International projectionist (Jan 1959-Dec 1960)

FIG. 1. The projection screen should be set up to face the rectangular seating area squarely for maximum audience capacity. When the room cannot be adequately darkened, however, it is best to place the screen in a corner facing away from the imperfectly shaded windows. The seating area then becomes diamond-shaped. ing distance," therefore, the projected picture should be 5 feet in width. Twice this picture-width is 10 feet, which in this example is the recommended distance between the screen and the front row of seats. A motion-picture screen should be an inch or two larger each way than the projected picture itself. This allows for a small margin around the picture and eliminates the possibility of "overshooting" the screen with attendant distractions for the viewers. Small discrepancies in projector-toscreen distance, or in the focal lengths of lenses, may result in pictures slightly larger than the calculated sizes. Rectangular or Square Shape? Should screens be rectangular or square in shape? Rectangular screens of the conventional 4:3 proportion are preferred for motion-picture and filmstrip projection for a neater, more professional effect. Square screens are nevertheless considered more serviceable for 2" x 2" slides, which may be oriented either vertically or horizontally, for 4" x 3*4" slides, which are generally projected through relatively square-shaped masks, and for opaque projection. Even the smallest school, for example, should have both kinds of screen for specific visual-education purposes. A projection screen should be placed high enough to prevent the heads of observers in the front rows from obstructing the sightlines of the persons behind them. Equally important, the portable screen should be located to prevent extraneous light from imperfectly-shaded windows from falling upon the projection surface. It may be necessary to place the screen in a corner of the room facing away from the windows, as shown in Fig. 1; but even this expedient has its advantages. The extreme front-row viewing angles which foreshorten the picture and cause eyestrain are eliminated by the diamond-shaped viewing area. The required screen size calculated by dividing the maximum viewing distance by 6 poses difficulties when the luminous power of the projector is too low for adequate picture brightness. A dim, hard-to-see picture is not an effective communication tool. A blank-screen brightness of 9 or 10 foot-lamberts of surface brightness when the projector is run w'thout film is the minimum standard for 35-mm movies in theatres. This level of screen brightness is also satisfactory for 16mm projection, inasmuch as the photographic densities of both 35-mm the atre-release prints and 16-mm instructional films are similar. As a concession to 16-mm tungsten-bulb projectors, which are much less powerful than the high-intensity carbon-arc projectors used in theatres, a minimum-brightness level of 5 footlamberts is considered acceptable in well-darkened rooms. Brightness Level vs. Size How large a picture will your 16mm projector give at the minimum brightness level of 5 foot-lamberts? Although different projectors and lenses have different luminous efficiencies, Table II provides at least an approximate estimate of the picturesize capacities of 16-mm machines having 110 — 120 volt bulbs of various wattages. A 750-watt 16-mm projector, for example, is capable of throwing a picture 5 — 6 feet wide on a matte screen at the minimum acceptable brightness level of 5 foot-lamberts. A classroom in which the last row of occupied seats is from 30 to 36 feet from the screen can be satisfactorily served by a 750-watt machine. Larger pictures from this particular projector will be too dim (unless a beaded screen be used with severe restriction of the angular width of the viewing area), but smaller pictures will be brighter and, in most situations, more effective. A 4-foot picture, for example, conforms to the professional 10-foot-lambert standard; while a picture 2% — 3 feet in width represents the 20-foot-lambert level which prevails in the better modern theatres. Still smaller pictures will be too bright. Excessive picture illumination raises the perception of shutter flicker above the threshold and thereby TABLE II: Recommended picture sizes for 16-mm projection on matte screens. POWER OP PROJECTOR LIGHT MINIMUM PICTURE WIDTH 20 footlamberts RECOMMENDED PICTURE WIDTH 10 footlamberts MAXIMUM PICTURE WIDTH 5 footlamberts 300-W Bulb 1« !£■ 2' 2i' 3' 500-W Bulb 1£' 2' 3' 3i* ^ 750-W Bulb 2i' 3' k> 5' 6' 1000-W Bulb 3' 3*' 5' 6' 7' 1200-W Bulb 3i' V 6' 7< 8' 30-amp . Arc 6' 7' 10' 12' IV INTERNATIONAL PROJECTIONIST FEBRUARY 1959