American cinematographer (Jan-Dec 1924)

Four AMERICAN CINEM ATOGR APHER November, 1924 Handling Cine Film at High Temperatures Important Formulae Enumerated and Research Matter Is Given Out. By J. I. Crabtree Eastman Research Laboratory Difficult Subject Discussed in Detail. How to Combat Excessive Heat. Special care is necessary when handling motion picture film at high temperatures when high humidities usually prevail in order t insure that the sensitiveness of the film emulsion shall remain unimpaired hefore exposure and that the latent image shall be retained after exposure and before development. Special treatment is also necessary if development of the exposed film must be conducted at temperatures above 70°F. while adequate precautions must be taken during storage of the developed film in warm climates, otherwise the keeping qualities of the film will be affected. ^ Effect of Temperature and Humidity on Motion Picture Emulsions. A. Unexposed Emulsions. Although definite data are not available on the effect of temperature on the speed of dry unexposed emulsions, experience has shown that if the temperature of a dry unexposed film is raised, for example, from 65°F. to 10OF., a considerable period of time must elapse before the speed and fog are affected to a sufficent extent to be of practical importance. Renwick1 has investigated the effect of humidity at constant temperature on the speed of motion picture emulsions and states that at a temperature of 65°F. the speed of a film emulsion in equilibrium with an atmosphere whose relative humidity varies between 0 and 80% may vary 50%, depending upon the particular emulsion. Experience has shown that with unexposed negative film stored in an atmosphere at high relative humidity, say 90%, and at a high temperature, say 90°F. or 100°F., the speed of the emulsion rapidly falls off and the emulsion becomes fogged to an extent depending upon the period of storage. B. Exposed Emulsions. In the case of exposed negative emulsions it is important that the latent image shall remain unimpaired and that the still unexposed portions of the emulsions shall not develop fog. At high temperatures and humidities certain chemical reactions occur in the film which destroy the latent image, and the image is said to "fade," that is, if the film is stored for any length of time the developed image is very much weaker or less dense than if the film was developed immediately after exposure. Apart from this fading of the latent image, under the above conditions of storage, the emulsion becomes fogged with time, which destroys the quality of the developed image. High temperature, however, has a surprisingly small effect on the latent image if the exposed film emulsion is dry, that is, if it is in equilibrium with an atmosphere of low relative humidity. The experience of numerous explorers in tropical countries who thoroughly dried out their film after exposure and before packing but who did not develop the film for several months afterwards thoroughly establishes this fact. ^ Effect of a Sudden Change of Temperature on Moist Film. If moist film, that is, film which is in equilibrium with an atmosphere of high relative humidity is suddenly cooled, moisture is apt to condense as droplets on the film emul sion which will cause spots on the developed film. Such spots are termed "moisture spots"2. There is very little danger of the formation of moisture spots due to sudden cooling of the original unopened film cans. The most frequent cause of moisture spots is sudden cooling of the film when in a moist condition in the camera and retorts. tf The Care of Amotion Picture Film Before Exposure. From the above discussion it is seen that film must be kept dry and as cool as possible before exposure. The film, however, must not be too dry, otherwise static may be produced in the camera with negative film, and in the printers wth positive film. On leaving the factory, motion picture film as contained in the cans is in equilibrium with an atmosphere having a relative humidity of 70 to 75% , but the paper wrapping absorbs moisture so that the film which is a few months old is relatively dry. The film cans, however, are not perfectly water-tight and in a very moist atmosphere the packed film would absorb moisture very slowly so that it is preferable to pack film which is liable to be exposed to damp atmospheres in hermetically sealed cans. Film packed in this way can be secured on special order from the manufacturers. Five small cans are contained in an outer can with a soldered inside cover and after exposure the film can be repacked in this outer can and again sealed by soldering the outer cover. If the film is liable to be roughly handled it is preferable to construct carrying cases of heavy sheet metal or fibre impregnated with bakelite and fitted with gasketed covers which can be clamped down with suitable screws or clamps. The containers should not be unduly exposed to the sun, and a layer of white felt around the carrying cases serve to insulate the film from sudden heat changes. Positive film in the laboratory should be stored in the original cans at a temperature at as near 65°F. as possible. Negative films should not be retained for too long a period either in the retorts or in the camera because in a damp atmosphere moisture spots are apt to be produced as explained above. Film should be removed from the original container only as required and in the field a changing bag will eliminate the necessity of carrying an excessive number of charged retorts. If at the end of the day any unexposed film still remains in the camera this, together with the exposed film, should be packed away in cans so that it will not absorb moisture. ^ The Care of Motion Picture Film After Exposure and Before Development. If any considerable period of time must elapse after the negative film is exposed and before it is developed, the film should be dried out as thoroughly as possible in order to retard the fading of the latent image, and fogging of the emulsion, and to prevent the possible formation of moisture spots as explained above. The film should not be dried to the point where it becomes excessively brittle, otherwise it would be apt to crack or develop static when unrolling previous to development. Two methods of drying the film are possible, as follows: 1. F. W. Renwick, Trans. Soc. M. P. Eng. Vol. 20. 2. "Markings on M. P. Film Produced by Drops of Water." by J. I. Crabtree and G. E. Matthews, Trans. Soc. M. P. Eng. Vol. 17, 29. 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