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L. Marran, L. Liu, G. Lau; Desktop Publishing & Validation of a Custom Near VA Chart for Measuring Accommodative Amplitude.. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4555.
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Customized visual acuity (VA) assessment can greatly facilitate basic and clinical vision research. Two obstacles have to be overcome in making a customized VA chart: to produce it up to specification and to validate it against an accepted standard. We used desktop publishing to produce near VA charts for repeated measures of subjective accommodative amplitude in patients implanted with the Synchrony ®AIOL lens (Visiogen Inc. Irvine, CA). There were six charts ranging from 20/20 to 20/63 in 0.1 log unit steps. Each chart had multiple lines of the same acuity level to prevent memorization (e.g. the 20/20 chart presented several unique lines of 20/20 optotypes in different order and no other optotype size). Each chart had a random noise background- to maintain a constant spatial frequency profile of the peripheral accommodative stimulus, a possible improvement over standard VA charts with multiple acuity levels, where neighboring spatial frequencies are dependent on the acuity line being viewed. We validated our custom charts against a commercially available (standard) chart.
A 1200 dpi canvas was used to take advantage of the high pixel resolution of modern desktop printers. Pixel accurate bitmaps of Sloan optotypes 20x20 to 90x90 in size were derived from a set of vector graphs and printed on a laser printer. The physical sizes of the printed optotypes were measured under 15x magnification. Pixels sizes that closely matched the required sizes of 20/20 to 20/63 optotypes (in 0.1 log unit steps) at a 40 cm viewing distance were selected. Bitmaps of selected optotypes were pasted into a canvas with customized format and background. The functional equivalence of the customized near charts to a standard chart were compared on 10 normal phakic subjects under the age of 40.
At all acuity levels, the physical sizes of the printed custom optotypes deviated no more than 0.034 log units from that of the standard, satisfying the 0.05 log unit ISO criterion and demonstrating physical equivalence. Also, at all acuity levels, log unit differences in the mean target distance for which reliable recognition of letters first occurred for the custom optotypes compared to the standard were found to be below the 0.05 log unit ISO criterion, demonstrating functional equivalence.
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