Heterochromatic modulation photometry.

Heterochromatic modulation photometry is a method for obtaining equiluminance for a pair of heterochromatic lights presented in temporal alternation. A series of fixed standard luminance/test luminance ratios are presented, and at each ratio the modulation depth of the pair is reduced in tandem until the observer reports that flicker disappears. The data can be described by a luminance contrast template that appears V shaped when plotted on log-log coordinates. In the fitting of individual data, a free vertical scaling factor reflects the observer's sensitivity to luminance modulation and a free horizontal scaling factor reflects the observer's similarity in spectral sensitivity to the CIE standard observer. Data for red/green flicker photometric matches demonstrate the technique. Heterochromatic modulation photometry offers several advantages over flicker photometry: (1) a single fixed perceptual transition occurs on each trial series, namely, the transition from flicker to steady, and (2) luminance matches can be obtained at fixed frequencies at a number of luminance levels. The same procedure can be applied to the measurement of the minimally distinct border and to the identification of tritan pairs (stimulus pairs that differ only in their stimulation of short-wavelength-sensitive cones).