The light emitted by the sun is a mixture of wavelengths and the eye registers this mixture as a colour we call white. Since the refractive index of a transparent substance varies with the wavelength of light.

It is possible to separate the wavelengths. When light passes into glass, each wavelength is bent by a different amount. As the light rays pass out of the glass plate, they are bent back and converge. But if the two sides of the glass are not parallel, as in a prism; the rays diverge. The arrangement of colour produced is called a spectrum.

Because colour is dependent upon the light source and you cannot guarantee a standard light source from the sun (dependent upon the weather/time of day/ time of year), a standard light source for colour matching is used. This uses the light given out by a block of platinum heated to 67UO °K (6H67°C). The sun has a temperature of around 5900°C.

The platinum glowing white hot is used to produce daylight tubes, north light tubes used for colour matching, Tungston filament bulbs have a yellower light from a element temperature of around 2850°K.