ColorAcademy 2005

Light

Spectrum

Colour is an optical and cerebral event caused normally by physical vibrations of Light. A range of hues that the human eye is capable of perceiving is called a Spectrum.

Wavelengths

The complete spectrum spans a set of vibrations of Light from a wavelength of under 400 nanometres (a nanometer measures 2 billionth of a metre) to over 700 nanometres (abbreviated to "nm").

Primary Colours
Above are the positions of the Primary colours within a spectrum of sunlight. Primary colours have special properties that make them useful in the mixing of colours.

		B		G		R			Additive Primaries
Subtractive Primaries			C		Y		M

Additives

The additive primaries are three hues taken one from the Blue, one from the Green and one from the Red band of the spectrum. When these are correctly chosen and used as coloured light-sources, they will mix to produce pure white. More important to a TV engineer, if they are mixed in different proportions they will cover more perceptionally different colours than any other three coloured light sources. Hence they are used for the colours of the phosphors in Tv sets.

Subtractives

The Subtractive primaries are three hues taken one from the Cyan, another from the Yellow and a third from the Magenta band of the spectrum. When these are correctly chosen and used as the colours of paints, inks, dyes or filters they can be mixed to give Black or a neutral greyness. Far more important, however, the subtractive primaries will mix in different proportions to give far more perceptible intermediate colours than any other three colours. Trichromatic printing, for example, is based entirely on percentage mixtures of the three subtractive primaries, Cyan, Yellow and Magenta.

The additive and subtractive primaries placed alternately around a circle make a useful figure for indicating complementary colours. These are:

(1) Three or more hues that will mix additively to create White (using, for example, three coloured lights, Blue, Green and Red as with TV). On the diagram any equilateral triangle is likely to point out complementaries.
(2) Two hues mixed additively that produce White. On the diagram the hues opposite each other are the complementaries. These opposites are what artists and designers call "complementaries", not realising that they are complementary because one of them is an additive and its opposite hue is a mixture of two additives.