Understanding disability glare: light scatter and retinal illuminance as predictors of sensitivity to contrast E Patterson, G Bargary, J L Barbur |
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Applied Vision Research Centre, City University London, United Kingdom
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| Forward light scatter within the eye causes a reduction in retinal image contrast, which can be debilitating in the presence of bright light sources. The concurrent increase in retinal illuminance can, however, improve retinal sensitivity under some conditions. The combined effect of reduced image contrast and increased retinal sensitivity remains poorly understood. The effects of glare-source intensity, surround luminance and test target location on the retina are investigated. The aim is to provide a new, more accurate model of contrast sensitivity in the presence of glare. A psychophysical flicker-cancellation test (M. L. Hennelly et al., 1997, Ophthalmic & Physiological Optics, 17, 171) was used to measure the amount and angular distribution of scattered light in the eye. Contrast thresholds were measured using the ‘Functional Contrast Sensitivity’ (FCS) test (C. M. Chisholm et al., 2003, Aviat. Space Environ. Med., 74, 551-559). Pupil plane, glare-source illuminances (0, 1.35 and 19.21 lm/m²), eccentricities (5°, 10° and 15°), and background luminance levels (1, 2.6 and 26 cd/m²) were investigated. In general, predictions based solely on scattered light overestimate the detrimental effect of glare on visual performance. Prediction accuracy is, however, improved significantly by incorporating into the model changes in retinal sensitivity. |
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