Stephen Westland
Professor of Colour Science and Technology
University of Leeds
The scientific literature often defines colour using phrases such as: colour is the neurophysiological response to electromagnetic radiation that strikes the retina in the eye after it has been reflected from objects in the world. In this short essay I will unpack the meaning of this phrase in a way that I hope will avoid technical jargon and, in so doing, will provide an explanation as to the nature of colour and how it affects us physiologically and psychologically.
We must start with what we mean by electromagnetic radiation. This is a term that describes many types of energy including radio waves, microwaves, light, X-rays and gamma rays. The difference between these different types of energy is simply the wavelength. The energy travels as a wave and the wavelength is the distance between the peaks of the waves. The term ‘light’ is used to describe the radiation that we can see. This corresponds to a very narrow band of wavelengths (see Figure 1) between infrared and ultraviolet radiation. Some writers use the term ‘visible light’. However, I believe this is not a helpful term since light – by definition – is visible. Light is that part of the electromagnetic spectrum that we can see and therefore ‘visible radiation’ might be a more accurate term.
Figure 1: Light is that part of the electromagnetic spectrum to which our eyes are sensitive.
There are several commonly occurring sources of light in nature of which the most common – and by far the most important – is, of course, the sun. However, there is also light from fires (and candles) and light from biological sources such as the bioluminescence that is seen in some fish. Today we are also very familiar with a variety of man-made sources of light such as incandescent light bulbs, fluorescent tubes, and more recently LED (light-emitting diodes) lights. The light that is emitted from each of these sources – natural or man-made is exactly the same in that it is visible radiation of wavelengths that we are sensitive to. The difference between the different sources is simply the prevalence of different wavelengths present in the emission. Candle light, for example, contains predominantly longer wavelengths and therefore has a reddish/yellowish appearance and gives a room a warm glow. By contrast, many fluorescent tubes contain more short-wavelength light and therefore appear bluer and give a room a colder feeling.
Sometimes we view emitted light directly such as when we watch television or use a computer display. But more often we see light after it has been reflected from surfaces such as printed pages, clothes, walls, furniture etc. When light interacts with inanimate objects in the world there are a number of different events that can take place and a full description of these is beyond the scope of this essay. However, in simplistic terms we can think that an object that appears white (such as a piece of paper) reflects all the wavelengths of light that are incident upon it and the distribution of wavelengths that reaches the eye is virtually the same as in the light source itself. For an object that appears black (such as a piece of black velvet) then almost none of the light is reflected. The black object absorbs the light (irrespective of wavelength) and therefore there is very little to be reflected. Incidentally, energy that is absorbed has to go some where; it cannot just disappear. Most of the absorbed light is converted into heat which is why one will feel much warmer dressed in dark clothes on a sunny day compared with white clothes. Most objects that appear coloured do so because they reflect some of the wavelengths of incident light more than others. An object that appears red, for example, might absorb the short wavelengths but reflect the longer wavelengths.
The reflected wavelengths of light are detected by the human visual system. This is a term that describes the eye and parts of the brain that are responsible for processing signals that the brain receives from the eyes. The inner lining of the eyeball contains a collection of cells that are sensitive to light. There are two classes of these cells; rods and cones. The rods are responsible for vision in low levels of illumination (so-called night vision) whereas the cones are responsible for vision in everyday situations. Within the class of cones they can be further categorized into those that are sensitive mainly to long-wavelength light, those that are sensitive mainly to medium-wavelength light and those that are sensitive mainly to short-wavelength light. These three classes of cone are responsible for colour vision and are sometimes called red, green and blue cones. The cones convert the light energy that is incident upon them and converts it into an electrochemical signal that is passed to the brain via the optic nerve. The signals from the retina are processed in the brain and ultimately result in the sensation of vision.
Figure 2: Schematic diagram of the human eye showing the retina that lines the eyeball and the optic nerve that transports the signals to the brain.
Colour is not the property of an object. During the 25 years I have worked in colour science I have often being asked the question; what is colour? However, just as relevant – and illuminating – is the question: where is colour? Colour is a psychological construct that exists in the brain. It results from the physical response of the brain when it (through the visual system) ‘looks’ at colour. A number of scientific studies have demonstrated the dissociation of colour perception from objects.
Figure 3: Simultaneous colour contrast (top row) showing two small grey squares that despite being physically identical have different colour appearances. The bottom image contains one red ink and one green ink but several different reds and greens can be perceived.
The colour that is perceived for an object depends upon the background upon which it is viewed and, to a lesser extent, on the light under which it is seen. Figure 3 demonstrates the former phenomenon which is known as simultaneous colour contrast. The grey square that is placed on the yellow background contrasts with the yellow and is therefore seen as less yellow than an identical grey square would on a neutral background (the word simultaneous is used because the yellow also contrasts with the grey). Notice the different appearances of the two grey squares in the top row of Figure 3. If these two squares were removed from their different backgrounds they would have the same colour appearance. Indeed, if one takes a small colour patch it is possible – by a careful arrangement of the background, the lighting, and the state of the observer – to create a situation where it has almost any colour appearance. It is clear that the appearance is something that we generate in response to seeing the colour patches of objects rather than colour being a property of the objects.
It may seem curious, however, that it certainly seems as if colour is the property of an object. When we look at a red book, for example, it certainly seems as though the redness is a property of the book. To understand this apparent dilemma it is necessary to consider the purpose for which vision – indeed, not just vision, but all of our senses – has evolved. We enjoy the sensations of vision, touch, smell, taste and sound. These sensations are generated in our brains as a response to signals that results from the stimulation of our sense organs. Together these sensations constitute what cognitive psychologists call an internal representation of the world. This is the world that we see, feel and hear as we move around. The usefulness of this internal representation is that it provides us with information about the world and allows us to navigate around the world, to find food and to avoid predators etc. It is clearly of benefit if the internal representation provides useful information about the world and seems to us to be the world. However, it is clear that if we had different sense organs the world would seem a very different place. Many species have more than or less than three cones and hence the colour appearance of their world will almost certainly be very different. Some species have totally different sense organs such as the bat, whose internal representation includes a sense similar to radar.
The complex nature of colour has been understood by many scientists and artists over the centuries. For example, the impressionist artists such as Monet understood that a yellow, for example, would appear more vivid if it was placed in a painting near to blue. The contrast phenomena demonstrated by Figure 3 have been the focus of many scientific studies and the nature of colour vision has been rigorously pursued for more than 100 years. We understand that it is the brain that produces colour and we can even identify that part of the brain that seems to be more concerned with colour. However, there are many questions that remain unanswered such as the precise mechanism by which the indirect stimulation of the cells in the brain by light actually produces the sensation of colour. This is a complex problem whose solution is probably at least a century away; indeed, if we could solve this problem we would understand the nature of human consciousness itself.
Now that we understand that merely looking at colours causes a physical change in the brain it is easy to reconcile ourselves with studies that have demonstrated the wider psychological and physical effect that colour has on us. There is a clear link between colour and emotion. We know that certain colours – and colour combinations – can cause us to feel cold, warm, happy, or sad, and that the colour of our environment can have a profound effect on our mental wellbeing and on our health. One of the most dramatic examples of this is the condition known as seasonal affective disorder which is a type of winter depression that affects up to half a million people in the UK each year. Exposure to high-intensity light has been shown to be effective in more than half of diagnosed cases. The colour of our environment is also known to affect our psychological state. Respectable scientific studies have shown, for example, that the use of pink can have a profound positive effect on the aggressive nature of some people and pink has been used to paint the interiors of holding cells in certain prisons. The use of red as an interior colour for a nursery has also been shown to negatively affect the way in which young children interact with each other. However, there are also examples of claims that are made about colour that have no scientific basis and which gain their credibility from a rise in mysticism in our society.
Professor of Colour Science
Head of School
School of Design
University of Leeds
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