NIHR Biomedical Research Centre for Ophthalmology, Moorfields Eye Hospital NHS Foundation Trust & UCL Institute of Ophthalmology, London, UK.
Ophthalmic Physiol Opt. 2011 Jan;31(1):17-22. doi: 10.1111/j.1475-1313.2010.00806.x.
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 variability of acuity measurements using these letters compared to conventional letters, and in particular how acuity is affected by the number of alternatives available to the subject.
Acuity was measured using high contrast letters of both conventional and Vanishing Optotype design for three experienced 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. Three measurements were made for each condition.
Threshold letter size was always larger for the Vanishing Optotypes than conventional letters, although the size of this difference (0.11-0.34 logMAR) 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 legible than conventional optotypes. Variability was also lower for the Vanishing Optotype sets (0.01-0.03 logMAR) than the conventional letter sets (0.03-0.06).
The smaller effect of the number of letter alternatives, combined with more equal discriminability and lower threshold variability, implies that Vanishing Optotypes may be appropriate targets from which to design letter charts to measure small clinical changes in acuity.
消失视标字母具有伪高通设计，因此目标的平均亮度与背景相同，并且在达到分辨率阈值后，字母很快“消失”。我们希望确定使用这些字母进行视力测量的可变性与传统字母相比，特别是在可供受试者选择的替代方案数量如何影响视力的情况下。
使用高对比度的传统和消失视标设计的字母，对三名经验丰富的正常受试者进行了视力测量。在中央视力的强制选择范式中，使用 QUEST 程序为两种替代方案（2AFC；AU 和 OQ）、4AFC（AQUO）、6AFC（QUANGO）和 26AFC（整个字母表）确定了阈值。对于每种情况，每个条件都进行了三次测量。
消失视标字母的大小始终大于传统字母，尽管这种差异的大小（0.11-0.34 logMAR）取决于替代方案的数量及其内容。消失视标对 AFC 数量和使用的个别字母的影响较小，这意味着它们比传统视标更易读。消失视标集的变异性也较低（0.01-0.03 logMAR），而传统字母集的变异性较高（0.03-0.06）。
字母替代方案数量的影响较小，加上更均匀的可辨别性和较低的阈值变异性，意味着消失视标可能是设计字母图表以测量视力小临床变化的合适目标。
