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means of these adjustments, it is possible to adjust the position of the lighting fixtures to any desired distance from the ceiling, to vary its distance from the projection screen by a considerable amount and further to vary the inclination at which the fixture hangs thus controlling to a great extent the distribution of the light from the fixture.
Complete diffusion of the light from the incandescent lamps mounted in the fixture was obtained by inserting a sheet of 10 x 12 opal glass in the position provided for holding the safelight. This material is of such nature that the light transmitted is very completely diffused. In the projection booth is situated a projection machine of the ordinary type. An arc current of 25 amperes was used throughout the tests recorded in this report. With this current the screen brightness as measured with the machine running but without a picture in position was found to be approximately 20 milli-lamberts. This measurement was made from a point very near to the axis of projection and due to the character of the screen was much higher than the brightness measurement made from points a few degrees from the axis. From measurements made previously in several of the motion-picture theatres in Rochester, an average value of screen brightness under similar conditions was found to be approximately 15 milli-lamberts. The screen brightness used in these experiments therefore is somewhat higher than is ordinarily met with in practice.
The lighting fixtures having been installed, the procedure followed was to determine by trial and error the maximum amount of light which it was possible to use without causing an appreciable loss of quality in the picture. After several preliminary trials it was decided that the best result was obtained by placing 60-watt lamps in the four central elements of the fixture and 40-watt lamps in each of the end boxes, the entire 10 x 12 surface of the diffusing glass being uncovered and used as effective source area. The position of the fixture which was found to give a very satisfactory distribution of the light was as indicated in Fig. 11. This position was approximately five feet from the ceiling and eleven feet from the rear wall of the room, and with the fixture inclined forward so as to give an evenly graduated distribution of the light on the ceiling. It was found necessary in order to prevent excessive illumination of the screen by the light from the fixture to place a cardboard screen along the front of the frame carrying the safelight fixtures. This was of such dimensions that no direct light from the opal glass was permitted to fall upon the. projection screen, and served also to reflect some of the light backward, so that the ceiling brightness at the rear of the room was brought up to a value more nearly equal to that of the brightness of the ceiling directly above the fixture.
This arrangement having been arrived at and considered satisfactory, several experienced observers were asked to express opinions as to whether or not the quality of the projected picture was seriously affected by the presence of this general room illumination.
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