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484 XIII. PROJECTION AND PROJECTORS
nor more than 14. If the 10-hour, 1000-w. lamp is used, and the projector emits the expected increase in light, a screen 5 1/2 ft. wide may be used at approximately the same screen brightness.
Good tonal quality of the picture is impossible if the room in which the audience is situated is not adequately darkened. A general room light of about 1/10 ft. -candle is not harmful ; this is the illumination level at which it is difficult but not impossible to read ordinary newspaper type. If the walls of the room are light-colored and the room is small, this general level of illumination may be reached by the light "sprayed about" by the screen itself during projection. Aside from making provisions for excluding light from the room until the general level of illumination is of the order indicated, it is particularly necessary to make sure that no narrow beams of light (such as sunlight) enter the room to produce bright spots on the walls near the screen or strike other objects in the room from which noticeable reflections may be obtained. For audience comfort, the screen should be by far the brightest object in the room. It is also preferable for the screen to be placed at least several feet away from a nearby wall so that the light "sprayed about" by the screen does not reveal nearby wall detail.
The Non-Theatrical Equipment Committee of the Society of Motion Picture Engineers published a report in the Journal of the Society in July 1941 that provides specific and authentic recommendations relating to the use of 16-mm projectors in classrooms. Much of the data presented is applicable to any kind of 16-mm projection and may be used as a guide to equipment selection and use. One very important point discussed is the location of the audience relative to the screen ; for convenience, audience location is described in terms of screen widths. In the case of a machine with a 750-w. lamp, the proper screen width is 5 ft. for 10 ft.lamberts screen brightness.
(1) The nearest row of seats shall be no closer than 2 w from the screen; a preferred minimum distance is 2.5 iv. (In terms of the 5-ft. screen, the first row of seats should be not less than 10 ft. away, with 12.5 ft. as the preferred minimum.)
(2) The farthest row of seats shall be no farther than 6 w from the screen; a preferred maximum distance is 5.5 w. (In terms of the 5-ft. screen, the last row of seats should be not more than 30ft. away, with 27.5 ft. as the preferred maximum.)
(3) No seat shall be farther away from the line between the projector and the screen than its distance from the screen. This is equivalent to an angle of 30° on either side of the projection axis.
Although it is definitely undesirable to do so, many screens are operating considerably below the 10 ft.-lambert brightness level. In a number