Data on perceived brightness and visual acuity were obtained under essentially identical conditions of steady and intermittent illumination, using long exposures (up to 45 sec) as well as brief ones (250 msec). Flicker frequencies down to 8 cps and light-time fraction to 0.083 were sampled. By taking into account the exact duration of the test exposures, a more general form of Talbot’s law was derived, which fits all the brightness matches obtained. When this new equation is used as a yardstick, intermittent illumination turns out to be less efficient than steady illumination so far as visual acuity is concerned for brief exposures. For long exposures it is more efficient, as was previously reported by Senders.
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