International projectionist (Jan-Dec 1950)

transfer from film to air is considerably increased. 4. Restrict Film Motion Mechanically As will be described later, some of the high-intensity effects limiting the radiation intensity on film appear as an undesirable film motion during the projection interval when film is supposed to be steady in the gate. In some cases, this motion can be restricted or prevented by mechanical means, such as the glass plates used on each side of the film in some viewers, or the forces exerted by streams of air directed at the film in the projector aperture. This report covers only the application of the latter two methods of increasing permissible screen illumination, that is, increased cooling of the film, and mechanical restriction of film motion. Heating of Film Film is heated during projection because the photographic image has a net absorption for radiation, both visible and infrared; in addition, it transmits some radiation and scatters some radiation, and, as its temperature rises above that of the surroundings, it loses energy by convection to the air and radiation to the surroundings. All of the net absorbed radiation acts to raise the film temperature, so that 1 watt of visible radiation absorbed by the film is as effective in raising film temperatures as 1 watt of infrared radiation or 1 watt of mixed radiation. Additional evidence for believing that only the image absorbs energy is given by the behavior of dye-image films which are relatively transparent in the nearinfrared region, even for relatively high absorption and density in the visible. Such dye-image films show less heating and less of the thermal effects than silverimage film does when projected at the same intensity. We have found this advantage of dye-image films to be roughly proportional to their transparency in the infrared. What actual temperature the film reaches during projection is an interesting question that has never been answered satisfactorily. Obviously, the limitations of pull-down time, together with the limitations of the size of the film, make direct experimental measurement extremely difficult. High-Intensity Effects When film is projected at these higher intensities, a series of phenomena occur, depending upon the particular projection intensity being studied. Listed in order of their appearance with increase in radiation intensity, these effects are summarized in Table I. Of these phenomena, negative drift is an entirely normal film characteristic which is present to greater or lesser ex TABLE I. HIGH-INTENSITY PROJECTION OF FILM Projection Intensity Threshold Phenomenon Mean Net Watts/Mm2 1. Negative drift 2. Embossing 0.20-0.30 3. Change in reflected image tone 0.30 4. Focus drift 0.35-0.40 5. Image flutter 0.40 6. In-and-out of focus 0.50 7. Blistering 0.60-0.65 tent in all cine projection. Embossing and the change in reflected tone have never, in our experience, contributed any deleterious effect to screen quality. Focus drift is perhaps a necessary evil that must be tolerated if maximum projection intensities are to be used. Image flutter is usually visible only to a critical observer, but in-and-out of focus and blistering are two phenomena which make it impossible to obtain a satisfactory screen image: they take the control of image quality completely out of the hands of the projectionist. Accordingly, it may be said that the first four of these phenomena are necessary or at least harmless occurrences, while the fifth is borderline, and the last two must be avoided. Negative Drift It has been shown1 that film in the aperture is almost never flat and that its position bears no relationship to the curl or other physical shape it may have either entering the top of the projector gate or leaving the bottom of the gate. Film in the aperture under the influence of the light beam behaves as though the emulsion surface were expanding with reference to the base dimension, so that each frame is distorted into a pincushion shape with the emulsion surface on the convex side; since the edges are held, the center of the frame is displaced toward the arc* This is a perfectly normal phenomenon occurring in all cine projection. High-speed pictures taken into the aperture show the center of the frame beginning to move toward the light source (which in our convention is called motion in the negative direction) from the first instant of its exposure to light. During the passage of the flicker blade, a partial recovery is effected, and with the second exposure to light the negative drift resumes and is carried to its maximum amplitude. Just how far negative the film drifts depends upon the intensity of incident radiation: the greater this intensity the greater will be the film motion. Under projection conditions approximately that of a first-run, de luxe house, we have observed film motion of approximately 0.020 to 0.025 inch from the flat plane as a reference. With intensities in excess of what film will stand for repeated projection, we have observed a maximum negative drift of approximately 0.045 inch. Embossing of the Film At higher levels of projection intensity (and the first-run, de luxe houses have now reached this value), embossing of the film is observed. This is a permanent film deformation, resembling an incomplete recovery from the negative drift just discussed. Actually, two types of embossing are observed: (a) frame embossing, wherein the entire area of each frame appears to be raised slightly above the normal surface of the film — as a row of separate pincushions — and (b) image embossing, wherein the dark and light areas of the image are seen by reflected light to be at slightly different elevations from the nominal film surface. These two types of embossing probably begin simultaneously somewhere in the range of mean net radiation intensity 0.20 to 0.30 watt per square mm. Of course, at the lower intensities the embossing is minute, and can be observed only with difficulty when the film is examined under low-angle reflected light. With increasing intensities, the amount of embossing increases until it can be seen when the film is examined under normal illumination. Even when the film is severly embossed so that the center of each frame is displaced as much as 0.005 to 0.010 inch from the film plane, we have never observed any deterioration of image quality or any visible indication on the screen that this film is different from unembossed film. Change in Reflected Image Tone Some types of film, after projection at higher intensities, show a change in reflected image tone even though the image tone by transmission is not affected. This phenomenon appears somewhere in the region of 0.30 mean net watt per square mm. Film thus affected shows a warmer tone when the film itself is examined by reflected light, and the tone * This conclusion assumes standard emulsion position for theater projection. If the emulsion position is reversed and the film is threaded emulsion to lens, as in some process projectors, and much 16-mm projection, it is still the emulsion surface that expands with reference to the base; accordingly the center of the frame is now displaced toward the projection lens. This paper assumes theater standard emulsion position throughout; if the results are to be applied to projection with the nonstandard emulsion position, the direction of film motion with respect to the projector will be reversed for negative drift, focus drift, image flutter, in-and-out of focus, and positive drift. 10 INTERNATIONAL PROJECTIONIST January 1950