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HANDBOOK OF PROJECTION FOR
as shown in the ratio column under 20°, table 3. Likewise the reflection factor for this screen at 30° is 71. Dividing 414 by 71 we obtain 5.83 as shown for that screen in the 30° column of table 3.
This value then in all cases gives the ratio of the screen brightness when viewed along the axis of projection to the brightness as observed at the angle indicated at the top of the various columns. These numbers are valuable in determining how great a difference in picture brightness exists between the seats along the center of the auditorium and those at the extreme sides. For instance suppose that in the par
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SURFACE F
ticular theatre under consideration the extreme angle of view is 40°. Now if screen No. 12 were installed in this theatre the picture as seen from the seats at the maximum angle would have a brightness only 1/7 as great as when the picture is observed from a seat on the center line of the auditorium.
The values tabulated in the columns designated as R give the mean (or average) value of reflection factor within the angle as indicated at the top of the column. Thus for screen No. 8 the average reflection factor for all angles out to 20° is 306. Now since it is desirable that there be as little change