International projectionist (Jan-Dec 1947)

Complete Projection Data Charts FORM 2, presented herein, includes the salient features of the arc lamp and the characteristics of the carbon trim. Fill out the form in pencil, not in ink, as changes may be desired later. Enter the most obvious and easily determined facts first; then attend to specific details. 1. ARC LAMPS. Under "Type" mention whether the unit is a high or lowintensity arc, and if "high" whether it is of the Suprex-type. Strictly speaking, there is no such thing as an "intermediate intensity" arc lamp. The term "intensity" as applied to an arc refers to the manner of light production and to the brightness of a unit area of the positive crater. In the low-intensity arc the light comes from white-hot carbon; while in the high-intensity type it emanates mostly from a crater-covering film of ionized carbon gas. A square millimeter of the crater has approximately the same intrinsic brilliance in all arcs of any one type, regardless of amperage and size of trim. Contradictory as it may seem, a high-amperage, low-intensity lamp may give more light than a low-amperage, high-intensity lamp. "Optical system" refers to the lightcollecting and converging system: mirror, mirror and converger, or condensing lenses, as the case may be. Under "Position of Carbons" mention the inclination of the negative to the positive, if any, and also the inclination of the positive from the vertical or horizontal. The remainder of Section 1 requires no special comment. 2. OPTICAL. The "true focal length" of a reflector may be learned by writing to the manufacturer. It may be determined in the projection room to within x/4 inch by the following method; Focus the arc on the projector aperture in the usual way. Now move the arc forward or back until the spot on the aperture cooling plate is as small and sharply defined as possible. Switch the arc off, and when the carbons are cool, very carefully measure the distance (1) from the positive crater to the center of the mirror, and (2) from the center of the mirror to the cooling plate (not the aperture, itself). Then use the following rule or formula: Rule: To find the focal length of a mirror divide 1 by the crater-mirror distance in inches. Divide 1 by the mirror-cooling plate distance in inches. Add the two quotients. Divide By ROBERT ALLEN MITCHELL The second in a series of seven articles providing data forms which, when filled-out to a reasonable degree of completeness, afford a comprehensive and accurate description of the individual installation. II. Lamp and Carbon Data 1 by the sum so obtained. The result is the focal length of the mirror in inches. Formula: d is the distance from positive crater to mirror; D is the distance from mirror to cooling plate. 1 F = + 1 D [NOTE: In mathematics 1 over a quantity is called the reciprocal of that quantity; thus the value of F in the above formula may be defined as the reciprocal of the sum of the reciprocals of d and D.] The curvature of projection lamp mirrors is not strictly spherical, for the curvature decreases slightly toward the edges. Such a "figure" is called parabolic. If mirrors were figured spherically, we should not have to be very fussy about the mirror-aperture distance, but the spot would be blurry, and the fadeaway of light toward the edges of the screen would be intolerable. To overcome this defect (spherical aberration) the reflector manufacturer "parabolizes" his mirrors. Now, the degee of parabolization required depends on the focal length of the mirror and its working distance from the aperture. For this reason the mirror must be positioned to within one millimeter of its proper distance if the best light is to be obtained. If the mirror be too far away from the aperture, the effect is that of an under-parabolized mirror; if too close, the effect is that of an over-parabobzed mirror. In the latter case, the picture may even be brighter at the edges than in the middle of the screen, a highly unsatisfactory condition. The focal length of a mirror combines with the diameter of the positive crater and the size of the aperture to limit the mirror's maximum distance from the aperture; for every projectionist knows that if the mirror be too far back, it is impossible to focus the spot down to a reasonably small size. Too large a spot causes an enorm ous waste of light and heats the head excessively. Because focal lengths and degrees of parabolization of different mirrors of the same size sometimes vary, the competent projectionist should not hesitate to readjust the mirror distance by "trial and error" until the point is reached where the brightest and most even illumination is obtained on the screen. The equivalent focus of a glass lens condenser assembly is determined in the same way as the focus of a mirror. Employ the distances crater-to-collector and converger-to-cooling plate. It may be remarked here that a condensing lens system is not so sensitive to slight errors of adjustment as a mirror system. Section 3 requires no explanation. It may be used for recording data on either rheostat or relay-type arc controls. 4. ARC AND CARBON TRIM. Under "Carbons" mention whether cored, type of coating, etc. Arc gap length may be expressed either in millimeters or fraction of an inch, whichever is preferred. Numerous recommendations have appeared in the literature regarding correct gap length, but the projectionist will know from experience what length gives the best results with his particular trim and current. Methods of determining arc voltage, resistance, etc., were discussed in the first article of this series. This information may be taken from Data Form 1A or IB. Carbon consumption in inches per hour may be determined in the following way: (A) Measure the length of a positive carbon in inches. Note the exact number of minutes it is burned during the running of a film. Measure the length of the carbon again. Then apply the following rule or formula: Rule: To find the hourly consumption of a carbon subtract the final length of the carbon in inches from the initial length. Multiply the difference by 60. Divide the product by the number of minutes of burning. The INTERNATIONAL PROJECTIONIST May 1947 13