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Fig. 3. 35mm optical system.
whose function is to form a zero-focus air gap between the rotating polygon and the other optical components which constitute the core. Second, there are two prismatic rhombs whose function is primarily to serve as path-length extenders in the optical sense. Without such path-length extension the diameter of the polygon could not be reduced sufficiently to enable it to be contained within the diameter of a film sprocket. Third, the core contains an adjustable optical shrinkage compensator the nature of which will be described later. Certain of the optical elements of the core are made of glass having a different dispersion from that of the polygon, which serves the further useful purpose of enabling the designer to correct the polygon for lateral chromatic aberration as a function of angular rotation. Other prismatic abberations, incidentally, can be readily controlled in design in a composite prismatic polygon of this type and can be kept within highly acceptable limits. Certain others, such as the spherical and longitudinal chromatic aberrations, can be corrected by means of a projection lens specifically designed to operate through the requisite glass path length.
A sectional view of the entire optical
system is shown in Figure 3. The pathlength extension created by the rhombic prisms is readily recognizable. The nature of the shrinkage compensator is also revealed.
It is an inherent feature of all machines using continuously moving film that their operation depends on the degree of shrinkage of the film. Therefore, in all such machines provision for shrinkage compensation of some kind must be made. In a prismatic-polygon type of optical compensator the best way to compensate for film shrinkage is effectively to vary the optical diameter of the polygon. This is, of course, quite impracticable in the case of the classical solid polygon, but is quite possible with the type of compensator here described. It is accomplished by introducing the optical equivalent of a parallel plate of variable thickness into the optical path between opposite pairs of facets of the polygon. The physical embodiment of this parallel plate of variable thickness is achieved by the subterfuge of sliding a thin wedge against a stationary wedge having a similar slope angle. The in-and-out movement of this wedge against its corresponding mate, separated by a very small air gap, then varies the optically effective path length
Traub: Television Film Scanner
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