The visible spectrum

If we consider the spectrum of 400 - 700 nm in detail, the relative energy distribution of various illuminants can be considered in terms of recognized colour names.

The above scale shows the approximate wavelengths of commonly recognized colour names.

The distribution of energy of the artists and colour matchers preferred light - north noon daylight can now be compared with, say, the light from an ordinary tungsten filament electric light.

The retina of the eye contains photoreceptors or light-sensitive cells of two kinds; rods and cones. The rods can sense only differences in light and dark and are very sensitive. While, according to recent researchers, being in use all the time, they alone provide night vision when, of course, no colour is perceived. Everything is seen in shades of grey alone. About 123 million rods are dispersed towards the periphery of the retinal area while about 7 million cones occupy the central areas. The cones with their bases facing forward respond only to strong light and are colour sensitive. This sensitivity may be divided into three groups - red, green and blue. The cones which respond to green are far more sensitive than those which respond to blue or red which gives the eye the following sort of responce sensitivity throughout the spectrum.


How we actually perceive the colour of the light falling onto the retina is the subject of several theories. Newton, who discovered in 1666, that sunlight contains all the colours of the visible spectrum, in 1704 published a series of speculations in which he implied that the retina contained receptors, each of which was sensitive to a single colour stimulus and when excited sent the appropriate signal to the brain. The analogy, with musical tones, was an over-simplification as Newton himself guessed but it was a start for other workers to follow.

Thomas Young in 1801 suggested that only three different kinds of receptor were involved and that they were sensitive to red, green and blue. In the mid nineteenth century, Hermann van Helmholtz, clarified Young's 'trichromatic' theory with experimental work and determined that the total colour sensation received by the brain originated from stimuli from the red, green  and blue sensitive cones of the retina. At the the same time, Edward Hering, who was a researcher in human colour blindness, found he could not accept the 'trichromatic' theory and formulated his theory of four primary colours in which there red, green, blue and yellow receptors with a coding system in the optic nerve between the eye and the brain which sent red/green; blue/yellow and black/white signals to the brain. It is now thought that a combination of the two theories might be near the truth.

In a simplified form this is that red, green and blue sensitive cones form the initial stimuli which are then coded into red/green, blue/yellow and light/dark signals, both in the retina and the optic nerve before transmission to the visual cortex in the brain.

It can be seen that daylight is stronger at the blue end of the spectrum than the red and so may be termed a bluish light. Tungsten light on the other hand is gradually increasing in energy as the wavelength increases and so has a strong yellowish appearance. If a viewing cabinet is so arranged that one half of a piece of white paper is illuminated with light of the energy distribution of daylight, and the other with tungsten light, the paper appears to be of two totally different colours; one a pale blue the other quite distinctly yellow. So whatever we look at is going to be coloured by the light by which it is illuminated quite apart from its own peculiar colour or light absorbing properties, For that is how, apart from self-luminous objects, that colour is obtained.