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turer. The angle of distortion is usually small, but depends on the type of material used.
The Perforated Screen
Perforated screens may be made from quite a number of different materials, but are usually of a woven cloth specially treated. The only advantage of the perforated screen is that the loud-speaker may be placed behind it, so that the audience may hear the sound direct from the screen. The sound may be impaired, however, if the perforations are allowed to fill up with dust and dirt.
The perforated screen "may sometimes be cleaned with a vacuum-cleaner," but this is seldom successful after the third or fourth time, as the weave tends to lose its geometrical structure. It is best to have the screen serviced by a firm specialising in this work.
The Screen to Choose
For the 16-mm. operator, a screen which can be erected without assistance is the ideal, and many kinds of self-erecting screens may now be purchased. In one example, the screen is rolled up within a long wooden box, and is erected merely by taking hold of the top of the screen and lifting it up — the screen remaining upright by means of spring-tensioned folding supports and the roller acting in the same manner as the familiar roller-blind.
Other screens may be of the type which are stretched by wooden frames and then hung or stood in position, but frames are liable to warp or bend after being in use for a time, and when this happens they should be changed to prevent damage to the screen by having its surface pulled out of shape.
When the screen is not in use it must be protected from both dirt and damp by the use of suitable covers. Screens are often made dirty by fitting them together on the floor, so always ask for assistance to keep the screen away from it, and remember that the back of the screen will convey dirt to the front surface when rolled up after use.
The correct screen can be selected to suit the size of audience and conditions from the table on page 80, which gives the dispersion of the various surfaces.
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