Stimuli which subtended a 61° visual angle were used to measure ΔI against a concentric field at 273 trolands. Peak intensity was adjusted to threshold as the stimulus duration was reduced in steps from 1 sec. to 100 microsec. Data from 10 observers allowed the calculation of a variance measure and a slope constant. Reciprocity held only at very short durations. The critical period for this stimulus was about 1 msec. Large errors may be encountered if linear intensity-time correction techniques like the troland-second are applied to large-area brief flashes.
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