Human contrast acuity variability

Abstract

Background: Army vision standards have varied little from Aviation’s nominal birth. On the basis of classic Snellen acuity, we simply cannot predict threshold skill levels of any one individual(s). A growing number of Army Flight Surgeons, clinicians, and vision scientists have argued for the inclusion of contrast acuity metrics within flight physical standards. Methods: Previous monitoring of operational contact lens utility in 223 Apache pilots, visual acuity data were gathered under two conditions: high illuminance; low illuminance combined with low contrast. Spectacle, contact lens, and aging influences were evaluated. Results: The high-contrast Snellen acuities clustered at 20/15 and 20/20. Low-contrast acuities stretched from 20/25 to 20/125. LogMAR analysis highlighted statistical significance between the two acuity sets (p < 0.001) to an unanticipated data spread. The known underlying mechanisms possibly related to this effect are poorly documented; all such variables collectively explain <30% of the known variation in low-illuminance vision. Discussion: Some pilots possessed the capacity to resolve 20/25 lettering under obfuscating conditions; others were adversely influenced by those same conditions. Snellen acuity involves target recognition; contrast acuity detects threshold differences; both aspects can be important. Conclusion: Prescreening under both vision assessment conditions will help identify and select superior vision performers. The validity and predictability of documenting this effect is targeted within planned future research efforts.