Reflection and contrast
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Contrast
Once a minimum luminance of the brighter of the foreground and background has
been reached, contrast becomes the most important issue in ease of use. The generally
accepted minimum for luminance is 35 cd/m2 (Shurtleff, 1980). Most
of the studies in this area compare performance with characters of different
sizes and contrasts. The inference is that if a person can perform faster with
a higher contrast image, it is because that image is easier to see. Studies (Crook,
Hanson and Weisz, 1954; Howell and Kraft, 1959; Snyder and Maddox, 1978) show
that visual performance increases with improved contrast over a range of contrast
ratios from 2:1 to 40:1. Preferred contrasts are often above 15:1 so long as
the display is capable of presenting a clear, sharp image at these higher contrasts
(Kokoschka and Haubner, 1983).
In recommending a minimum or optimum contrast, character
size and task should be considered. A minimum contrast
ratio of 3:1 is frequently recommended. That contrast
ratio is often satisfactory for relatively slow-paced
tasks. Contrasts should be much higher, however, for
tasks where the rapid and accurate perception of the
symbols is important. Under these conditions, a contrast
ratio of 20:1 or more, or modulation contrast of 0.9
or more, is frequently recommended.
There is a relationship between contrast, character
size and perceptibility. There is a certain amount
of blur in the optical system of the eye, and that
blur reduces the contrasts of the retinal image contrast.
Therefore, as character size is reduced, contrast should
be increased to provide an equivalent contrast in the
retinal image.
If a contrast ratio of 3:1 is considered a minimum
for a character size of 16 minutes of arc, smaller
character sizes should have higher contrasts to result
in equal perceptibility.
The following equation (Cm) was derived from data
by Crook and his associates (1954) and may reasonably
represent the higher contrasts required for characters
smaller than 20 minutes of arc to give them about the
same perceptibility as a 20-minute high character with
a modulation contrast of 0.3.
Cm = 0.3 + 0.06 (20 - SH)
where SH equals symbol size in minutes of arc.
ISO 9241, Part 3, requirement on contrast is:
"The minimum luminance contrast of character details,
within or between characters, that are relevant for
legibility shall be;
Contrast modulation: Cm=0.5
Contrast ratio: CR=3:1"
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