Computer monitor pixellation and Sloan letter visual acuity measurement

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

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.

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

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.