It is fairly clear that the requirements will be far more stringent for the design of characters and symbols on a colour screen than for those on a monochrome display with a dark background and light text. Research has shown that colour sensitivity increases as the colour field increases in angle, up to 10 minutes of arc. Small light fields have a reduced colour saturation which means that in practice light colours tend to be seen as white.
On this basis, characters, symbols, or critical details in the picture that subtend less than 20 minutes of arc should not be used. On larger fields, where greater accuracy in colour discrimination is required, this angle should be at least 1°. The lines that make up the characters should be thicker on a colour screen than on a conventional one. If there is more than 1 minute of arc between lines, colour separation will be distinguishable. The characters on a colour screen should not be placed more than 15 minutes of arc from the line of sight. Characters that appear towards the periphery of the screen will not be colour-coded to the same accuracy.
Research has also been performed on legibility and luminance contrast of colour screens under different environmental lighting conditions (Silverstein, 1982). This has shown that there is no further improvement in legibility when the luminance contrast is increased beyond 5. It should also be remembered that the colours seen on the screen will change depending on the room lighting. The room lighting should therefore not be changed once the colour screens are installed.
