UEC Visual Acuity System Compared To The Visual Acuity Measurement Standard.

Background: The purpose of this study is to compare the liquid-crystal display (LCD) screen method of visual acuity measurement using the Canela Visual Acuity System (CVAS) at the University Eye Center (UEC) with the Visual Acuity Measurement Standards (VAMS) set forth by the Visual Functions committee of the International Council of Ophthalmology. Specific inquiries in this study include: 1) optotype size progression, 2) consistency with Sloan letters, 3) number of letters presented per line and spacing between optotypes, 4) accuracy in presentation of varying contrast levels, and 5) effects of polarization at various viewing angles.
Methods: The letter “E” and Landolt “C” will be isolated at each visual acuity line for comparison. The number of pixels on the LCD monitor for each letter and spacing between letters will be counted horizontally and vertically via a loop magnifier. The calculated visual acuity will be derived using the standard test distance employed in the clinic. A photometer will be used to measure the luminance under bright and dark conditions.
Results: The calculated LogMAR deviates outside 5% tolerance between -0.50 to 0.30, or 20/6.3 to 20/40, for both letters “E” and Landolt “C”. The CVAS uses 16 letters, half are Sloan letters and half are not. Spacing between letters scale with letter size. Measured contrast levels fall outside 5% tolerance for letter contrast less than about 0.80, and a 5% difference exists between contrast levels in bright and dark conditions. Mean
contrast also decreases as a function of viewing angle.
Conclusions: The CVAS does not accurately display the stated visual acuity in some of the most commonly used sizes, including 20/20. In most cases, the displayed contrast level is much higher than stated. The CVAS requires further review to assess the reliability and accuracy in clinical use.