Mixture of coloured lights
Before proceeding further it must be pointed out that mixing coloured paints is an entirely different thing from mixing coloured lights. Paints will be dealt with later. The effect of mixing coloured lights may be investigated by using three projectors fitted with slides of various coloured gelatine sheets, and arranged so as to produce overlapping images on a white screen.
In this way it may be shown that a mixture of the three primary colours, red, green, and blue, gives a white patch on the screen. However, a successful result is obtained only by using the right kind of red, green, and blue gelatine for producing the colour and by having each light of the correct intensity. This can only be done by experimenting with different types of gelatine and by having lamps of the appropriate brightness.
By using two projectors only the following facts may also be verified:
Red + Green = Yellow
Red + Blue = Magenta
Blue + Green = Cyan
These results, together with the knowledge that a mixture of the three primaries, red, green, and blue, gives white, lead us to expect that:
Red + Cyan = White
Green + Magenta = White
Blue + Yellow = White
A further experiment with two projectors using appropriate gelatines shows that these inferences are correct. Two colours such as those described above which give white light when added together are called complementary colours. To sum up, Fig. 25.6 shows the result obtained when three circular overlapping patches of red, green and blue light are formed on a screen by three projectors respectively.