182 BIOLOGICAL EFFECTS OF RADIATION 



those of the black body, which is the underlying basis of all radiation 

 measurements. The first column gives the actual temperature in degrees 

 Kelvin. The second, spectral emissivity, is the ratio of the radiance of 

 tungsten to that of the black body at the same temperature for 0.665 n 

 or 6650 A. Thus a tungsten filament at 1000°K. emits 0.456 as much 

 radiation at 6650 A in the red as would a black body of that temperature. 

 From the third column, we find the corresponding value of 0.486 for 

 0.467 IX or 4670 A in the blue, i.e., tungsten emits 0.486 as much in the 

 blue as the black body at the same temperature. The fourth column 

 gives the average relative emissivity e^ throughout the visible. Thus 

 tungsten at 3000°K. emits 44 per cent of the number of lumens radiated 

 by a black body at that temperature. As we have seen from Fig. 3, 

 the wave-length distribution of tungsten corresponds to a higher tem- 

 perature of black body. The term color temperature (eighth column) 

 gives the temperature of a black body which would most nearly match 

 that of tungsten at the given temperature, in relative distribution or 

 color, but not in magnitude (or brightness). Since tungsten emits less 

 for all wave-lengths than the black body at the same temperature, it 

 emits still less in comparison with the black body at the color tempera- 

 ture, which is higher. Thus, we find the color emissivity ec (Column 5) 

 which is relative to the black-body emission at the color temperature less 

 than the luminous emissivity e„. As has been noticed, tungsten emits 

 much less relatively in the infra-red than in the visible. Hence compar- 

 ing the total emission with that of a black body at the same temperature, 

 one finds a much lower value for the total emissivity et than for the lumin- 

 ous emissivity e^. These are given in Column 6. The brightness 

 temperature (Column 7) for 6650 A is the temperature for which the 

 black body would emit the same luminous intensity as tungsten for that 

 wave-length. These values are, of course, always lower. 



The radiation temperature is that for which the black body would 

 jdeld the same total radiation. From Column 9 we see that this is much 

 lower than the given temperature. The radiant flux density, sometimes 

 termed total radiant intensity (Column 10) enables us to compute the 

 total power emitted from any tungsten surface of known temperature and 

 area. In a similar way, from Column 11, (normal brightness in candles 

 per square centimeter) we can compute the candlepower for known 

 temperature and area. It is also convenient to remember that this 

 normal brightness is equal to the number of lumens/steradian/cm.^ in 

 a direction normal to the surface. The last column gives the lumens 

 output for 1 watt input. Since most of the electrical energy is dissipated 

 by radiation, this is close to the value for the relation of lumens to watts 

 in the radiation. 



It is interesting to compare these values with those of solar radiation, 

 which we found to be about 103 lumens/watt. A high-temperature 



