Connolly DM, Barbur JL. Low contrast acuity at photopic and mesopic luminance under mild hypoxia, normoxia, and hyperoxia. Aviat Space Environ Med 2009; 80:933–40. Introduction: Visual acuity is compromised under low luminance, low contrast viewing conditions that occur in contemporary night flying, but the effect of hypoxia on low contrast acuity in dim light remains poorly defined. This study examined the effects of oxygenation state on the contrast thresholds required to maintain visual acuity in 12 subjects at low photopic (12 cd · m⁻²), upper mesopic (1 cd · m⁻²), and mid-mesopic (0.1 cd · m⁻²) luminance. Methods: The Contrast Acuity Assessment test (City University, London) was used to measure the contrast thresholds needed for gap orientation discrimination using a Landolt C stimulus. The thresholds were measured at the fovea and at eccentricities up to ± 5° from fixation under mild hypoxia (breathing 14.1% oxygen), hyperoxia (100% oxygen), and normoxia (air). The diameter of the pupil was measured using infrared imaging and the influence of oxygenation state on pupil size assessed. Subsequently, normoxic thresholds were measured in detail under conditions of constant retinal illuminance to enable prediction of the effects of oxygenation state in the mesopic range. Results: Relative to normoxia, hypoxia caused the contrast thresholds to increase at all light levels, but particularly at mesopic luminance. Hyperoxia decreased contrast thresholds, but only at the lowest light level. In general, hypoxia caused a reduction in mean pupil size while hyperoxia caused the pupil to dilate. Subjects with naturally smaller pupils tended to exhibit greater hypoxic impairment. Conclusions: Mild hypoxia degrades low contrast acuity progressively with decreasing mesopic luminance. At 0.1 cd · m⁻², supplementary oxygen enhances low contrast acuity, implying that visual performance is oxygen-dependent in the mid-mesopic range. Relative to performance under mild hypoxia at 3048 m (10,000 ft), supplementary oxygen can extend functionally useful vision to lower light levels. The findings are relevant to contemporary military night flying, viewing the external scene directly or through night vision devices, or viewing dimly illuminated flight deck instruments.