Transactions of the Society of Motion Picture Engineers (1916)

under (a and b) with the result that the light patch on the screen, the so-called circle of confusion, will have colored fringes, red towards, and blue away from the center of the field. The spherical aberration will add distortion to the image in the way explained above. The patch of confused light which appears on the screen in place of the image of the object point caused by the size of the light source and by the chromatic aberration, can be converted into a true image of the object point by the interposition of a (d) Projection Lens. Figure 6 shows the arrangement and also offers an explanation why in spite of the chromatic aberration of the condenser the image of the white object on the screen is white. We insert here that the image of the source must always fall in the aperture of the projection lens, which has to be large enough but need not be larger than necessary to pass all the radiation going from the condenser through the image of the source. The differently colored rays which intersect in the object point belong to white rays which fall upon different zones of the condenser lens. For instance, of the white ray from the source (i) a red component is refracted in the direction of the object point, while of a white ray (2) a blue component and of another white ray lying between (i) and (2) the green component leaves the condenser in the direction of an object point. This multitude of colored rays passing through the object point must, therefore, comprise all of the colors composing the light of the source and when gathered by the projecting lens and all brought to the same image point on the screen, form a white image'of the object point. It is very evident also, that the image on the screen is white only if all the colored rays which may possibly go through the object point are united in the image on the screen. If, for instance, the diameter of the condensers were too small to pass the ray (2), the image would appear reddish yellow instead of white, because the blue is missing, or if the diameter of the projection lens were to be too small to let the red ray pass the image of the point would be bluish. As long as we take care not to rob the pencil forming the image on the screen of any of its colored components, we shall have no difficulty on account of the chromatic aberration of the condenser. Of much greater influence is the effect of the spherical aberration. To obtain an even illumination of the screen, the section through the cone at the location of the stencil must be evenly illuminated. The cone of a perfect condenser offers sections with even distribution of light anywhere between the condenser lens and the image of the source — figure 7. There is a slight falling off of the illumination towards the margin for the same reason as stated at the beginning of this paper. A condenser which has spherical aberration will produce a cone in which the distribution of light is even only in sections near the condenser lens. The sections approaching the image of the source show a more and more increasing accumulation of light near the margin of the section because here the rays are more crowded together than in the central part of the field as may be seen in the figure. This is of no significance in lantern slide projection because the slide on account of its size is always located near the condenser, but very disturbing when, as in motion picture projection, the stencil is small and moved nearer to