Theory of wavelength discrimination in tritanopia

Many theories of color discrimination predict a discontinuity in the wavelength-discrimination function of a tritanope at the point in the spectrum at which the rate of change of the visual signal constrained to an equiluminant plane passes through zero (near 460 nm). The predicted discontinuity follows from the use of a first-order approximation for which the reciprocal of the slope of the response function that generates the visual signal is proportional to the discrimination limen. In view of the good discrimination shown by such observers elsewhere in the spectrum, however, such a singularity is impossible. I show that the inclusion of the higher-order terms produces a finite value in the 460-nm region that falls in the range of values from the literature that have been obtained experimentally.