Projection engineering (Jan 1932-Mar 1933)

APRIL, 1932 'age ing a compression spring on the outer face of the drag roller. The film is held close to this drag by a roll of impregnated fabric tensioned by a flat steel spring. The sound aperture is of the double roller type which insures a smooth, steady movement of the film past the scanning beam, and a minimum likelihood of its moving out of the focal plane. These rollers, also of stainless steel, are very accurately ground and rotate in jewel bearings. They are rigidly mounted and doweled in place so that they may be removed for inspection or cleaning. The position of the two rollers on opposite sides of the film gives a pivoting action of the film around a point midway between the two rollers. This tends to maintain the scanning point on the focal plane while if both rollers were on the same side of the film, changes in the stiffness of the film due to variations in thickness, temperature, or age would throw it out of the focal plane. The optical system is shown in the photograph of Fig. 2 and diagrammatically in Fig. 4. It is mounted on a separate base which is secured to the casting of the head. It has three major elements. On the left is a collimator lens for focusing the light passing through the film onto the sensitive part of the photoelectric cell. In the middle is the reproducing lens unit for projecting the reduced image of the narrow illuminated slit on to the sound track, and at the right a lens used for adjusting lamps in the field. The reproducing lens unit consists of an objective lens, at the left a slit, and at the right a condensing lens. The slit is on the optical axis and midway between the two lenses. Its opening is held accurately to the proper width and its edges are selected for smoothness and parallelism throughout their entire length. A s i x v o 1 1 , nine-ampere ribbon-filament lamp is employed, and the condensing lens, with a magnification of unity, focuses an image of the filament on the slit, gives a \l/2 to 1 suits in an image Fig. 2. The three lens units of the optical system are mounted on common support which also carries the double-roller sound aperture. The objective lens reduction which rein the plane of the film .085 inch long and .001 wide. Special care is taken to insure an image evenly illuminated throughout its length and with clearly defined edges. Two bearings support the lens unit, one of them being slotted to permit clamping the unit in place. Between them is a collar which, by an adjusting screw, allows the unit to be rotated sufficiently to permit obtaining the correct azimuth adjustment. Focusing may be done in the field by a knurled head on the objective lens mounting which is then held in place by a lock nut. When first set up the various units are accurately adjusted with the aid of single-frequency films. When this has been done, the lamp adjusting lens is focused so that a sharp image of the lamp filament is projected on a ground glass screen at the rear of the lamp housing:. This screen carries cross lines Fig. 3. A double-roller sound aperture in the re the producing head maintains the film at the focal plane of lens unit. and is adjusted so that the lines correspond with the center of the projected image. To adjust subsequent lamps it is necessary only to center the filament image on these cross lines. This arrangement also makes it possible to tell when the filament sags and thus when the lamp needs replacement. The sound lamp is adjustable in all directions as indicated on Fig. 2. Contrary to normal expectation, adjustment of the lamp is not very critical. An out-of-focus condition in the horizontal position by as much as 2>l/2 turns of the adjusting screw results in only a half decibel loss in volume. Although the range is not so great for the other directions, adjustment of the lamp is possible in a comparatively short period of time since the width of the lamp filament is 30 times that of the slit. Projecting through the rear wall of the sound head, the photoelectric cell has its terminals adjacent to those of its coupling transformer which is mounted on springs in a separate housing as shown in Fig. 5. The spring suspension insures that extraneous vibrations will not be picked up by the unit. One of the prime requisites for high quality reproduction is a steady motion of the film through the light aperture. As an aid in securing this the sound sprocket has on its outer end a flywheel driven by a damped spring as shown in the illustrations. The motor, rated at Y\ h.p. and running at 1200 r.p.m. also carries a flywheel on the outer end of its shaft. The uniformity of rotation of the sound sprocket may be checked stroboscopically by a shutter with 36 slots mounted on the motor drive shaft, and 120 lines equally spaced around the rim of the flywheel. Both the slots and lines are accurately spaced by microscopic measuring devices at the factory. The combined er