Relation of Threshold Criterion to the Functional Receptors of the Eye*

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Abstract
Luminance thresholds for the resolution of parallel line grating test patterns were determined by the method of constant stimuli. Measurements were made in the dark-adapted eye. Thresholds were determined with each of eight selected color filters and also with Wratten neutral tint filters. Seven gratings were used which required visual acuities from 0.042 to 0.625. With gratings requiring a high order of visual acuity, minimum luminances for resolution of a grating are very nearly the same for all the color filters. As visual acuity requirements are decreased, however, luminance thresholds determined with red filters become higher relative to thresholds determined with other filters. Luminance thresholds with blue filters become lower relative to thresholds obtained with other filters. These results are interpreted to indicate that changes in the threshold criterion may result in changes in the character of visual function from rod function through mesopic function to cone function even though the eye remains dark-adapted. In situations where an individual is adapted to a visual field of low over-all luminance and must periodically, in short glimpses, read visual displays at higher levels of illumination, the appropriate specification of the relative effectiveness of illumination wavelengths will depend on the visual acuity required to read the display.

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