International projectionist (Jan-Dec 1950)

Record Details:

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depend greatly upon the type of nozzle used, its exact location in the gate, the angle which the air stream makes with the film, and the level of radiation intensity. Examination of Fig. 1 and of related data indicates two important conclusions about the use of air jets for cooling: (1) The heat transfer coefficient (or the cooling efficiency) depends primarily upon air velocity and increases approximately as the 0.9 power of the velocity. (2) Cooling efficiency is only slightly affected by the volume of air used, and at constant velocity the heattransfer coefficient increases only about as the 0.3 power of the air volume. Elect to Use Air Cooling This conclusion has been verified throughout our experimental work, and accordingly the problem of cooling film with air during projection becomes a matter of cooling with high-velocity air. That this should be true is apparent when we realize that the air immediately adjacent to the film surface is relatively stationary and sheltered from the air currents which might be set up at the back of a projector, and yet all the heat lost to the air must be conducted across this stationary layer and into the many air currents of the projection booth. By increasing the velocity of air from the jet, a scrubbing action is produced on this layer of stagnant air, reducing its thickness and reducing the insulating effect that it has in limiting the cooling of the film. The higher the air velocity the better the scrubbing action and the less stagnant air is present to impede the rapid loss of heat from the film. In order to supplement this work on determination of heat-transfer coefficients for stationary film with actual projection data, a different method of determining film temperature became necessary. It was found possible to make an experimental film — totally unsatisfactory as release positive but adequate for test projection— which could be made to indicate when the film had exceeded a certain temperature threshold. Normal Projection Temperature For these experiments, such an experimental film was used, adjusted to give an indication at a temperature level approximating that reached under normal projection with present de luxe equipment. This makes it a convenient experimental tool, since if the conditions can be so arranged that this experimental film can be projected with no change, one is reasonably sure that conventional cine positive can be projected with none of the more serious high-intensity image defects. Figure 2 shows the results of projecting this experimental positive at high intensities with the incorporation of highvelocity air cooling. It will be seen that this experimental positive can be projected at intensities up to 0.35 mean net watt per square mm, and no change is experienced even without cooling air. To exceed this limit, however, air must be applied — and in our particular equipment, the use of air velocities at the nozzle up to 400 feet per second permits increasing the mean net radiant flux to 0.50 watt per square mm, or an increase of over 40% with no increase in film temperature. An increase slightly beyond this limit can be obtained by raising the air velocity still higher, although the returns are diminishing. The data of Fig. 2, therefore, show that it is possible to obtain approximately a 50% increase in radiant-energy flux on the film and still maintain acceptably low film temperatures through the use of a rather simple air-cooling arrangement. In the higher air-velocity range, the data of Figs. 1 and 2 are not entirely consistent, for one shows a continuing increase in cooling with increasing air velocity while the other shows only minor increases beyond 600 feet per minute. This is not surprising, since they were obtained under widely different experimental conditions. It also may not be of great practical importance, since in our equipment the very high velocity range is accompanied by an intolerable amount of noise at the projector. Measurement of Film Position Before going on to discuss the problems of in-and-out of focus and the positioning of film in the aperture, the methods for determining this film position should be reviewed. In the original arti (Continued on page 26) d u </> u I > U Q IOOO 800 600 400 200 OOO 0.20 0.40 0.60 0.80 AVERAGE NET FLUX-WATTS/MM.2 FIG. 2. Reduction in temperature of experimental film accomplished by high-velocity air. Graph shows number of projections of a loop of experimental film before it reached an unsafe temperature, and points out how much the region of normal behavior can be extended by air cooling. -0.020 + 0040 + 0.020 0.020 CONDITION A 3£ 6 NUMBER OF PROJECTIONS CONDITION B NUMBER OF PROJECTIONS FIG. 3. Projection history of film as measured by changes in the point of best focus. If this point is determined for each projection of the film, the resulting curves can be related to film performance. This is particularly valuable for preliminary rapid testing to predict film behavior in the trade. A slow rate of change of focus, as in condition A, is typical of good screen quality; a rapid rate of change of focus, as in condition B, is always accompanied by poor image quality. 18 INTERNATIONAL PROJECTIONIST February 1950