220 RADIATION BIOLOGY 



temperature, and other factors, so that for critical work frequent cali- 

 bration is required. 



THERMAL DETECTOR 



The receiver of the thermal detector consists of a thin highly absorb- 

 ing, or "black," film whose temperature changes may be determined by 



(1) the change in electrical resistance of the receiver, as in the bolometer, 



(2) the electromotive force or voltage produced at a thermoj unction in 

 contact with the receiver, as in the thermocouple and thermopile, or (3) 

 the change in gas pressure of a small sealed chamber surrounding the 

 receiver, as in the pneumatic or Golay detector. The general character- 

 istics of thermal detectors have been discussed by several authors (Bell 

 et al, 1946; Ellickson, 1947; Fellgett, 1949; Harrison et al, 1948; Hornig 

 and O'Keefe, 1947; Jones, 1947, 1949a,b). Williams (1948) has reviewed 

 the development of thermal detectors for infrared spectroscopy. 



The selection of the most useful temperature detector for any particu- 

 lar application requires evaluation of its performance in regard to the 

 following principal characteristics: 



1. Radiant-power sensitivity or responsivity So, the ratio of the out- 

 put signal to the radiant power incident on the receiver; usually expressed 

 in volts per watt, which is equivalent to microvolts per microwatt. 



2. Irradiance sensitivity Sh, the ratio of the output voltage to the 

 incident irradiance in microvolts per microwatt centimeter-^, which is 

 equivalent to So A, where A is the sensitive area of the receiver; expressed 

 in volts per watt centimeter"^. 



3. Noise equivalent power i/,„, or noise equivalent intensity NEI, the 

 minimum flux detectable when the detector is operating into an amplifier 

 that has the same time constant as the detector; expressed in watts. 



4. Time constant r, the time in seconds required for the receiver to 

 attain 63 per cent (1 — l/e) of its equilibrium temperature following 

 irradiation; expressed in seconds. 



5. Resistance R of the bolometer or thermocouple detector; expressed 

 in ohms. 



The radiant-power sensitivity of a thermal detector is approximately 

 inversely proportional to the area of the receiver. Since response is pro- 

 portional to the rise in temperature induced by the radiant flux and since 

 the dissipation of heat from the receiver is roughly proportional to receiver 

 area, any increase in size of the receiver necessitates a corresponding 

 increase in total flux to produce the same response. Therefore the 

 response of the thermal detector is proportional to the average irradiance. 

 When the detector is used with a monochromator, the flux can be con- 

 centrated into a small image; i.e., the irradiance is made as large as 

 possible. The dimensions of the bolometer and thermocouple receivers 

 are made approximately equal to those of the slit image, which are usu- 



