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Legibility Index for Examining Common Viewing Situations: A New Definition Using Solid Angle

机译：用于检查常见观看情况的易读性指数：使用立体角的新定义

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The legibility levels of text and graphics can be assessed using the Legibility Index, conventionally defined as the distance at which material can be read with perfect accuracy (the legibility distance) divided by the character height. The ratio equals the inverse tangent of the visual angle subtended by the character. The definition assumes that the viewing material is perpendicular to the observer, which is not always true. Although text and graphics are, in reality, often viewed not perpendicular to the display, they are rarely studied. To examine the legibility of characters viewed not perpendicular to the display, this paper has redefined the Legibility Index as the inverse square root of the solid angle subtended by the target, based on a hypothesis that the three-dimensional solid angle, rather than the two-dimensional visual angle, both subtended by the character, captures how people recognize text and graphics that usually have two significant dimensions (width and height). This hypothesis proves consistent with how retinal images of text and graphics activate cones in thefovea of a viewer's eyes, and has also been verified in a laboratory using legibility data collected from 20 human subjects. Using these data, the calculated steady redefined Legibility Index closely matches the fact that the 20 subjects have similar recognition performance (critical legibility level with 100 percent accuracy) of text (same target) under the same lighting conditions (L_b=120 cd/m~2, C_%=97.9, zero ambient light) at different viewing angles (perpendicular or not perpendicular to the displays), while the calculated ratio of legibility distance to character height rapidly decreases with increased viewing angles.
机译：可以使用“易读性指数”来评估文本和图形的易读性水平，“易读性指数”通常定义为可以以最佳精度读取材料的距离（易读性距离）除以字符高度。该比率等于字符所对应的视角的反正切值。该定义假设观察材料垂直于观察者，但并不总是如此。尽管实际上通常不垂直于显示器查看文本和图形，但很少对其进行研究。为了检查不垂直于显示屏的字符的可读性，本文基于以下假设：将三维立体角而不是二维立体角重新定义为目标所包围的立体角的平方根。角色都具有的三维视角捕获了人们如何识别通常具有两个重要维度（宽度和高度）的文本和图形。该假设证明与文本和图形的视网膜图像如何激活观看者眼睛中央凹的视锥细胞一致，并且也已在实验室中使用从20位人类受试者收集的易读性数据进行了验证。使用这些数据，计算出的稳定的重新定义的可读性指数与以下事实非常匹配：在相同的光照条件下（L_b = 120 cd / m〜如图2所示，C _％= 97.9，环境光为零（垂直于显示器或不垂直于显示器）时，计算的易读距离与字符高度之比随视角的增加而迅速减小。

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