Young Laura K, Love Gordon D, Smithson Hannah E
Department of Physics, Durham University, Durham, UK.
J Vis. 2013 Jun 20;13(7):12. doi: 10.1167/13.7.12.
We previously showed that different types of aberration defocus, coma, and secondary astigmatism affect reading performance via different mechanisms. In this paper, we show the contrary result that, for identification of isolated letters, the effects of rendering different types of aberration can be described by a single cross-correlation-based metric. Aberrations reduce the effective resolution of an optical system, quantified by the high-frequency fall-off of the modulation transfer function. They additionally cause spatial-frequency-dependent phase and contrast changes, which have a size-dependent effect on letter forms. We used contrast threshold as our performance measure, instead of distance acuity, to separate the effects of form alterations from those of resolution limits. This measure is additionally appropriate in comparing single-letter-based performance to reading at a fixed distance. The relationship between a cross-correlation-based measure of letter confusability and performance was the same for all three types of aberration. For reading, we had found a different relationship for coma than for defocus and secondary astigmatism. We conclude that even when two tasks--letter identification and reading--use the same component stimulus set, the combination of multiple letters in a reading task produces functional differences between the effects of these aberrations that are not present for isolated letters.
我们之前表明，不同类型的像差（散焦、彗差和继发性散光）通过不同机制影响阅读表现。在本文中，我们展示了相反的结果，即对于孤立字母的识别，呈现不同类型像差的影响可以用单一的基于互相关的指标来描述。像差会降低光学系统的有效分辨率，通过调制传递函数的高频衰减来量化。它们还会引起空间频率依赖性的相位和对比度变化，这些变化对字母形状有大小依赖性的影响。我们使用对比度阈值作为我们的性能指标，而不是距离视力，以将形状改变的影响与分辨率限制的影响区分开来。该指标在将基于单个字母的性能与固定距离阅读进行比较时也同样适用。基于互相关的字母混淆度测量与性能之间的关系对于所有三种像差类型都是相同的。对于阅读，我们发现彗差与散焦和继发性散光的关系不同。我们得出结论，即使字母识别和阅读这两项任务使用相同的组成刺激集，阅读任务中多个字母的组合也会在这些像差的影响之间产生功能差异，而这些差异在孤立字母的情况下并不存在。
