Cone spectral sensitivities and colour matching

    The trichromacy of individuals with normal color vision is evident in their ability to match any test light to a mixture of three "primary" lights. The relative intensities of the primary lights required to match equal energy test lights of wavelength λ are referred to as the red, green and blue colour matching functions (CMFs), respectively, and written  ,  and . If the CMF is negative, the primary light in question must be added to the test field to complete the match.


    CMFs can be linearly transformed to other sets of real and imaginary primary lights, such as the X, Y and Z primaries favored by the CIE, or the L, M and S cone fundamental primaries that underlie all trichromatic color matches. Each transformation is accomplished by multiplying the CMFs by a 3x3 matrix. The goal is to determine the unknown 3x3 matrix that will transform the CMFs, ,  and , to the three cone spectral sensitivities, , , and .


    Colour matches are determined at the cone level. When matched, the test and mixture fields appear identical to all three cone classes. Thus, for matched fields, the following relationships apply:




    , and




where ,  and  are, respectively, the L-cone sensitivities to the R, G and B primary lights, and similarly ,  and  are the M-cone sensitivities to the primary lights, and ,  and  are the S-cone sensitivities. We know  ,  and , and we assume, for the red R primary, that  is effectively zero, since the S-cones are insensitive to long-wavelength lights. (The intensity of the spectral light of wavelength, λ, which is also known, is set to be equal in energy or quantal units throughout the spectrum, and so is discounted from the above equations.)


    There are, therefore, only eight unknowns required for the linear transformation:




    Because we are usually unconcerned about the absolute sizes of ,  and , the eight unknowns collapse to just five:




where the absolute values of  (or  ),  (or  ), and  (or  ) remain unknown, but are typically chosen to scale three functions in some way: for example, so that ,  and  peak at unity.  In one example (Smith & Pokorny, 1975),  sum to , the luminosity function.


    The above equations could be for an equal-energy or an equal-quanta spectrum. Since the CMFs are invariably tabulated for test lights of equal energy, most workers, use an equal-energy spectrum to define the unknowns in the equations and to calculate the cone spectral sensitivities from the CMFs. They then convert the relative cone spectral sensitivities from energy to quantal sensitivities (by multiplying by λ-1).



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