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1950 INFRARED WITH ELECTRIC-FLASH 487
and types of gases. To obtain some indication of the effect of voltage and capacity on infrared output, three types of tubes were investigated. Infrared light measurements were made with a special integrating-type light-meter5 designed for use with electric-flash, which employed a red-sensitive photocell and a Wratten 88A infrared filter.6 Figure 2 shows the sensitivity curve of the photocell and the transmission curve of the 88A filter. This filter has a transmission of less than 0.1% below 7,200 A. The overlapping of the two curves (shown by the shaded area) represents the region of response of this combination of filter and photocell. This region extends from 7,200 to 12,000 A.
Using the methods of measurement described, three different types of flashtubes were investigated to determine the effect of voltage and, to a limited extent, energy loading on the infrared output. The General Electric Co. FT-110 is a new flashtube designed for 1,000-v operation in portable electric-flash equipment. Its infrared effi
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FT-214' ~"T~-
50 WATT SECOND? j
Fig. 3. Effect of operating voltage on the
infrared efficiency of several flashtubes.
500 1000 1500 2000 OPERATING VOLTAGE
ciency, as shown in Fig. 3, is nearly three times as high at 500 as at 2,000 v when operated at 50 w-sec (watt-seconds), and approximately twice as high when operated at 12.5 w-sec. The CAA (Civil Aeronautics Administration) flashtube shows an infrared efficiency more than twice as high at 500 as at 2,000 v. This tube is a very small quartz lamp designed specifically for an infrared instrument recorder described later in this paper. The GE FT-214 is a standard 2,000-v flashtube used for portable and semiportable flash equipment. Its infrared efficiency remains approximately constant over this voltage range. From these limited data, it is probably safe to state that for maximum infrared efficiency a flashtube should be designed for as low voltage as is consistent with proper starting characteristics and flash duration.
The other design factors of tube dimensions, gas pressure and type of gas also may very well affect the infrared efficiency and should be investigated as they have been for the visible region.2