Discrimination of Short-duration (two Pulse) Flashes as a Function of Signal Luminance and Method of Measurement

The recent introduction of strobe lights for anticollision purposes raises the possibility of using temporal patterns of short duration flashes as information carrying signals. The current experiments are concerned with the detection of the minimum duration dark interval between signal light pulses as a function of signal luminance and the psychophysical method of measurement. Experiment I tested the theory of signal detectability (TSD) prediction that obse~ver sensitivity is independent of the psychophysic?l method used in measurement. Discrimination of a constant duration comparison stimulus (three msec) and a variable duration test stimulus (seven to 32 msec) was measured with a two-alternative Forced-Choice (FC) procedure and a Yes-No procedure. Sensitivity was comparable under the two psychophysical procedures, thus supporting the application of TSD to the sensory, processes involved in discrimination of two-pulse stimuli. Experiment II measured discrimination with the FC procedure at three luminance levels: 31.8, 318, and 3183 candelas per square meter. Discriminability increased with luminance. Thus, pulses separated by a dark interval short enough so that only a single flash is seen over the entire scotopic intensity range may, however, be seen at photopic intensities as two pulses, or appear to flicker, or otherwise appear to be of different character. To maintain a homogeneous flash appearance over the entire range of signal intensities, the dark interval duration for multi-pulse flashes should be determined for the highest intensity condition at which operational observation is anticipated. I. Introduction. The recent introduction of strobe lights for anticollision raises the possibility of using temporal patterns of short duration flashes as information carrying signals. An i:::1dividual flash can be produced by multiple brief light pulses, simulaneous or successive, as long as the component pulses are sufficiently close in time and space as to not be perceptually resolvable. The duration and intensity of a particular flash thus produced can be varied by ma:::1ipulation of the temporal spacing and the number of component pulses. Variation of signal intensity will also normally occur with varying visual range, windshield transmissivity, atmospheric conditions and any other factors which affect the amount of light reaching the observer's eye. 1 In order to design multi-pulse signal flashes it is essential to know how the minimum detectable dark interval between brief pulses varies with signal intensity. The effect of luminance on the discrimination of duration differences between multi-pulse flashes is examined for the limiting case of two-pulsfl stimuli presented under night flight conditions.