British Kinematography (1952)

10 11 Rill SH KINKMATOGRAPHY Vol. 20, No. 1 tive ly small angular movement of the eyes will bring the screen surround into nearly direct view. Therefore, unless the surround brightness has been properly adjusted, the viewer is faced with a considerable variation in adaptation brightness which can do nothing but detract from his pleasure and comfort by providing an undesirable visual environment. When designing lighting, it is necessary to consider the various characteristics and requirements of the visual task. While the viewing of projected pictures usually involves prolonged periods, the task involves some factors that are different from those pertaining to other tasks such as reading. Table I The Visual Size and Area of a Screen when Viewed at Various Distances Viewing distance Angular subtense in screen of screen degrees widths Width Height Solid angle subtended Per cent by screen of steradians visual field w 53.1 41.1 0.75 15.0 2W 28.1 21.2 0.19 3.8 3W 18.9 14.3 0.083 1.7 4W 14.3 10.7 0.047 0.94 5W 11.4 8.6 0.030 0.60 6W 9.5 7.2 0.021 0.42 1W 8.2 6.1 0.015 0.30 %w 7.2 5.4 0.012 0.24 Much of the information of the story is obtained by words and the gestures, facial expressions and actions of the performers. Therefore, visual acuity is less important than the discrimination of a wide range of brightness contrasts. The viewer is not confronted with the problem of resolving small details near the threshold in size. However, while discrimination of the characteristics of the visual task may not be critical, the eyes and attention are focused steadily with but brief respites. Adaptation Brightnesses In any specific situation, the desirable surround brightness is dependent upon the brightness level to which the eyes are adapted. Therefore, it is necessary to determine the relationship between the picture brightness and the surround brightness. However, the former varies over a considerable range, depending upon projection equipment, theatre and screen sizes, film characteristics, etc. It may range from a very low level for the opaque projectors used in educational work to the high levels obtained with slide projectors. Nevertheless, it is possible to develop a concept in terms relative to the screen brightness obtained with the projector running without film. Furthermore, since a motion picture, or any sequence of projected still pictures, presents a continuously variable brightne-s pattern, it is difficult to arrive at any specific brightness that can be considered representative of all conditions. One method is to record the variation in integrated or average-picture brightness for a typical film and to determine the mean brightness over an extended period of time. However, the range of average brightness, especially the minimum values, are of importance. A typical record for a black-and-white him is shown in Fig. 2. The film used included photographs of almost completely white areas to extremely dark night scenes and can be considered to be representative. In order to make the record of Fig. 2 more universal in its application, the ordinate is shown as per cent, of clear-screen brightness. Thus, it is a simple matter to convert the relative values to actual brightness. For example, in terms of the clear-screen brightness, the maximum average brightness recorded for the lightest scene was about 25%; the minimum brightness was 1.0 ; and the mean value for the entire film was about 10%. Therefore, when the clearscreen brightness is 15 ft-L, the picture brightnesses are approximately 3.8 maximum and 0.15 minimum with a mean of 1.5 ft-L. It is interesting to note that these values compare favourably with those obtained by Logan. A similar record taken with an industrial colour film gave a mean brightness of about 5% of the clear-screen brightness with maximum and minimum values of 16% and 1%, respectively. For a clear-screen brightness of 15 ft-L, these brightnesses are a mean value of 0.75, a maximum of 2.4, and a minimum of 0.15 ft-L. Obviously, there will be quite a wide variation among