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In full dark adaptation the human visual system makes very good use of the light that enters the pupil, for it can be shown that, under favorable circumstances, about 6% of the entering quanta are used effectively in arriving at a decision of the type "A is brighter or dimmer than B" (Barlow, 1962). The proportion of quanta actually absorbed is approximately 10% (Rushton, 1956), hence one can say that the neural, decision making processes are 60% efficient.
H. B. Barlow (Fri,) studied this question.