Sine wave mesopic contrast sensitivity – defining the normal range in a young population

Abstract

Limited information is available regarding the normal range of mesopic sine wave contrast sensitivity (CS). The aim of this study was therefore to define a reference range and in addition to identify factors related to mesopic CS in a young population. Individuals with normal visual acuity (VA) were included from a population of air crew members visiting the Institute of Aviation Medicine for routine medical examinations. CS was measured using the Optec 6500/FACT and evaluated by (i) the CS curve with five frequencies and (ii) the index of contrast sensitivity (ICS). ICS was defined as the sum of the weighted differences from the median CS value for each frequency. Multivariate analyses were performed to identify factors related to CS. In a total of 197 subjects included, the mean mesopic CS score was significantly lower compared to the corresponding photopic measurements for all five frequencies tested. Median CS score for frequencies 1.5, 3, 6, 12 and 18 cycles per degree was 50, 114, 180, 60 and 33 for photopic and 71, 80, 90, 22 and 12 for mesopic (- glare) conditions, respectively. Better binocular VA was positively associated, while the use of corrective lenses was negatively associated with CS performance. We have provided reference values for mesopic CS and identified factors important for CS. The difference between mesopic and photopic measurements highlights the need for separate population norms. The new ICS score may be a useful collective descriptor of the traditional CS curve with five frequencies.