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The effect of pixel nonlinearity on cathode-ray tube-based visual acuity tests
1Arlene R. Gordon Research Institute, Lighthouse International, New York, New York 10022-1202, USA. lliu@lighthouse.org
Summary
Pixel nonlinearity in cathode-ray tube (CRT) displays causes gap width anisotropy, affecting visual acuity tests. This anisotropy, where raster-parallel gaps appear wider than raster-perpendicular ones, can lead to inaccurate results in CRT-based vision testing.
Area of Science:
- Ophthalmology
- Display Technology
- Visual Perception
Background:
- Cathode-ray tube (CRT) displays exhibit pixel nonlinearity, causing discrepancies between displayed and intended images.
- This nonlinearity can lead to anisotropy in pixel interactions, impacting tests using small horizontal and vertical gaps, such as visual acuity tests.
Purpose of the Study:
- To evaluate the impact of CRT pixel nonlinearity on visual acuity testing.
- To quantify the anisotropy of gap width perception on CRT displays.
Main Methods:
- Two high-quality CRT monitors were used to test gap width perception and visual acuity.
- Stimuli included a square ring with variable white gaps; luminance adjustments were made to equalize perceived gap widths.
- Visual acuity for horizontal and vertical gaps was measured at various distances, with and without monitor rotation.
Main Results:
- Gaps parallel to raster lines (raster-parallel) appeared wider than those perpendicular (raster-perpendicular).
- Matching perceived widths required adjusting luminance, indicating a significant difference in apparent gap size (e.g., 4-pixel horizontal vs. 5-pixel vertical).
- Visual acuity tests showed higher correct response rates for raster-parallel gaps, translating to a two-letter acuity difference, independent of monitor orientation.
Conclusions:
- CRT pixel nonlinearity creates a measurable gap width anisotropy at visual acuity sizes.
- This anisotropy can introduce significant uncertainties in visual acuity measurements performed on CRT displays, particularly when comparing horizontal and vertical gap performance.