Figure 52 shows a set of modulation curves of contrast sensitivity for the human eye. The separate curves are for different light densities and are labeled in terms of average retinal illumination, or trolands. Acuity, or contrast sensitivity, is measured by first establishing the angular size of the pattern, stated in cycles per degree. Then, the minimum modulation required to detect the pattern is determined. There are two conclusions that may be drawn from the curves in Figure 52.
Contrast is the important factor after that point.
In fact, there is little practical difference between the 9 and 90troland curves. Improvements in acuity produced by light levels higher than those may be attributed to a reduction in refractive error caused by using a smaller pupil. Second, for the luminances of interest in an office environment, contrast sensitivity is best for a spatial frequency of about 5 to 7 cycles per degree. That would be a bar pattern, with each bar being about 4 to 6 minutes of arc in width. That corresponds to letter heights of about 12 to 18 minutes of arc.