Journal of the Society of Motion Picture and Television Engineers (1950-1954)

conduction, radiation and convection. In order to use conduction the film must be in contact with a hot surface. Film driers have been built in which the support side of the film is brought in contact with hot, rotating drums while dry air is blown over the emulsion side. There are serious difficulties in obtaining uniform contact on the drums, but the principal objection to the system is that in order to maintain a reasonably small drum for high film speeds the drum temperatures must be above the softening point of the support. Because the drums cannot be cooled quickly, accidental slowing or stopping of the machine will damage the film. Damage is also apt to result if the film dries too quickly while in contact with the drum. Radiation methods of heating the film are frequently used. These usually take the form of strip heaters or "infrared" lamps. For the rapid drying of motion picture film there are serious objections to both of these. With any type of radiant-heat source the heat absorbed is a function of the film density. It has been shown that film subjected to infrared radiation will absorb more energy in the dark areas than in the light. Under accelerated drying this can lead to differential drying and film distortion is apt to result, particularly with silver-image films. The other factor which must be considered is that the internal temperature of the film is a function of the radiation absorbed by it. The evaporation of the water from the emulsion will cool the surface and make its temperature approach the wet-bulb temperature of the drying air, but the internal temperature of the film will be higher and may be above the critical limit of 240 F. This can be prevented, of course, by limiting the radiation to the value which the film will stand with the air conditions which are to be used while operating at the chosen film speed. These conditions could be determined experimen tally for each film, but if the film speed accidentally decreased or the air temperature rose, the film would be damaged. Briefly, then, the principal objections to the use of infrared heat sources for motion picture film rapid driers are that careful control of film speed and drying air conditions are required to avoid damage either by overheating or differential drying. Internal heating can also be obtained by high-frequency currents. This is dielectric heating and is quite similar to other forms of radiant heating. This system can be self-regulating if properly installed. The energy absorbed by the film will be roughly proportional to the water content, if the correct wavelengths are used. In order to do this it would be necessary to use multiple stages of different wavelengths. In addition, air circulation is required to carry off the moisture. With this type of equipment very rapid drying can be obtained. However, because of the cost of the equipment for such an installation its use can probably not be justified except under very special circumstances. Convection, which is the third general method of heat transfer, seems to be the most suitable for this application. It has been shown that evaporating the moisture rapidly requires high-velocity air with its water-vapor pressure considerably lower than the water-vapor pressure at the film surface. This can, of course, be done by dehumidifying the air, but it can also be done by heating it, which is usually less expensive. Heating the air, then, serves the dual purpose of increasing the drying potential and supplying the heat required to balance the evaporation. The equipment required is simple and lends itself to inexpensive, automatic control. Furthermore, measurements of the wetand dry-bulb temperatures will determine the actual emulsion temperatures during the constant-rate phase of drying and the maximum possible F. Dana Miller: Rapid Film Drying 89