276 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



(thermopiles), as shown in Fig. (5-1)40. One set of thermoj unctions 

 is exposed to the incident radiation, the other side is protected 

 from it. A series-parallel linear thermopile has been described by 

 Crane and Blacet. 1 Whether or not the use of thermopiles offers an 

 advantage over the single thermoelement depends upon the level 

 of the incident radiation, its cross-sectional area, and the input 

 impedance of the subsequent stage. 2 Roess and Dacus 3 have shown 



that the signal-to-noise ratio of the 



radiation thermoelement is a maxi- 



Radiatmn — ~. mum for the single thermoelement. 



3. REDUCTION OF CONVECTION AND 



CONDUCTION TO ATMOSPHERE SUR- 

 ROUNDING THE THERMOELEMENT. 



Fig. (5-1)40. Radiation thermopile These losses can be reduced by 



made from several thermoelements, enclosing the tliermo junction ill an 

 alternate junctions exposed to radi- -, i ^^^ •*.• -j. 



,. J ' evacuated envelope. 1 he sensitivity 



rises in the form of an S-shaped curve, 

 starting at about 1 mm Hg, and reaches a final value at about 10~ 4 

 mm Hg. The increase of sensitivity through evacuation depends upon 

 the construction of the element and amounts to a factor varying be- 

 tween 10 and several hundred. 4 Of course, the enclosure envelope 

 causes absorption of radiation in the glass wall and limits the spectral 

 range for which the thermoelement can be used. This difficulty 

 can be remedied, to some extent, by the use of thin windows or 

 windows of quartz and other infrared or ultraviolet transparent 

 materials. 



4. REDUCTION OF RADIATION LOSSES FROM THERMOJUNCTION. 



Since these losses depend upon the temperature of the junction, an 

 operation of the element at low temperatures can increase the sensi- 

 tivity of the system. Cartwright, 5 who used the radiation thermo- 

 element at the temperature of liquid air, found an increase of 

 sensitivity by a factor of 10 as compared to operation at room 

 temperature. (For other possible advantages of operation of the 

 element at low temperature, see the paper by Cartwright.) 



Different methods have been used for blackening the exposed 

 surface in order to absorb as much as possible of the incident radia- 

 tion. Soot, turpentine smoke, and camphor smoke are useful in air 



1 R. A. Crane and F. E. Blacet, Rev. Sci. Instr., 21, 259 (1950). 



2 See F. Kerkhof, Arch. tech. Messen, J 2404-1, October, 1940. 



3 L. C. Roess and E. N. Dacus, Rev. Sci. Instr., 16, 164 (1945). 



4 G. Rosenthal, Z. Instrumentenk., 59, 432 and 457 (1939). 



5 C. H. Cartwright, Rev. Sci. Instr., 4, 382 (1933). 



