All types of lenses, including the lens in the human eye, focus light of different wavelengths to different extents; blue is focused nearer the lens and red farther away from the lens. This means that if light containing a large difference in wavelengths is presented to the eye—for example, violet-blue and red on the same VDU screen— the eye will find it difficult to focus at a suitable ‘distance setting’ from the screen. The colours may be perceived to lie in different planes, and the eye will have to work
FIGURE 10.6 Sensitivity of the rods and cones to different wavelengths of light.
constantly in order to focus and achieve optimal clarity. One must therefore avoid having colours widely separated in wavelength adjacent to each other.
The perceived colour from a directly radiating light source such as a lamp or a cathode ray tube (TV/VDU screen) will not be affected much by the colour of the surrounding light. It is, however, important to remember that the colour of light perceived on a VDU screen will also be affected by the intensity and colour of the room lighting even if this effect is not very marked. This is because the human eye is affected directly and indirectly by the light in the room, and will become adapted both to its intensity and colour. In addition, there will always be a certain amount of light reflected towards the surface of the VDU screen, and this will be ‘mixed’ with the light from the screen itself.