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152 RICHARDSON'S BLUEBOOK OF PROJESTION
high, but the distribution is very poor. They are obviously suited only for use in long, narrow auditoriums. Of the surfaces measured, C and E best serve wide theatres. Surface B is still better for light distribution but quite low in reflecting power.
Fig. 48 sets forth ratios of distribution of reflection power. Observe that vertically at the left are figures from 0 to 100 which represent reflection power of the surface. The extreme left edge is the line representing the reflection power of the surface when viewed from straight in front of the center of the surface — in line with the axis of projection. There the reflection power reaches to 370, one hundred representing its brightness where the light is evenly distributed. This added brilliancy is taken from the margins, which drop, you will observe, very low.
At the bottom are figures representing angles of view, or "viewing angles." The slightly curving white line represents the reflection power of the surface at every possible viewing angle up to 50 degrees. If you follow the vertical line immediately over 20 until it meets the white line, it does so at the horizontal line extending to the left of 100. This means that if the screen is viewed at an angle of 20 degrees it will only be as bright, compared with brightness when viewed from straight in front, as 100 is to about 370, while at a 50 degree viewing angle the illumination will drop until the brightness, as compared with a straight front viewing angle, will be only in the ratio of 30 to 370. This indicates the condition known as "fadeaway."
Fig. 48 and table No. 6 shows the necessity of careful investigation of reflecting and distributing powers of surfaces before installing a screen. Projectionists and theatre managers should require the manufacturer to supply such measurements and to check through with an expert testing organization.
(25) A study of the foregoing tabulation makes it evident that, important as reflection power is, the distribution of light is of even greater concern.