Spectrophotometric curves uniquely determined by every set of color specifications can be based on a Fourier-type analysis, using normal and orthogonal color-mixture functions. Normal and orthogonal color-mixture functions are convenient for the investigation of the propagation of errors in spectrophoto-metric colorimetry. The space based on normal and orthogonal color-mixture functions serves as a suggestive model for the as yet unavailable visually homogeneous color space, in which visual thresholds of color shall be represented by equal-size spheres, at least for some fixed conditions of adaptation. Recent puzzling results of investigations of color discrimination and heterochromatic photometry can be explained in a straightforward manner in terms of general properties of visually homogeneous color space. Those properties are independent of the detailed structure of that space and are exemplified, though probably in a simplified manner, by the color space based on normal and orthogonal color-mixture functions.
D. L. MACADAM, "Orthogonal Color-Mixture Functions," J. Opt. Soc. Am. 44, 713-724 (1954)