The Basis of the Dependence of Visual Acuity on Illumination

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Abstract
The relation of irradiation or optical spreading to visual acuity was investigated as a function of illumination. The two-bar test object and its background were separately illuminated. In the experiments on acuity, it was found that, with an illuminated test object on a light-free ground, visual acuity increases with intensity until a maximum acuity is reached; thereafter, visual acuity decreases with rising intensity. With a light-free test object on an illuminated ground, visual acuity increases with rising intensity,[long dash]the improvement in acuity becoming progressively less as the intensity is increased. The visual acuity depends, as far as illumination is concerned, not only on intensity but also on the particular distribution of the light in the test object and its background. In the experiments on irradiation the difference between the apparent and physical widths of the bars of the test object gave a numerical measure of irradiation. It passed through the zero value at some particular intensity both with the illuminated test object on a light-free ground and the light-free test object on-an illuminated ground. The visual acuity found in the absence of irradiation at one particular intensity added to the amount of irradiation found at any other intensity gave the theoretical visual acuity. The theoretical visual acuity values, determined from the irradiation measurements, show a close agreement with the empirically determined visual acuity.

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