The alternating frequency in flicker photometry is normally high enough to go beyond the critical fusion frequency, thus avoiding the interference of color alternation. If, however, the alternating frequency is purposefully lowered, we have new method, a modified direct heterochromatic matching technique, which we have called successive brightness matching. A luminous efficiency function was obtained with this method by an equal brightness criterion at the alternating frequency of 0.5 Hz. The function showed a favorable agreement with the normal direct brightness matching, as we expected. With a similar low frequency, 2 Hz, another luminous efficiency was obtained according to the newly introduced criterion, the minimum-flicker perception, at the transition point of one light to another. The function agreed perfectly with flicker photometry, implying that the criterion detected only the activation in the achromatic channel proposed in the opponent-colors theory. At increasing the alternating frequency successive brightness matching converges to the normal flicker photometry, which was confirmed by the change in luminous efficiency. The transient frequency was about 4 or 6 Hz, which indicated the cutoff frequency of the chromatic channel of the visual system.
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