Transactions of the Society of Motion Picture Engineers (1916)

The utilization of the radiation not intercepted by the lens was attempted many years ago. About 1840, Prof. Petzval described a construction which I feel should be given the publicity it deserves, because it marks the limit of what can be accomplished in this direction. I was not able to refer to Petzval's original description and had to use a diagram in Hugo Schroeder's Elemente der Photographischen Optik. There is no interpretation of the diagram given and if the following should not do justice to the construction, I have to apologize to Prof. Petzval. In figure 13, s is the source placed in the center of curvature of a hemispherical mirror, which throws all the light radiating in the direction away from the condenser back into the source. At its equator the hemispherical mirror is joined coaxially by a paraboloid which collects the rays of the solid angle B and sends them in a direction nearly parallel to the optical axis towards the condenser lens which forms an image of the source at s\ All the light radiation within the solid angle B passes after reflection by the mirror through the condenser lens to the image point j' while the light radiating in the remaining solid angle d passes through the condenser lens directly, forming another image of the source at s\ The drawback of the construction lies in the technical difficulty of producing a perfect paraboloid and joining the two mirrors so that no shadow is cast within the cone and in the fact that two images of the source are produced. A paraboloid with badly-formed surface and also a decentration of the two mirrors are apt to cause uneven distribution of light in the cone even when the stencil is located near the lens. This apparatus may work well when used for lantern slide projection, but hardly for M.P. projection. Whether its efficiency is greatly superior to a good condenser made up of lenses in the usual way is somewhat doubtful if we consider the loss by reflection on the parabolic mirror and the fact that the greater part of the radiation is reflected by the hemisphere and then reflected by the parabolic mirror and, therefore, will suffer this loss twice. Because of the similarity with Petzval's construction, we may mention here another one, which has been quite in vogue as an automobile headlight a few years ago and which is almost identical with the proposal made by Mr. Jenkins at the Rochester meeting in April, 191 8. It contains (Fig. 13) a small condenser in whose focal point the source is placed and the usual arrangement of the concave mirror. The angle taken in by the condenser B and which is, at least, geometrically speaking, doubled by the mirror, amounts to about 50 degrees. This is thrown forward by the condenser in a beam of parallel rays by the condenser. A paraboloidal reflector is joined coaxially with the spherical mirror, so that its focal point coincides with the focal point of the lens and the center of curvature of the spherical mirror. This paraboloid intercepts the greater part of the radiation which is not used by the lens and spherical mirror and like the condenser lens, projects it forward in a parallel beam. By adding a condenser lens of the diameter of the reflector, an image can be formed of the source not far from the condenser and the combination be used for projection work. A similar effect can be produced by moving the source and the center of the spherical mirror outside of the paraboloid 58