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Types of Visual Aids and Their Uses 127
Translucent Screens. The translucent screens, as mentioned above, are screens which transmit the light from the projector to the audience. The projector stands behind the screen and projects through it. There are some advantages to this type of screen and several disadvantages. It is used ordinarily for the projection of opaque materials where it becomes advisable to have both the projector and the screen at the front of the room. Some have arranged a dark box for projection by covering the space between the screen and the projector with a dark cloth supported by two or three small rods. This keeps interfering light from the back of the screen and provides projection possibilities in a well-lighted room, which could not be accomplished otherwise. However, if it becomes necessary to use such a screen temporarily, it will be less expensive to construct the screen than to buy it. The chief requirements are a frame of the proper size and enough architect's tracing cloth to cover that frame. The frame can be made by any manual-training department or carpenter shop, and the cloth can be purchased at a bookstore or architect's supply house. A screen approximately 3x4 feet in size can be constructed at a total cost of from $3 to $5, whereas the same screen of the special types prepared for the commercial field would cost $15.
When translucent screens are used, short focal-length lenses of good quality are usually required and this adds to the cost of projection equipment.
If a translucent screen is to be used regularly and the best possible results are desired, a screen which has been built for the purpose should be purchased. The newer types of prepared translucent-screen materials are superior to the tracing cloth which is recommended for temporary use.
Auditorium Screens. The preceding discussions of screens have dealt entirely with screens for classroom use. The general principles of reflection should be applied in selecting a screen for a school auditorium. A much larger screen will be needed for satisfactory results in the auditorium and, in most cases, it will be desirable to purchase a screen which has been perforated for sound. The sound screen is very much the same as an ordinary screen except that it has been perforated with small holes through which the sound from the loudspeakers will pass when sound pictures are being projected. The perforated screens cost approximately the same as the ordinary screens and the small perforations will not affect the brightness of the picture