CONTRIBUTOR: Shahjahan Alhassan - 13th Oct 2001

Let us consider the anatomy of the human eye, as it provides some information relevant to the way we perceive colour. When we look at a scene, a picture of that scene is projected onto the back of the eyeball, the region known as the Retina.

The retina is covered with tiny receptors which pick up the intensity of light falling on them, and convert this light into signals which are processed in the retina, and then transmitted to the brain. The receptors come in two types, Rods and Cones, the majority being rods.

• Rods - these are only sensitive to intensity and not to colour, but work in very low light levels.
• Cones - these detect colour as well, but only in high intensity light.

So what does this mean? Well, AWPA (1996) suggests designers need to take into consideration that when colour must be distinguished, low intensity light should not be used. Furthermore it is not possible for the user to see detail in both dark and light areas at once.


There are three different types of cones, red, green and blue. Each one refers to their increased sensitivity to certain wavelengths of light. The lack of one or more cone type can result in colour blindness.

Although only one person in three thousand is totally colour blind, many do have problems with certain colours. Eight percent of men and 0.4 percent of women possess the inability to distinguish between red and green. The majority of us are blue-blind in the centre of our field of vision where we perceive fine details. As we get older the lens of the eye yellows, thus making it increasingly difficult seeing pure blue (Horton, 1994).