International projectionist (Jan-Dec 1950)

AIR COOLING of Motion Picture Film for Higher Screen Illumination By F. J. KOLB# Jr. Eastman Kodak Company The third and concluding installment of this series of articles which detailed the steps necessary to effect efficient air cooling of the film. IN DISCUSSING in-and-out of focus, it was pointed out that this phenomenon occurs when the best compromise film focus is near zero, permitting some frames to drift negatively and others to drift positively. Actually, in-and-out of focus has been produced in film under radiation conditions that normally do not result in in-and-out, by displacing the film with an air jet to bring its focal position near zero. Thus, it becomes apparent that, for in-and-out of focus, air directed only at the emulsion side has two conflicting contributions : (1) it cools the film and by lowering the film temperature delays the onset of in-and-out of focus, and (2) it forces the film more nearly toward zero and therefore hastens the beginnings of in-and-out of focus. In order to obtain the full benefit of the cooling action, it is necessary, therefore, to counterbalance the .mechanical force of the air jet on the emulsion surface (where most of the film cooling must be accomplished) by directing a similar jet at the base side. The optimum procedure is to use sufficient air on the emulsion side materially to reduce film temperature, plus sufficient air on the base side to produce an unbalanced air force holding the film on the negative side of zero. The combination of these two effects is more efficient than either one alone in permitting projection at higher radiation intensities. The mechanical force holding the film negative and preventing its going in-andout of focus must be applied before inand-out of focus is actually experienced. Our experiments have shown that such a counterbalancing force can be quite effective in delaying in-and-out of focus or + J. Soc. Mot. Pict. Eng., Dec., 1949. in preventing it entirely if the projection circumstances are proper. However, it has less influence in correcting in-and-out of focus once it has occurred. Projection Lens Design In this connection it is important to mention the relationship between film position and uniformity of focus on the screen. Most projection lenses are designed so that the image surface is not truly a plane but is a curved surface concave toward the projection lens. Therefore, film which is positioned in such a surface concave toward the projection lens can be imaged sharply over the entire picture area. Film whose curvature is greater than that of the image surface, or film that is convex toward the projection lens (and therefore contrary to the image surface) cannot be focused sharply over the entire picture area. A compromise must be taken, focusing part of the image and leaving the rest unsharp. It is fortunate, therefore, that the best film performance is obtained with film restrained on the negative side of zero — since in this position it lies in a curved surface concave toward the projection lens and in best agreement with the image surface of the lens. If projection conditions are chosen that permit the film to assume a surface convex toward the projection lens — even though the steadiness and image quality are adequate — it will be tound that the best definition extends over so small a portion of the entire picture area as to be unacceptable in most circumstances. Results of Air Cooling In the latter part of this paper we have described the results of a series of experiments aimed at increasing the safe maximum projection intensity, and permitting an increase in screen illumination without loss in image quality or damage to the film. Such an advance has been made possible through use of high-velocity air in what for simplicity has been called "air cooling," although from the discussion it has been apparent that this is actually a matter of both cooling and positioning. These improvements have been discussed in a more or less general way so far, and a summary of what they mean in the theater may be gathered from Table II. These figures have been called "probable limits" because it will take considerable practical experience to determine what is the maximum limit for air cooling without encountering difficulties in the theater. Therefore, Table II may be said to present three levels of illumination : (A) the present maximum. (B) a readily obtainable increase, and (C) the probable maximum with air cooling alone, subject to more detailed confirmation. From our experiments on air cooling, we are certain that condition B should be TABLE II. PROBABLE LIMITS FOR SATISFACTORY FILM PROJECTION STANDARD 35-MM THEATER PROJECTION (B) (C) (A) Moderate Maximum No Air Cooling Air Cooling Air Cooling OUTPUT : Mean net radiant flux, watts/mm2 0.45* 0.58 0.74 Screen lumens No shutter 17,000* 22,000 27,000 50% shutter 8,500* 11,000 13,500 Per Cent Light Increase 0 29 59 SOURCE : High-speed condenser-type lamp Positive diameter 13.6-mm 13.6-mm Amperage 170 265 Heat-absorbing glass 1.2-mm Aklo* None * Heat-absorbing glass is necessary to reduce the radiant output of this trim to a value that is safe for film without air cooling. These light and energy values were measured with the glass in place, INTERNATIONAL PROJECTIONIST • MARCH 1950