The precise effect of wavelength on the judgment of critical flicker frequency (CFF) has been a controversial issue for a number of years. Some studies report no effect of wavelength; other studies indicate that wavelength is a basic determiner of the CFF threshold. The purpose of this study was to attempt a resolution of the controversy by means of a systematic and thorough investigation of the problem. The apparatus used was the Fordham calorimeter. By means of monochromators it was possible to deliver various spectral colors with a constant passband of 10 mμ to the subject. The intensity of the light source was controlled by means of neutral tint filters and an optical wedge. Flicker was produced by intercepting the beam of light with a sector disk driven by a Graham variable-speed motor. Eight wavelengths covering most of the range of the visible spectrum were used, and CFF thresholds were obtained at seven different luminance levels in eight experimental sessions for each wavelength. Three subjects with normal color vision were employed.
The results showed that the wavelength of the stimulating light changes the slope of the curve relating CFF to log I. The curves at the blue end of the visible spectrum are steeper than those at the red end. Furthermore, the greater the separation between wavelengths, the greater is the probability of a significant difference between slope values. The fact that the slopes of the curves differed with wavelength does not mean that the Ferry–Porter law is invalid, but rather that one should adjust the “constants” of the equation for the particular wavelength being used. Consequently, it can be stated that CFF is a function of the wavelength of the stimulating light. The most explicit interpretation that could be made was that this change reflected some sort of change in the receptor system of the eye.
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