Color discrimination ellipsoids were obtained for several colors throughout the range of luminances, 10 to 0.03 ft-L. The ellipsoids were determined from three-color matches by two observers, using a colorimeter with a two-part circular field subtending an angle of two degrees, and a dark surrounding field. It was found that sensitivity to color differences remains constant for a normal observer until the field luminance drops below about one foot-lambert. Below this level, discrimination decreases rapidly and a normal observer appears tritanomalous. Sensitivity to luminance differences is essentially independent of chromaticity and decreases with luminance in the same manner as the data obtained from measurement of the Weber fraction with monochromatic or white fields.
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