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MANAGERS AND PROJECTIONISTS 492c
Following the same reasoning if the maximum angle of observation is 50° then only two screens of class B are suitable, while all of those of class C fulfill the requirement. There is little point, however, to discussing the requirements of a 50° maximum angle of observation since the geometrical distortion at 50° or greater is so enormous as to render practically useless seats placed at such angles from the screen.
The accompanying cuts, lettered from A to H inclusive, offer plain and somewhat startling proof of the necessity for careful selection of a screen surface to "fit" the auditorium.
Surfaces A, B and C are respectively plaster wall, cloth (Lockwood sheeting) and white pebbled wall paper.
In reading performance curves such as these you must remember that up and down represents reflection power at the various angles at which the screen surface is viewed, 100 presumably representing the reflection powers of the surface used for comparison in the tests, viz. : Magnesium Carbonate.
Surface A, kalsomine, page 484 is to all intents and purposes a perfect diffusing surface. With it the screen will look just as bright at a fifty degree angle as at a position straight in front of it — 0° angle. True there is just the least bit of drop in the "curve" line, but it is so slight as to be negligible. You will also observe that it has a reflection power of a little more than 80% when one stands straight in front of it, which drops to only a bit below 80% at a 50 degree angle. EXCEPT FOR CLOTH (Surface B) you will find that all surfaces having high diffusing power have pretty close to 80% for their straight-in-front reflection power. Cloth, B, Page 485. has a bit more than 60% for its straight-in-front reflection power, and drops to a bit less than that figure at 50° angle — that is to say, when you move to the side of the auditorium until you view the screen surface at an angle of 50°.
Kalsomine, therefore, is a much better screen surface than cloth, because it not only has a much higher power of reflection, but also a somewhat better diffusing power than has cloth, assuming both surfaces to be in good condition as to cleanliness, of course.