Another way to estimate the cone spectral sensitivities is to make direct measurements in colour-blind individuals. For estimating the M- and L-cone spectral sensitivities, protanopes and deuteranopes can be used, since these dichromatic observers behave as if they possess only a single visual pigment in the long-wavelength spectral range. This makes possible a direct and straightforward measurement of the longer-wavelength cone spectral sensitivity, merely by the use of conditions under which the S-cones (and rods) do not contribute to sensitivity (e.g. by using an auxiliary violet adapting field). If dichromacy reflects the loss of one of the three cone types in the normal observer (Young, 1807; König & Dieterici, 1886), then, with the S-cones disadvantaged, the protanopic spectral sensitivity should be that of the normal's M-cones, and the deuteranopic spectral sensitivity should be that of the normal's L-cones. This approach is valid only if protanopia and deuteranopia are truly reduced forms of normal trichromacy, in which one of the two longer-wavelength photopigments is absent and the other is identical to that in the normal observer.
The so-called pseudo-isochromatic plates use printed dots of different chromaticity to produce patterns that can be identified as numbers (see below), letters or shapes. By choosing chromaticities that are confused by red-green dichromats, patterns can be produced that are visible to color normals, but invisible to dichromats, or that appear different to color normals and dichromats (as below). The first examples of such plates were those by Stilling (1st edition, 1875), but the more popular set of plates for color vision testing are those by Ishihara (1st edition, 1917), four examples of which are reproduced below. Color normals should see A=8, B=5, C=29, and D=74. Color deficients may see A=3, B=2, C=70, or D=21. Please note that tests displayed on color monitors may not produce accurate results, since the chromaticities produced by different monitors can vary considerably. If you suspect that you are color deficient, we suggest that you have your color vision professionally evaluated.
Ishihara, S. (1954). Tests for colour-blindness. Tokyo: Kanehara Shuppan.
König, A. & Dieterici, C. (1893). Die Grundempfindungen in normalen und anomalen Farbensystemen und ihre Intensitätsverteilung im Spektrum. Z. Psychol. Physiol. Sinnesorg. 4, 241-347.
Stilling, J. (1875). Pseudo-isochromatische Tafeln zur Prüfung des Farbensinnes. Kassel: T. Fischer.
Young, T. (1807). Lectures on Natural Philosophy. London: Johnson, Vol. II.