Computer monitor pixellation and Sloan letter visual acuity measurement.

Purpose: To assess the effects of changing computer monitor pixel density on visual acuity measurements made using Sloan optotypes.

Methods: Acuity was measured on 10 participants aged 19 to 38 years (mean 27.9 ± 7.0) measured binocularly wearing their best spectacle correction. Stimuli were eight lines of five Sloan letter optotypes in logarithmic progression, ranging in size from -0.4 to 0.3 logMAR. Test distance was varied so that pixels on the monitor ranged in size from 0.125 mins of arc to 1.97 mins of arc. Two sampling approaches were used: (1) unfiltered sampling, with each pixel rendered either black or white; and (2) filtered sampling with pixel luminance averaged across a pixel aperture, giving grey-scale smoothing of letter edges.

Results: A broken line fit was made to each data set, with acuity being stable at an asymptotic threshold VA for small pixels sizes, with thresholds increasing linearly when pixel sizes exceeded a critical pixel size P . For unfiltered stimuli, P averaged 1.1 mins of arc and for unfiltered stimuli averaged 0.69 mins of arc. For filtered stimuli, P was 1.79xVA , and for unfiltered P was 1.05xVA .

Conclusion: The results show that grey-scale filtering makes acuity measurement more resistant to the effects of pixellation. Based on a conservative interpretation of these findings, we make the recommendation that charts be constructed with, for filtered optotypes, a maximum pixel size of 0.6 x of the smallest MAR used and, for unfiltered optotypes, 0.35 x the smallest MAR used.