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Properties of ADDITIVE MIXINGAs indicated earlier, coloured lights are

easy to define and hence seem to be suitable for

use as primaries in a system of colour specification.

The properties of additive mixturesof coloured light have been studied for many years [1–5], and those that are

particularlyrelevant to their use in systems of colour

specification are considered here.

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We can match a wide range of colours using a mixture of, say, red, green and blue primaries.

Suppose our primaries are single wavelengths and we use them to matchwhite light consisting of a mixture of all the wavelengths in the visible region

using an arrangement such as that shown in Figure 3.

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WHITE LIGHT

Although the mixture is physically quite different from the white light,

by carefully adjusting the amounts of the primaries wecan match the white light: that is, we can produce with the mixture a white that looksidentical to the white light.

If we change the colour or lightness of the surround, Our white colours change in appearance.

Over a wide range of conditions, however, thematch holds: if the colours change, they both do so by the same amount.

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Grassman’s law

This was recognised by Grassman, who stated in 1853: ‘Stimuli of the same colour(that is, same hue, same brightness, and same saturation) produce identical effects in mixtures Regardless of their spectral composition’.

Hence we can deal with colours without consideringtheir spectral composition, at least in many applications. Grassman’s law also implies that if colour A matches colour B and colour C matches colour D,

then colour A additively mixed with C matches colour B mixed with D.

This is vital when we consider that normal colours are additive mixtures of all the wavelengths in the visible spectrum.

We need to consider the effect of the additive mixture of all the wavelengths.

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Modern Colorimetry Based on EXPERIMENTIt cannot be stressed too strongly that modern

colorimetry is based on

the properties of additive mixtures of coloured lights, and that these properties have been determinedby experiment.

The main properties were established well over a century ago.

Subsequent work has confirmed that the simple properties described above are indeed valid,

but has defined much more closely the range of experimental conditions Under which the simple laws hold.

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COLOR VISION THEORYTheories of colour vision must attempt to explainsuch laws, and also their exceptions.

The fact that colours can be produced using onlythree primaries implies that there are three types of receptor among the cone cells of the eye,

and that variations in the magnitude of the responses from the three types produce the range of colour sensations that we call colour vision.

This theory was originally put forward by Young and elaborated by Helmholtz.

Modern theories suggest that the visual mechanism is complicated, particularly with respect to the ways in which the receptors are linked to the brain.

Modern colorimetry and the CIE system are based on the experimental facts, not on any particular theory of colour vision.

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Mixture of PrimariesSuppose we represent our red, green andblue primary light sources by [R], [G] and [B].

If we use these to match a colour using amixture of the primaries, we can represent the amounts of our primaries by R, G and B respectively. We can then write Eqn 3.1:

which is equivalent to saying that C units of the colour [C] can be matched

by R unitsof the red primary [R] additively mixed with G units of the green primary [G] together

with B units of the primary [B].

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Mixture of PrimariesIt is important to distinguish carefully between the primaries themselves, such as [R], and the amounts of the primaries used ina match, such as R.

The amounts used of each primary, R, G and B, are known as the tristimulus values of the colour [C]. These values depend on the colour [C].

If the values are known, they give an indication of the colour. Thus if R and B are high and G is low,the colour can be matched using a lot of the red and blue primaries and only a little of the green primary: thus the colour is some sort of purple.

The exact colour obviously depends on the exact nature of our primaries [R], [G] and [B], and if these are very purethe colour is likely to be a saturated purple.

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MIXING PRIMARIESIn most respects, equations such as Eqn 3.1 can be treated as ordinary algebraicequations. Thus if we write Eqn 3.2: then an additive mixture of C1 units of [C1] with C2 units of [C2] can be matched byR1 + R2 units of the red primary [R], additively mixed with G1 + G2 units of the green primary [G], together with B1 + B2 units of the blue primary [B] (Eqn 3.4):