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Modulated Air Blast
for Reducing Film Buckle
By WILLY BORBERG
Present-day demands for high-intensity light sources point up the need for a suitable technique for reduction of excessive film buckle. Air jets which direct a continuous air flow against one or both of the film faces have been proposed. This technique does not, however, take into account the cyclical nature of film surface deformations during projection. It is found that improved performance can be obtained with a modulated air blast which is synchronized to the frame cycle. This paper describes the cyclical effects involved and shows why the modulated air blast is to be preferred over continuous air blast. It presents experimental data regarding buckle magnitudes in 35mm film and describes the experimental equipment.
_L HE TYPE OF BUCKLE with which WC
are concerned in the present discussion is a deformation which takes place during the frame cycle while the film is in the aperture. It may leave no record of its existence on the film after projection. It can be made visible by stroboscopic or high-speed photographic techniques, appearing as a rythmic — almost breathing — motion of the film surface in the aperture. It produces deterioration of image focus during part of the rapidly recurring projection cycle.
The causes of film buckle have been investigated and described before. Carver, Talbot and Loomis,1-2 as well as Kolb,3 have done considerable work on
Presented on April 25, 1952, at the Society's Convention at Chicago, 111., by Willy Borberg, General Precision Laboratory, Inc., Pleasantville, N.Y.
the subject in connection with broad studies of film performance. They have developed the terminology needed for presentation and their usage will be followed. The present discussion will be concerned with those effects which vary during the film frame cycle. However, a brief statement of the basic factors will not be amiss.
Each single picture frame goes through a cycle which starts with pulldown into the aperture, proceeds through the first exposure, the flicker blade cutoff and the second exposure, and ends with the pulldown of the next frame. During the two exposure intervals the film arrests some of the radiant energy from the light source and transforms it into heat. This causes the film to buckle (or bulge) in a manner very similar to that observed in the operation of a bimetallic element.
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August 1952 Journal of the SMPTE Vol.59