The ranges of stimuli that are perceived as white were investigated in a dark surround. Stimulus light was supplied by three gas-filled tungsten lamps modified with three glass filters and an opal glass. Chromaticity and luminance were changed by adjusting distances between the opal glass and each of the lamps. We employed the method of minimal changes and the constant-stimulus method. In the former case, the luminances of two of the stimulus components were kept fixed, while the observer was allowed to change the luminance of the other component. Ranges of chromaticities reported as white with a probability of 0.5 or more by three observers were located along straight lines near the blackbody locus, at color temperatures higher than 5500 K. With the constant-stimulus method, 45 observers judged 18 stimuli located at different positions in the neighborhood of CIE standard source C in the chromaticity diagram, all of which had luminances equal to 20 asb. The result was that an oval range of points reported as white with a 0.5 probability was obtained around CIE standard source C.
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