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SCREENS 149
Testing Reflection Power
(19) At the time of purchasing the screen insist upon an extra sample of not less than six inches wide by six feet long. A piece 18 inches square will serve equally well. Wrap this sample in at least two thicknesses of soft black cloth. Lay it away on the shelf of some dark closet where it will remain dry. Cared for thus, it will undergo little or no change for a long time unless its surface contains chemicals that discolor without exposure to light. To test the screen surface, suspend the sample, by means of small wire hooks as nearly as practicable in the center of the screen. Project a reel of pictures and observe the effect. A sheet of white, rough-finish cardboard will serve in place of the screen sample if that is not available. White blotting paper may also be used. Paper is not always satisfactory because its properties are not identical with the screen, but it will give a fairly good idea of the condition of the surface. The white paper test is valueless against a tinted screen surface.
It is better to make the test with a reel of pictures rather than with the light alone because actual working conditions yield a better comparison.
(20) No object or substance known to man reflects all the light striking its entire surface. There is always some loss. It cannot be truthfully said that the reflection power of any screen surface is in excess of 100 percent. It is always less than 100 percent.
However, some surfaces show more than 100 percent reflection power through a comparatively narrow angle, which only means that the other angles have been robbed of a portion of their light. (21 ) The situation is shown in the accompanying tabulation. Surface G, for example, shows 163 percent at 0 degrees angle, but at a 50 degree angle it sinks to 31 percent. The angles from 20 degrees down have been robbed of light to make the space within 20 degrees very brilliant. While the amount of total light reflected by a screen is important, it is not so important as the directions in which it is reflected.
(22) Despite statements made by screen salesmen.