Simple Method of Measuring Brightness Threshold of Dark Adapted Eye at All Ages

Abstract

The relation between the vitamin A status of the body and the eye's ability to adapt to the dark is now well established. 1 2 3 4 5 The chemical mechanism underlying this relation has been studied by Wald. 6

While several instruments for measuring dark adaptation are available, 7 8 9 10 none is applicable to infants and very young children. In the course of an investigation 11 of the vitamin A requirements of infants, necessity impelled us to devise a method for measuring their ability to adapt to the dark. Fortunately, our task was simplified by a number of recent studies of this function in adults. It has been demonstrated that whereas the dark adaptation of persons with certain diseases is more or less delayed, 2 , 12 alterations in the vitamin A status of clinically normal individuals are accompanied by a simple rise or fall of the visual threshold, the speed of adaptation remaining constant. 3 , 4 , 5 Hence, where it is not possible or convenient to measure the complete time course of dark adaptation, it is sufficient for purposes of dietary studies of normal subjects to determine the final or equilibrium threshold alone. 4 The present apparatus and procedure accomplishes this in a satisfactory and simple manner.

In measurements of the precise course of dark adaptation, fixation of the eye in relation to the direction of the stimulus is essential to insure that the portion of the retina measured is the same throughout the test. If only the final or equilibrium threshold of the most sensitive portion of the retinal periphery after complete dark adaptation is to be determined, fixation is unnecessary.