The Effect of Flash Frequency on the Apparent Intensity of Flashing Lights Having Constant Flash Duration

Abstract

Observations were made in the laboratory, on the effect of varying the flash frequency on the apparent intensity of flashing lights having durations of 1/40th second and 1/10th second. The majority of the readings were taken at an illumination level of 0.5 mile-candle on the eye. This is the desirable lower limit of illumination on the eye for adequate conspicuity of marine and aviation lights. Observations had been reported indicating that when distant lights were flashed at frequencies above that at which the flashes were distinct, the intensity appeared greater than Talbot's Law would suggest. It is shown that these observations, which were not made under controlled conditions, were evidently in error. Up to frequencies of about 5 flashes per second the light appears as flashing and its apparent intensity can be obtained from the fixed light intensity by the application of the Blondel-Rey correction. At higher frequencies than about 13 flashes per second (with the shorter flashes) the light appears continuous and the Blondel-Rey correction is no longer applicable; instead Talbot's Law applies. Between about 5 and 13 flashes per second the light appears to flicker and the apparent intensity is greater than either Talbot's Law or the Blondel-Rey correction would indicate. This flickering light is not thought to be of any practical value as its apparent intensity is less than a flashing light consuming the same amount of energy but giving a readable flashing indication.