Motion Picture Herald (Jan-Mar 1956)

i I The wire that's BUILT to be BURIED In drive-in theatres TELESEAL gives top performance underground because it is built for the job. TELESEAL is jacketed with rugged neoprene that stands up in all locations. TELESEAL is dependable, it gives constant uninterrupted performance because from start to finish it is made for direct burial, without conduits. Be sure to specify TELESEAL by name when you order from your theatre supply house or local electrical dealer. WELL BUILT WIRES SINCE 1899 © 1955 WHITNEY BLAKE COMPANY NEW HAVEN 14, CONNECTICUT indication of the projection system speed by observing and measuring the diameter of the “aerial image’’ in the light beam in front of the projection lens. The aerial image in front of a lens is actually the image of the reflector, or the condenser, created by the lens. At this point the optical “stop aperture” of the projection system may be measured. If the diameter of this image is smaller than the diameter of the last clear glass opening of the lens, then something in the system is “stopping” down the speed. The speed of the projection lens is equal to the focal length divided by the lens aperture diameter. For a well-designed lens, the aperture diameter should be equal to the diameter of the last glass element. If the aerial picture in front of the lens is smaller in diameter than the last glass element, it is possible that the lamp optics (either reflector or condenser) are too small or out of proper position. The lamphouse optics are not filling the lens and are acting as a smaller “stop aperture” for it. Testing System Speed Let us assume that we are using a 4-inch focal length lens of f/2.0 speed. This lens “stop aperture” should be a minimum of 2 inches in diameter, therefore the aerial image in front of the lens should never be less than 2 inches in diameter. If we hold a dark card in front of the lens and move it back and forth, there should be a spot where the image of the reflector or condenser is extremely sharp. At this point the image should not be less than 2 inches in diameter. If it is smaller, then the reflector or condenser is not filling the lens properly and is reducing the optical speed of the entire projection system. Since a sharp image of the reflector, the reflector-holder and carbon guides may be obtained at this position, it is possible to see whether the reflector is filling the lens completely with light. Hold a graduated metal strip in front of the mirror face, from the rim towards the center of the mirror (like a radius line). Or\e way ‘t’o postfedl'^ be a, coupon clipper The F & Y Building Service is the outstanding agency in Theatre Design and Construction in Ohio and surrounding territory. THE F & Y BUILDING SERVICE 319 East Town Street Columbus 15, Ohio "The Buildings We Build Build Our Business" Detecting the Speed The image of this strip will be visible on the card in front of the lens. Cut notches in the metal strip every inch to coincide with the radius of the mirror. By counting the notches visible in the image ft will be easy to determine exactly hoAv much of the mirror is seen by the lens and if the lens is faster or slower than the reflector. The effective speed of a complete projection train can be only as high as its slowest point. There are many locations where mechanical and optical stops, or restriction, will cause reductions in maximum speed. These faults should be located and, if possible, removed. 34 Motion Picture Herald, March 24, 1956