International projectionist (Jan 1959-Dec 1960)

introduces an element of discomfort in the viewing of motion pictures. As a general rule (to which there are numerous exceptions) 16-mm projectors for school use should have bulbs at least as strong as 750 watts. A 750-watt projector will serve small classrooms; while a 1000-watt machine will serve large classrooms and small assembly halls. Larger auditoria requiring screens wider than 7 feet must resort to the use of carbon-arc projectors connected to fume-exhaust flues, powered by rectifiers, and housed in a projection hooth. These, like powerful theatre projectors, should be operated by skilled projectionists. Ambient Light Level A classroom screen, as stated previously, should be protected from extraneous light while projection is in progress. The excellent contrast-range of professional movies is due to the very low level of ambient light which reaches the screen in a motion-picture theatre. While many visual-educaltion authorities offer several good reasons for disapproving the taking of notes b\ students during the exhibition of films, there may be special occasions, as during repeat showings, when notetaking and group discussion may assist the learning process. In such cases there must be sufficient ambient light to permit the student to see his notehooks and to identify other members of the class by sight. The intensity of ambient-light levels during projection may be specified. For "dark-room" projection, when no notes are to he written, the illumination of the room should not exceed 0.1 foot-candle. This will permit good \ isual results to be obtained with FIG. 3. The practicable viewing area for matte screens is very wide. With this "diffusive" type of screen, only the effect of foreshortening due to perspective limits the width of the viewing area. The matte screen is the best allaround type for classroom use. screen-brightness levels of 5 — 10 footlamberts. For note-taking and group discussion during projection, room illumination should be approximately one foot-candle. If the screen can be protected at least partially from the full intensity of the ambient light, well and good; if not, then the minimum recommended picture width (Table II) should be utilized to increase screen brightness to about 20 foot-lamberts. This is easily possible with relatively small classes, or divided classes. The size of a projected picture can be altered by two methods. ( 1 ) The projector may be moved closer to the screen for a smaller picture, farther away for a larger one. (2) The lens of the projector may be changed to one of shorter focal length for a larger picture to one of longer focal length for a smaller picture, the projection distance remaining unchanged. 450 55 50 45 40 35 30 25 20 15 10 5 Angle of view in de£r*ees FIG. 2. This graph gives the reflection characteristics of non-perforated projection screens. Apparent reflectivity may be several hundred per cent on the center line in the case of directional-type screens. The pictures on these screens accordingly look very bright when viewed on the center line, but uncomfortably dim at extreme angles. Only the plain white matte screen furnishes uniform light distribution over all viewing angles. An assortment of several lenses is highly desirable, and the focal length selected for a specific set-up should be such as to give the desired picture size when the projector-to-screen distance is at least as great as the maximum viewing distance. It is always a good idea to have the projector behind the audience, not in its midst. Relative Brightness Level Never Forget: the relative brightness of the picture is an inverse function of its total area, the amount of light issuing from the projector remaining the same. Halving the width of a picture increases the brightness 4 times; doubling the width results in only ^ of the original brightness. The standard 16-mm projection lens 1 *+W-*\ i ' f A ^v 1 i i l'l V ^^ /l\ ' 1 \ 1 1 > 1 1 \ / ; \ ^ Beaded Screen 6W / \ / Seating \ f area. \ ' r I \ FIG. 4. The recommended viewing area for glass-beaded screens. Because of their excessive concentration of light close to the axis of projection, the picture appears very dim even at moderate viewing angles. The same effect occurs with "high-gain" aluminum-surfaced ("silver") screens. has a focal length of 2 inches. The widely available l^-inch lens increases picture-width 1.3+ times; while the 3-inch lens reduces picturewidth 1.5 times; and the 4-inch lens 2 times. Table III (page 7) gives the actual sizes of the pictures with a wide variety of lenses at different distances from the screen when the projector has the standard 16-mm aperture measuring 0.380" x 0.284". Use this table to select needed lenses. Table IV (page 26) indicates the size of screen for each lens and projection distance, the screen being just slightly larger than the picture. This table, which includes only commercially available {Continued on page 25) INTERNATIONAL PROJECTIONIST • FEBRUARY 1959