International projectionist (Jan-Dec 1947)

without allowance for shutters, filters, etc-, since these more general light values can readily be corrected for the transmission of the particular accessories incorporated in a specific assembly of interest. The data in Table 1 show screen lumen values at maximum light ranging from 2500 to 21,500 and illustrate the wide range of possibilities offered by the various combinations of lamps, carbons, and optical systems. It will be noted that the side-to-center screen distribution at maximum light is generally 60 or 65 per cent. If a more uniform distribution of light be desired, this can be obtained at a sacrifice in intensity. Values of screen lumens are quoted at 80 per cent distribution as an example of a more -uniform distribution and are approximately 10 to 15 per cent lower than maximum values. The importance of adequate lens speed in utilizing the light provided by the projection lamp is also illustrated by the data. The F: 2.0 treated lens gives approximately 50 to 60 per cent more light than the F: 2.5 untreated lens with the condenser lamp, and 30 to 35 per cent more with the mirror lamps. Much of the gain with the mirror systems results from the greater transmission of the treated lens. There is an additional gain with the condenser system because the F: 2.0 lens transmits light from the ' F : 2.0 condenser system which is not passed by the slower speed F: 2.5 lens. Factors Affecting Results In order to compare the screen light intensity in motion picture theatres it is necessary to take into account such other factors as projector shutter, heat filters, port glasses, draft glasses and screen size. Values of light intensity in footcandles at the center of the screen are plotted for a range of screen widths in Figs. 1 to 4. These are based on the various projection systems and conditions of Table 1, and allowance has been made for: (1) A projector shutter of 50 per cent transmission, (2) A projection room port glass of 90 per cent transmission. The values obtained in a particular theatre installation may differ from those reported here for numerous reasons, among which are the following: (1) Departure from the optical characteristics specified in Table 1. For example, many of the older lenses are of slower speed than F : 2.5. (2) The possible presence of absorptive factors in the optical train other than the ones assumed here. For instance, shutter transmission may be other than 50 per cent, a heat filter is sometimes employed, a draft glass is often used, and the present assump UJ < O i O 25 20 ' 10 Z U 5 rr u (/) u. O cc UJ z" » UJ O 20 5 ,5 5) l0 z K 5 — i — i — ' — i — n — I FIGURE I •[.' '5.5*E.F. f-.ZJS UNTREATED LENS' 80% DISTRIBUTION — I 1 1 1 ! FIGURE 2 J "5.5*E.F. f:2.5 UNTREATED LENS MAXIMUM LIGHT 10 20 30 40 50 60 10 20 30 40 50 60 FIGURE 3 "5.0* E.F. f:2.0 TREATED LENS " 80X DISTRIBUTION "ITEMS \ I I FIGURE 4 | "5.0"E.F f:20 TREATED LENSMAXIMUM LIGHT FIGS. 1, 10 20 30 40 50 60 10 20 30 40 50 60 Screen Width — Feet 2, 3 and 4: Light intensity3 at center of various sized screens with typical carbon arc projection systems. ITEMS1 (1) 12-8-mm Low Intensity, 32 amp., 55 volts; (2) 7-6-mm "Suprex," 40 amp., 27.5 volts; (3) 7-6-mm "Suprex," 42 amp., 33 volts; (4) 7-6-mm "'Suprex," 50 amp., 37 volts; (5) 8-7-mm "Suprex," 60 amp., 36 volts. (6) 8-7-mm "Suprex," 70 amp., 40 volts; (7) 13.6-mm-A" High Intensity, 125 amp., 68 volts; (8) 13.6-mm-%" High Intensity, 150 amp., 78 volts2; (9) 13.6-mm-%" Super-High Intensity, 170 amp., 75 volts2. NOTES (1) Refer to Table 1 for details on items; (2) May require heat filter and result in 10 per cent decrease in width at given intensity; (3) Footcandle values assume — (a) 50 per cent shutter transmission; (b) 90 per cent projection port glass transmission; (c) No film or filters other than port glass. tion with respect to the port glass may not apply. (3) Degree of cleanliness or misalignment of the optical system. (4) Departure from the specified operating conditions for the arc. Figures 1 and 2 show the foot-candle intensities at the center of the screen under the aforementioned conditions for 80 per cent distribution and maximum light with the F: 2.5 untreated lens. Figs' 3 and 4 show similar data with the F: 2.0 treated lens. These curves show the screen widths that can be illuminated to intensities of 5 to 25 foot-candles. The screen widths involved range trom about 10 to 60 ft. It is perhaps worthy of note that in Fig. 2 with the F: 2.5 lens Item 8, the 150-amp arc, gives higher screen intensity than Item 9, the 170-amp arc, whereas this is not the case in the other three illustrations. The explanation for this anomaly rests on the relatively greater radiation of the 170-amp carbon at wide angles. It is necessary to use the higher speed of the F: 2.0 lens in order to obtain the higher light from the 170amp arc. In order to ensure a sufficient screen brightness for proper viewing conditions, the A.S.A. Standard Z22.39-1944 has specified that "The brightness in the center of a screen for viewing 35-mm motion pictures shall be 10 — j foot-lamberts when the projector is running with no film in the gate." The screen light intensities in Figs. 1 to 4 can be converted to screen brightness in foot-lamberts by multiplication by the screen reflectivity. This has been done using a screen reflectivity of 75 per cent and the resultant data have been used to plot Figs. 5 to 8. These block 8 INTERNATIONAL PROJECTIONIST • April 1947