American cinematographer (Jan-Oct 1925)

Sixteen AMERICAN CINEMATOGRAPHER August, 1923 Static Markings on Motion Picture Film Data as to their Nature, Bij J. I. Crabtree From Transactions, Society Cause, and Methods of and C. E. lues of Motion Picture , (Research Laboratory _ . Prevention. > ,-•„ Kodak company.) Engineers. (Continued from July issue, in which romplete illustrations appea red. ) 8. Electrification of negative film in the cutting room. Since electrified film has a powerful attraction for dust particles, it is important to maintain a fairly high humidity in the cutting room in order to minimize the propensity of the electrified film to attract dust. Such humidification also tends to prevent printer static. J,. Printer static. The largest proportion of static markings encountered in the laboratory are produced during printing, and especially with step printers. Static is rarely encountered with all-metal continuous printers. In a step printer the film is subjected to excessive friction during the pull-down movement, especially with shrunken negatives. Static markings may, however, be prevented: 1 . By avoiding friction. All sprockets should be of correct dimensions and in alignment with the take-up roll. If the sprocket teeth are staggered, or if the take-up roll is in malalignment, excessive tension is exerted on one edge of the film. Too much tension should also be avoided at the take-up roll, while the loops should be adjusted to prevent any possibility of the film rubbing against itself or any part of the machine. The printer should also be correctly "timed," that is, the pressure plate should be released before the pull-down movement commences and should not return in place before the filn^comes to rest. Although glass is not an ideal material for pressure plate construction in view of its nonconductivity, metal plates are unsatisfactory where a transparent plate is otherwise desired, while glass produces a minimum of scratches on the film. The pressure plate should be renewed whenever the surface becomes roughened. 2. By humidifying the film. When motion picture positive film leaves the factory it is in equilibrium with an atmosphere of 70 to 75% relative humidity, but if the laboratory conditions are favorable for the production of static markings the quantity of moisture which the raw film contains is not sufficient to positively insure the absence of static during processing. It would be dangerous, however, to humidify the film further during manufacture, owing to the danger of the formation of moisture spots when the film is stored.11 Since a certain lapse of time is necessary for moisture to affect the emulsion, it is possible to humidify film immediately previous to or during processing to an extent which would be dangerous if the film was to be subsequently stored. 3. By humidifying the air in the printing room. If the printers were always in perfect adjustment and not run at too high a speed, a higher relative humidity than 75'/ at 70" to 75"F. would not be necessary in the printing room. In order to take care of the excessive friction to which the film is liable to be subjected if the printers get out of adjustment it is advisable to maintain the relative humidity at from 80% to 90% at 70" to 75"F. At such a high relative humidity the air feels uncomforably cool to the worker at temperatures below 68"F. and oppressively warm above 75°F. The exact relative humidity to be maintained depends on the particular machines used, the condition of the film, the temperature of the air, and time during which the film is exposed to the air before it is subjected to friction. The higher the temperature the lower is the relative humidity necessary to overcome a given tendency for static. Usually the film is exposed to the air for only a few seconds before reaching the printer gate. This period may be prolonged by looping the film over several idler rollers before it reaches the gate. Such a procedure, however, is usually unnecessary if the negative is humidified as described below. Methods of humidifying the air supply have been fully described in a previous communication.4 Since the air in the printing room is at a higher relative humidity than that in any other room, it is necessary to boost the humidity of the air supply locally, and this can be readily accomplished either by means of water spray jets or steam jets. A series of water spray jets operated by compressed air and inserted in the air line serve to immediately change the relative humidity and have the advantage of cooling the air in hot weather. In winter both steam and water sprays are often necessary. 3. "A Study of the Markings on Motion Picture Film Produced by Drops of Water, Condensed Water Vapor and Abnormal Drying Conditions," by J. I. Crabtree and G. E. Matthews, Trans. Soc. M. P. Eng., Vol. 17, p. 29. American Cinematographer, January, 1925, p. 12. 4. "The Development of M. P. Film by the Reel and Tank Syatems," by J. I. Crabtree, Trans. Soc. M. P. Eng., Vol. 16, p. 10 3.