International projectionist (Jan 1963-June 1965)

vertical aluminum screen in a drive-in wastes li^ht by reflecting it up into the sky. Projection building" QJor maximum picture brightness, an aluminum screen should be tilted to race the projection axis sauarely. Drive-In Projection . . . from page 6 tilted, of course), the corresponding maximum widths are 60 and 70 feet. Just imagine indoor projection quality on a drive-in screen! It is easily possible to have brilliantly lighted pictures in the smaller drive-ins and to fill these theatres every night with satisfied patrons. Although we have implicitly specified the use of modern projection mechanisms, modern "fast" lamps, and coated "fast" lenses for drive-in use in order to achieve satisfactory light levels, we have purposely avoided being too specific or dogmatic. Slight departures from optimum lamp or lens speed can usually be compensated by the use of projector shutters having the blades trimmed especially for drivein use, a small increase in arc current, the elimination of light-wasting heat filters in favor of cold mirrors, the elimination of projector-port glasses, the use of a more highly reflecting or slightly directional screen surface, etc. Lumen and Lamp Calculations The lumens of projector light output required for any particular footlambert light level (e.g. 4.5 or 10 f ootlamberts ) may be easily calculated when the width of the projected picture and the reflectance of the screen are known. A safe figure for the reflectance of a good white-painted matte screen is 0.85, and that for the on-axis reflectance of an aluminum screen is 1.5. (These correspond to reflectivities of 85% and 1 50 % , respectively . ) For the reflectance or gain of an unusual screen paint or surface, consult the manufacturer of the material. The "screen area" ( in square feet ) needed in the following formula is found by multiplying the width of the projected picture by three-fourths of the width of the projected picture. This gives the projectedpicture area on the basis of the standard Academy 35-mm aperture (0.600" x 0.825"), and should be used in this formula even though a widescreen aperture is used in actual projection. The reason for this is simply that lamp lumen ratings are nearly always specified on the basis of the Academy aperture. To repeat, just ignore the fact that you are using a widescreen aperture, and carry through the lumen calculation on the basis of the standard Academy aperture, as directed. Area x Desired footlamberts Lumens = 5 j, — : screen retlectance When you have thus calculated the required projector light output in lumens for the desired footlambert brightness level, you are ready to select an arc lamp, carbon trim, and amperage to supply this number of lumens. But note that the lamp and carbon screenlumen ratings published by manufacturers and trade papers are for projectors without any shutter (that is, not running ) , whereas the above formula gives the number of lumens with the shutter running. Screen brightness standards always specify that the projector be running, but without film, when the light is measured. Therefore, to correlate the lumen result you get by using the above formula with manufacturers' screenlumen ratings, multiply your result by 2. This is valid because the average projector shutter halves the amount of light reaching the screen — a shutter transmission of 50%. Lamp, carbon trim, and arc-current light output data in lumens are furnished by the Strong Electric Corp., 87 City Park Ave., Toledo 1, Ohio for lamps of their own manufacture, by the National Carbon Co.. 30 East 42nd St., New York 17, N. Y. (Bulletin No. 3 K and by the very complete and trustworthy tables in the article titled "Screen Light Requirements in Modern Projection, Part I" in IP for June 1959. Consult any or all of these sources, but reject all suspicious or unverified advertising claims. Example Shows How Simple It Is Suppose that you are operating in a small drive-in having a 60 ft. white-painted screen of about 0.85 reflectance, (a) How many lumens of light (projector running without film ) are needed to give the drive-in minimum of 4% footlamberts of brightness? (b) What lamps, carbon trims, and currents will serve to give this screen brightness? The area of this 60-ft. screen ( on the basis of the Academy aperture, as explained above ) is 60 x ( 0.75 x 60) = 60 x 45 = 2,700 square feet. Therefore: 2,700 x 4.5 Lumens = ^ = 12,150/0.85 = 14,294 lumens (a) This result may be "rounded off" to 14,000 lumens. Because lamp and carbon light outputs are measured without the projector shutter running, we should select a carbon-trim and current combination rated at approximately twice 14.000 lumens, that is. 28,000 lumens. ( b ) Consulting any of the available trustworthy lamp and carbon tables, we find that 28,000 lumens ( approximately ) are supplied by ( 1 ) an 18-inch mirror lamp burning 11-mm regular positives at 120 amps., (2) by an 18-inch mirror lamp burning 10-mm Hitex positives at 125 amps., or (3) by an 18-inch mirror lamp burning 13.6-mm regular positives at 145 amps. The projection lens should be coated and of f/1.9 — f/1.7 speed. The accompanying table gives suggested carbon sizes ( regular carbons only except for blown arcs ) and currents which, burned in modern large-mirror lamps used with projectors having f/1.9 or f/1.7 coated lenses, will give 4% and 10 footlamberts of brightness on 85% white and 150% aluminum screens. This table is intended to be only suggestive, but may nevertheless be used as a trustworthy guide in the correction of the serious lighting deficiencies which prevail in far too many of the nation's drive-in theatres. Because no account was taken of side-to-center screen light distribution in the computation of this table, it favors a slightly brighter-than-minimum-standard light level at the center of the screen, which is all to the good. iP {TO BE CONTINUED) 18 International Projectionist April 1963