On the Physical Behavior of Flashing Incandescent Light Sources

Abstract

It seems to me that the physical behavior of flashing electric light sources is not sufficiently understood by some kinetic artists who use them. I am thinking in particular about Robert Baldwin's recent discussion in Leonardo in which he claimed that he had projected images on a screen having durations of about 1/100 sec. by means of passing electric current during a short time interval (1/100 sec.) through the filament of an incandescent lamp in a slide projector [1]. I doubt that images produced in this way and sufficiently bright to be seen can be of such short duration. I shall give the physical reasons below but I shall not discuss the important psychophysical aspects of subliminal stimulation of visual perception. There are many types of electric light sources but there are only a few common ways of obtaining a uniformly repeatable short high-intensity light flash. [2]. The most reliable way involves the use of mechanical shutters in front of a continuous light source. A desirable feature is that the mechanism can be calibrated accurately and the color of the source controlled. It can obtain a flash duration down to the range of 1 to 10 msec. (1 millisecond or msec = one-thousandth of a second). Another way involves a gas discharge lamp, which is capable of obtaining a flash duration down to the range of 1 to 10 / sec. (1 microsecond or j/ sec. = onemillionth of a second). The lamp requires a highvoltage supply, the color is peculiar and it is expensive. For direct viewing, images can be displayed on a cathode ray tube (CRT), as in television, for exposures as short as a few microseconds. Color is not controllable and the method is expensive. A way not generally recommended consists of briefly energizing an incandescent lamp, which Baldwin used, causing it to give a brilliant flash but, as I shall explain below, the duration of a flash will not be below 20 msec, because 20 msec represents the delay required to heat the filament to luminosity. The temperature of a glowing tungsten filament