Sec. 5-1] RADIATION TRANSDUCERS 275 



disk by radiation, conduction, and convection is equal to the in- 

 cident radiation power. For most practical applications, the output 

 voltage is a linear function of the incident radiation power density 

 in microwatts per square centimeter. 



The receiver disk is usually made from a thin sheet of gold leaf, 

 silver, or platinum foil, and the thermocouple wires are soldered or 



F^ ' 



E 



Fig. (5-1)38. Radi- Fig. (5-1)39. Radi- 

 ation thermoele- ation thermoele- 

 ment: F, disk; T, ment: T, thermo- 

 thermoelement. j unction . 



welded to the foil (e.g., by a capacitor discharge). A different con- 

 struction is shown in Fig. (5-1)39; it consists of a flat strip of two 

 different metals forming a thermoelement. 



Numerous radiation-thermoelement constructions have been 

 developed, primarily with the aim to increase the sensitivity and 

 shorten the response time of the element. The following means 

 have been employed: 



1. use of very thin thermoelements. Thin elements have a 

 small heat capacity and, therefore, a fast response; also the heat 

 conduction from the thermoj unction through the thermoelement 

 wires or foils diminishes with a decrease of their cross section. Moll 

 and Burger 1 have soldered together relatively heavy sheets of the 

 two metals which form the thermoj unction. The resulting sheet 

 was then rolled down to a foil of 5 /u thickness, from which strips 

 were cut. Another construction uses a thin insulating carrier, on 

 which, on slightly overlapping adjacent areas, two different metals 

 are deposited by cathode sputtering or evaporating. 2 The elements 

 produced in this way have high mechanical stability, and the method 

 permits a wide choice of metals. Similar radiation thermoelements 

 (0.1 /Li thickness) can be made by electrolytic deposition. 3 



2. multiple thermoelements. The output voltage can be in- 

 creased by the use of multiple thermocouples, all connected in series 



1 W. J. H. Moll and H. C. Burger, Z. Pkysik, 32, 575 (1925). 



2 H. C. Burger and P. H. van Cittert, Z. Physik, 66, 210 (1930); L. Harris 

 and E. A. Johnson, Rev. Sci. Instr., 4, 454 (1933), and 5, 153 (1934). 



3 C. Muller, Naturwissenschaften, 19, 416 (1931), and R. V. Jones, J. Sci. 

 Instr., 14, 83 (1937). 



