142 



BIOPHYSICALLY ACTIVE LIGHT 



the core of the carbon, selective emission in the hot gases may be utilized 

 to enrich different parts of the spectrum. If the lower terminal of such 

 an arc, set in a vertical position, is made of a large cylinder of iron with 

 a shallow depression in its center, the arc is termed an " iron arc " and 

 is extraordinarily rich in ultraviolet radiations. In the commercial 

 types of carbon arc the amount of ultraviolet radiation between 2800 



o 



and 3100 A varies considerably. For example, the energy distribution 



15 



10 



2000 



3000 4000 



Wavelengths in A 



5000 



6000 



Fig. IV-7. Emission of " Sunshine S " carbon and " Therapeutic B " carbon 

 arcs as compared with quartz mercury arc. S and B, bare arc without reflector, 

 30 amp, alternating current, 50 volts across arc. Hg, quartz tube mercury arc, 

 no reflector, average voltage 140 and 170, alternating current, and 4 amp. Each 

 square represents 250 microwatts of radiant flux per square centimeter at 1-meter 

 distance from the arcs. (Data by courtesy of the National Carbon Company.) 



of the radiation obtained from the " therapeutic carbons " of the Na- 

 tional Carbon Company as determined by Greider and Dowries are 

 shown in Fig. IV-7. The " Eveready therapeutic B carbons " and the 

 " Sunshine carbons " are marked B and S, respectively. The B carbon 

 supplies more biologically active radiation than the S carbon. The B 

 carbon contains iron. It gives light with a slightly bluish tinge but in 

 the visible range has a candlepower less than one fourth that of the S 

 carbon. 



The spectrum of B carbon is characterized by many lines that extend 



o 



from the visible through the ultraviolet region to 2300 A or shorter. It 

 is very intense in the long-ultraviolet region. In order to avoid con- 

 junctivitis during exposure with this type of carbon arc it is necessary 

 to protect the eyes with glass goggles. 



Finsen Unit 



Inasmuch as interest in the biological effectiveness of ultraviolet 

 radiation arose first in therapy, powerful sources of ultraviolet radiation 



