Projection engineering (Sept 1929-Nov 1930)

Page 8 Projection Engineering, May, 1930 ill « i ^ \\ 1 i ^ '" 'v. NhL^SP ^HHh fr s* ^ i Sk. jSUHHk^I Tp^SP^^ "-"^"^» ^ -*S*I^S "J ^JH i,„„ . Fig. 2 . The Mechau Projector. rate. This would have increased our frequency band by some fifty per cent, without noticeably improving the character of the picture. The Mechau Projector, a photograph of which is shown in Fig. 2, helped greatly in the solution of this highly complex problem. The horizontal film magazines are readily identified but the principle of the optical intermittent employed is best shown in schematic as in Fig. 3. The figure is, in a large measure, self explanatory. The set of rotating mirrors oscillate in such a manner as to hold each frame stationary on the screen. As one frame fades in intensity the next takes its place. Hence, there is always a picture on the screen and there is no motion of the picture across the field of vision. In order to avoid the use of a spiral disc in scanning the projected image, we employ a rotating multiple mirror to create a steady motion downward, thus obtaining an optical equivalent of the spiral disc. These projectors fortunately were available at a time when we found it necessary to go "on the air" with standard sound film. Mr. C. Francis Jenkins developed and manufactured a similar and much simplier "optical intermittent" some years ago and this is now undergoing a process of adaptation in the laboratory. This is based on Jenkins' prismatic discs and his prismatic rings of similar character. One of these discs can be rotated so as to follow one frame of a film during its course of motion and thus hold it steadily on the field until it is displaced by the succeeding frame just as in the Mechau system but with vast simplification. As an illustration, Fig. 4 shows two of these discs arranged to function as a spiral scanning mechanism in conjunction with a lens disc. We obtain a steady downward motion of a light beam by means of the rings iiinl throw it across the field by means of the lens disc, thus obtaining another optical equivalent of a physical spiral. This mechanism is adaptable to scanning at the transmitter or to the projection of an enlarged image at the receiving point, in conjunction with a high intensity glow lamp. These film transmissions are known as radio movies (or radio talkies, as the case may be), as opposed to "direct pickup" where the actual subject (or the projected image) is scanned. There are two possible methods of scanning in this case, either by scanning an image, where the complete illumination of the subject, either by brilliant artificial light or by sunlight, is necessary, or by scanning the subject with a "flying spot" of intense light. The two methods are outlined schematically in Figs. 5 and 6. Development Difficulties The principal difficulties encountered in the flying spot system are : first, that a shadow effect giving an unnatural cast to the image is introduced by the spot and by the angle at which the cells are required to work ; secondly, that the subject must work in semi or complete darkness and, too, the subject is required to face an exceedingly intense light spot. The system has the advantage of requiring cells of comparatively low sensitivity and large area and has given fair results with minimum equipment, though with low over-all efficiency. The direct system had the disadvantages of requiring highly sensitive photocells and noiseless amplifiers together with highly efficient light gathering lenses of high speed. While these disadvantages were great they were offset by advantages and promising possibilities. Under favorable circumstances, a normal degree of light could be employed. Pickup could be made out of doors by sunlight alone. The maximum illumination required could be obtained without discomfort to the subject. By proper lighting values a considerable stereoscopic effect could be obtained. The equipment was efficient and compact, lending itself readily to portability. Fig. 3. Illustrating the principle o f the optical intermittent. In the final arrangements incandescent lamps and arcs were unsatisfactory because of troubles arising in film pickup. The best source of illumination seems to be the d-c. mercury vapor lamp, supplied from well filtered sources. The scanning mechanism is at present an accurate 48-line spiral disc upon which the image is thrown by a camera lens of extremely high speed. Because of the' small value of light obtainable it is necessary to effect every possible conservation. This is done through the use of quartz lenses wherever possible. Because of the low input voltage obtained our chief difficulty has been in the construction of amplifiers in which the initial noise level is sufficiently low. We have been able to avoid disturbance from magnetic induction, mechanical vibration and acoustic vibration by proper shielding and suspension. It is necessary to choose the resistances employed with care so as to avoid the trouble encountered through thermal agitation in the resistive material itself. Improved Photoelectric Cells In obtaining the photoelectric cells for this work it was necessary to do considerable research in this field. ■Happily we have been quite successful and have been able to greatly improve picture detail by using the new Fig. 4. Two discs arranged to function as a spiral scanning mechanism. cells which we have developed, two of which are shown in Fig. 7. While the output from the cell enters into the problem, the other factors are also important. High sensitivity must be coordinated with linear operation, quietness, and freedom from frequency restriction. The gas filled type of cell has an output greatly in excess of that obtained from the hard vacuum type but suffers from "lag" and excessive frequency restriction at light frequencies beyond 10,000 cycles. We have, therefore, developed a "compromise" cell in which a fairly high degree of sensitivity has been attained together with marked decrease of high-frequency cutoff. Mechanical production difficulties, such as the attainment of a uniform light sensitive surface, have been overcome. These cells employ a