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44
BRITISH KINEMATOGRAPHY
Vol. 16, No. 2
Fig. 5. Consequently the temperature throughout the emulsion layer rises rapidly. It may be calculated that only a negligible part of the heat escapes from the emulsion to the air while the frame is in the gate, and consequently the heat flows through the emulsion layer and into the film base. Emulsion is a poor conductor of heat, and film base is even worse, which means in practice that the heat does not have time, during the brief exposure period, to travel far through the base.
The full mathematical treatment of this is complex, and the solution of the problem is given in the appendix. From this it is possible to construct the full picture of what happens within the film.
Mode of Heat Absorption
The temperatures throughout the thickness of the film at the end of the exposure period are as sketched in Fig. 5. The following points may be noted :
(i) There is a small drop in temperature through the emulsion layer.
(ii) The temperature of the emulsion is much higher than the average temperature of the base. This causes a curl or buckling of the film in the gate, as in a bi-metallic strip of a thermostat, the emulsion side curling outwards. This explains the effect photographed with an ultra-high-speed camera,9 where it was found that the buckling occurs very shortly after the beginning of the exposure, and increases only slightly thereafter ; the objective lens has therefore to be focused back slightly towards the arc to give the best average focus.
Fig. 5. Enlarged cross-sectional view of film, with graph showing distribution of temperature within the emulsion and base at end of exposing period in the gate. Only I / 1000 in. of the thickness of the base is heated significantly.
ROOM TEMP —
(iii) The heat travels only a very short distance into the base during the exposing period and
only about 1/1000 in. of the base is heated significantly. (iv) The rise of temperature of the far surface of the base during the exposing period is less
than 1/100 that of the emulsion side of the base.
Temperatures in the Base
Due to the fact that overheating of film damages the base rather than the emulsion, by causing " embossing " of the picture area, it is necessary to consider more precisely what happens to the base, and particularly the part of the base next to the emulsion, which has already been shown to be the part which will be first damaged. Fig. 6 shows the temperature of the emulsionside of the base during the exposing cycle. In a theatre projector the temperature rises rapidly at first as the light comes on to the particular frame considered, and then falls somewhat as the flicker-blade obscures the light — the heat is spreading deeper into the base. During the second exposing period the temperature again rises ; in the figure this part of the curve is to some extent estimated. After exposure is completed the heat now in the film (and mainly concentrated in the first thousandth of an inch of the base) spreads throughout the thickness of the base. This spreading is virtually completed about four frames after the gate (i.e. 4 24 of a second later) when the whole film is at about one-fifth of the maximum temperature previously reached. The whole film then very gradually cools down to room temperature after being wound up on the take-up spool.