Colour is not a physical quantity but a psychological one (see Figure 10.3). Electromagnetic waves are converted by the eyes and the visual nerve centre, including the visual parts of the brain cortex, and are experienced by humans in different ways. These different experiences are called colour. In the human retina there are thought to be three different types of sensitive bodies that convert electromagnetic radiation of different wavelengths and brightness, and thereby bring about different experiences of colour.
Table 10.4 gives the approximate relationship between the different physical characteristics of the radiation and the parameters in colour vision. The wavelength of the light is related to the colour experienced (for example, red, blue, yellow, green). The luminance is related to the brightness and the purity of the colour to the perceived colour density. But it is important to remember that there is no simple relationship between the physical characteristics of the light and the mental perception of colour. Colour perception is influenced by a large range of different factors and some of these are examined below.
Pigmented surface (absorbed and reflected radiation)
FIGURE 10.3 Relation between light radiation and subjective response to colour.
Approximate Relationship between Physical Characteristics and Colour perception
physical Characteristic Colour perception
blue, yellow, etc.
‘Purity’ Colour density
There are three main sources of colour generation:
1. Light reflected towards and absorbed by a pigmented surface.
2. Electromagnetic radiation (including invisible ultraviolet [UV] light) that is reflected and absorbed by a fluorescent material and thereby generates new light.
3. Direct light, for example, a lamp or a cathode ray tube (TV or visual display unit [VDU] screen).
All these different types of colour generation depend on whether the lighting in the room is coloured. Table 10.5 shows how surfaces with different types of colour pigment are experienced when they are exposed to red, blue, green, and yellow lighting, respectively. A pigment that is experienced as blue (that is, has a ‘blue colour’ pigment) in ‘white’ light will be experienced as reddish-purple in yellow light.
A similar but less marked effect is also obtained when fluorescent material is illuminated with coloured light.
As colour is to a large extent a psychological experience that is formed within the human visual system, there are a number of physiological and psychological factors that determine how we perceive colours (see Figure 10.3 and Figure 10.4). The following relatively important factors will be discussed in more detail:
1. Central vision
2. Focusing of colours in the lens
3. Colour blindness
4. Meaning and perception