Measurements of the quantum efficiency of discrimination in human scotopic vision.

In the previous paper (Barlow, 1962) a method for determining the quantum efficiency of vision was described. In the present paper some results obtained on human parafoveal scotopic vision are reported. There are many experimental parameters involved in an estimate of overall quantum efficiency: for the stimulus these are the area, duration, intensity, retinal location, colour, and the relative intensity of the pair of stimuli. Since it is here the ability to discriminate two stimuli which is being used to estimate efficiency, one is not restricted to intensities lying close to threshold, so that an additional stimulus variable is introducedone which turns out to be important. There are further variables associated with the state of adaptation of the eye. It would be a formidable task to investigate the effects of each variable, and rather a futile one when so much is already known about the effects of each upon the threshold. The experiments reported here were intended primarily to confirm that the highest attainable figure for quantum efficiency is in the neighbourhood of 5 %, and to find whether this high figure can be maintained when the eye is light-adapted. Exploratory observations are reported indicating that, in any given conditions, the highest efficiencies are obtained when discriminating between intensities near to the subjective threshold. It is also shown that light adaptation shifts to higher intensities the range where discrimination is most efficient.