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Effect of High-Intensity Arcs upon 35-mm Film Projection

1943; Volume: 41; Issue: 7 Linguagem: Inglês

10.5594/j09841

0097-5834

E. K. Carver, R. H. Talbot, H. A. Loomis,

Advancements in Photolithography Techniques

In the study of the effects of high-temperature arcs on 35-mm motion picture projection, it was noticed that the sharpness of the image on the screen was materially affected by changes in the heat intensity. This indicated that film does not always lie in a flat plane in a projector gate but takes different positions at different temperatures. In order to study this phenomenon more carefully, a portion of the projector gate was cut away permitting high-speed Ciné Kodak pictures {about 1500 frames/second) to be taken of the film as it passed by the aperture of a projector. The pictures show that most films enter the gate in a state of slight positive curl (curl toward the emulsion) and then change to a state of negative curl during the instant they remain exposed to the heat of the arc. This change in curl is due to the expansion of the emulsion layer by the heat. This effect is especially pronounced with the new high-intensity arcs. — The effect on the quality of the screen image of this change in curl of the film in the gate was studied by means of high-speed Ciné Kodak analysis of the screen image. The pictures show that at high heat intensities and with the projector focused to give the sharpest image on the screen, the images are in sharp focus for only a portion of their duration on the screen. Each screen image comes into view out of focus and gradually becomes sharper until just before the pull-down when it reaches its maximum sharpness. Such pictures are of good screen quality if the projector is focused carefully. — Under certain conditions, when the film is in a very moist state and when lamps of the highest heat intensity are used, the screen images may not all be sharp. Occasionally a few frames may be entirely out of focus. The high-speed analysis of the action of the film in the gate shows that these out-of-focus frames behave in an abnormal manner. In these frames the normal change in curl from positive to negative in the gate is interrupted by a reversal back to positive curl. Thus at the end of the pull-down cycle these frames lie in a plane slightly toward the lens of the projector, whereas all of the normal frames lie in a plane slightly toward the lamp from the plane of the gate. The distance between these two planes is greater than the depth of focus of the lens and thus these abnormal frames appear out of focus. It is believed that this sudden reversal to positive curl is due to a contraction of the gelatin due to loss of moisture. — We recommend that the heat intensity at the aperture, as measured by a thermocouple which we have described, should not exceed 1250°F and that films should be dried thoroughly.