Journal of the Society of Motion Picture Engineers (1930-1949)

248 GILLETTE September The amplitude factor has been adjusted to yield an intensity of 10 per cent of the initial intensity at 500 microseconds, a value which fits the data of several manufacturers. Clearly the simple inverse power law cannot apply in the neighborhood of zero time since infinite values of intensity would result. For this region, extending out to 250 microseconds, linear decay has been assumed. The total light output of any element of the phosphor is proportional to the integral of this curve, that is, to the area under the curve. That portion of the total light which is effective in exposing the corresponding element of the film is determined by carrying out the Fig. 2 — Decay characteristic of Pll phosphor. integration for just that interval of time, different for each line of the scan, during which light from the phosphor is permitted to reach the film. The effective exposure of the element of film is assumed to be proportional to this integral. That assumption is not really justified in view of the well-known failure of the reciprocity law when applied to exposures of such short duration. However, no information is available on which to base a more accurate statement. The mechanism that prevents light from reaching the film, thus limiting the period of integration, is obviously the closing of the shutter if the equipment contains a mechanical shutter. If the shutter is electronic there is actually no interruption of the passage of light from phosphor to film. Here the limiting action is the motion of the film during its transport period. While the film is in motion