International projectionist (Jan-Dec 1949)

serious only in the larger theatres and in drive-ins where unusually large screens are used, but the constant improvement in light sources and equipment generally make this topic of industry-wide interest. It should be borne in mind that the film absorbs, mainly in its emulsion, a part of the energy contained in the light beam. It is this absorbed energy which causes the film to deform or buckle and, at too high a light level, produce blisters or even charred areas in the darkest portions of the photographic image. Heat-absorbing light filters, arresting a high percentage of the invisible heat rays, are used to good advantage in many installations having high-intensity arcs. The filter has, of course, the disadvantage of a loss of visible or useable light in the order of from 10 to 20%, varying with the type of glass and its thickness ; however, it will prevent the excessive absorption of energy by the film emulsion, thus reducing the deformation of the film image exposed in the aperture. Excessive Heat Occasions Strong Negative Buckle This deformation, usually referred to as "buckling," has a direct bearing on the quality of the screen image. A slight buckle toward the light source is tolerable, but a severe curvature of the image from the normal plane will cause an out-of-focus effect on the screen whereby either the screen center is relatively sharp in focus and the outer portions are not, or vice versa; or only a poor average focus over-all may be obtained. A good picture can never be projected with heavily buckled film, since the film in the aperture is never flat during the exposure time and each film frame moves from the normal plane to its maximum buckled position twice. This movement is riot instantaneous, it takes time, thus the film is in focus only for a fraction of the time that it is projected on the screen. Such momentary deformation of the film along the optical axis is readily detected by observers with a trained eye, and this might explain IP's reference to "projectionists who insist that they can detect with the naked eye a great improvement in the screen image" after heat filters are installed. A heat filter, used in connection with equipment currently available, reduces film buckle to an acceptable level and, furthermore, protects the film from permanent injury due to high heat levels, such as lasting buckle or embossing, which renders them inferior for future use, even when projected at comparatively low light levels. Modern heat-absorbent glass has a useable light transmission of from 85 to 90% and a high rejection rate for light in the infra-red region. The greatly improved screen image and the better preservation of the film itself more than compensates for the relatively small reduction in light on the screen. Film Behaviour With and Without Heat Filter The accompanying chart shows the typical difference in behaviour of film in the aperture under the influence of light and heat, when used either with or without heat filter between the light source and the film. Displacement measurements were made during each revolution of continuously-running loops — one reading at the center of the film and another reading at a point near the edge of the aperture. The figures show the displacement in reference to the zero deflection point, i.e., a perfectly flat piece of film in the film trap. The light source for both loops was a 175ampere, high-intensity arc with F:2 condensers. A rearshutter with 53% light transmission was used. The light reduction on the screen due to the filter was 15%. It should be noted from the curves on the chart that in projection without a filter a considerable difference exists between the points observed and the reference plane, to an extent that even a total reversal takes place after a few runs of the loop, from a negative to a positive buckle. With filter, the displacement of the observation points to each other, and to the zero plane, is considerably reduced, and during the entire run of the loop the buckle remains on the negative side. The cooling of the carbon jaws by water jackets can hardly have any effect upon the heating or cooling of the film in the aperture, and on this point we are in full accord with the opinion expressed in the IP article. Air Blast Directed Upon Film at Aperture The air-cooling of the film in the aperture has been widely used for about 25 years. European manufacturers adopted this method mainly to promote safety rather than to obtain an increase in the amount of light flux through the film. A pistol-range target projector, built about 35 years ago in this country, stopped after the firing of a shot and, in order to determine the marksman's skill, the image was kept stationary for the inspection of the bullet hole on the paper screen. A series of fine holes around the aperture directed a stream of air onto the film emulsion, thus keeping the film cool enough to prevent ignition. The screen illumination did not even approach today's values, nor was picture definition up to the present standard, otherwise these pioneer designers would have had to wait for the recent paper by Dr. F. J. Kolb, of Eastman Kodak Co.* Dr. Kolb shows that a dual air-jet arrangement utilizing highvelocity air is necessary to do the job of removing the absorbed heat from the film emulsion and simultaneously directing another stream of air against the film base. This method, according to Dr. Kolb, permits operation with lamps delivering from 50 to 60% more light than is now feasible. The air-jet design must provide a fine bal * "Air-Cooling of Motion Picture Film for Higher Screen Illumination,*' presented at the Spring 1949 Convention of the SMPE. Comparative degree of buckle when film loop is run with and without heat-absorbent glass filters. Buckle figures are in inches. Drawing by International Projector Corp. £+.030 UJ z 5 10 15 20 NUMBER OF RUNS PER LOOP — •>► 22 INTERNATIONAL PROJECTIONIST September 1949