Society of Motion Picture Engineers : incorporation and by-laws (1919)

If now the eye is placed near the screen and looks back at the objective one sees a bright spot of Hght which may or may not fill the entire area of the objective. In order to do this with the arc or incandescent lamp burning at full brilliancy it is necessary to use either smoked glasses or to place a piece of heavily smoked glass between the light and the condenser or just in front of the condenser. The last position is preferable as it enables the observer to obtain a general view of the apparatus at the same time that the size of the light spot is observed. The result of such observations* is shown in Figs. 10-12. The apparatus used in the experiments here described is illustrated in Fig. 9. The different parts are the same as shown diagrammatically in Fig. 4 and consist of an incandescent lamp backed by a spherical reflector. The condenser serves to concentrate the light from the lamp upon a standard aperture. The objective serves to form a real image of a picture located at the plane of the aperture upon a screen in front of the apparatus. In the apparatus as illustrated a movable screen has been placed in the position occupied by the condenser image. With this set-up, which is used in testing all condensers, it becomes readily apparent that if the condenser contains an optical defect so that light from some portion of it does not pass through both the aperture and the objective the image will show a shadow at this point. Condensers which do not show all of the rings properly functioning are eliminated. The tests illustrated in Figs. 10 to 12 were made by setting a camera with a telephoto objective of 50'' equivalent focus (x8) about 20 ft. in front of the apparatus. Two exposures v/ere necessary, a 5-minute exposure to show the general outlines of the apparatus and an extremely short exposure to show the light spot coming through the objective. In Figs. 10 and 12 is illustrated the larg<e objective with 2Yg-'' free opening and in Fig. 11 is shown the smaller objective free opening. Both are marked 53^" equivalent focus, and are supposed to be identical except in diameter. Figs. loa and iia show the appearance with the camera objective in the direct axis of the beam, that is, located in the position of the exact center of the picture. The separation between the condenser and the aperture is 634" a-S is called for in the theoretical design for this combination illustrated in Figs, i and 2. Note that in the cases of both loa and iia the entire area of the objective is filled with light, and hence both objectives are contributing as much light to the center of the picture as they possibly can but that on account of its greater area, the larger objective is able to contribute more light than the smaller one. In Figs. lob and lib the same separation is used, but the camera was so placed that it was at the extreme edge of the picture, i.e., if it were even one inch farther towards the side, no light from the lamp would reach it. This shows both the large and small objectives still completely filled with light. If instead of using the corrugated condenser, as previously illustrated, it is desired to use the ordinary plano-convex condensers, a separation between the front of the condenser and the aperture plate of much more than the 63^2'' previously referred to is 72