Computer monitor pixellation and Landolt C visual acuity.

Abstract

To investigate the effects of computer monitor pixel density on Landolt C visual acuity measurements and to provide appropriate pixel density recommendations for Landolt C vision chart design. Participants were 10healthy observers aged 18-31years (mean 21.7±3.6). Logarithmic progression charts were used, consisting of eight rows of five Landolt C optotypes ranging from -0.40logMAR to 0.30logMAR. Monitor pixel angular subtense varied from 0.10min arc to 1.97min arc, achieved by changing the chart test distance. Testing took place with two filtering conditions: unfiltered optotypes, (pixels rendered either black or white) and anti-aliased optotypes with pixel luminance averaged for a pixel square and rendered as grey levels. Each participant's acuity versus pixel size data sets were fitted by a nonlinear relationship with acuity equal to an asymptotic threshold visual acuity (VAas )for small pixel sizes below a critical pixel size (Pcrit ). For pixel sizes larger than Pcrit there was a linear relationship between acuity thresholds and pixel size. For anti-aliased Landolt Cs, mean Pcrit was 1.23min, and for unfiltered Landolt Cs average Pcrit was 0.65min. For anti-aliased LandoltCs, Pcrit was 2.01xVAas , and for unfiltered Landolt Cs Pcrit was 1.05xVAas . These results are quantitatively very similar to previous research on pixellation and Sloan optotype acuity. We have demonstrated that spatially filtering Landolt C optotypes acts as anti-aliasing, to make them more robust to the degradation effects of pixellation. Previous recommendations for maximum pixel size on Sloan letter vision charts can be applied safely to Landolt C charts.