64 Applied Biophysics 



is an appreciable contamination with ultraviolet radiation. 



In practice, the hot bodies used as sources of radiation are 

 all at fairly low temperatures, and so provide radiation which 

 is all in the infrared with a little visible red light. An excep- 

 tional case is provided by the arc lamp, where the hottest part 

 of the carbon rods may be at a temperature as high as 3500° C 

 (3730° K) and gives a considerable proportion of ultraviolet 

 radiation, even though the greatest part of the energy emitted 

 is in the infrared. Even an ordinary incandescent filament lamp 

 actually emits a small proportion of ultraviolet radiation, but this 

 is all absorbed in the glass of the lamp bulb. 



At all temperatures, therefore, which may be acquired by the 

 dull emitter heating elements, or the metal shields and reflectors 

 of radiant heat apparatus, the radiation emitted is in the infrared, 

 and, because of absorption by glass, the actual radiation which 

 reaches the patient from an incandescent filament lamp is also 

 infrared, accompanied by a small proportion of energy in the 

 visible region. 



On the other hand, low-temperature sources are relatively 

 inefficient, and thus, if we wish to secure a large total emission 

 of radiation from the source, we must use extended sources, 

 such as heated metal sheets, or groups of point sources. To 

 illustrate this point, the example of a heated metal sheet radiating 

 to surroundings at room temperature may be quoted. At a 

 temperature of 100° C this emits only one calorie per minute 

 from each square centimeter of surface. 



Calculation of Dosage 



The X-ray worker always has to deal with a point source 

 of radiation, so that the radiation comes to his patient as a 

 beam. For him it is a comparatively simple matter to calculate 

 the dosage received by the patient from the strength of the 

 source, the distance of the patient, the area irradiated, and 

 similar data. In radiant-heat therapy with extended sources, 

 such as, for example, radiant-heat cradles, there is no single 

 beam of radiation, and each part of the patient's skin receives 



