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492 HANDBOOK OF PROJECTION FOR
of ratio and a maximum value of R is the most desirable from the standpoint of distribution of reflected light.
In table 4 are tabulated the values of ratio and R for the screens classified as semi-specular, class B. It should be noted that the values of ratio are in general much lower for this class than in the case of the specular screens (class A). Likewise the values of R, the mean reflection factor, for various angular limits are lower in general.
In table 5 are given the values relating to screens of the diffuse type, class C. The very low values of ratio for the screens of this class indicate that the screen brightness varies but little with angle of observation. Unfortunately the values of average reflection factor, R, are also much lower in general than for the screens of the semi-specular and specular types, In theatres where the maximum angle of observation is great, however, it is necessary to use screens of the diffuse type in order to avoid obtaining a much more brilliant picture when observation is along the axis of projection than when the picture is observed from the side seats.
The use of a specular type screen in a theatre will necssarily result in a variation of brightness across the screen when observed from a point relatively near the front of the house. It is therefore necessary to use a screen of the diffuse or semidiffuse type in order to obtain a picture of uniform brightness.
The separation of the various screen surfaces into classes facilitates to a certain extent the analysis of the results from the standpoint of their practical application. The screens classified as specular in type, class A, are suitable in general for use in theatres where the maximum angle of observation is relatively small, less than 25° or 30°. The surfaces classified as semi-specular, class B, are suitable for use in an auditorium where the maximum angle is somewhat greater than in the previous case. For all cases where the maximum angle of observation is greater than 40° a screen of class C type should be chosen.
From a careful consideration of the subject it seems that the value of the ratio of the reflection factor measured at normal observation to that measured at maximum angle of observation in any particular installation serves as the most logical criterion from which to select the most suitable screen in any case. This value represents the ratio of brightness of the screen as observed by a person in the center of the auditorium to that of the brightness observed by a person occupying a seat at the side and near the front. By keeping this