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. 232 HANDBOOK OF PROJECTION FOR
to the thickness of the glass, there is tendency to out of focus when a mirror screen is viewed at a wide angle, because of a double reflection — one from the silver back and one from the ground front. The satin finish mirror screen is an ideal installation for the long, narrow theatre, by reason of the fact that the audience will all be seated practically directly in front of it, under which condition a very brilliant picture may be had with a comparatively low projection light value; also the satin finish mirror screen has the peculiarity that the further you get away from it, within reason, of course, the more brilliant the picture appears. It is, therefore, particularly of value in a very deep, narrow house.
One of the principal objections to the mirror screen is that it is costly, difficult to install and subject to some, though slight, risk in the item of breakage. Once installed, however, barring very improbable accidental breakage, it should require no attention, except an occasional washing, for many years.
TRANSLUCENT SCREEN.— The translucent screen is either made of translucent material, such as tracing linen, or it is of ground glass. With this type of screen the projector may be located on the side of the screen opposite from the audience, which will view the picture through the screen. The image appears on both sides of the screen, but is reversed to the projectionist, to whom all titles and other reading matter will read backward.
When projecting through a translucent screen (called "rear projection") the film is placed in the projector with the emulsion side toward the screen, instead of toward the light as in ordinary projection.
It is possible to use ordinary cheese cloth or thin muslin cloth for a translucent screen, but if this be done the projection lens must be sufficiently below the center of the screen so that a straight line from the eye of the spectator to the lens will not pass through any part of the picture. In practice this means that such a screen cannot be used for rear projection at all if there is a balcony in the theatre. The reason for this is that any spectator who sits in such position that the eyes will be in line with any portion of the picture and the lens will see the brilliant lens spot through the screen. The ground glass, tracing linen and screens of similar characteristics break up this bright spot and render it invisible.
If a cloth screen be used the result will be greatly improved if it is kept wet with water.