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January 2018



LUMENIA WHITE PAPER ON HUMAN CENTRIC LIGHTING Paradigm shifts usually come with different interpretations of the rules and new concepts, until one interpretation prevails and the general rules and standards are formed. INTRODUCTION In the last years, the lighting industry has seen tremendous and rapid changes, especially regarding the production of light and the understanding of different effects of light radiation: The fast development and adoption of the LED technology; The acceptance of light’s non visual effects on people or more correctly - Non Image Formation (NIF). As opinions about the future of HCL vary at the moment, it is important to form the basic and conceptual thinking, which must be based on reliable evidence and research. In this paper we will discuss the following: • BACKGROUND & HISTORY - a historical background to what we today named HCL - Human Centric Lighting; • MELANOPSIN AGE - about the research which lead to the Melanopic Age; • HUMAN CENTRIC LIGHTING - what does it really mean and where are we now; • LUMENIA HCL LIGHTING INSTALLATION - a statement of how Lumenia interprets and intends to implement the HCL concept, the basic design and ergonomic principles.

BACKGROUND - HISTORY The sun and daylight have been worshipped in three glorious periods: the classical antiquity, the heydays of the Arabic culture and the period between 1860 and WW2. The sun and daylight have from very early history had a central place in people´s life and thought. The understanding of how the sun and daylight can improve the habitability of the environment has allowed people to survive in various climates.

From the oldest days the Sun has been worshipped and carefully studied, for example in Ancient Egypt (2000-1000 BC) and other ancient cultures. The sun is however not only essential for life, its radiation also has healing effects, which were recognised early, and actively used in Greek medicine starting with Hippocrates around 400 BC. The Ancient Greeks were very curious about the nature of light, and most of the Greek philosophers like Plato (400 BC), Aristoteles (350 BC) and others held the notion that light was a physical matter, but they all had different explanations. Although most of the classical Greek notions of what light is turned out to be wrong, the research continued and Euclid (300 BC) formed the basics of geometric optics, part of which is still valid. Some 350 years later, around 50 AD, the Greek mathematician and engineer Hero from Alexandria was exploring the laws of reflection and refraction, which was one of the last contributions to optics of the time.

The role of the brain in visual processing

The optical science was dormant until the Arabs took over and started developing the knowledge based on the Greek heritage. Especially Alhazen (1000 AC) who explored how the optical system in the eye worked. How the image was projected upside down inside the eye, which was very confounding. This showed that visual processing was more than pure Euclidian optics and pointed to what was later understood as the role of the brain in visual processing.

Science of light in the Renaissance Europe

Another 250 years later, monks studying old Arabic scripts brought the science of optics to the Renaissance Europe. Some of them even made their own contributions to the science, like the Franciscan Roger Bacon.


Classical optics prevailed until the Danish astronomer Roemer (1670) determined the speed of light at about 200,000 km/ sec. This was the first tangible result of using mathematics and physical observations in situ, comparable with Kepler’s (1620) explanations of the movement of the planets. Descartes (1600-1650), a French philosopher, had great influence on the optical science for a while, but most of his explanations of the light phenomena turned out to be wrong, and his importance and influence in this field of science today is very small.

Understanding the physics of light

The Dutch mathematician Huygens (1629-1695) was much more important for the understanding of the physics of light with his wave theory. Huygens made another important contribution to the light science, which would not be understood fully until our time, namely Snell’s law of refraction. This had in a way already been described by Alhazen, but not as clearly as by Snell (1580-1626).

Time for Newton

Isaac Newton (1648-1727) rocked the boat with his corpuscular theory, which made Huygens Wave Theory obsolete. Newton showing that white light was a spectrum of colours was one of the great and important discoveries not only in optical science, but in science overall. The Newtonian science was dominating the science society during the 18th century, until Thomas Young (1773-1829) was able to revive the wave theory, showing that light also behaved like waves in 1804. It took a long time, nearly 100 years, until scientists agreed that these theories were complementary.

Based on Snell’s law, the French mathematician Pierre de Fermat (1607-1665) formulated the principle that light (later understood to be the photons) travels not the shortest but the quickest way through different media. This is Fermat’s principle, which later proved valuable in explaining certain phenomena in quantum physics, such as Feynman’s (1918-1998) principles. At the time, this principle had difficulties gaining root, since it seemed unfathomable that light could make an intelligent choice.

Wave theory of light Even the modern understanding of light as partly waves goes back to scientists In the Antique like Euclide. After Newtons Corpuscular Theory which dominated the 18th century physics and made Huyghens wave theory obsolete, Thomas Young showed with his split experiment that light also was moving like waves.

Light influenced behaviour

Light chooses the quickest/fastest path. Just as the safeguard on the picture will do. He will choose the path with minimum water on his way, becasue he moves faster through the air. This very early insight of refraction (and reflection) was described early by several greek and arabic scientists like Ptolemy and AlHazen and later by the dutch Snell and Huyghens and refined mathematically by Fermat showing that light travels not the shortest but the quickest way.

At around the same time, the beginning of the 19th century, scientist started observing that light could influence the behaviour of plants, that sunlight could darken skin, cure rickets and treat skin ulcers, concentrated through a lens accelerate wound healing and destroy tumours. In the mid-19th century, Hippocrates’s ideas that sunlight positively affected mental health was revived.


During the 20th century, this treatment called phototherapy would be rediscovered several times as an effective means for treating Seasonal Affective Disorder - SAD.

The first two periods ended because of the cultural decline. Reasons FOR almost ending the third period are more complex; the extensive research in the visual science and optics after WW2 resulted in ground-breaking findings on how the eye, vision and brain function.

Later Dr. Finsen demonstrated in 1897 that sunlight or light from an arc lamp had antibacterial effects and could cure skin tuberculosis, a discovery that earned him the Nobel prize 1904.

Perhaps too much focus was placed on the visual function of the eye and brain, while the health aspects were left to the pharmacy.

In 1876 Ponza reported that light therapy was beneficial in treating patients with mental illness.

In the same period the understanding of what light is developed. In the mid-1860s Maxwell, another important physical researcher, demonstrated that light is an electromagnetic double wave with two components oscillating perpendicularly to each other.

THE MELANOPSIN AGE The renewal of interest in and study of light’s effects on health and healing seems to have been in the minds of people exploring lighting at the turn of this century, when Berson was able to identify the melanopsin molecule in the ipRGC ganglion cell. The start in the thirties was slow, when two lighting engineers Luckiesh and Moss had the audacity to suggest that artificial light could have medical and biological impact on people, and the idea was met with condescending comments from medical circles.

Polarisation During the 19th century the scientists were very confused about how light could be both particles and waves.  The electrical field vector is vibrating in a perpendicular direction to the magnetic field vector. Thus, a light pulse by any radiator - atom or molecule - in a single radiation event is  polarized between the electro and magnetic fields.

3 The next giant step in the understanding of light and radiation came with Planck’s quantum theory (1858-1947), and later Einstein (1879-1955) found that quanta were massless particles of energy, later named photons, that are released from molecules upon the absorption of light. Further development of these theories by Bohr (1885-1962) and Schrödinger (1887-1961) provided a new conceptual framework for the coming nuclear age.

Step by step the effects of melatonin and other hormones are discovered

As a matter of fact, the new revival came already around the year 1960, when it was discovered that the hormone melatonin is connected with sleep, and that its production depends on light. The hormone was discovered already in 1917, it was however believed to have a function more related to the skin. Wurtman and Waldhauser proved that melatonin secretion was controlled by light in 1984 at MIT in Boston. This was an important step to understanding the human circadian rhythm, and that the human species was no exception in nature, and no different from animals or plants in this regard. All life has a circadian rhythm.

The understanding of light’s importance for health and how it affects human physiology was quite extensive, and different treatments and cures with both artificial and natural light were provided until World War II. Many public movements supported by governments were initiated in several countries during this time, based on Light and Health (Licht und Gesundheit) principle.

Pharmacy prevails

After World War II the booming pharmaceutical industry killed the interest for using sunshine and daylight as medicine, since pharmaceuticals were regarded as more efficient, giving faster results and with effects, verified in laboratories by medical doctors. The worship of sun and daylight has had three glorious periods in history: the Greco-Roman era, the heydays of Arabic culture, and the period between 1860 and WW2.

Organs producing hormones which are reacting to light.


For the remaining part of the century, the research in lighting was mainly focused on visual problems and brain’s involvement in visual processing, and the effects of light on the body were left to endocrinology specialists and other medical professionals like chronobiologists, who were met with some scepticism from the rest of the medical society. The attention for the hitherto neglected hormone melatonin was increasing from the 1960s, when Aaron Lerner started studying its effect on sleep, but soon suggested that it was important for several other body functions, and not just sleep, namely the immune defence and calcium metabolic processes, as well as in restraining tumour growth. 4 Furthermore, low melatonin levels caused by working night shifts could be linked to higher rates of breast cancer among women and colon cancer among men.

The interplay between day and night is the basic control of the human circadian rhythm During the night, the body’s pineal gland and brain should produce melatonin, alongside adenosine and orexin. At dawn, our eyes transmit light to a centre in the hypothalamus called the Suprachiasmatic Nucleus (SCN) or body clock. This is how active hormones like serotonin and cortisol are produced. Research by teams gathered mainly around Berson, Brainard, Czeisler and Foster further discovered that the signal was sent to the SCN by a rare receptor in the retina, containing a light sensitive molecule called melanopsin, which has the action spectrum of 450-490 nm i.e. bluish light. This is the spectral region where the sky radiates the maximum of light. These ganglion cells are called intrinsically photosensitive Retinal Ganglion Cells (ipRGC). Compared to other types of photoreceptors, numbering up to more than a hundred million for the rods and about five million for the cones, the ipRGCs are very few, numbering only about 3-5000. They are only found on a small area of the retina, on the lower hemisphere where the sky is projected onto the retina. This means that light within the 450-490 nm spectral band projected onto other areas of the retina does not send relevant signals to the SCN. It is also important to note that it is not the solar spectrum or sunshine, but the light scattered from the sun into the atmosphere that gives the effect.

A proper light supports human natural 24 body rythm Without important parts of the light spectrum between 10 a.m and 3 p.m. , human body produces sleeping hormones when it should be in best and most productive state. It is like driving a car with a pulled-up handbrake.

Concept of staying out during the day

This important discovery improved the understanding of how to support or control a more harmonic and healthier circadian rhythm. But the recommendation of spending your day under the blue sky is very difficult or impossible to follow for the majority of people, working and living in modern built environments with sparse amounts of daylight and their retinas not exposed to the sky. Furthermore, the limitations of the light sources and lack of important parts of the spectrum make the usually installed artificial lights unfit for controlling melatonin entrainment, since these are designed to efficiently improve visibility or stimulate emotions.

Dialogue with all stakeholders is necessary for a successful lighting policy 

Chronobiology, namely stimulating the circadian rhythm to follow the natural day-and-night rhythm is one of the keys to a healthy life. However, a number of questions remain to be answered and more specifically; agreement needs to be found within the lighting industry, and manufacturers of lighting equipment and light sources.


We however know a lot about the Non-Image-Forming effects of the light from the old traditional research of light and vision, and we know how the eye as a light transformer reacts to different light environments and various light stimuli.

If you read the phrase word by word, the suggested meaning is that light should support human needs. This means not only the need to improve visibility, but also to support the biological needs.

It is now time for HCL.

What still needs to be solved and understood is how to measure the impact of light entering through the cornea and lens towards the ipRGC cells. Furthermore, the angle of incident light must be determined, since the most efficient angle is not vertically to the cornea, but rather a bit above the horizontal plane.

HCL - HUMAN CENTRIC LIGHTING You cannot open a lighting fixture manufacturer’s catalogue today without finding a set of fixtures with a HCL functionality. Obviously this phrase was coined parallel to the development of LEDs with a high CCT. What does it really mean? After reading a number of brochures and slogans, you can become quite confused about what HCL is supposed to mean, as the explanations contain contradictions, misunderstandings about spectrum, etc. To many it seems to simply mean fixtures equipped with LEDs with the CCT above 5000 K. Some seem to equate it with Tuneable White or Dynamic Lighting, while others define it as all of the above. No differences are made between table lamps, down lights, recessed LED line or large surface lights, hanging or ceiling mounted.

This gives you some idea how unreliable the claims given in many of the brochures are.

LUMENIA To put it very simply: the perfect HCL installation would be an artificial sky with the spectral distribution, luminance intensity and pattern mirroring the real sky. Of course this is impossible to execute indoors. But what could be done is what Lumenia did. We developed a solution with surfaces large enough to cover a solid angle from the eye, making sure that enough light of the appropriate spectrum is projected onto the sensitive part of the retina, where the ipRGCs are concentrated. The fixture produces no glare and its background is illuminated to minimize the contrast between the fixture and the background, not forcing the pupils to contract with the contrast glare. The luminous distribution satisfies all the visual requirements and the fixtures create no flicker. The balance between the horizontal and vertical distribution of light is also essential for natural modelling and perception of people and objects.

Human Centric Lighting solution has a lot to do with the evolution of the human eye, and the optical system which also includes the non-image forming visual processing function.

At the moment there is no generally accepted and reliable HCL standard. The same goes for the circadian metrology. How to measure the efficiency of the spectrum based on its effects on the melatonin entrainment? There is no generally accepted understanding of how good HCL lighting should be designed or executed.

It is time for Human Centric Lighting Circadian Rhythms


SKY LUM Series Human Centric Lighting

Visual Acuity

Improved Productivity Energy Savings & Sustainability

A GOOD AND FUNCTIONAL HCL LIGHTING INSTALLATION WOULD BE: • A large surface with low luminance but enough light; • No glare; • Good light distribution; • Low contrast between the fixture surface and the background, avoiding contrast glare;

• The appropriate spectrum with enough light in the 450-490 nm spectral band and > 5000 K; • No flicker. A good HCL luminaire satisfies both VISUAL and NIF requirements.

LUMENIA white paper on human centric lighting  

Paradigm shifts usually come with different interpretations of the rules and new concepts, until one interpretation prevails and the general...

LUMENIA white paper on human centric lighting  

Paradigm shifts usually come with different interpretations of the rules and new concepts, until one interpretation prevails and the general...