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N. Shah, S. C. Dakin, T. Redmond, R. S. Anderson; Vanishing Optotype Letters as a Clinical Acuity Test: Repeatability and Effect of the Number of Alternatives. Invest. Ophthalmol. Vis. Sci. 2010;51(13):967.
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Vanishing Optotype letters have a pseudo high-pass design so that the mean luminance of the target is the same as the background and the letters thus ‘vanish’ soon after the resolution threshold is reached. We wished to determine the repeatability of acuity measurements using these letters compared to conventional letters, and how acuity is affected by the number of alternatives available to the subject.
Acuity was measured on a standard CRT monitor using high contrast letters of both conventional and Vanishing Optotype design. All measurements were made on three occasions for three normal subjects. Thresholds were determined for central vision in a forced choice paradigm for two alternatives (2AFC; AU and OQ), 4AFC (AQUO), 6AFC (QUANGO) and 26AFC (whole alphabet) using a QUEST procedure.
Threshold letter size was always larger for the Vanishing Optotypes than conventional letters, although the size of this difference (1.3 - 2.2 times) depended on the number of alternatives and what they were. The effect of the number of AFC, and the individual letters employed, was smaller for the Vanishing Optotypes implying that they are more equally discriminable than conventional optotypes. Variability was also lower for the Vanishing Optotypes (3.8 - 6.8%) than the standard letters (6.2 - 14.6%).
The smaller effect of the number of alternatives, combined with more equal discriminability and better repeatability implies that Vanishing Optotypes may be better targets from which to design letter charts to measure small clinical changes in acuity.
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