The tests which are commonly employed for estimating visual function are carried out with the light-adapted patient. The acuity of the fovea is determined under optimal conditions of illumination, which means that the test chart is illuminated by at least 20 or more foot candles. Similarly, the isopters of the peripheral field are determined with white or colored test objects which reflect sufficient light to stimulate the peripheral portions of the light-adapted retina. Throughout the field areas of depressed function are detected in the photopic retina. The result of these tests allow us to evaluate the functional capacity of the photopic retina.
The retina behaves differently in the dark-adapted state, however, and there are well known diseased conditions in which it is normal when light adapted but is grossly abnormal in semidarkness. Numerous tests have been devised to measure its function in the dark-adapted state, and the literature is now
McDONALD R, ADLER FH. CLINICAL EVALUATION OF TESTS OF DARK ADAPTATION. Arch Ophthalmol. 1940;24(3):447–461. doi:10.1001/archopht.1940.00870030023002
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