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156 RICHARDSON'S BLUEBOOK OF PROJECTION
the screen stretched tight and flat and permits it to be taken out conveniently for restretching.
Cleaning Screens
(36) Do not accept any statement from a salesman or manufacturer that a screen surface can be cleaned to regain its original reflection powers. It cannot be done with any screen surface. Some surfaces when cleaned for the first time do regain a large percentage of their original values, but each time the cleaning process is repeated those values diminish. This is true even with glass bead screens. The beads themselves may be cleaned perfectly but there is the material in which the beads are embedded which is also a part of the reflecting surface and which loses reflecting power with each successive cleaning.
(37) Except glass, any substance now used for screen surfaces undergoes a chemical change with the passage of time. The change may be slow but it is steadily progressive.
(38) Some patent screen surfaces can be refinished successfully — but only by the manufacturer who makes them, and it is usually as costly as a new screen. In any case the theatre owner should make sure about such claims by the manufacturer.
Perforations
(39) From twenty to forty holes are punched in every square inch of screen surface. Thirty holes per square inch (144 X 30) totals 4,320 holes per square foot, or 1,049,760 holes in an 18 X 13.5 foot screen surface.
(40) Screen surface area cut down by perforation runs between 9 and 10 percent. To compensate for the loss of so much reflecting area, at least 9 percent more electrical energy must be used to secure screen illumination equal to that provided by a solid surface of equal reflection power. There is no compensation, however, for the loss in picture details, which is estimated to be about 10 percent.