THERMAL INTERCHANGE WITH ENVIRONMENT 



281 



received. Thus we may write for this 

 transfer: 



(5) Hu = So €1 €2 (T* - To') A, 



in which Hr = heat transfer in gm. cals./ 

 sec, So = Stefan Boltsman constant = 

 1.37 X 10-12 gjjj cal./sec./cm.2, T and To = 

 absolute temperature of the hot object and 

 its environment, ci and €2 = the emissivity 

 of the surfaces of the radiator and environ- 

 ment, with maximum values of unity, and 

 A = the effective radiating area of the hot 

 object. 



Emissivity: The Black Body Radiator 



By definition, a "black body" is one which 

 reflects none of the radiation which strikes 

 it. That is, it absorbs completely radiation 

 of all types and reflects nothing. The term 

 "black body" is thus quite appropriate. 

 No physical object is completely black in this 

 sense, since every object reflects some hght, 

 even though it be a small amount. In 

 contrast to the "black body" is the perfect 

 reflector, a type of surface which is 

 approached in nature by highly poUshed 

 metalhc surfaces (see Table VI). If an 

 object reflects a small but equal amount of 

 light of all wavelengths, it is termed a "gray 

 body." Most surfaces are black, or almost 

 so, for some radiations and not black for 

 others. Such surfaces are colored, and it is 

 to this class of surfaces that the human skin 

 and most clothing belong. The white skin 

 reflects visible light well and does not reflect 

 at all well the invisible infra-red light. 

 Neither negro nor white human skin is by 

 any means "black" for aU types of radiation. 

 In fact, the white skin reflects about 30-40 

 percent of the sun's radiation, whereas the 

 dark skin of the negro reflects less than 18 

 percent of these rays, according to Oppel 

 and Hardy (32). However, in the far 

 infra-red, the region in which the skin 

 reflects httle, the skin is a good radiator. 

 Therefore, for all practical purposes, human 

 skin, regardless of its color, can be taken to 



be approximately black body for the range 

 of wavelengths in which it radiates. 



Another important factor is the emissivity 

 of the environment. For example, if one 

 were in the center of a large sphere which 

 had highly reflecting walls, nearly all of the 

 radiation emitted from the body would be 

 reflected back to be absorbed by the sldn. 

 Therefore, even though the wall of the 

 sphere were very cold, the skin could lose 

 little heat to it by radiation. 



For indoor environments in which the 

 walls are at air temperature and are painted, 

 a survey of the waUs with a radiometer (19) 

 wiU give their temperature, and €2 may be 

 assumed to be unity for practical purposes. 

 In more complicated environments, in which 



TABLE VI 



Table of Emissivities 



(Low Temperature) 



(X max ^ 9-10 m) 



hot objects or other radiators are present, 

 resort must be made to instruments such as 

 the thermointegrator (39), or the Globe 

 thermometer (4). Because of the simplicity 

 and utility of the Globe thermometer as a 

 means of estimating the combined theimal 

 effect of air temperature, wall temperature, 

 and air movement, a reading of this refer- 

 ence (4) is particularly recommended for 

 those concerned with air conditioning in 

 undersea craft. 



The Radiating Surface Area 



It is the body area that is open to the 

 environment that is effective in contributing 

 to heat loss by radiation, since the skin 

 areas between the fingers, under the arms, 

 between the legs, under the chin, etc., 



