Contrast and color
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Contrast and color
Some standard charts on the subject of contrast come from viewing situations
that are quite different from the VDT workplace. These charts imply a strong
dependence on "threshold" contrast on background luminance. In fact,
these data can best be understood if very low background luminances (less than
0.01 cd/m2) are considered a separate class, class 0. In this range,
the key issue is a minimum target luminance of 0.1 cd/m2.
There is a transition range (0.01...0.1) which could be called class 1. Normal
offices and VDT viewing fall into class 2 (more than 0.1). This is shown in
Figure 20.
Values of the standard visibility response function (CIE, 1972).
Luminance/contrast sensitiviy data of Figure 20 plotted differently.
Figure 20 is the typical figure shown in most handbooks. Figure 21 is the
same data plotted as task luminance for a given background luminance. This
figure better illustrates how two functions have been confounded, absolute
threshold (A) and relative threshold (C), and the transition range between
them (B).
Figure 21 represents these data re-plotted to show the relationship between
threshold task luminance level and background luminance level.
If you can detect color, you are in class 2. In class 2, threshold contrast
and perception of luminance difference follows a ratio "law." This
is called the Weber-Fechner law. (The original formula was published by Weber
(1834) and was extended by Fechner in 1860; hence, the formula is known as
the Weber-Fechner Law. As the first is in Latin and the second in German, the
English-reading reader may prefer to consult the discussion of the work in
Woodworth and Schlosberg, 1938.) A "just noticeable" ratio of target
to background is about 1.05:1. About seven of these ratios (1.05:7) is called
a "shade of gray." The reader may be familiar with resolution charts
that are used by photographers that display gray scales in shades of gray.
VDTs are designed to present visual images which differ by at least a "shade
of gray," seven "just noticeable" steps or more. Only in very
poorly illuminated VDT installations is contrast a concern.
For two shades of gray, the luminance ratio doubles. For the calculation-minded:
Shades of Gray = log x (Lobject) - log x (Lbackground)
where x = square root of 2.
For example, six shades of gray is a luminance ratio of 8:1.
There are two readily confused terms: "shades of gray" and "gray
scale." Gray scale refers to how small a step in gray level equipment
is capable of producing. That capability is best achieved when the equipment
uses ratio steps. In that case, seven or eight ratio steps will make a shade
of gray difference detectable. If, however, the application is such that small
differences in shades of gray are not desirable, then eight or none ratio steps
are useful. Since the levels are encoded, the number will usually be a power
of two (64 or 128 levels of gray are common).
For non-vision science reasons, linear expression of the code points in brightness
is common. In the linear case, even more levels are useful since the visual
effect is more detectable at low brightness (where a linear step represents
a greater ratio). These multiple levels are useful for shading. An important
recent development is the use of these levels to suppress the jagged appearance
of angled lines in graphics and symbols. The literature refers to this as "anti-aliasing" after
an analogous effect in communication theory.
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