Color matching: (continued)
Consider a mixture of two colors S1 and S2 i.e S=S1+S2
If S1 is specified by ( Rs1, Gs1, Bs1) and S2 is specified by (Rs2, Gs2, Bs2)
This implies, S is specified by (Rs1,+Rs2,Gs1,+Gs2,Bs1,+Bs2)
The constraint of color matching experiment is that only non-ve amounts of primary colors can be added to match a test color. In practice this is not sufficient to effect a match. In this case, since negative amounts of primary cannot be produced, a match is made by simple transposition i.e. by adding positive amounts of primary to the test color
a test color S might be matched by ,
S+3G=2R+B
or, S=2R-3G+B
The negative tristimulus values (2,-3,1) present no special problem.
By convention, tristimulus values are expressed in normalized form. This is done by a preliminary color experiment in which left side of the split field shown in Fig (2.7), is allowed to emit light of unit intensity whose spectral distribution is constant wrt
i.e. (equal energy white E).Then the amount of each primary required for a match is taken by definition as one “unit”.
The amount of primaries for matching other test colors is then expressed in terms of this unit. In practice equal energy white ‘E' is matched with positive amounts of each primary.
Figure(2.9): The color-matching functions for the 20 Standard Observer , using primaries of wavelengths 700(red), 546.1 (green), and 435.8 nm (blue), with units such that equal quantities of the three primaries are needed to match the equal energy white, E .
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