Sloan Letter Visual Acuity Charts and Computer Monitor Pixilation

Title Sloan Letter Visual Acuity Charts and Computer Monitor Pixilation
Author, Co-Author Andrew Carkeet, Lucas Lister
Topic Functional Vision/Pediatrics
Program Number
Ballroom A-B
Abstract Purpose: This research investigates how pixilation of computer monitors affects visual acuity measurement.

Methods: Stimuli were presented on a computer monitor as 8 lines of 5 Sloan letter optotypes in standard logarithmic progression format, ranging in size from -0.4 to 0.3 logMAR.  Test distance was varied so that pixels on the monitor subtended different angles: 0.125, 0.200, 0.315, 0.50, 0.79, 1.25, 1.97 minutes of arc.  Two pixel-sampling strategies were used: unfiltered sampling in which each pixel was rendered either black or white; or filtered sampling, in which pixel brightness was taken as the average letter brightness integrated across a pixel-sized aperture (i.e. grey-scale smoothing of letter edges). Binocular acuity was measured at each distance on 10 participants aged 19 to 38 years (mean 27.9 ± 7.0) using their best spectacle correction.

Results: Each observer’s logMAR acuity v log pixel size data was fitted with a broken line function, in which LogMAR acuity was described by a horizontal straight line below a critical pixel size (Pcrit) and by a linear relationship between acuity and pixel size above Pcrit.  For small pixels sizes (below Pcrit), average asymptotic thresholds were a mean of -0.209 logMAR (SD 0.06) for the filtered letters, just slightly but significantly better than for the unfiltered letters with a mean -0.184 logMAR (SD 0.06) (t9 = 2.26, p = 0.050). Average Pcrit was significantly (p < 0.001) larger for filtered stimuli (at 0.0445 log minutes of arc or 1.1’) than for unfiltered stimuli (at -0.1628 log minutes of arc or 0.69’). Pcrit for Sloan letters were 1.79 and 1.05 times threshold for filtered stimuli and unfiltered stimuli respectively.

Conclusion: For the purposes of visual acuity measurement, Sloan letters can be well rendered by relatively coarse sampling.  Filtered letters (i.e. with grey-scale smoothed edges) can be rendered with much coarser (apparently sub-Nyquist) pixel sampling than unfiltered letters.
Affiliation of Co-Authors QUT