The concept of colour

Colour is a basic human experience. From infancy onwards most of us are aware of an ever changing pattern of shapes and colours which we learn to interpret as blue, green, red, yellow etc,. Colour is not an illusion - something having no real existence - a mere by product of our senses. Colour is something which is impressed directly upon our consciousness. We are directly aware of it. Light and colour are as real as anything else in the universe.

Since light and colour are impressed directly on our consciousness, they are to a large extent incommunicable and private to each individual. It is impossible to convey an adequate idea of light and darkness to one who has never experienced them, such as a man born blind. It is related that someone was once trying to explain the sensation of bright red to a man blind from birth, and after a great amount of discussion the blind man suddenly exclaimed: 'Now I know what you mean - like the sound of a trombone!' whereupon the first felt he had succeeded. Such description by analogy, poor as it is, is probably as good as can be given in the circumstances. Light must be experienced to be appreciated.

It is not certain that those of us who are gifted with sight see in the same way. The green of the grass means something to one person, but we cannot borrow the visual apparatus of another to check whether their sensation of green is the same as that of their neighbour. All that can be said is this: that whereas a minority of people certainly do not see the same colours as the rest, it is fairly probable that the majority see in about the same way though small individual differences in perception are frequent. Mistakes in colour matching made by the minority are so gross that there is no doubt that they must be classed as abnormal; they are said to have defective colour vision, though very few are colour blind in the true sense. Among the so-called normal, however, greater or smaller variations in colour vision are common and no sharp distinction between normal and abnormal can be made. We cannot be certain even that 'normal' subjects receive similar sensations of colour from the same objects, all we can say is that it seems fairly likely that they do.

The visual sensation is found to be closely connected with the action of certain electromagnetic waves which have the property of acting on our eyes in such a way that electrical impulses are sent to the brain. These impulses give rise to the sensation of light, white or coloured according to the nature of the impulses. According to the sensation of light that we experience, we derive information about the objects sending these electromagnetic waves to us from the outer world. Our eyes are sensitive only to a limited range of waves. A vast expanse of electromagnetic radiation exists outside this range, and if our eyes were sensitive to any other part of this radiation instead of the visible light, we would gain quite a different impression of the outer world. Thus the development of radio-astronomy has revealed the presence of great stars which are completely invisible to our eyes and which mankind has been up to now quite unaware. If our eyes were sensitive to radio waves instead of to light, the pattern of the starry sky would assume a totally different appearance. Although the process of seeing normally requires the action of suitable electromagnetic radiation upon our eyes and nervous system, we can at times be aware of light and colour without the intervention of radiation and our eyes at all. Some subjects certainly dream in full colour and the light and colour which they experience do not differ. It may be supposed that in the dream state the brain cells responsible for vision are excited from other brain cells instead of from the eye via the optic nerve. Blind persons can still dream in full light and colour, but as far as is known, persons born blind can never be aware of light or colour since the necessary nerve patterns have never been established anywhere in the brain.

Since electromagnetic radiation of certain wavelengths normally enters into the process of vision and is indeed an essential part of it, this radiation is commonly spoken of as 'light' to distinguish it from similar radiation which is incapable of affecting our eyes. Such usage of the word 'light' is strictly speaking incorrect, but it is so well established that it would be unavailing to attempt to displace it. Consequently 'light' may mean either a visual sensation or the electromagnetic waves which are capable of exciting this sensation. The latter usage is exemplified in the phrase 'a beam of light', In practice it is rare that any confusion arises.

There are both psychological and physical aspects of colour. For example, we associate green with coolness, and might paint the walls of a room with green paint to give the sensation of coolness, yellow walls would give the sensation of warmth.

An example of psychological and physical aspects of colour is the effect of surrounding colour on what we see. A person looking at the two rectangles above sees each centre panel in relation to its border. The right hand panel appears darker because it is seen in contrast with its lighter border. Actually, the panels are the same.

Physicists had long believed that colour perceptions came from specific wave lengths of light, but experiments indicate that colours can be perceived even in the absence of their specific wavelengths. It was mentioned earlier that a person could experience colour without the presents of electromagnetic waves. One way this can happen is in dreams, another way in which we can see colour is in after images. After images, may give rise to illusions as they may mistakenly be thought to be due to external objects. Thus a mother who had been sewing a red blanket glanced at her infant and noticed that its face had turned green.

After images come in complementary colours. If the eyes focus for 30 seconds on the small white dot in the centre of the blue, black and yellow flag below, then turn to look at a white area, a fleeting after image of the same flag in red, white and blue will be perceived. This is caused by the fact that your eyes become accustomed to the colours in the flag and is therefore oversensitive to the complimentary colours. By suddenly shifting to a white background, which is essentially reflecting all colours equally, the eye gives emphasis to the complimentary colours.

To perceive colours from nature there are three required items: A light emitting source, a coloured object and the eye/brain mechanism.

The first is the light which falls upon the coloured object. Light is that part of the electromagnetic radiation spectrum to which the eye is sensitive. It is a minute part of this spectrum being only from about 380 to 760 nanometers in wavelength range. (Where 1 nanometer = 10^-9 metres). The whole spectrum ranges from 10^-17 metres for the shorter cosmic rays to 105 metres for very low frequency radio waves. Even in this small range the sensitivity of the eye is so low to the two extreme ends of the visible spectrum that it is usual to include only the range 400 to 700 nanometers in colour prediction and colour control work.