Transactions of the Society of Motion Picture Engineers (1916)

the image of the source. If we place the stencil so that its diagonal is equal to the diameter of the cone, we shall have its corners in the marginal concentration of the light and, therefore, an uneven illumination of the stencil (see figure 7). To avoid this, the stencil has to be moved towards the condenser until its whole area lies in the evenly illuminated part of the cone. This means, of course, the loss of a very great percentage of light. The mere fact that a condenser has spherical aberration does not entail any loss of light if we are satisfied with an uneven illumination as long as the aperture of the projection lens is large enough to receive all the light passing through the image of the source. The sharpness of the image on the screen is not influenced by the spherical aberration of the condenser, if only the illuminating system is so centered that the image of the source, the spot, lies symmetrically to the center of the projection lens. If a slight decentration of the spot, without the spot getting outside of the aperture of the projection lens causes a deterioration of the image on the screen, the fault lies in the projection lens. Another factor of influence upon the efficiency of a given combination of light source, condenser and projection lens is the size of the source and the location of the mat forming the border around the picture, the so-called aperture plate of the apparatus. The actual amount of light traveling from the condenser to the image of the source is embraced in a cone, the base of which is the condenser and the apex the image of the source. If the source is not point-shaped, the cone will be truncated. We consider first a point-shaped source. If the stencil is of rectangular shape and its diagonal equal to the diameter of the light cone at the position of the stencil, the four segments of the illuminated source outside of the stencil do not contribute to the illumination. We shall refer to this in the following as diaphragm action (I). This is perfectly self-evident and there is no remedy for this loss unless we make the stencil circular. See figures of the half rectangles inscribed in the sections through the lower cone. Rectangular condensers have been suggested and are resuggested now and then, but instead of saving light, they cut out the unused light at a diff"erent plane of the optical system. They serve no purpose and are more expensive to make and to mount than the ordinary round condenser. In case of a source of extended area the aperture plate does not only cut off" these four segments, but also screens off" parts of the condenser or of the light source in such a way that, while the radiation from the central parts of the source, which fills the condenser aperture, reaches the image of the source and, therefore, is utilized, the radiation from the extra axial points of the source, although it passes through the condenser, only partly reaches the image of the source and contributes to the illumination of the object point. We shall call this in the following diaphragm action (II.) To simplify the drawings the light source and the aperture plate in the following are assumed to be circular. Figure 9 shows the path of the light rays from the center and from the two ends of the source as they pass through the condenser towards the image of the source s and s' are two conjugate points of the source and its image and the rays 52