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160 RICHARDSON'S BLUEBOOK OF PROJECTION
ers' eyes and detracts from the enjoyment of the picture. Again, when placed too close to the screen the theatre patron must cover an excessively wide viewing angle, which is very hard on the eyes. He is also subjected to a foreshortening effect as illustrated in Fig. 50, if seated in one of the end seats.
(47) As screen image dimensions are increased the amount of light must also be increased to maintain the brilliancy of illumination.
(48) A study of percentage factors of illumination for various sized screens reveals a startling differential which must be counterbalanced by a proportional increase in light energy. The accompanying table tells the story at a glance. Let us take a screen size of 10 X 12 feet as a unit of measurement. We will assume that the illuminating factor of this screen under proper conditions is 100. If the same amount of light is projected on screens of increasing sizes the drop in illuminating values is as follows :
10 X 12 = 120 sq. ft. 100 percent
12 X 16 = 192 sq. ft. 39
12.75 X 17 = 216.75 sq. ft. 34
13.50 X 18 = 243 sq. ft. 29
14.25 X 19 = 270.25 sq. ft. 27
15 X 20 = 300 sq. ft. 25
(49) Moreover, increase in picture dimensions require a very rapid magnification of the film photograph. (50) The linear magnification may be determined by dividing the width of screen image, in inches, by width of the projector aperture.
(51) The larger the screen the more quickly defects in the film will be sighted by the audience, if a brilliant illumination is maintained.
(52) There is no advantage gained from mere size or from brightness of illumination unless it is required by the conditions of the theatre. A very bright picture is unquestionably very hard on the eyes. Subdued illumination may be just as hard because of the straining after the darker details in the screen image. A properly illuminated 16-foot picture has better visibility than an