International projectionist (Jan-Dec 1948)

Screen Data: Types, Sizes, Illumination for 35 and 16-mm Film Projection THE respective reflectance characteristics of motion picture screens determine the conditions under which the four general classes can be used to satisfy approved brightness recommendations.'-' 1. Matte Surface Screen. Matte surface screens reflect incident light in such a way that their brightnesses are substantially the same at all angles of view — hence they are recommended where the viewers occupy a wide angle. A surface coated with a flat white paint has this characteristic. Several screen materials are available which produce a similar result (Fig. 1 curve) . This type is required in practically all theatres because of the wide viewing angles and is recommended for other auditoria for the same reason. 2. Beaded Screen. The surface of a beaded screen is covered with small glass spheres which reflect the major part of the light back in the direction from which it came (Fig. 1 curve) . To observers sitting near the axis of projection, pictures on beaded screens are several times brighter than pictures on a perfectly" reflecting matte screen. To observers about 22 degrees off the axis of projection, pictures seen on both types would appear equally bright, except that the far side of the beaded screen would appear somewhat brighter than the near side. This brightness difference This compendium of data anent the charactersitics of the four general classes of motion picture screens should prove to be a valuable aid in insuring proper presentations in all situations. These data have been approved by all recognized authoritative bodies including the SMPE, American Standards Assoc, and the Illuminating Engineering Society. is greatest at the shorter viewing dis however, such screens are used to obtain tances. higher picture brightnesses when viewSuch screens rarely are used now for ing positions can be kept within about theatre projection because they do not 22 degrees of the projection axis. It is TABLE A. Lumens-at-Screen for Non-Theatrical Projection SCREEN SIZE (feet) MATTE SCREENS BEADED SCREENS Lumens for Lumens for Lumens for 5 Footlamberts 20 Footlamberts 5-20 Footlamberts 2.5 x 3.33 55 210 45 3 x 4 75 305 65 3.75 x 5 120 475 105 4.5 x 6 170 690 150 5.25 x 7 235 940 205 6 x 8 305 1,225 265 6.75 x 9 385 1,540 340 7.5 x 10 480 1,915 415 9 x 12 690 2,750 600 10.5 x 14 935 3,745 815 12 x 16 1,230 4,920 1.070 satisfy the brightness requirements of the entire normal seating area, particularly where the projection room is well above the audience evel. Where the axis of projection is only slightly above eye level, 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 i i i \ \ \ \ i i i i \ BEADED \ i / / \ \ \ / / / / / / / / \ \ \ \ ' / i i \ \ / 1 / 1 .A \ t. METALIZED ''SURFACES /7 / \ 1 r / / / s MATTE \ \ \ s ™ — — ■ 35 30 25 20 15 10 5 0 5 10 15 20 VIEWING ANGLE IN DEGREES (FROM AXIS) 25 30 35 FIGURE 1. Reflectance characteristics of screen surfaces. The reflectance values shown are expressed as a per cent of the reflectance of a magnesium carbonate block. The reflectance of a fresh clean magnesium carbonate surface is approximately 98 per cent. particularly important to observe the minimum viewing distance recommendations given elsewhere herein. 3. Metalized Screen. The surface of metalized screens is coated with fine particles of metal, usually aluminum, each of which reflect light specularly (Fig. 1 curves). Such screens show a pronounced "hot spot" which is near the center of the screen for those near the axis of projection. The hot spot moves toward the near side of the screen as the observer moves away from the axis. Brightness differences increase with viewing angle and with reduction in viewing distance and are excessive for theatre projection. Metallic screens are necessary, however, for viewing polarized projected pictures. 4. Translucent Screen. As its name implies, the characteristic of the translucent screen is to transmit light. The material used must be sufficiently thin so that there is a minimum loss of definition in the projected image and yet sufficiently diffusing to satisfy the requirements of brightness uniformity through the desired angle. When completely diffusing, their properties are essentially the same as those of the matte screen, but the brightness is less for a given amount of incident light. Translucent screens of high trans INTERNATIONAL PROJECTIONIST May 1948