U.S. Army sunglasses: issues and solutions

Some military aviators have reported difficulty discriminating the color of cockpit warning lights when wearing plastic sunglasses. This difficulty could be due to lenses that are too dark or are nonneutral and thus alter color vision. A study was performed to identify the origin of this problem, recommend solutions, and determine a sunglass transmittance that optimizes visual performance.

Five pairs of plastic sunglasses (-4 to +4 D) were ordered from each of seven military optical laboratories (70 lenses total). Each laboratory was instructed to dye the lenses neutral gray with 21 percent transmittance. Light transmittance and color distortion were evaluated across laboratory and lens power. Spatial and color vision were assessed through a range of sunglass transmittances.

There was no systematic effect of lens power, but light transmittance and color distortion varied widely across laboratories (transmittance = 1 to 30 percent; p < 0.001).

Because light transmittance and color distortion were related inversely, it was believed that both factors could be corrected by accurate verification of transmittance, but commercial transmittance meters proved to be inaccurate. The high transmittance of deep red and infrared light through plastic lens dyes is read as visible light by transmittance meters, making readings too high. A filter was identified that provides accurate readings when used with transmittance meters. A sunglass transmittance of 23 percent resulted in minimal decrease in visual performance relative to normal clinical test conditions.