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488b HANDBOOK OF PROJECTION FOR
An examination of the reflection characteristics of the screens examined and tested shows that they may be roughly grouped into three general classes :
A, specular
B, semi-specular
C, diffuse
Class A includes all of those surfaces which reflect a large proportion of the incident light within a relatively narrow angle. Class C includes those screens which have a high diffusing power and therefore the reflected light is distributed approximately uniformly over a wide angle. Class B contains those screens which are intermediate between the extremes represented by class A and C. It should be emphasized however that the line of demarkation between these three classes is not definite. The classification is somewhat arbitrary and is made only for purposes of practical convenience.
In examining and considering the data here tabulated the exhibitor and projectionist should remember that two items only are considered, viz.: the reflecting power of the surface and its relative reflection at varying angles, and that while these two things are of first importance, still there are other very desirable physical characteristics to be considered when the final selection of a screen is to be made. It may be found, for instance, that two or more surfaces are equally efficient for a certain auditorium, insofar as concerns reflective power and evenness of light distribution, but that one of them is found to have other characteristics superior to its competitors, such as a more substantial foundation (cloth backing) or the mounting or guarantee given with it. In such a case the final selection would of course be made on these latter points, reflection and distribution being equal.
In table 3 are tabulated the screen surfaces which have been classified as of specular type, class A. For convenience in analyzing the characteristics and in order to show the essential peculiarities of screen performance under various conditions the values of "ratio" and "R" have been computed for angles of 20°, 30°, 40°, and 50° as indicated at the top of the tabulation. The value of "ratio" is obtained by dividing the reflection factor at 0° by the reflection factor at the angle as indicated at the top of the table. For instance in the case of surface No. 8 the reflection factor at 0° (see table 2), is 414 while that at 20° is 153. The ratio of 414 to 153 is 27