International projectionist (Jan-Dec 1956)

Curved Gate for Simplex Projectors By HERBERT BARNETT General Precision Equipment Corp. Fluttering of film in-and-out-of-focus under high projection light levels is greatly reduced by the new curved film gate designed by General Precision Laboratory for XL mechanisms. FOLLOWING a year's engineering study of the projection problems created by the use of more powerful arc lamps, General Precision Laboratory has designed a curved gate for the Simplex XL projector. The gate will be manufactured by International Projector Corporation and marketed by National Theatre Supply. It will be available both on new XL mechanisms and as a field modification for XL's already in service. Recent trends toward larger screen sizes and the necessity for increased illumination to maintain adequate brightness have exposed certain limitations in the quality of the projected image when a flat film gate is used. One of the most serious problems is the maintenance of sharp focus over the screen area and throughout the exposure cycles of each picture frame. High quality projection demands that the film be accurately positioned within the focal plane of the lens and that it remain fixed as nearly as possible within the depth-of-focus range. The lens setting tolerance within which a film image will remain in sharp focus is what is meant by depth-offocus range. Hand-tooled experimental model of the new trap and gate for the Simplex XL projector. Note the shape of the rails on the projector gate. Flexible sheet pressure runners are attached to the trap. Contour of the rails determines the attitude of the film frame at the aperture. The principal deterrent to sharp focus is the condition of momentary buckle due to heat absorbed by the film from the projection light source. When heat is absorbed in this manner, unequal expansion takes place between the film base and photographic emulsion. Consequently, film at the aperture is distorted with a tendency to deform away from the lens. The process becomes continuously more pronounced at temperature increases during the projection cycle. Film held in a flat gate and under the influence of high light levels will buckle or flutter at the center of the picture frame for a distance that is several times the depth-of-focus range of the lens. When this occurs only a compromise focus can be accomplished. This focus is neither constantly sharp over the projection cycle nor uniform from center to side of the screen. Where light levels are not extremely high, momentary buckle does not result in serious permanent distortion of the photographic print. However, repeated passage through the projector at very high levels of illumination frequently causes the film to be permanently buckled in the sense that it no longer returns to its original shape. When this state has been reached, attempts to project such prints in the standard flat gate result in serious inand-out-of-focus jitter due to buckling of the film that is opposite in direction to the expansion that occurred before the film was permanently damaged. The effect may be so extreme as to render the print unusable. A curved gate properly designed will materially reduce the magnitude of film movement under the influence The two graphs shown here deserve careful examination by all projectionists. The curve in Graph A shows the excursion. or movement of the center of a film frame during the two projection periods. During these exposure periods temperature rise due to heat absorption from the light beam causes the film base and emulsion to expand unequally, resulting in what is normally called buckling. This curve shows the PULL FIRST FLICKER SECOND NEXT DOWN EXPOSURE BLADE EXPOSURE FRAME u O 10 < D a. o u o > L. < E CENT \N INCI 5 i 111 o o 1 f ± " /// ninT^^ ///////// J < , iHJff niriT X r L. o LULU LI J DTK 'sittL IJ L 1 t tn /// y/Jy) '/// ^*^_^ u < / / / 5 "> /// DEPTH OF FOCUS RANGE OF LENS <° yy, ^ y// litl B extent of excursion of the film in a straight gate. The curve in Graph B shows the degree of reduction of the film excursion when a properly designed curved gate is employed. Restriction in buckling, afforded by the curved gate, additionally improves»center-to-side focus as indicated by corresponding figures for average focus shown in each diagram. INTERNATIONAL PROJECTIONIST • JULY 1956 13