International projectionist (Jan-Dec 1951)

B 298557 SRaanNmamnHH 'QTi PJjJj INTERNATIONAL PROJECTIONIST VOLUME XXVI APRIL 1951 NUMBER 4 Honeycomb-Condenser Lamp Optics The appended article, in two sections, comprises a translation from the German by Robert A. Mitchell of an article "Die Wabenkondensor" is Bild Und Ton (Bd. 3, Nr. 4, S. 127) and supplementary comment by the translator. This material is published herein in line with the long-established policy of IP in providing a forum for the free and full expression of views by anybody having anything interesting to say anent the projection process. Comment on the appended article is invited. By A. R. SCHULTZE Zeiss-lkon, VEB, Dresden CRITICAL examination of picture reproduction in motion picture theaters unfortunately often forces one to the conclusion that the quality of the projection leaves something to be desired. Even though the necessary picture brightness can be attained in all cases by means of modern mirror lamps, insufficient uniformity of illumination and of the color of the picture are ever-present defects, the latter being especially troublesome during the presentation of color films. The cause is to be sought in several factors which lie as much in the lamp optics as in the quality of the carbons used and in the attention given to the operation of the lamp. The claim is here advanced that the disadvantages of conventional lamps may be considered obviated by the honeycomb FIG. 1. Representation of an arclamp optical system using a concave mirror. condenser optical system. With this system the slight falling-off of illumination at the edges of the picture, the sensitivity of the lamp optics to trim-position and burning errors, as well as color variations in the picture with the use of Beck [high-intensity] carbons, are overcome. In the conventional mirror lamp the luminous crater of the positive carbon is imaged on the picture-aperture by the mirror, the path of the rays being shown in Fig. 1. Irregularity of Illumination Every irregularity in the crater and every alteration in the position of the crater image [the "spot"] cannot help but result in an irregularity in the illumination of the picture, especially when high-intensity carbons are used. Slantwise burning of the carbon shows up on the screen as colored corners and edges, the reason being that the crater, when viewed across the white gas-ball, has a yellowish to reddish color. And especially when low-current intensities are employed, the correct imaging of the crater and its position relative to the mirror must be maintained very precisely, and to a degree of accuracy not feasible in practice. Hence screen illumination free from defects is no longer attempted with currents under 40 amps because of the small area of the positive crater. These disadvantages are avoided with the familiar condenser-utilizing lamp optical-system, the so-called Kohler arrangement, because with this system it is not the picture-aperture but an interposed auxiliary condenser upon which the crater of the positive carbon is imaged. And the condenser, in turn, images the "principal condenser" [a large condensing lens analogous in function to a lamp mirror] upon the aperture. With this system, also called the "intermediate image formation arrangement," no influence upon the illumination is exerted by incorrect position or oblique burning of the positive carbon, as the image of the principal condenser, not that of the crater, bes on the pictureaperture. In consequence of the low efficiency of a condenser system, the application of this type of arc-imaging for projection in motion picture theaters has not succeeded until the present time. New Condenser a Requisite It might be supposed that we could at once set about applying this system to a mirror lamp, as shown in Fig. 2. But this is not possible because a dark shadow, originating from the carbon-holder and carbon which naturally lie between the FIG. 2. Optical system utilizing concave mirror and intermediate-image formation. INTERNATIONAL PROJECTIONIST April 1951