International projectionist (Jan-Dec 1936)

September 1936 INTERNATIONAL PROJECTIONIST 21 FIGURE 2 Magnification is 24:6 — 6 times; speed is 24:101/i — f/2.3 inside of the nose casting will not be projected to the screen. "A larger diameter reflector increases the speed of the optical system, consequently a different setting is required for the new reflector equipment. HALFSIZE LENSES, having large front and rear factors (except in cases where the focal length is exceedingly short) are almost compulsory to 'obtain the total amount of light reflected by the new mirrors, because the angle at which the light beam strikes the film aperture is so great that only a small portion of the beam can enter the rear factor of the small quarter-size lens. Be sure that you have HALF-SIZE LENSES. "Slide the entire lamphouse ahead until the back surface, or vortex, of the reflector measures about 25" from the film aperture. The exact distance will depend upon the focal length lens you are using. If the lens is 4" to 4%" E. F., the vortex should be approximately 24" from the film line. At 43,4" to 5" E. F., the vortex should be approximately 25". At 5" approaching 6" E. F., 26", etc. "This setting can best be determined by the projectionist, various results be ing obtained for different lenses. Be sure, however, that you have not placed the reflector so close to the film line that you are more than filling the lens. This will result in light loss inside the lens barrel, and it will become excessively warm. By placing a white card in front of the lens and moving the entire lamphouse ahead and back, you can readily find the spot or setting at which the lens is just filled."] Consideration of the foregoing contribution by Mr. Kneisley opens up a vast field of speculation relative to lamp design, lightpickup angles, reflector diameter, lens speed, etc., — all of which is of absorbing interest to the practical projectionist. The appended comments, and accompanying illustrations, are an attempt to clarify some aspects of this problem. — Editor. THERE is nothing new or startling about the Kneisley conversion process. It simply utilizes a lO^-inch reflector which is exactly the same as those used in all late-type, low-intensity lamps, and applies it by means of various accessories to early-type lamphouses. Several of the Warner theatres, cited as references, report a 40% light increase through use of this equipment, a figure which Mr. Kneisley subsequently revised upward by 20%. Discussion of percentages, however, will lead nowhere; what is required is a delving into known fundamentals of light projection. When the first low-intensity lamps were designed and put on the market, the average projection lens had a working speed of //3. Accordingly, the lamps were designed to match the lenses in common use at that time. Figure 1 shows the optics of the old Stanard low-intensity lamp which was the original production of Strong Electric Corp. This lamp has not been manufactured by Strong for several years. It will be noted that this reflector has a light-pickup angle of 110 degrees and a working speed of //3, which, as mentioned previously, was correct for the average projection lenses in use at that time. Figure 2 shows the optics of the current model Strong Utility lamp. This lamp has a reflector lO^-inches in diameter, a light-pickup angle of 140 degrees and a working speed of //2.3, which is designed to match the modern projection lenses having a like working speed. This increased light-pickup angle and increased lens speed does result in some increase of light on the screen, although the figure of 72%, or even 40%, is much more than could be hoped for on anything like the average installation. This is the reflector that Mr. Kneisley is substituting for the 8-inch reflectors in his conversion process. Comparative Test Inconclusive For the convenience of the reader and as an interesting comparison, there is included herein Fig. 3, which shows the optics of the Strong high-intensity Mogul lamp. Fig. 4 is an unofficial layout of the Peerless (McAuley) lowintensity lamp, which has a pickup angle of 110 degrees and a working speed of //2.6. Figure 5 is an accurate representation of what would happen if a 10%-inch re FIGURE 3. Magnification is 32: :53/8— 6 times; speed is 32:14— f/2.3