International projectionist (Jan-Dec 1935)

EFFECT OF APERTURE LENSES ON ILLUMINATION Wilbur B. Rayton THE illumination of a projected motion picture image for any given combination of light source, light collector, and projection lens is a subject so complex that any approach to full comprehension of all its details can be attained only after prolonged study. This fact may be responsible for the proposal of numerous schemes for improving illumination, either by increasing its total quantity or by improving its distribution, that at first sight look plausible, but do not always work out as expected. It is to the application of one such proposal in the projection of standard 35-mm. film that attention is here directed. It is a suggestion the principal aim of which is to improve evenness of illumination. Since relatively few have occasion to keep in mind all the details of motion picture illumination, it seems advisable first to outline briefly the two types of illuminators in common use. The ideal illuminator for motion picture projection would be a light source of uniform and sufficient brightness and as large as the aperture in the film gate, but not so hot as to damage film when placed practically in contact with it. If such a light source were available, a motion picture projector would reduce to the form shown in Fig. 1. Illumination would be even over the area of the picture except for two facts, one of which we shall ignore, and the other of which is demonstrated in the figure. Reversing the Light Path It is much more convenient in studying this problem to think of the light sometimes as traveling from the screen to the film; at other times it may be more convenient to think of it as traveling from the light source to the screen, in the normal manner. The reversibility of a light path makes such a procedure entirely justifiable. At present we shall Projection optics is the most generally misunderstood link in the reproduction train and the topic of much misinformation. The accompanying paper, originally read before the S.M.P.E., reflects the opinion of an acknowledged authority on optics on a means for improving evenness of screen illumination. think of it as traveling from the screen toward the light source. In Fig. 1 two beams of light are shown, one focusing at the center and the other at the corner of the aperture. The one that focuses at the center of the film aperture fills the front of the projection lens, but the oblique beam does not. Any light belonging to the latter beam that falls on the front lens higher than the upper ray shown will be stopped by the mounting of the rear lens. In a few cases there may be none of this vignetting of the oblique beams, but it is generally found in lenses of any considerable length. This reduction in the effective aperture of the lens for oblique beams results in a reduction of brightness at the margin as compared with the center, even if the source of light were ideal, as here assumed. Unfortunately, no source of light even remotely approaches such conditions. Available sources that are bright enough are not only tremendously hot, but are also too small, so the science of optics is called upon to make the available sources of light serve our purpose. To get away from the danger due to heat, : it is obvious that it is necessary to move the source of light farther from the film. To overcome the difficulty due to the insufficient size of sources the wellknown optical law is applied; stating, that by means of a collective element, such as a condenser lens or a concave mirror, a source of light can be made to behave as though it were of any desired size, without any more than a minor reduction in its intensity. With a given light collector, the maximum size the source may be made to appear is the diameter of the collector (condenser lens or mirror) . In practice it is not always possible to make the light source appear as large as desired because practical considerations limit the size of the light-collector. The laws of optics set a definite limit to the size attainable in condensers and mirrors of a given focal length; and, generally, the laws of economic limitations become effective before the optical limitations do. Larger condensers and mirrors could be made and some improvement in illumination achieved thereby, but the cost would become prohibitive. Since light collectors can not be made as large as they should be made, certain undesirable consequences follow, which will now be examined briefly. There are two cases. The simplier one is presented by customary practice in reflector arcs and will be considered first. Fig. 2 represents an assembly of a 5-inch projection lens of the Cinephor type, a standard aperture, an arc with an 11-mm. positive carbon, and an elliptical mirror 11.5 inches in diameter. The arc is imaged in the plane of the film or very near it. To study this case it is helpful again to consider the light Right: FIGURE 2 PROJECTION LENS FIGURE 1 APERTURE LIGHT SOURCE [18]