Confusion points and constant-luminance planes for trichromats, protanopes and deuteranopes

The confusion points of dichromats are derived from the constant-luminance planes of trichromats, protanopes and deuteranopes experimentally defined by heterochromatic-flicker photometry: (1) the zero-luminance planes of the observers considered in this experiment intersect almost exactly in a line that crosses the plane of the chromaticity diagram in the tritanopic-confusion point and confirm that the short-wavelength sensitive cones can be considered to have no contribution to luminance; (2) protanopic- and deuteranopic-confusion points are taken as being defined by the intersection of the tangent line to the long-wavelength region of the spectrum locus and the zero-luminance plane for protanopes and deuteranopes, respectively.