International projectionist (Jan-Dec 1954)

cracking due to the heat of the arc. When used in conjunction with the Wabenkondensor the side-to-center distribution of these lamps is 88% 90%. The theory of the Wabenkondensor was discussed in our translation and commentary beginning on page 5 of the April 1951 issue of IP. Now that the Wabenkondensor has undergone further development and has enjoyed wide and successful use in the field, a review of this ingenious optical system is again in order. The Wabenkondensor In principle, the Wabenkondensor is a special application of the Koehler intermediate-lens system used in German portable projectors to provide perfectly smooth screen illumination with incandescent lamps. Fig. 5 illustrates the Koehler system. When an ordinary condensing lens is used, a more or less fuzzy image of the bulb filament is focused upon the projector aperture, and the field of illumination on the screen is accordingly marred by traces of the filament image. The Koehler system eliminates these inequalities of light by intercepting the beam with a second lens which forms an image of the evenly illuminated condenser upon the aperture. Now, the center of the positive crater of a high-intensity arc is considerably brighter than the edges, the brightness-difference increasing with decreasing current. For example, the brightness of the edges of the crater is but 65% that of the middle at 50 amps., and only 45% at 25 amps., the lowest practical current for HI arcs. When this type of luminous crater is imaged upon the projector aperture, as by a conventional arc-lamp, we get a field illumination on the screen which is bright in the center and dim at the edges. Secret Is "Honeycomb" This disadvantage of HI projection could be overcome by the Kohler arrangement, imaging the uniformly illuminated mirror upon the projector aperture by means of a lens placed in the light-cone of the lamphouse. But, unfortunately, the hole in the center of the mirror, as well as the interposed positive carbon holder, would cast shadows upon the aperture, making the light even more uneven. To make the Koehler principle applicable to the mirror arc, therefore, a new kind of lens had to be devised — the "Honeycomb" (Waben) condenser. Fig. 6 is a diagram of the Wabenkondensor setup. The ordinary imageforming lens of Fig. 5 is replaced by two "lens-array plates" {Linsenrasterplalten) which consist of a large number (about 150) of single lenses which join each other without any gap (Fig. 7). The number and arrangement of these lenses is the same on both plates, but their sizes and FIG. 7. The Wabenkondensor assembly In place in an arc-lamp. Pictured below it are the two lens-array plates: (left) the rectangular-lens plate, (right) the hexagonal-lens plate. shapes are different. Each single lens of the lenticular plate A in Fig. 6 focuses an image of the mirror upon its corresponding lens in plate B. The lenses of plate B are hexagonal to conform to the circular form of the mirror image. Each individual lens of plate B, in turn, images its corresponding lens in plate A upon the projector aperture. Accordingly, the lenses of plate A have an oblong shape of the 3 :4 proportion to conform to the shape of the aperture. The total optical effect is the superposition on the aperture of about 150 different images of the oblong lenses. The partial shadowing of a portion of plate A by the positive carbon-holder can have no effect on the illumination of the aperture, which is completely even. The Critical Distance The distance between the mirror and the Wabenkondensor is not particularly important, but the correct, and very critical, distance separating the two lenticular reseau plates is maintained by their mount, which replaces the light-cone of the lamp. The distance between the projector aper ture and the outer, smooth surface of the hexagonal-lens plate must be adjusted to 136 mm. (5.3.54 in.), with a permissible leeway of 1 mm. (0.039 in. ) . Under no circumstances should this distance be altered, for any change shows up on the screen as a loss of light or shadows at the edge of the picture. It may seem that a little light might be lost by reflection from the surfaces of the two lenticular reseau plates, which is true; but these losses are minimized by the use of high-grade glass and antireflex coatings. But whatever the residual losses, they are more than compensated for by the efficient oblong shape of the spot of light on the aperture plate. A circular spot, as every projectionist knows, not only wastes a great deal of light, but heats the film gate ( if not watercooled) to an uncomfortably high temperature. It has long been this writer's opinion that uniform screen illumination of the order of 90% 95% side-to-center distribution is a necessity for firstclass projection, and that the projectionist is severely handicapped in the practice of his art by the ordinary HI arc-lamp. There are several ways to produce uniform HI screen illumination, and the Wabenkondensor system is one of the more practical methods. Efficient Illumination What of the illumination efficiency of the Wabenkondensor? Does it increase the brightness of the picture or does it waste light? These questions can be answered on the basis of tests made with the Ikosol II lamp burning various HI trims with and without the Wabenkondensor. In making these comparative tests the projector was run without film, a F/1.9 coated lens was used, the plate glass in the projection port was coated, and the lamp was adjusted for 75% -80% side-to-center distribution of screen light without the antireflex-coated Wabenkondensor, 88% 90%) with it. With a 6/5-mm trim burning 35 amps, screen light with the Wabenkondensor is 114% of the light without it. With a 6/5-mm trim at 40 amps, the light is 110%: with a 6V2/ Syo-mm trim at 40 amps. 113%; with a 6y2/5^/^-mm trim at 45 amps. 109%; with a 7/6-mm trim at 45 amps. 115%; and with a 7/6-mm (Continued on page 28) INTERNATIONAL PROJECTIONIST JANUARY 1954 13