66 Applied Biophysics 



fore, as being of clinical importance, is not the absolute amount 

 of radiation energy received from a clinical source, but the excess 

 of radiant energy received on the skin, over that which would 

 normally arrive from the surroundings. That is, we must com- 

 pare the incident flux with that from surroundings at normal 

 room temperature. 



The fundamental physical method of measuring radiation flux 

 is to absorb all the incident radiation on the blackened surface 

 of known area of, say* a block of metal, and to determine the 

 energy received from the rise in temperature of the receiver. 

 Corrections must be applied for the cooling of the receiver 

 which will lose heat by ^ radiation and by conduction to the sur- 

 rounding air. To eliminate the latter and to secure a rapid 

 reading, the receiver is made of small heat capacity, is placed 

 inside an evacuated glass envelope, and its temperature is 

 measured by thermoelectric methods. Estimates of intensity of 

 infrared radiation made with a vacuum thermopile are, however, 

 liable to be very misleading in clinical practice, because the glass 

 envelope absorbs all radiation beyond about 3.5u, and we have 

 found,"^ that in certain clinically important cases, two-thirds of 

 the incident flux may be beyond this limit. 



The Thernioradionieter 



We have developed an instrument for the clinical measure- 

 ment of radiation flux (thermoradiometer) which dispenses 

 with such an envelope. It consists of two receiver plates 

 which are blackened and carry a pair of thermo junctions on 

 their reverse faces. The upper one receives the radiation 

 flux to be measured, while the lower one receives radiation 

 from a surface maintained by water cooling at room tem- 

 ])erature, and which, therefore emits the radiation characteristic 

 of our normal temperature surroundings. The two receiver 

 })lates are screened from one another by a small metal block, 

 which has the efl"ect of smoothing out random fluctuations of 

 temperature. On the other hand, the two plates are very close 

 together, and, therefore, the air temperature for each of them 



