180 



BIOLOGICAL EFFECTS OF RADIATION 



Table 13. — Illumination Equivalent of a Gram Calorie per Minute per 

 Square Centimeter of Radiation with the Sun at Different Zenith 



Distances (43, cf. page 479) 



Thus we obtain for 1.5 gm. cal./min./cm.- 10,500 foot-candles as the 

 highest vakie commonly attained at sea level. He arrives at an average 

 value 6700 foot-candles for 1 gm. cal./min./cm.^, which is somewhat 

 low for high sun, and high for low sun. From this value for 1 gm. 

 cal./min./cm.-, we obtain 103.0 lumens/watt for the average luminous 

 efficiency of solar radiation (the range being 98 to 107 lumens/watt). 



Obviously, no black-body or tungsten-filament source could simulate 

 the solar distribution. Daylight glasses and also solution filters have 

 been developed which give requisite color temperature in the visible, but 

 they generally fail in both ultra-violet and infra-red. For the sake of 

 comparison, the relative distribution has been plotted, shown by dotted 

 curves, Fig. 20, on an arbitrary scale for a tungsten filament, tempera- 

 ture 2755°K., together with curves for combination with water filter and 

 heat-absorbing filters. 



TUNGSTEN FILAMENT 



Undoubtedly, the most convenient artificial source is the tungsten 

 lamp filament. It differs from the black body as a result of the reflection 

 of the metallic surface of the filament which of course contributes to a 

 correspondingly lower emission of radiation. For convenient comparison 

 with the radiation from a black body, the radiation from tungsten at 

 the same temperature has been plotted in dotted curves in Fig. 3. It 

 will be seen that while the radiation is less for all wave-lengths, thus 

 corresponding in magnitude to a lower temperature black body, the rela- 

 tive distribution of intensity according to wave-length corresponds to a 

 higher temperature, i.e. the maximum occurs at a shorter wave-length, 

 and the intensity at short wave-lengths is greater in comparison with the 

 long wave-lengths. This arises from the increase in reflectivity with 

 increasing wave-length. Table 7 gives the values for the radiance in 

 erg/sec. /cm. ^/steradian/cm. wave-length in a direction normal to the 

 surface, for both black body and tungsten. 



Other radiation properties of tungsten have been gathered together 

 in Table 14 (29, 30). Most of these data deal with the convenient conver- 

 sion factors by which one obtains the values for tungsten radiation from 



