Reflection and contrast
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Contrast Specification
Various conventions have been used to characterize the difference between the
image and its background. These have included the contrast ratio, modulation
and various formulas to define contrast and percent contrast. Contrast ratio
is the simplest, that is, the higher luminance divided by the lower luminance.
Contrast ratios and percent contrast values, however, do not result in scales
that reasonably represent the response of the visual system. The perceptual
difference between contrast ratios of 2:1 and 4:1 is not the same as the
difference between 4:1 and 6:1. Modulation contrast, however, does have the
attribute. For example, over a range where added contrast will make the item
easier to see, an increase in modulation of equal steps will produce about
equal improvements in performance. Log scales are used in acoustical work
for the same reason. An example of this function may be seen when plotting
data from a legibility study (Shurtleff, 1980) in terms of contrast ratio
(Figure 45) and modulation (Figure 46) with respect to performance.
Character size (minutes of arc) in terms of contrast ration.
Character size (minutes of arc) in terms of modulation.
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