title: Human Contrast Acuity Variability.
abstract: 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.
  
  
  
  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.
  
  
  
  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.
  
  
  
  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.
  
  
  
  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.
authors.affiliation: Vision Performance and Protection Division, U.S. Army Aeromedical Research Laboratory, 6901 Farrel Road (Building 6901), Fort Rucker, AL 36362.
authors.forename: Morris R
authors.initials: MR
authors.lastname: Lattimore
mesh_terms: D000328:Adult; D001359:Aviation; D000073537:Biological Variation, Population; D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D008889:Military Personnel / Q000706:statistics & numerical data; D018579:Patient Selection; D014787:Vision Tests / Q000379:methods / Q000592:standards*; D014792:Visual Acuity / Q000502:physiology*; D000078329:Workforce