Two methods of measuring foveal spectral sensitivity were compared: (1) the absolute-threshold method and (2) a homochromatic-contrast method. In the second method, the subject saw a fixed-contrast homochromatic stimulus consisting of a steady background field and a superimposed 32% increment flash. The overall radiance level of the stimulus was varied systematically to determine the minimum radiance required to detect the flash. The spectral-sensitivity curve obtained by this method was somewhat narrower than the curve obtained by the absolute-threshold method. The homochromatic-contrast method was also used for a retinal region 8° above the fovea. A curve resembling the common photopic curve was obtained. It is concluded that the homochromatic-contrast method may serve as a useful addition to the standard methods of spectral-sensitivity measurement.
DWIGHT A. BURKHARDT and PAUL WHITTLE, "Spectral-Sensitivity Functions for Homochromatic-Contrast Detection," J. Opt. Soc. Am. 57, 416-420 (1967)