Journal of the Society of Motion Picture and Television Engineers (1950-1954)

mid-30's for the first time defined clearly in a theoretical paper the interrelationship between the continuous film projector (proper) and the television film scanner using optical compensation. Lens Methods. Generally speaking, lens methods of compensation have been less successful than mirror or other methods for a variety of reasons. The optical aperture of such systems is usually somewhat limited by aberration considerations and the problem of identity of a large number of lens elements is and remains a serious one. Moreover, there are the problems of precision adjustment of the lenses and of cost. Prism Methods. These are generally polygonal derivatives of the planeparallel glass-plate type of optical compensator. They are based on the physical-optical phenomenon that a ray passing through a parallel plate of finite thickness will be displaced proportionally to the angular rotation of such a parallel plate, at least for a reasonable number of degrees of rotation, beyond which, however, the law of operation may be said to break down and the displacement becomes nonlinear. The well known optical designer H. Dennis Taylor, who will be remembered for his invention of the now famous triplet lens, was the first to publish a theoretical analysis of this type of compensator. The prismatic-polygon type of compensator has a number of features which make it highly attractive: It is simple, consisting essentially of a one-piece optical component whose precision can be "built in" once and for all. There are no adjustments either during assembly or during operation. Moreover, the device is both small and light, and can be manufactured at very reasonable cost. Until very recently, however, the theory of operation, and in particular the theory of prismatic aberrations, was not too fully understood, nor were the results obtained with these devices too encouraging. In the past few years, however, improvements of a very substantial nature have been made in the field of prismatic polygons, these improvements being based on a much better insight into the theory of operation and on a number of new inventions. It is the purpose of this paper to describe a new Television Film Scanner which uses such an improved prismatic-polygon type of optical compensator. Description of Optical Compensator* Space does not permit a detailed technical description of the reasons underlying the various improvements and changes that have been made to the prismatic-polygon system, but the following description will serve to convey the nature of the device as incorporated in the 35mm television film scanner presently in production. The rotating element is shown in Fig. 1. Several distinguishing features are readily apparent. The polygon is 24-sided, rotating once per second. It has a diameter of about 5| in. The polygon is no longer a solid one, but has a large cylindrically polished hole cut in it. The polygon may be said to have sprocket teeth on it, or more correctly speaking the polygon is now so dimensioned that it can be fitted within the confines of a film sprocket. This permits the film to be partially wrapped around the polygon in such a manner that each film frame is optically and mechanically registered with each corresponding facet of the polygon. This is a most important feature of great practical significance. It is referred to as isotransport. In the absence of isotransport, when film velocity and polygon velocity are separately established, four dynamic conditions must be ful *An invention of Dr. John C. Kudar. Philco has the exclusive license under Dr. Kudar's applicable patents and applications, but only for the television field. Traub: Television Film Scanner