Abstract
In this study a psychophysical experiment was conducted in order to determine to what extent changes in background reflectance affect the selection of equal appearing gray tone intervals. This is important to know because cartographers frequently use a series of gray area tones to represent quantitative data, and it is essential that the map reader perceive a direct correspondence between the value of the gray area tones and the quantities they represent. The results of the experiment indicated that changes in background reflectance had no significant effect on the selection of the darker gray tones, but as the value of the gray tones increased, so did the effects of background. The power formula developed by Stevens and Galanter provided a good approximation to the experimental data for white and black backgrounds, and can be used to construct a series of equal appearing gray tone intervals. The potential of a reformulated power function based on adaptation-level theory to predict background effects was considered. Some constraints in using the equal value gray scale with screened gray area tones also were discussed.
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