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of finely divided metallic silver imbedded in a matrix of gelatine. These deposits absorb varying amounts of the incident light depending upon the mass of silver present per unit area. In those parts of the film representing the shadows and darker half tones of a picture, a very large proportion of the incident radiation is absorbed. Following this absorption of energy, a rise in the temperature of the film must necessarily occur. Although the absorption of energy from the light incident upon the film during projection is responsible for by far the greater part of the temperature rise, there are minor contributing factors. Some heat is absorbed due to the film being in contact during projection with heated parts of the mechanism and undoubtedly some heat is generated by the friction arising from the pressure plates which hold the film in the focal plane during projection.
It is evident that the total amount of heat absorbed and the resultant temperature of the film as it leaves the projector will depend upon many factors. First of all, the temperature rise will depend upon the heat absorbing, transmitting, and reflecting characteristics of the film itself and it is to be expected that film having relatively high average optical density (since it absorbs more of the incident radiation than one in which less silver is present) will leave the machine at a higher temperature than film having a low average optical density. The amount of heat absorbed will depend upon the energy flux-density incident upon the film and this is dependent upon such factors as the nature of the light source used, the size or wattage of the light source, the type of condenser system employed to concentrate the radiation on the gate of the projector, the mechanical details of construction, and the perfection with which the optical and mechanical adjustments are maintained. The amount of heat absorbed due to the contact of the film with heated parts of the machine will naturally depend upon the temperature of those parts and the length of time with which the film is in contact therewith.
A search through the literature reveals very little reliable information as to the amount of energy absorbed during projection or to the temperature of the film as it leaves the gate of the projector. Some more or less qualitative measurements have been made, but no data which appear entirely reliable have been found. Since the temperatures to which a film is subjected have such a profound influence upon its durability and upon the satisfactoriness with which it can be projected, it seems worth while to make a rather careful study of this subject.
In the work reported in this paper, the influence of certain of the factors mentioned above on the magnitude of the heating effect has been studied. Thus, the dependence of film temperature upon the mean density of the film, the speed of projection, and arc current has been determined. No attempt, however, has been made to separate the total heating effect occurring under any given condition of projection into the various contributing factors such as that due to absorption of radiant energy, absorption by contact with heated parts, and generation of heat by friction.
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