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January, VJo'l
GUTH: SUKKi)i:\l) brightness
II
films. However, these brightnesses appear to be within the range of what is obtained in representative theatres.
EXPERIMENTAL ARRANGEMENT
In order to isolate and to control independently the brightness of the various areas in the visual field, the experimental arrangement illustrated in Fig. 3 was adopted. This is a modified scale model of a theatre in which 1 in. equals 1 ft. The screen was 20 in. wide, thus corresponding to a 20-ft. screen. The observers were located at a distance of six times the screen width from the plane of the screen, or a distance of 120 in. Immediately surrounding the screen was a transilluminated diffusing glass, the brightness of which could be adjusted by
In the present investigation, which was intended only to be exploratory, clear-screen brightnesses ranging from 1.1 to 60 ft-L were used. These include brightnesses that are obtainable with various types of projection equipment such as highly efficient projectors used at a relatively short projection distance, opaque projectors, slidefilm projectors, etc. The brightnesses were obtained with a standard 16 mm. projector in which were used lamps of 200, 300 and 750 W for screen brightnesses of 1 1 , 25 and 60 ft-L, respectively. By means of a neutral-density filter, these brightnesses could be reduced to one-tenth of these values for a lower range of 1.1, 2.5 and 6 ft-L. These clear-screen brightnesses corresponded
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Fig. 2
A record illustrating the variation in integrated or average brightness of a typical motion picture in terms of the clearscreen brightness.
the observers. This area corresponds generally to the area on a stage surrounding a motion picture screen and is termed the screen border. Between the observer and the screen was a panel, the brightness of which could be independently controlled. The observer viewed the projector screen through a rectangular aperture in the panel. This aperture was of such a size that the transilluminated screen surround could be seen by the observer. This arrangement enabled control of the two surround brightness without permitting any stray light to reach the screen. While this experimental arrangement does not duplicate exactly the visual situation of a theatre, it is considered to be sufficiently typical for the present purposes.
to mean picture brightnesses ranging from 0.1 to 6 ft-L,
The observers viewed the motion picture and, for each value of clear-screen brightness, adjusted the brightness of the border until they deemed it most desirable for viewing the projected picture. Their judgment was based upon viewing comfort and upon the appearance of the projected pictures. A group of five observers made a series of five observations on each of two sittings for the various screen brightnesses. Each observer was permitted as long a period to make each observation as he felt necessary. Each series of observations included representative portions of the motion picture.