Transactions of the Society of Motion Picture Engineers (1916)

Record Details:

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usually a decided appearance of flicker on the screen. With a gasfilled incandescent lamp on an alternating current circuit the light is steady and therefore a three-wing shutter can be used with the result that the frequency of cut-off of a picture is sufficiently high so that there will be no noticeable flicker. In the use of wide aperture objective, to secure as much illumination on the screen as possible, the shutter should be considered. If the shutter is not adapted to the wide aperture lens, it will not cut off the light from the screen during the period of picture travel and there will be the appearance of a travel ghost. This may be seen especially when titles are being shown as it tends to cause a stringing out or fuzziness of the tops and bottoms of the letters. Most of the shutters for machines have been designed for use with the smaller lenses and the wings are not wide enough to take care of the wide aperture lenses. If the shutter diameter remains unchanged and the lens is very greatly increased, the angle through which the shutter must move in order to cut off the light from the screen is decidedly increased, thus resulting in a slower cut-off and a corresponding reduction in the time that the full lens aperture is utilized while the film is stationary. The use of a shutter with a diameter correspondingly larger is desirable where a wide aperture lens is used, though in most cases such a change in the shutter size cannot be made conveniently. The only thing which can be done to minimize the difficulty is to proportion the wings of the shutter so as to eliminate the travel ghost and at the same time get as much light on the screen as possible. SCREEN ILLUMINATION The useful light which is delivered to the screen is the final test of the effectiveness of any system of condensers and lenses which may be used. The object is always to produce the brightest and most uniform screen possible with a given source of light. It is no disadvantage to have the illumination in the center of the screen somewhat greater than near the edges because the greater part of the action in the pictures is well toward the center; however, there should be sufficient light at the edges to show up details and the screen should not be so uneven that it is noticeably brighter in the center than at the edges especially when titles are being shown. If the difference between the center and the edge illumination is too great, titles will show up clearly in the center and will fade off toward the ends of the lines so that it is difficult to read the complete message. A typical screen illumination is shown in Fig. 6. The figures in the center of each square represent the actual foot candles on a small screen having an area of 27.75 square feet. The total illumination delivered to this screen is 693 lumens. The intensity of illumination in the corners is approximately two-thirds the intensity in the central portion of the screen and this ratio is not too great for the satisfactory projection of pictures. The color of the light delivered by an incandescent lamp has a warmer tone than the light from a carbon arc and therefore the pictures do not have that hard chalky appearance which is sometimes noticeable in pictures projected by high-powered arcs. As the illumi 43