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BETTER THEATRES
August 2 1, 1943
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WORLD'S LARGEST MANUFACTURERS of PROJECTION LIGHTING EQUIPMENT
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THE STRONG ELECTRIC CORPORATION
87 City Park Avenue • Toledo, Ohio PROJECTION ARC LAMPS • RECTIFIERS • REFLECTORS
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MOTION PICTURE SOUND ENGINEERING
FROM THE WORKSHOPS OF THE HOLLYWOOD STUDIOS
■ This up-to-the-minute manual belongs in the kit of everyone working with sound. Containing 547 pages, it covers every method and process in sound engineering. Principles of electronic action are fully treated, equipment is minutely explained and profusely illustrated with diagrams, tables, charts and graphs. It provides a complete course in advanced sound engineering.
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QUICLEY BOOKSHOP, Rockefeller Center, New York
rectly the screen illumination? I believe that several points should be considered, but some other projectionists with whom I have discussed this matter seem to think that the center reading is all that is necessary. Can you prove to us which is right?
"Question 2 — What is considered a good distribution of screen light? That is, how bright should the sides be compared to the center?
"Question 3 — Is there ever a case in which the sides of the screen are as bright as the center?
"Question 4 — With the same arc current, is it always true that when the sides of the screen are focused brighter, the center becomes dimmer?
"Question 5 — Does the size of the opening in any given lens have any effect on the distribution of the light on the screen?
"I hope you can clear up these points for me once and for all as they have been the subject of many discussions."
These five questions are extremely interesting and I feel sure that they have been the subject of many friendly battles beside those in which you have taken part. Here are the answers in the order in which you gave the questions.
Answer 1 — The light on a screen is correctly designated by an average of at least three readings taken at the following points :
One reading at the center of the screen; and one reading at each side about 8 inches in from the edge and halfway between the top and bottom of the" screen.
In cases where a highly accurate average is required, seven readings are taken — one at the center, one at each side, and one at each of the corners. These seven readings are then averaged.
Three readings, however, are usually sufficiently accurate for general use. The average reading is computed by adding the foot-candle values of all the readings together and dividing by the number of readings taken.
To prove the inaccuracy of using the center reading alone to designate screen illumination, let's examine the possible combinations of error with some arbitrary figure— 15 foot-candles for instance — at the center of the screen. The following table shows the average light for five different conditions in which the light has a distribution from 30% to 80%, but a constant center illumination of 15 foot-candles.
Humiliation Per Cent
Average Distribu
in Ft. tion Candles
lumination in Ft. Candles
This clearly demonstrates how misleading the use of the reading for center illumination alone can be.
Answer 2 — A good light distribution is 70%, in which the light at the sides is equal to 70% of the light at the center.
Answer 3 — In theatre projection there