The technical requirements for a colour screen are considerably greater than for an ordinary monochrome VDU screen. One must, for example, set considerably greater demands on sharpness of edges and the ability of the ‘guns’ to send their rays to the correct place at the edges and corners of the screen, so that no convergence problems occur. It has been shown that increased luminance on a VDU screen increases the colour contrast up to a luminance on the screen of 3000 cd/m2. Increasing the operator’s light adaptation level also increases his or her sensitivity to colour (Silverstein, 1982). A light background gives the effect of a higher degree of colour saturation. In practice this means that a higher level of environmental light can be used when working with colour screens. In many cases it may also be desirable to select a light background colour on the VDU screen in order to achieve an even higher level of saturation which, in turn, reduces the risk of both diffused and reflected glare.