International projectionist (Jan-Dec 1954)

LENSES: Projection Heart (Continued from page 16) the speed of the mirror may be reduced until it corresponds with the effective speed of the lens. This expedient reduces screen illumination, of course, but it improves the quality of illumination and decreases heating of the film. A German Suggestion Another method has been devised by Zeiss Ikon of Germany, manufacturer of the Ernemann projectors. Instead of increasing the speed of the projection lens or decreasing the speed of the arc-mirror, the light-rays which emerge from the aperture are all directed into the projection lens by means of a special single-element lens placed directly behind the aperture. This special lens is called a Bildfensterlinse, a German word meaning "picture-aperture lens." Figure 4 illustrates diagrammatically how this lens forms a small, intensely brilliant image of the arcreflector inside the projection lens. The special lens is placed as close to the aperture as possible in order to form the smallest possible image of the mirror, insuring that all of the light coming from the aperture goes into the lens for projection to the screen. Picture— Aperture Lens The "picture-aperture lens" impresses us as a worthwhile contribution to projection technology. Made of special heat-resistant optical glass, it is inserted into the Ernemann projector as shown in Fig. 5. Actual tests show that this lens considerably increases the brightness of the picture when employed in conjunction with the W ahenkondensor lamp (See IP for January 1954). It is evident from Fig. 4 that the Bildfensterlinse produces the effect of matching the projection lens with the lampoptics without requiring the use of lenses of extreme speed and unsatisfactory optical characteristics. Cleaning of Lenses A fine lens should be accorded even better care than a priceless jewel. A diamond, being harder than optical glass, better withstands careless scrub EXPERIENCE A PRICELESS ASSET OF ALTEC STEREOPHONIC SEJIVICE! BttEElWt^'^ The finest service experience in stereophonic sound is yours when you sign an Altec contract. Experience bought and paid for in countless hours of pioneering effort and research . . . ■ experience in supervising critical CinemaScope industry showings and first run premieres, experience with an unceasing flow of stereophonic installation and service orders, experience implemented by . Altec-devised precision tools. YOUR experience with an Altec stereophonic service contract will prove pleasant and profitable! j_ tebM i SEB-VICE .„„I««J.",'' 161 Sixth Avenue New/ York 13, N.Y. SPECIALISTS IN MOTION PICTURE SOUND bing and scratching! Coated lenses require special thought, if not special care, because the anti-reflection coating J I © ----Ahens [ Aperture © e» iBildfcn|terltn|c image of \ mirror Aperture FIG. 4. How the Zeiss-lkon Bildfensterlinse (picture-aperture lens) defies the laws of optics. In (A) is shown a conventional mirroraperture-lens projection system. Note that much light is wasted because the lens isn't big enough to intercept all of it. In (B) we see what happens when a small, powerful lens is placed in the lamp beam at aperture distance — an intensely brilliant reduced image of the mirror Is formed in mid-air a few inches away from it. In (C) such a "pictureaperture lens" is placed behind the aperture to route all of the light rays into the projection lens. The effect of perfect optical matching Is thus cleverly produced without using impractically large projection lenses. is a film of magnesium fluoride only 1/8000 mm thick — about 800 times thinner than this sheet of paper. So, even though the coating on a coated lens is somewhat harder than most glasses, it is so thin that it can easily be scratched off. And scratches, as well as fingermarks, oil-spots, and FIG. 5. Projectionist inserting the "Bildfensterlinse" behind the aperture of an Ernemann projector — the picture-aperture lens. Other stains, are extremely conspicuous on a coated lens because they destroy the anti-reflection properties of the coating. As for removing dirt from lenses, both coated and uncoated lenses 32 INTERNATIONAL PROJECTIONIST APRIL 1954