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divided between two successive film frames. The third view shows the polygon a small instant later yet (To -j1/24 sec -f 5). This moment in time in the rotation of the polygon is a particularly interesting one, as it clearly demonstrates that when precisely one facet is illuminated by the projection lens the chasing action of the optical compensator causes the raster to move potentially over a height of three film frames in extent; in other words, a "gate" three film frames high is required even though no more than two film frames are utilized simultaneously at any one moment.
The heart of the television film scanner, which is the optically compensated "picture head" or film-traction mechanism proper, is shown in Fig. 6. The ganged shrinkage control knob is easily recognizable as well as the polygon surrounding the stationary-core assembly and the refocus-compensator device. The mechanical simplicity of the picture head requires no emphasis, except to point out that the main large picture sprocket is not motor driven, but is driven by the film. Figure 6 also shows the relationship between picture head and the tri-color photocell housing. The special 4-in. flying-spot scanning tube is shown in Fig. 7. Its face is almost completely filled by a raster 2£ X 3 in. in size which is of course a perfectly conventional 525-line, 60-field interlaced raster whose vertical and horizontal return traces are completely suppressed for the establishment of black level. The tube has a special fine-grain, short-persistence phosphor developed by the Lansdale Tube Company. It is operated at about 30 kv and at a beam current of 250 fia. The complete equipment is shown in Fig. 8.
Summary
The special features of the 35mm television film scanner may be summarized as follows:
(1) Continuous film motion, which
Fig. 5. Operational Diagrams: Position 1 (top); Position 2 (center); Position 3 (bottom).
has the advantage of greatly reducing film wear, implies a simpler and more reliable film-traction mechanism than is possible when operation is of the intermittent type.
(2) The optical compensator, which is a specially corrected type of rotating prismatic polygon, is a relatively small component revolving at a very low speed. This device has its own precision permanently "built into" it. Once manufactured it cannot change or distort in any manner, thus never requires further adjustment or maintenance.
(3) A film shrinkage correction device is provided as an integral part of
Traub: Television Film Scanner
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