There are three basic types of cone receptor. They respond to red, green and blue light respectively. It is the relative output of these three receptors which determine what particular colour we see.

Thus if a mixture of two or more lights can stimulate the red, green and blue sensitive cones in the same proportion as a single light, then the two colours appear to be the same, even though one is a mixture. Mixing red, green and blue light in equal proportions gives white light since all three colour receptors are balanced.

Red and green light will give yellow; red and blue light, magenta; blue and green light, cyan. Red, green and blue are called the light primaries, the other three the secondaries. A mixture of the three primaries or the three lighting secondries will give white light.

For example, green and magenta in the correct proportions will give white light. This type of colour mixing using light is called additive colour mixing and should not be confused with the other more familiar subtractive colour mixing when using paint, for example.

How the Eye Sees Colour

There are three types of cone sensitive to red, green and blue light respectively. The signals generated in these cones are transmitted through optical nerves to the brain which usually interprets them in terms of previous experience.

Any change in either the spectral distribution of the source colour or the colour sensitive mechanism of the eye affects the final sensation of colour.

Colour Rendering

Since a colour object possess the power to reflect certain parts of the spectrum of the incident light better than others, its colour appearance will depend on the spectral emission of the light source.

For example, a 'blue’ sample will only look blue if there is blue energy in the incident light.

The effect that a light source has on the appearance of coloured objects is known as colour rendering and this has importance in a number of practical lighting applications. Since the large number of different types of fluorescent and high intensity discharge lamps available of their own characteristic colour rendering properties.

Traditional fluorescent lamps made using halophosphate phosphor have their emission peak in the yellow part of the spectrum which is close to the wavelength where the eye is most sensitive. Hence their luminous efficacies are high. However they emit little red radiation and their red colour rendering is poor.

Deluxe lamps, on the other hand, are made using mixtures of phosphors, one of which is a good emitter of red light. These types of lamp are not as efficient as halophosphate lamps but they are useful for applications where good colour rendering is important e.g. colour matching.

The high output of Pluslux and Polylux series of lamps use a conventional halophosphate and three phosphors that emit over wavelengths bands that are very narrow compared with those for conventional phosphors. By varying the combination the three red, green and blue phosphors different shades of white are produced.