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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 259
above manner, is plotted against the approximate age of the film in Fig. 18. The initial shrinkage on each curve was obtained from new prints, which in the case of nitrate is practically zero due to the moist condition of the film at the time of measurement. The number of projections on the balance of the samples is not known, but probably increases with the average age of the different samples tested. It may be seen that the nitrate prints released to the trade for a year have a shrinkage of about 0.4 per cent. This agrees reasonably well with the curve for rolls in untaped cans (Fig. 15), determined in the laboratory under controlled conditions. The safety 16-mm film released to the trade for a year shows a shrinkage of about 0.8 per cent. However, this safety positive film is the older, higher-shrink type (No. 8, Table 4) and the present film (No. 9, Table 4) will shrink much less under the same conditions.
The shrinkage encountered in the trade in individual cases may vary considerably from that shown in Fig. 18, depending on the actual projection and storage conditions in different parts of the country. Nevertheless, it is apparent that the deviation from standard pitch of present-day motion picture films is much less than that for which the projector sprockets were originally designed.
FILM CURL
The tendency of a photographic film to curl is well known. Curl is usually most pronounced at low relative humidities owing to the contraction of the gelatin emulsion. For convenience we have called the curl positive when the film is bent toward the emulsion side, and negative when the film is bent away from the emulsion side. In the case of motion picture film a slight positive curl is generally preferred. This is believed to reduce scratching of the emulsion in handling. The curl of the film should not be too high at any relative humidity likely to be encountered in practice and should not change too much with age. Film with excessive curl is difficult to handle, is more susceptible to the distortion called "spokiness,"12 and may not focus sharply when projected. On the other hand, film which is flat is sometimes regarded as too limp, although this may not be a serious objection.
The cause of curl and the effect of relative humidity on curl are illustrated in Table 5. It may be seen that when the relative humidity is reduced from 70 per cent to 20 per cent, stripped emulsion contracts approximately 7 times as much as uncoated nitrate base.