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LEE
December
color temperature, no matter what type of photosensitive element is used as a transducer in the sound system.
Since the lead-sulfide-cell output is less sensitive than the Si and 84 surfaces to light-source color temperature, one would expect it to be less sensitive to alternating-current ripple in the lamp power supply. Fig. 5 shows the hum-to-signal ratios obtainable with the lead-sulfide cell used with a number of different lamps when the lamp power is pure 60-cycle alternating current. The figures do represent an improvement over the Si and S4 tubes, and the low-resistance filaments are almost good enough. These are high-power lamps, however, with large envelopes, and not suitable for use in 16-mm equipment.
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EXCITER LAMP VOLTAGE (AC)
Fig. 5 — Hum-to-signal ratio obtained with the lead-sulfide cell and a number of standard exciter lamps. The lamp excitation was raw 60-cycle alternating current.
The use of an indirectly heated exciter lamp has been prominently mentioned. Tests have been made of experimental lamps of this sort used with lead-sulfide cells, and signal-to-hum ratios of greater than 90 decibels have been obtained with raw alternating-current voltage on the lamp. However, the color temperature of these lamps is quite low; consequently the total radiated power and the signal output are also low.
The considerations affecting the choice of an exciter lamp for use with the lead-sulfide cell may be summed up as follows: The advantages claimed for the cell have included increased signal output and a very much simplified exciter-lamp supply. Upon close scrutiny it appears doubtful that these two advantages can be realized simul