The luminosity curve of the deuteranomalous fovea.

Analogous to protans, the two types of deutan color-defectives-the dichromats (deuteranopes) and the anomalous trichromats (deuteranomalous)-do not differ in spectral sensitivity in the red-green range at threshold (either in the dark or against bright colored backgrounds). However, luminosity curves obtained by heterochromatic brightness matching show the latter to be slightly more sensitive in the blue-green, and slightly less so in the red, than the former. Experiment proves that these differences are due (at least in part) to contributions of cones containing the deuteranomalous anomalous pigment which are missing from the deuteranope's eye. The absorption spectrum of the anomalous pigment can be inferred with assumptions (analogous to those already made with protanomalous trichromats) about how the different cone mechanisms pool their responses to yield luminosity. Two alternatives thus revealed are (a) the normal red pigment in dilute solution or (b) a spectrum very similar to that of the normal red pigment but shifted slightly toward the short wave end of the spectrum. Since the spectrum inferred by (a) has the same lambda(max) as the normal red pigment, (a) predicts that deuteranomalous observers will require a negative red primary when matching monochromatic lights of wavelengths near the lambda(max). This is not observed.