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V. DIMENSIONS AND STANDARDS
nearer trace of the track. Measure the distance D6 from the edge of the film to the far trace of the track. The Avidth of the track (D6-D5) should be 0.071 in., if the machine is built within the standards, the track width will fall between 0.070-0.072 in. The distance of the center line from the edge of the film should be 0.058 in. ; the method of measuring is described in Part 1 of this chapter. See Figure 17.
If the traces are quite fuzzy due to overexposure, it may be wise to use EK 5365 (fine-grain duplicating positive film) for the test, or some other relatively slow film of high resolving power. Other films that may be suitable are EK 5372 and Eastman or DuPont microcopy (nonpanchromatized). Incidentally, some microcopy film is panchromatized to increase its effective speed; because panchromatic emulsions fog under
Fig. 17. Measuring width of projector scanning beam. Film is measured after exposing raw stock in the projector and developing it.
D-D.
: 0.071 in. (nominal)
red light, positive-type emulsions without color sensitizing are best suited to the purpose. A really desirable film would be a chromatically unsensitized microcopy emulsion coated on a clear base. It may be difficult to obtain such a film commercially; if it is, other types somewhat less desirable may have to be used. At least 16 sets of readings of D5 and D6 should be made along the central 50-ft. portion of the 100-ft. roll, both readings being made at the same point along the film. The center-line reading is the arithmetic average obtained from the 16 sets.
In general, it is a good plan to take a relatively large number of readings, since the probable error of the average is proportional to the square root of the number of observations. Generally speaking, the number of reading sets should be 4, 9, 16, 25, 36, 49, 64, etc. ; the actual number taken being dependent upon the probable error desired. For most purposes, 16 sets will usually be sufficient.