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Figure 4. Approximation of the contrast sensitivity curve with a parabola. Heinemann (4) measured human contrast sensitivity at different levels of adapting luminance. Examples of this relationship at two adapting luminance levels are shown (solid lines). In reality, there is a whole family of curves of similar shape that have a minimum that shifts with the adapting luminance. Consider the lower curve, which corresponds to an adapting luminance of 100 cd/m2. The eye is maximally sensitive at a display luminance of 100 cd/m2, with an MDC of about 0.05. However, an object located in a dark part of the image at 10 cd/m2 would have to have a contrast of 0.1 to be seen. The practical solution in radiology is to use a spotlight to raise the luminance to 100 cd/m2 and improve the contrast sensitivity. As the adapting luminance decreases, the curves shift upward and maintain roughly the same shape. Attempts have been made to fit the curves from Heinemann's experimental data with simple equations (5). The algorithm of Liu and Nodine (8) required advanced information about adaptation level and was computationally intensive. We simplified that algorithm by assuming that a parabola (dashed lines) could be used to approximate contrast sensitivity at different levels of adapting luminance. The fit is reasonable at high adapting luminance (100 cd/m2), where radiologists prefer to operate. The fit for a lower adapting luminance (10 cd/m2 [upper curve]) is not very good. However, this luminance is well below a practical average viewing luminance.
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