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246 PROJECTION SCREENS COMMITTEE [J. S. M. p. E.
The lower limit of screen brightness should therefore be determined by the light reaching the screen from the auditorium. There is no criterion for the maximum desirable amount of illumination corresponding to the highlights. We do know, however, that with the auditorium in a darkened condition it would not do to have too bright a screen as this would be physiologically harmful.
Desirable screen brightness is dependent on all these variables. Only by analysis of judgments drawn from many observers subject to varied, controlled conditions will it be possible to determine the optimum relations.
In an endeavor to obtain more information on this subject, we conducted our tests at the meeting of the New York Section. This meeting afforded an excellent opportunity in as much as there was present a body of trained men who would readily understand our aims. We did not expect conclusive results from our tests, but regard them as a preliminary step in the investigation. Obviously, a complete study of all the factors would require the time of many men over a period of months.
In these tests we used two projectors, one a hi-lo and one a low intensity arc, setting these to produce known values of screen illumination. Two types of arc were employed to determine whether different color characteristics affect the amount of light judged desirable. There was no illumination in the auditorium other than that supplied by screen reflection. It would have been interesting to vary the lighting also, but the time at our disposal necessitated restriction of the variables. Two reels of film were used, one with a large percentage of brilliant scenes in it, such as outdoor shots, the other consisting of interiors, emphasizing human features and shadows. We wished to learn whether different amounts of light would be found desirable for different types of subject-matter.
The arc light intensity was varied by means of wire filters inserted in the projection machine behind the condenser lens. Four settings were used. The first setting was 68 per cent of the maximum, which was the second setting. The third and fourth settings were 50 and 25 per cent, respectively. The low intensity machine was first used for both reels and was followed by the hi-lo intensity arc. There were present 61 observers, most of whom commented on the projection on questionnaires which were distributed among them. Their findings are summarized in the following table. The brightness values are without film and with the shutter running.