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2.5 foot candles is sufficient for the projection of a dense film on a dusky muslin screen; neither do I think that an intensity as high as 30 foot candles is desirable for a light film projected on a highly selective or spread reflecting screen. I know of cases where theatre managements have been forced, through the complaints of their patrons, to reduce the intensities they were employing from a value near the upper limit of the range I have given to a considerably lower one. If muslin or plaster screens are used, and these are faithfully kept clean, I believe an intensity of 5 foot candles should be ample for the proper projection of all except very dense films. If an aluminumized or a groundmirror screen is employed a value of 2^ foot candles should be ample. A great many theatres are projecting pictures with less than 3 foot candles, using a plain screen, while one of the largest and best known theatres is obtaining less than 10 foot candles on a plain screen.f The distinction between dense films and tinted and toned films is of interest. In the former, the density is due to variation in exposure and development and is not as a rule obtained with definite purpose; in the latter, the purpose is to lend color to the picture and to control its brightness. I believe that an intensity of 5 foot candles on any good, clean screen will be sufficient for the projection of any film sent out by a competent producer, and I also believe that no discomfort will be experienced from intensities in the neighborhood of 10 foot candles. In giving these values I presuppose the intensity due to the general illumination of the theatre to be of a very low order and assume the measurements to be made with the shutter open and no film in the machine. As I have previously mentioned, the effect of extraneous light in decreasing the brilliancy of a picture is very marked.
(Mr. Caldwell, by means of demonstration apparatus, showed various photographs and prints illuminated at intensities of 1, 5, and 25 foot candles. The demonstration showed that 1 foot candle was too low an intensity for good vision, that 25 foot candles was so high as to be glaring, and that color distinction and details were largely lost, and that at 5 foot candles the subject appeared to the best advantage.)
You may be interested in a device which has recently been developed which makes it possible to measure screen intensity and screen brightness very quickly and with accuracy.
In this meeting we are interested in projection apparatus only in so far as the component parts of the two systems, arc lamp and incandescent lamp, affect screen intensities. I shall discuss these two systems very briefly from this standpoint.
Figure 5 shows the arc lamp system as it is now and as it has been for a number of years. The light intensity at the screen with any system is of course dependent upon the light given off by the source, and with both arc lamp and incandescent lamp systems the light which the source gives off is dependent, but not in equal ratio, upon the current which flows, The intensity
tOptic Projection — ^J. A. Orange. See Bibliography.