The principle that things are easier to find if they contrast with their environment is expressed quantitatively. Two indices of conspicuousness, relative fixation rate and search time, are shown to be related to the color difference between the target and background objects. The color difference equations used are CIELAB, CIELUV, and the Judd modification of the index of fading. None of the equations is superior to the others. Furthermore, neither Judd’s modification nor Semmelroth’s adjustment accounts for effects of surround lightness. It is suggested that color difference be used as a tool for design and evaluation of visual displays, for construction of color codes to optimize search time, and as a generalization of achromatic contrast in psychophysical research.
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