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1954-55 Theatre Catalog, 12th Edition, Page 313 (277)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 313
Page 313

1954-55 Theatre Catalog, 12th Edition, Page 313

FIGURE 2 illustrates the stray-light effects on the tone reproduction of the screen image of a reversal color print, including the reaction of the theatre audience to the varying conditions.

observed when the print is viewed in front of an illuminator. The other curves show the degradation in tone reproductionAwhich occurs to the picture on the screen as the level of stray-light brightness is increased to 0.3 per cent, 1 per cent, 3 per cent, and 10 per cent of screen brightness.

The general location of shadow, midrange, face and highlight regions in an average scene have been added at the bottom of the graph to make it easier to locate these positions on the tone reproduction curves. It can be noted from studying these curves that stray light lowers the density in the dark shadow areas of the screen image by a much greater amount than in the lighter regions represented by the face and highlight densities. The higher the percentage of stray light reflected by the screen to the audience, the less pleasing and crisp the picture looks upon the screen. Audience reaction to stray light is recorded in Figures 1, 2, 3, and 4,

Tone reproduction curves similar to those on the previous graph were obtained (Figure 3) when the straylight brightness was held constant at 0.1 ft-L, while the screen brightness was varied from 20 ft-L to 1 ft-L. This is the screen-brightness range found in the screen-brightness and stray-light survey of theatres and drive-ins. It is important to note that it is necessary to consider both the stray-light level and the screen brightness in specifications relating to the projection of good quality films.


A rough reproduction of this experiment might be tried in your own home using a projector such as a 35mm Kodaslide projector equipped with a 6inch f/3.5 projection lens and a 200watt, 120-volt lamp. A 28-by 42-inch piece of white matte paper, such as art board or a white blotter, can be used for the screen. Do not use a beaded or aluminized screen. Obtain a lot-watt, 115-volt frosted light bulb and place it about 10 feet in front of the projection screen in such a manner that it will illuminate the screen without shining in the viewers, eyes. This lamp will now produce about 0.1 ft-L of stray light on the screen. Next, locate the projector so that the slide image fills the screen. This will produce a screen brightness of approximately 20 ft-L. Make a series of holes in black or opaque cardboard, graduating the size of the holes from one slightly leSS than the diameter of the projection lens down to a hole the size of a lead pencil; when one of these holes is held in front of the projection lens, screen brightness is reduced. When the small hole of lead-pencil size is placed over the center of the lens, the screen brightness is reduced to approximately 1 ft-L. If you project several of your favorite slides under these conditions, the importance of adequate screen brightness and the degrading effect of stray light will become obvious.


The 0.1 ft-L stray-light brightness on the screen was selected because it has been related by some people to the statement that 0.1 footecandle of miscellaneous illumination in an auditorium is not harmful to the quality of projected pictures. Usually, these articles make reference to a paper written by Loyd A. Jones and published in the 1920 Transactions of the SMPE.* In quoting this paper, many people have used his recommendations without qualification and haVe neglected to state that the information Jones reported was based upon data collected at a screen brightness level of approximately 20 fteL. Furthermore, Jones shielded his auditorium lights so that only 0.02 to 0.04 ft-L of stray light were reflected by the screen as a result of the auditorium illumination. Therefore, the amount of stray light on the screen, which resulted from the level of indirect illumination approved by Jones, represents approximater 0.2, per cent of the screen brightness.

Figure 3 is the result of a carefully controlled experiment with greater refinements than the suggested home experiment. It should be noted that the picture quality, represented by the shaded areas, as well as the tone reproduction, denoted by the curves, drops

when the level of screen brightness is decreased from 20 ft-L to the lower limit of 1 ft-L in the situation where stray light is held constant. The figure 5 ft-L of screen brightness is important to classroom users of standard 16mm incandescent projectors because little more than 5 ft-L can be expected on a matte screen from this projector equipped with a 1000-watt lamp, even when the screen is only 10 feet Wide.5 Increasing the picture size to 20 feet drops the brightness to approximately 1 ft-L. Higher brightness levels or larger pictures can be obtained with carbon arc projectors, but here again for 16mm pictures. the 5 ft-L level is reached at a picture width of about 20 feet. These curves illustrate the importance of maintaining an adequate level of screen brightness and the rapid increase in stray-light problems which occurs whenever the picture size is expanded beyond the light-producing capacity of the projector for either 16mm or standard 35mm theatre equipment.

Good picture quality on the screen is important to all who are concerned with the production and processing of films. This includes film and equipment manufacturers as well as those who labor toward bringing to the projection booth the best obtainable motion picture print.
1954-55 Theatre Catalog, 12th Edition, Page 313