Brightness, visual acuity and colour blindness
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Measurements have been made of the dark-adapted foveal threshold of normal and colour blind persons in five parts of the spectrum using a 1° circular test field.
Compared to normals, protanopes (red-blinds) show an elevation of the threshold which increases slowly from blue to yellow and rises rapidly thereafter until in the red the threshold is more than ten times as high as normal. Deuteranopes (greenblinds) do not show so high an elevation, their maximum in the green being only about 70% above normal.
These threshold elevations correspond to luminosity losses in the spectrum. For the protanope the total loss in the spectrum is nearly one-half of the normal luminosity; for the deuteranope it is nearly two-fifths of normal.
Measurements were made of the illumination necessary for equal visual performance in normal and colour blind subjects. It was found that protanopes require 119% and deuteranopes 55% more light to achieve a visual acuity of 0.29 reciprocal minutes. These values agree closely with those found in the luminosity measurements.
Such losses support the idea that colour blindness corresponds to the loss of one of the three receptor systems usually postulated to account for normal colour vision. However, the colour sensations reported by colour blind persons, especially monocular colour blinds, do not support the idea of a lost or inactivated receptor system. A fresh explanation for colour blindness is needed to reconcile these conflicting kinds of evidence.
KeywordsVisual Acuity Blindness Total Loss Receptor System Colour Vision
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