In three separate experiments it is shown that (i) heterochromatic additivity failures for foveally viewed trichromatic lights can be predicted using a vector model derived from bichromatic additivity data, (ii) near-threshold bichromatic additivity failures are not qualitatively different from threshold-level failures, and (iii) foveal spectral sensitivities obtained by direct brightness matching and threshold methods are greater in the long- and short-wavelength ends of the spectrum than sensitivities obtained by flicker photometry. A new opponent-colors model that is appropriate for threshold-level color vision is expressed as a transformation of the CIE standard observer. The model allows the derivation of a light unit that correlates with signal detectability and predicts (a) confusion lines for deuteranopic and tritanopic vision, (b) spectral sensitivity as measured by flicker photometry (i.e., a sensitivity function much like the CIE <i>V</i><sub>λ</sub>function), (c) spectral sensitivity as measured by threshold (or direct-matching) techniques, (d) threshold-level heterochromatic additivity failures, (e) the apparent saturation of a threshold-level spectrum, (f) wavelength discrimination for a near-threshold spectrum, (g) loci of constant lightness-to-luminance ratios Within the CIE chromaticity diagram, and (h) the essential quantitative differences between threshold and near-threshold heterochromatic additivity failures.
Sherman L. Guth and Howard R. Lodge, "Heterochromatic additivity, foveal spectral sensitivity, and a new color model," J. Opt. Soc. Am. 63, 450-462 (1973)