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

front condenser lens. A standard motion picrtufe projector aperture plate is mounted on a metal plate and is placed at the crossing point of the rays, hence at the point of greatest concentration of the condenser beam. This experiment shows that, with the aperture located at or ahead of the point of greatest concentration of the condenser beam, the beam does diverge beyond the aperture. In Fig. 4 we have still further proof of the divergence of the beam under the conditions named. Fig. 4 is the same as Fig. 3, except that the metal plate has been removed and the projection lens set in place. We have thus shown that the ray does diverge beyond the aperture when the aperture is at or ahead of the point of greatest concentration of the condenser beam, and that the divergence is in exact proportion to the diameter of the condenser and inversely proportional to its distance from the aperture. There is, however, a theory still tenaciously adhered to by some that the point of greatest concentration (often incorrectly called the crater image — incorrectly, because, due to the fact that the crater is neither a flat surface or parallel to the lens, it, the image, actually is in focus in its various parts through a considerable length of the condenser beam) of the condenser beam should be advanced to a point between the factors of the projection lens, as is the correct practice in stereopticon projection. Those who have given the matter exhaustive study, from the practical as well as the Fig. 5 A B AREA OF 2.75" CIRCLE 5.9 SQ.IN. AREA OF 1.5" CIRCLE L76 SQ.IN. LOSS OF LIGHT AT X 87% 13% UTILIZED LOSS OF LIGHT AT"B"57% 43% UTILIZED Fig. 6 .45