A test of visual contrast discrimination has been developed for demonstrating visual impairment due to anoxia at altitudes of 15,000 feet or above. It is made of photographically printed Landolt rings only slightly darker than their white background, and varying in contrast so as to cover the range just barely discriminated by different individuals at sea level and at altitude. Since moderate anoxia has little effect on vision in daylight, but causes a deterioration in contrast discrimination at twilight and night, the tests are designed to be viewed at a twilight brightness of about 0.1 millilambert. Trials with aviation cadets and other untrained persons have shown that the tests fulfill their purpose of demonstrating the effect of anoxia on vision. They also show the existence of large individual differences in the score at sea level and in the effect of altitude. The tests have been used in researches with skilled subjects on the effects of various procedures for producing anoxia, and have successfully given a quantitative measure of the resulting visual impairment.
SELIG HECTH, CHARLES D. HENDLEY, SYLVIA FRANK, and SIMON SHLAER, "Contrast Discrimination Charts for Demonstrating the Effect of Anoxia on Vision," J. Opt. Soc. Am. 39, 922-922 (1949)