International projectionist (Jan-Dec 1946)

VOLUME XXI FEBRUARY 1946 NUMBER 2 Some Physical Properties of Film Relating to Image Stability IT IS THE purpose of this article to consider the relation of a few film characteristics to certain problems in the laboratory and in the theatre projection room. The problems selected for discussion are image distortion and film defects resulting from insufficient drying. As a rule, a small amount of distortion of either the negative image or the positive image does not detract noticeably from the usefulness of the motion picture print. There are, however, certain special fields in which the matter of image distortion has become increasingly important. These fields include color photography, special-effects photography, and certain applications of photography for military use. An image on photographic film should not be considered as stable as one on glass or steel. Actually, the image on a piece of film is changing in size or shape continually throughout its life. Distortion of the photographic image is generally the result of shrinkage owing to loss of volatile materials, expansion or contraction produced by changes in moisture content, and failure of the printer to transfer the exact dimensions of the negative image onto the positive film. The magnitude of these distortions can be illustrated by selecting a test object of known dimensions, photographing it, and following the dimensional changes in the photographic image of the object throughout the life of the film. A suitable object may be a test target, as illustrated in Figure 1, constructed t /. Soc. Mot. Pic. Eng., Srpl., 1945. By R. H. TALBOT EASTMAN KODAK COMPANY Tolerances of the order of ten thousandths of an inch, and more, have been commonplace in motion picture engineering. Nowhere in the reproduction process are these exacting requirements more pronounced than in the relationship between filmwidth and projector performance. The accompanying article details in interesting fashion the magnificent job done by the industry's technical forces in maintaining high standards of performance. on a dimensionally stable material such as glass. The target is 1.378 in. (35-mm) wide and of any convient height. It consists of 4 fine lines A, B, C, and D. For convenience, line A is located 0.2000 in. from edge 0, and line B is located 1.0000 in. from edge O. Lines C and D are 0.5000 in. apart. The image of this target is now transferred to a piece of 35-mm negative film by contact printing, perfect alignment of target and film at edge O being maintained when the exposure is made. There has now been produced on the 35-mm film a latent image whose dimensions are identical with those of the test target. The film is then processed. It will be found that the dimensions of the developed image on the film do not coincide exactly with those of the target. If care is taken to bring the negative film to equilibrium with air at the same relative humidity after processing as that of the air with which it was in equilibrium at the time of exposure, any dimensional changes will be caused solely by processing. Processing Shrinkage Figures The permanent processing shrinkage of Eastman motion picture negative film (nitrate base) is of the order of 0.10 per cent in length and 0.15 per cent in width. Therefore, upon measurement of the image lines after processing, it will be found that line A has moved 0.0003 in. toward edge O from its original position, and that line B has moved 0.0015 in. toward edge O. Lines C and D are now 0.4995 in. apart. Ordinarily, it is not necessary to maintain the processed negative in equilibrium with air at the same relative humidity as that with which it was in equilibrium at the time of printing. However, in the special cases noted, the changes in dimensions which accompany changes in moisture content of the negative become an important factor. It may be assumed that most negative raw stock, when removed from the container, is in equilibrium with air at about 60 per cent relative humidity. If the negative, after processing, is stored in a dry place, there will be a further shrinkage of the temporary or reversible type caused by loss of moisture. Thus, if the negative were stored in a vault in which the relative humidity of the FEBRUARY 1946