FIG. 2.

Contrast sensitivity function. Contrast sensitivity varies with spatial frequency. The spatial frequency of a letter is determined by the cycles per degree (cpd) of the visual angle using sine wave gratings. Contrast is defined as the luminance of an object compared with its surround. The contrast of sine wave gratings is defined as the difference in luminance between the gratings over their sum and is expressed as a percentage (i.e., 1.25% contrast). Decreasing letter size (as demonstrated by the word acuity in the figure) is associated with increasing spatial frequency. The maximum of human detection is 30–60 cpd at 100% contrast (2,3). The peak of the curve, which represents the maximal viewing of low-contrast objects, occurs at 3–5 cpd, as indicated by the word contrast (2,3). High- and low-contrast letter acuity charts alter the spatial frequency (letter size) and hold the contrast for each letter constant, whereas contrast sensitivity charts alter the contrast of the letters at a single spatial frequency (letter size). Reproduced with permission from (3).