Human variation in dark adaptation facility

There is mounting evidence that the military’s emphasis on photopic visual performance standards in the absence of mesopic and scotopic standards is a major oversight. The lack of an individualized mesopic capability represents a scientific knowledge gap that could impact soldier survivability. Data from 24 subjects (who had participated in a project on “Facilitating the transition from bright to dim environments”) have been re-assessed with regard to their individual dark adaptation response variability. A reduced ‘bleaching’ stimulus of 378 foot Lamberts (fL), lasting only 5 minutes, often elicited a rod-cone break in a matter of seconds. The time required for each subject to subsequently detect and identify as many of 10 reduced–intensity light sources was measured. The 2° lighted targets ranged from 0.250 fL to 0.001 fL in luminance. The dark adaptation time to detect the 0.002 fL target was 60.81 +/- 26.9 seconds (mean +/- standard deviation) for the control condition. The clear spectacle lens group took 78.99 +/- 44.34 seconds. Grouped data analysis indicated the lack of a statistically significant difference between the data-sets by t-test (p ≤ 0.06). Yet, there is a difference in the degree of response variability by both grouped data sets, as well as by individual data response variability. Variation between the control and lens-wearing conditions can readily be explained by 8-10% transmission losses from surface reflections, and direct glare interference. Nevertheless, both iterations exhibited a varied facility or speed of dark adaptation, which further varied on the testretest condition, meaning there was no learning effect from the first test to the second one. This same visual sensitivity variability characteristic is duplicated within the realm of visual resolution.