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1949 AIR COOLING OF FILM 649
(7^ Blistering
With the increase in projection intensity to still higher levels, the film is observed to blister during projection. These blisters form in the film base immediately adjacent to the emulsion (where, as shown in the curves of Fig. 4, maximum base temperatures are reached). Depending upon the conditions of their formation, these blisters may be so small that the separate blisters are indistinguishable to the naked eye, or they may reach a diameter of approximately 0.005 inch. Once blisters appear, the film is useless for further projection, since the thermal isolation of the emulsion directly over the blister causes it to reach so high a temperature as to burn off and disappear. The unburned blisters show on the screen as dark spots resembling the sudden appearance of severe grain in the image, and the burned-through blisters show as white areas on the screen surrounded by obviously charred emulsion. The exact level at which blistering appears is dependent upon the type of film under consideration but for release positive, a mean net flux of 0.60 to 0.65 watt per square millimeter is required. It should be pointed out, however, that in-and-out of focus appears only after repeated projection, while blistering may be observed on the very first run. Accordingly, in an untested setup the inand-out of focus threshold may be exceeded and even the blistering threshold passed; film projected under such conditions will blister rapidly and nevertheless require a number of projections of the blistered film before in-and-out of focus is observed. This represents an extreme case, however, and it is safe to say that tested equipment already shown safe for in-and-out of focus will not blister release positive.
It is apparent that the high-intensity effects which depend almost entirely upon film temperature can be modified or prevented only by preventing the film from reaching these damaging temperatures. High-intensity effects that are influenced not only by temperature but by other factors sometimes can be postponed even though the temperature threshold is exceeded.
EQUIPMENT FOR AIR COOLING
For these experiments, a radiation source capable of producing more energy than the present commercial projection lamphouses was necessary. The Peerless Hy-Candescent lamphouse was specially modified5 to operate with water-cooled positive jaws and permit the burning of experimental trims7 at currents and radiation intensities up to 65 per cent in excess of present commercial equipment, providing an instantaneous net flux of 1.65 watts per square millimeter and a