Colorimetric data as revised by Judd can be transformed to König fundamentals L, M, S, representative of the long-(LWS), middle-(MWS), and short-(SWS) wavelength sensitive cone spectral sensitivities. The fundamentals are normalized so that two cone types, M and L, sum to the luminous efficiency function, YJ. The height of the S fundamental is undefined in this transformation. A constant luminance chromaticity plane can be derived by calculating L/YJ and S/YJ, with the area of S set equal to that of YJ. This chromaticity space is convenient for calculations of real stimuli. The axes of this space, when adjusted to reflect cone adaptation to the equal-energy spectrum have been shown to match the null axes of the major retino-cortical processing streams. The relative cone troland chromaticities can be multiplied by the retinal illuminance level to give L, M, and S trolands. In this metric, chromaticity data can be plotted as threshold-vs.-illuminance functions. Cone excitation is derived from cone trolands, by dividing by the maximal sensitivity of the fundamentals. Cone excitation units are used to derive models of retinal processing. The cone quantal excitation rate is a scaled version of the cone excitation.
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Smith et al. (1996) studied this question.