The luminance of tachistoscope lamps as a function of flash duration

The steady-state luminance of the lamps in a commonly used Iconix tachistoscope was shown to depend, in a complex nonlinear fashion, upon the length of time the lamps were lit. This was probably due to variations in lamp temperature with time. An integrating photometer was used to measure the light energy present in short flashes and in time periods of various durations with the lamps burning continuously. The equivalent luminance of short flashes was calculated by comparing values obtained by integration over a short flash with values resulting from integration, over an identical duration, of the lamps in a steady-state. With flashes up to 50 msec in nominal duration, luminous energy was less than one-half of that present in the same duration with the lamps burning continuously. The importance of this information for the interpretation of tachistoscopic experiments, as well as possible means of monitoring the effects, are discussed.