GENERATION, CONTROL, AND MEASUREMENT 221 



ally within the range 0.2-0.5 mm in width and 3-10 mm in length. The 

 sensitivity of monochromator detectors is usually specified in volts per 

 watt (or microvolts per microwatt) of continuous flux, with the dimen- 

 sions of the receiver specified. 



In biological research it is often necessary to measure low values of 

 irradiance produced by large sources whose flux cannot be concentrated 

 into a small image. For this application the maximum irradiance sensi- 

 tivity SoA or Sh is required, and relatively large receivers are used. 

 The power sensitivity So is usually less than with monochromator detec- 

 tors, but the area A is large, giving a large value for SoA. 



The irradiance sensitivity of a thermopile is proportional to the square 

 root of the total receiver area for any specified design, provided that the 

 area is increased without changing the resistance or other physical charac- 

 teristics. For example, in the case of a thermocouple of a given design, 

 if the area is increased four times by a series-parallel arrangement of four 

 elements which leaves the total resistance unchanged, the voltage sensi- 

 tivity^ is only doubled. 



The noise equivalent power sets the lower limit to the radiant power 

 that can be detected when an electrical instrument, such as a vacuum- 

 tube amplifier with an equivalent time constant, is coupled to the detector. 

 This is the radiant flux that produces a signal voltage just equal to the 

 noise voltage. However, this is not the lowest radiant power that can be 

 detected if longer periods can be tolerated. 



The time constant or speed of thermal detectors is controlled by the 

 thermal mass or heat capacity of the receiver and the rate of heat 

 exchange with the surroundings. The time constant is made small by 

 using thin receivers and by rapidly dissipating the heat. Fast detectors 

 of high sensitivity are achieved by the use of the thinnest receiver mate- 

 rials and by reducing the rate of heat flow from the receiver to a mini- 

 mum. The receiver loses heat by radiation, conduction, and convection. 

 The radiation loss per unit area is determined by the receiver emissivity 

 and temperature, and little can be done to reduce it. The conduction 

 losses are minimized by the use of fine wires in bolometers and thermo- 

 couples. Convection losses can be practically eliminated by evacuation. 

 Evacuation of fast bolometers and thermocouples frequently results in a 

 10- or 20-fold increase in sensitivity and an appreciable increase in the 

 time constant owing to the lowered rate of heat dissipation. 



The time constant is specified by manufacturers in various ways, such 

 as the time to attain some proportion of maximum deflection after the 

 beginning of irradiation, or the percentage of the zero-frequency signal 

 obtained with various frequencies of modulation or chopping of the inci- 

 dent flux. The most fundamental quantity is r, the time required for 

 the signal to rise to 63 per cent (1 — 1/e) of its equilibrium value. When 

 galvanometers are used, fast detectors with time constants of less than 



