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1945 Theatre Catalog, 4th Edition, Page 309 (285)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 309
Page 309

1945 Theatre Catalog, 4th Edition, Page 309

The Requirements of Projection lighting

Screen Size Determines the Type of Lamp Providing the Most Pleasing Illumination

The selection of projection lighting equipment best to meet the requirements of the various types of theatres is determined by the area of the screen which has previously been established by the style and size of the theatre itself.


In a survey by the Subcommittee on Screen Brightness of the Society of Motion Picture Engineers, it has been determined that the most pleasing projection, with a minimum of eyestrain, is attained with a screen brilliancy of between 9 and 13 foot lamberts as measured with the projector running, but with no film in the projector.

Candles, Lumens, and Things

For convenience in calculating, let us take the even figure, 10 foot lambert, as the desirable brilliance, and further assume the screen to be in perfect condition and possessing 100 per cent redectivity. This condition would require 10 foot candles projected to each square foot area of screen.

However, since the reflective efficiency of a screen is a variable factor beyond his control, the projection-lighting engineer prefers to calculate his equipment in terms of foot candles of projected light rather than in foot lamberts or screen brilliancy.

Accordingly, the light output of the projection system is generally measured in terms of lumens. The number of lumens which are projected to the screen surface have no real meaning until we associate them directly with the area of the screen surface that they are intended to cover.

Since various projectors, with their particular shutter designs, vary between 40 and 50 per cent in their ability to pass light, it becomes desirable in the calculations to eliminate these variables. For simplification, the projection-lighting engineer measures the amount of projected light when the shutter is not running and there is no film in the projector.

Since the average projector passes only 50 per cent of the light from the lamp and the average screen is only 75 per cent eiiicient as a refiector it becomes necessary to produce about 25 foot candles in the lamp so as ultimately to obtain the desired 10-foot lamberts at the screen when the projector is in actual use.

Actually, the lamp must, therefore, be capable of producing about 2% times as much light as seems necessary on first thought. If we are to secure a screen brilliancy of 10 foot lamberts, we must, therefore, produce 25 foot candles of

light to the film for every square foot of the screen area.




President, Strung Electric Corporation

Light Volume and Screen Area

It is interesting here to note how this required light volume must necessarily be increased as the screen area is enlarged. In other words, if we had only a 1 square foot of screen, we could secure 10 foot lamberts with a projection lighting system producing 25 lumens.

A 12-foot screen has 105 square feet of area and accordingly would require 105 times 25 lumens, which is 2,625 lumens of light necessarily projected to the film to attain the pre-determined brilliance of 10 foot lamberts.

An 18-foot screen has 236 square feet of area and consequently requires 236 times 25 lumens or 5,900 lumens of projected light to obtain this same level of screen brilliancy. Likewise a 24-foot picture has 419 square feet of area and necessarily requires 419 times 25 lumens, which is 10,080 lumens of light to achieve satisfactory screen lighting.


Since it is uneconomical to build a single lamp that is adjustable in light capacity to meet this wide variance in light requirements, the projection lighting equipment manufacturers have developed a series of lamps in a wide range of

capacities and prices to meet the pocketbook and satisfy the lighting requirements of various size theatres.

Small Theatres

For clarity, we will first discuss the small theatre of 200- or 300-seat capacity with its 10-foot screen, theoretically requiring 820 projected lumens.

Since this theatre must operate at the very lowest cost of current and carbons, the low-intensity arc, projecting 2,200 lumens, is often used, despite the fact that the quality of this low-intensity light is slightly yellow in color and, accordingly, not entirely suited to good projection. Operating cost, and not quality of projection is the controlling factor in this instance. However, if the volume of patronage allows, or the audience is inclined to be critical of the quality of the projected picture, even these small theatres should install nothing less than the 1-kw. high-intensity arc, projecting 4,200 lumens, and resulting in a projected picture of a color and quality comparable to that found in the deluxe houses.

The low-intensity lamp employs only low cost pure carbon electrodes. Accordingly, the carbon cost is but 3 cents an hour, and, since the electric current consumption is low, the over-all operating cost is only 13 cents an hour. This lamp requires only an economical 2-tube rectifier.

THE LOW-INTENSITY LAMP, using pure carbon electrodes, generally operates between 20 and 30 amperes; with a potential drop across the arc of 50 to 55 volts. While the light from this lamp is slightly yellow, it does throw 2,200 lumens of light on the screen and is the most economical of all proieciion lamps to operate, and, in theatres up to 300 seats, has proved a satisfactory illumination source. (Strong Electric Corporation photograph.)
1945 Theatre Catalog, 4th Edition, Page 309