Preferred density of sunglasses. 
Light- and dark-eyed observers, ranging in age from 17 to 67 years, compared six pairs of sunglasses whose neutral density filters transmitted either 0.8, 2.5, 4.8, 8.9, 15.8, or 91.2% of the light.  The sunglasses were worn in bright sunlight both in summer and winter and rated for comfort as to the light-level which they admitted to the eyes.  In addition, the observers took a test of resolution acuity with each pair of filters.  Observers of both eye colors and of every age group preferred, on the average, sunglasses which reduced the light level to 1000 to 1400 cd/m2.  These preferred filters are much denser than typical commercial sunglasses.  Resolution acuity declined significantly for the older observers with filters denser than the preferred ones.