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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 249
DIMENSIONAL CHANGES IN FILM
The dimensional changes which occur in motion picture film with changes in humidity, temperature, and age are important because they affect the distance between perforations (perforation pitch). Camera, printer, and projector sprockets are designed to accommodate film having a definite longitudinal and transverse perforation pitch and any marked change in the dimensions of the film causes more or less serious trouble. In the past only lengthwise shrinkage of motion picture film has been considered of importance, but of late certain photogrammetric applications have aroused interest in widthwise shrinkage which is not necessarily the same. Therefore, in the discussion that follows some attention will be given to differences between lengthwise and widthwise shrinkage.
In any discussion of film shrinkage it is important to distinguish between the different types of dimensional change which may occur. 'Dimensional changes in film are of 2 types — temporary or reversible, and permanent or irreversible. Temporary dimensional changes are caused by thermal expansion or contraction resulting from change in temperature, and humidity expansion or contraction resulting from change in moisture content. Permanent dimensional changes are due to a variety of causes. The magnitude of the dimensional changes which take place in a given film depends largely on the composition of the base, the treatment it received during manufacture, and the type and thickness of emulsion employed. Both temporary and permanent dimensional changes may occur simultaneously, and both may be complicated by hysteresis and related phenomena. The size of a piece of film at any instant is the resultant of all these effects.
The shrinkage measurements described in this paper were made in air-conditioned laboratories at accurately controlled temperature and relative humidity, using the pin-gage method described in the Bureau of Standards Research Paper No. 1051. 17 The humidity coefficient of linear expansion (sometimes called humidity amplitude) was determined by first measuring samples conditioned at 20 per cent RH at 70 F approached from above, and then remeasuring after conditioning at 70 per cent RH at 70 F approached from below. The change in length was then calculated for a 10 per cent relative humidity interval. This procedure does not exclude errors due to hysteresis which amount to approximately 0.005 per cent of the dimension for every 10 per cent change in relative humidity.