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702
LEE
December
measurements. The isolated points represent some rough alternatingcurrent measurements made as a check, with the output from the 400-cycle silver-track test loop taken as a reference. As was expected, the output from a metallic silver sound track is considerably greater than that from other types of track with the lead-sulfide cell, although the silver-sulfide track is nearly as effective. There is a considerable loss of signal (16 to 18 decibels) in the case of the composite dye
PbS CELL RELATIVE SIGNAL OUTPUT
CALCULATED FROM DC TRANSMISSION DATA
I 1 I
2600 2700 COLOR TEMP. (K°)
2800 2900 3000 3100 3200 3300
Fig. 7 — The relative performance of four different type* of sound track with the lead-sulfide cell; these figures are calculated from transmission measurements made using the lead-sulfide cell as a receptor.
track. Note that the dependence on the color temperature of the exciter lamp is approximately the same for all the tracks.
CONCLUSION
It is still not wise to make any sweeping statements about the probable future of the lead-sulfide cell. However, it would be worth while to outline some of the advantages and disadvantages connected with its use, on the basis of present knowledge. There are several dear-cut advantages:
(1) An increase in signal output of 15 to 30 decibels over ordinary phototubes for a given optical system and exciter-lamp color temperature.
(2) Increased signal-to-noise ratio, with silver sound tracks.
(3) Maximum sensitivity at fairly low values of load resistance — 1 megohm or less. Hence, circuit capacitance is not such a problem.