The eye is an extremely light-sensitive instrument capable of responding to almost unbelievably small amounts of light energy. However, a simple answer cannot be given to the question: How sensitive is the eye to light? The absolute sensitivity of the eye cannot be gauged by a single threshold value, since the minimum amount of light necessary for vision has been found to depend on the conditions of stimulation. Therefore~ the absolute sensitivity of the visual system is best understood by examining the functional relationships between th~ absolute threshold and the conditions that determine its value:
The value of the abs<;>lute threshold depends upon previous stimulation. Exposing the eye to intense light greatly decreases the absolute sensitivity of the eye. Sensitivity is recovered gradually if the eye is subsequently kept in darkness. Nearly complete recovery of sensitivity occurs after about one hour in the dark. The dark adaptation curve is traced out by measuring an observer's absolute threshold periodically
during the recovery period and plotting its value as a function of time in the dark. The threshold at the beginning of dark adaptation may be as much as 100,000 times as high (Slog units) as the threshold after complete dark adaptation.