278 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



b. Radiation Bolometers. 1 The radiation bolometer, Fig. (5-1)41, 

 consists essentially of a thin strip of a material which absorbs radia- 

 tion; the temperature increase causes a variation of the electrical 

 resistance of the strip. 



The bolometer strip is commonly used in a Wheatstone bridge or 

 a similar suitable network. The bridge is balanced while the bolom- 

 eter is protected from radiation. The ratio of voltage unbalance 



caused by exposure of the bolometer strip 

 to the unit radiation (in watts per square 

 centimeter) is frequently used as a measure 



^_ 



■ 



R, &R for the sensitivity of the bolometer. 2 



The transfer characteristics of metal 

 bolometers are essentially linear, at least 



Fig. (5-1)41. Radiation within the generally employed practical 

 bolometer. limits. The sensitivity is proportional to 



the current passing through the bolometer 

 at low current densities; as the current density increases, the 

 sensitivity reaches a maximum, since the radiation losses increase 

 proportionally with T 3 . 



Design considerations for bolometers of high sensitivity and short 

 time response are analogous to those for radiation thermocouples 

 treated above. The sensitivity can be increased by reduction of the 

 thermal losses, the speed of response by reduction of the heat capac- 

 ity of the strip. The bolometer is frequently made from a very thin 

 strip of metal, such as Wollaston foil (platinum), or evaporated on 

 a nonconducting carrier (nitrocellulose pellicles). The reduction of 

 the thickness cannot be driven too far, since the resistance-tempera- 

 ture coefficient of very thin metal layers is considerably less than 

 that of metal in bulk form. 



A number of older constructions are reviewed by T. Dreisch, in H. Geiger- 

 K. Scheel (eds.), "Handbuch der Physik," cbap. 26, pp. 842ff., 1928. Recent work 

 on metal bolometers may be found by E. B. Baker and C. D. Robb, Rev. Sci. 

 Instr., 14, 356 (1943); I. Amdur and C. F. Glick, Rev. Sci. Instr., 16, 117 (1945); 

 W. G. Langton, J. Opt. Soc. Am., 36, 355 (1946); C. B. Aiken, W. H. Carter, 

 Jr., and F. S. Philips, Rev. Sci. Instr., 17, 377 (1946); B. H. Billings, E. E. 

 Barr, and W. L. Hyde, J. Opt. Soc. Am., 36, 354 (1946), and 37, 123 (1947), 

 and Rev. Sci. Instr., 18, 429 (1947). A theory of the metal bolometer may be 

 found by R. P. Chasmar, W. H. Mitchell, and A. Rennie, J. Opt. Soc. Am., 46, 



1 The bolometer is also used for the measurement of physical phenomena 

 other than radiation, e.g., mechanical displacements or alternating currents. 

 Such applications are not considered in this section. 



2 P. Moon and W. R. Mills, Jr., Rev. Sci. Instr., 6, 8 (1935). 



