Sec. 5-1] RADIATION TRANSDUCERS 277 



but not in vacuo because of their low thermal conductivity. ( lolloidal 

 graphite (Aquadag) has been recommended. Platinum black, 

 electrolytically deposited from a solution of 1 part platinum chloride 

 and 0.00S part of lead acetate in 'M) parts of water at a current 

 density of 10 mA/cm 2 , furnishes good results. Blackening by evapo- 

 ration of bismuth and antimony in the presence of air at a pressure 

 of 0.5 to several millimeters Hg has been reported by Pfund 1 and 

 by Burger and van Cittert; 2 evaporation of gold in an H 2 atmosphere 

 by Roess and Dacus; 3 and evaporation of aluminum by Gilliam. 4 



With proper blackening the radiation thermoelement has an equal 

 spectral response over the entire spectrum from the ultraviolet to 

 the far infrared. The only limitation is that imposed by the window 

 material mentioned above. 



The sensitivity of radiation thermoelements is of the order of a 

 fraction of a microvolt for an incident radiation of 1 /^W/cm 2 . Data 

 furnished by different authors frequently do not permit comparisons. 

 The useful receiving areas of elements vary from a fraction of a 

 square millimeter to several hundred square millimeters. For opti- 

 mum sensitivity, the receiving area should not exceed the cross 

 section of the incident beam. The output impedance is between 

 10 and 100 ohms and can reach higher values with multiple thermo- 

 elements. The response time of radiation thermoelements may vary 

 from a fraction of a second to several seconds. Systems with very 

 short response time (~0.1 sec) are described by Roess and Dacus, 5 

 as well as by Cary and George. 6 



For references and bibliography, see the review papers by M. Czerny and 

 H. Roder, Ergeb. exact. Naturw., 27, 70 (1938); also H. Cary and K. P. George, 

 loc. tit.; F. Kerkhof, loc. tit.; L. Geiling, Arch. tech. Messen, J 2404-2, Novem- 

 ber, 1955, and J 2404-3, February, 1956. An extensive study of the ultimate 

 sensitivity is due to D. F. Horning and B. J. O'Keefe, Rev. Sci. Instr., 18, 

 474 (1947). A further study, primarily concerned with noise in thermal-radiation 

 receivers, is published by R. C. Jones, "Performance of Detectors for Visible and 

 Infrared Radiation," in L. Marton (ed.), "Advances in Electronics," vol. 5, p. 1, 

 Academic Press, Inc., New York, 1953. 



1 A. H. Pfund. Rev. Sri. Instr., 8, 417 (1937). 



2 H. C. Burger and P. H. van Cittert, Z. Phijsik, 66, 656 (1933). 



3 Roess and Dacus, loc. tit. 



4 E. J. Gilham, J. Sci. Instr.. 33, 338 (1956). For further information on 

 blackening methods, see A. H. Pfund, in W. E. Forsythe (ed.), "Measurement 

 of Radiant Energy," McGraw-Hill Book Company, Inc., chap. 6, pp. 210ff., 

 New York, 1937. 



5 Roess and Dacus, loc. tit. 



6 H. Cary and K. P. George, Phys. Rev., 71, 276 (1947). 



