222 RADIATION BIOLOGY 



100 msec are of little value. The fast detectors are of special merit for 

 rapid recording and where modulation of the incident flux is used to pro- 

 duce an a-c signal that is relatively uninfluenced by stray flux and readily 

 amplified. 



The resistance of the detector determines the method of coupling to the 

 measuring instrument to ensure maximum power transfer. In general, 

 the input resistance of the measuring system should approximately match 

 that of the detector within a factor of 2. 



The evacuated bolometer or thermocouple detector requires a rigid 

 window, and even those operating in air require a window to eliminate 

 drafts and convection currents. The window material should have negli- 

 gible selective reflection and absorption in the spectral regions to be used. 

 Although fused quartz and thin glass windows have adequate trans- 

 parency for many photochemical applications, both materials absorb the 

 infrared beyond about 5 n, which amounts to 10-20 per cent of the flux 

 emitted by radiometric standard lamps. This necessitates a large cah- 

 bration correction for window losses. Calcium fluoride (fluorite) is prob- 

 ably the most satisfactory window material, since it transmits freely from 

 the ultraviolet to beyond 9 m in the infrared, and errors due to uncer- 

 tainty of corrections for selective window absorption are negligible. Cal- 

 cium fluoride is also relatively nonhygroscopic as compared with other 

 halides, and drying agents are usually not required. 



Bolometer. The radiation bolometer is, in principle, a resistance ther- 

 mometer in which the resistance element consists of a thin blackened 

 strip of metal or other conductor with a high temperature coefficient of 

 resistance. If the receiver element is one arm of a balanced Wheatstone 

 bridge, the temperature rise due to absorption of radiant flux can be 

 measured as a change in resistance. The bolometer responsivity can be 

 expressed as a voltage change per watt of incident flux, wdth a specified 

 bridge current flowing through the receiver. The sensitivity of a bolome- 

 ter is roughly proportional to the current up to values that produce exces- 

 sive heating of the receiver element. The sensitivity can be varied over 

 a wide range by varying the current. Vacuum bolometers are frequently 

 operated at temperatures up to 50°C. 



When only one bolometer element is used, it is not possible to dis- 

 tinguish readily between changes in intensity of the radiant flux being 

 measured and random fluctuations in scattered flux and ambient temper- 

 ature. These fluctuations may be balanced out of the bridge response 

 by a compensating element or compensator that is identical in form with 

 the receiver and is made a second arm of the bridge (Fig. 3-25). By 

 careful construction as much as 95 per cent compensation can be attained, 

 and further compensation can be effected by shunting a large variable 

 resistance Rs across the more sensitive element. 



Bolometers designed for use with monochromators have the receiver 



