International projectionist (Jan-Dec 1938)

of the sound track with the required wide range of frequencies for high quality sound, the film speed was increased to 24 pictures per second or 90 feet per minute. To project the picture a device is required which will move a long strip of film consisting of a large number of still pictures so that each picture comes to a complete rest in the film gate at the aperture opening where the light source is projected and then moves on to make way for the next one. Motion is imparted to the film by punching holes in each side of the film and employing sprockets which consists of cylinders with teeth around the periphery on either side which engage with the holes in the film. The design of the shape and spacing of the holes in the film and of the sprocket teeth as well as the diameter of the sprockets and their location is critical as it is desirable to eliminate any slippage or chatter that might be imparted to the film. Furthermore, as the film ages and is subjected time after time to intense heat it shrinks and steps must be taken to compensate for this. • Intermittent Movement The heart of a motion picture projector is known as the "intermittent movement," which part is used to accomplish the desired motion of the film. It permits each still picture to remain stationary for approximately 1/32 of a second and then moves it out and the next picture into its proper position in 1/96 of a second. The rate of pull-down is governed to some degree by the tensile strength of the film, which must not be damaged or torn for suitable results. The intermittent movement consists of a sprocket on one end of a shaft whose teeth engage the film just beyond the point where the picture is projected. On the other end of the shaft, in an oilfilled sealed compartment, is a "star wheel" with four slots at right angles. On a separate shaft a "pin wheel" engages with the "star wheel". The second shaft is driven from the non-operating side of the projector at a constant speed of 1440 r. p. m. and any irregularities are damped by a flywheel. While the pin travels the length of the slot and back the star wheel, and consequently the sprocket, turns enough to move one picture out of the film gate and to move another into its proper position for projection. During the time it takes for the pin upon leaving the slot to travel around and engage the next slot of the star wheel, the star wheel and the sprocket stand still. Thus it takes three times as long for the pin to travel around the pin wheel from one slot to the next than it takes for it to travel twice the length of the slot. We have already noted that it is desirable to have the image of the projected still picture on the screen for approximately 1/50 of a second. It is also necessary to cut off the light projected to the screen while the film is actually moving to prevent any blur on the screen which would result if the light and pic ture image were continuously projected. For this reason a balanced two-blade shutter is employed to cut off the light during the period of film motion. The blades are 93 degrees each and the shutter revolves approximately 1440 r. p. m. Thus, each still picture projected is cut off twice for approximately 1/96 of a second each, but the film moves only during one of the cut-off periods. The eye then sees 48 separate still pictures per second, each for 1/96 of a second, each pair being the same view, but they all blend into a continuously moving picture. • Magnification Ratio The size of the picture on the 35-mm. film is .600 x .825 inch. It must be projected and magnified onto a reflecting screen that in a theatre will vary in size from 8 x 11 feet up to 37 x 50 feet, depending on the size and shape of the theatre. This means a magnification of from 25,600 to about 55,000 times in terms of area. To accomplish this obviously means that a tremendous amount of light is required. Two other factors contribute to this problem. There is a loss of light due to its being cut-off for a part of the time by the shutter, and the lens that is required to magnify this picture to such a large degree absorbs a lot of light. Light sources in general use consist principally of direct current arc lamps with reflectors which concentrate the light on the film aperture. Arc lamps of various intensities are available, each ISeiv S.M.P.E. President E. ALLAN WILLIFORD The head of National Carbon Co. will oversee film technical activities next year requiring power conversion apparatus of varying capacity dependent on the rating of the lamp. Provisions are made for automatically controlling the carbons so that as they burn they are kept in proper distance relationship for constant light intensity. Carbons are designed for a burning period of about 25 minutes. Mazda lamps of the 1000 or 1500 watt variety are sometimes used in small theatresThe color content of the light is important, depending on the type of screen used in order that the final result be satisfactory contrast. The delicacy and accuracy of a motion picture projector can be best emphasized by simply pointing out a few of the problems involved. Up to the present time it has been found to be most economical for the theatrical field to use film with a nitrate base which is highly imflammable. 16-mm. film and special nontheatrical 35-mm. film is made with an acetate base which is non-imflammable. This requires the use of a fireproof projection room with certain protective devices. It also means that the projector must reduce the heat at the aperture as much as possible and that means must be provided to prevent film from stopping too long or piling up at the aperture, in which case fire would occur. Also, means must be provided to prevent fire from reaching the film storage sections or magazines of the projector if fire does occur. By placing the rotary shutter between the light source and the aperture, the heat is reduced at the aperture about 50%. Even at that, the heat at the aperture may be of the order of 700 degrees F. The projector parts must be of materials that are not affected by that amount of heat. The shutter is a mechanically rotating device which has to be properly lubricated and must not vibrate if it is not to damage the machine. This is not an easy matter due to its proximity to the hot light source, which might cause expansion of the metal and vaporization of the lubricant. • The Film Stock The photographic image on a film consists of a very thin layer of emulsion on a celluloid base, the entire film being about 6/1000 of an inch thick. It is necessary to focus the image of the projected picture sharply on the screen due to the large order of magnification. Thus, while the film is moving through the film gate, means must be provided for accurately keeping the emulsion in one plane so that the film should not move out of focus at any time. At the same time, care must be taken not to scratch the emulsion in any way as the film must be used over and over again. Keeping in mind the large magnification, a slight motion of the film in the aperture while the shutter is open, or even a slight motion of the projector due to vibration, becomes a large motion on the screen, which is, of course, intolerable, due to eyestrain. Since we cannot tolerate picture jump on the screen of over TA of 1 percent, it means keeping the film steady at the aperture in the order of tenths of thousandths of an inch. These are but a few of the problems which make it necessary to adhere to tolerances of the order of 1/10,000 of an inch and to use especially selected metals (Continued on page 21) NOVEMBER 193 8 11