*-b* Colour Plane

Dimensions of 1976 CIELAB 

Reference white is always L* = 100

Lightness has typical L* values in range 10 to 95

Red-green Red green (a*) (a ) and blue-yellow blue yellow (b*) (b ) axes are unbounded, but typically have maximum values: a* [-100 to +100] b* [ -80 to +120]

Grey always has a*, b* values = 0 (achromatic)

Reference White

CIE LCH (L*C*hab) 90 0 Yellow +b*

XW,YW,ZW are the tristimulus values of a reference white for which YW is normalised to 100. The reference white can be chosen as: • the illuminant (theoretical, e.g. D65) • the th equi-energy i white hit SE • the actual light source • the media white point (e.g. white paper) • a perfect diffuser (referenced by a white tile)

C*

1800 Green -a*

For a display, media white is normally maximum output, namely the colour generated by signals R = G = B = 255

00 Red +a*

H*

2700 Blue -b*

 a* 

C*ab 

hab 

Remember this Colour?

Is it the same colour as before?

180

2

The LCH colour space is the polar co-ordinate version of CIELAB.

L* is lightness ranging from 0 (black) to 100 (white) – unchanged

C*ab denotes “chroma”, i.e. colourfulness relative to white.

hab represents the hue angle, ranging from 0 to 360 degrees.

hue

  b* 

 b*  arctan    a* 

2

(in degrees)

Colour Comparison The two colours are actually slightly different.

Original

Darker Hue more blue Higher chroma

If not, describe how it is different. The human visual system is much better at relative than absolute colour judgement.

Prof. Lindsay MacDonald, London College of Communication

17


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Colour Difference (2D)

Colour Differences

E* = √(a*2 + b*2) b* 

A colour difference is a quantitative representation of the visual difference between colours of two samples samples.  Small colour differences are used in industry to:

The Euclidean distance in 2D colour plane. plane

C1 E*

b* b

1. Compare production samples against standards for acceptability.

C2 a*

a*

2. Determine the magnitude of adjustment required to convert a product with unacceptable colour to one with acceptable colour. 3. Measure colour changes resulting from exposure of samples to weather, light, laundry or other real or simulated usage.

Colour Difference (3D) 

The magnitude of the total colour difference between two samples can be represented by a single number called Delta E (written E).  A scalar tells only the size of the colour difference, not the direction

E*ab = √(L*2 + a*2 + b*2) L*

The Euclidean distance in 3D colour space. space

Delta E

– For best results, differences of all three L* C* H*, should be monitored.

C1

Colour difference equations calculate E values, which ideally correspond with human visual perception of colour differences at each position of colour space space.  E values are usually scaled so that: 

C2 b*

E = 1 is just perceptible to the human eye (threshold);

a*

E = 2 is just acceptable for typical applications.

Computational Comparison Two colours: Difference or distortion (comparison against reference).

Reference colour Computational C t ti l metric

Measure of difference

Summary 

In the physical domain, the spectrum is key to the measurement of lights and materials.

In the visual domain, trichromatic response is key to colour perception.

The CIE system y of colorimetry yp provides a method of colour specification by definition of: – A standard observer under fixed viewing conditions – Computation of tristimulus values – Transformation into perceptual colour coordinates

Test colour Colour stimuli

Vision model

Source

Observer

Object

Prof. Lindsay MacDonald, London College of Communication

18


Introduction to Colour Measurement

AIC Meeting, Mar del Plata, Argentina, October 2010

Recommended Reading

Lindsay MacDonald Professor of Digital Media L.MacDonald@lcc.arts.ac.uk

Equipment for Demonstrations        

Spectrum generator Spectroscopes JETI TSR X-Rite eye-one Pro RGB/CMY filter gels Metamerism samples Red card Red filters

Prof. Lindsay MacDonald, London College of Communication

19

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Lindsay MacDonald: Introduction to colour measurement (power-point)

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Lindsay MacDonald: Introduction to colour measurement (power-point)

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