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Feb., 1950
ROSS : HEATING OF FILMS AND SLIDES
45
The rise of temperature of film in a process projector is also shown in Fig. 6. There is no flicker blade, and a 200° angle of shutter opening has been assumed.
Further results arising from the theory are that it makes little difference whether the light falls on the emulsion-side or the base-side of the film ; the absorption of radiation by the base material is negligible compared with the absorption by the silver ; it makes little difference whether the silver is distributed uniformly through the emulsion layer, or concentrated to some extent near the surface ; at the end of the exposing period about one-eighth of the heat is in the emulsion and the remainder in the base. The average density of the print makes little difference — only practically full whites are significantly cooler.
Light Output from Projector
The mathematical treatment relates the rise of temperature of the film-base to the intensity of heating of the film in the gate, which may conveniently be expressed in watts per sq. cm. If then, we use the information of Part I of this paper, and know the luminous efficiency of the radiation of a particular projector, it is possible to relate the rise of temperature to the total light passing
Rise Of Temperature (°Cj ioo
Process Projector
Theatre Projector
i8o° zoo'
Final Uniform Temp> 36o"
^T
•o+
■05
•OS
Seconds
Fig. 6. The heating cycle of film while in the projector gate. The rise of temperature is calculated for the emulsion-side of the base. The maximum value reached depends on the light output.
through the gate. The total light reaching the screen will be slightly less, due to the light-loss in the objective lens.
Fig. 6 has been drawn for a temperature rise of 100°C. above room temperature. This is only an assumed maximum value, but it is of interest to note that this corresponds to a light flux at the gate of about 9,500 lumens in the case of a theatre projector. (With shutter running, intensity of radiant energy at gate 62 watts per sq. cm. ; unfiltered arc light at 95 lumens per watt ; uniform illumination over the gate.) Bearing in mind the assumptions and simplifications which have to be made in such a theoretical treatment as this, this is in remarkably close agreement with experience, and with measurements made on various projectors.8
It is clearly of no use to try to relate the maximum temperature which the base will withstand in the gate to the temperature at which it is damaged in some laboratory test, valuable though this may be for other purposes. For only the " skin " of the base is heated, and only for a very short time ; and, further, it would be difficult to relate " damage " as a projectionist knows it to the results of a special test.
It is the author's opinion therefore that there would be little value in measuring the temperature of the " skin " of the film base while in the gate — even if a method of doing it could be devised. Reliance must be placed instead