ABSORPTION OF RADIANT ENERGY 143 



have been studied and exploited primarily for their therapeutic effective- 

 ness. It must be emphasized, however, that there is no necessary rela- 

 tion between erythemal effectiveness and therapeutic value. As a 

 result, the establishment of a system of units based on erythemal effec- 

 tiveness must not be viewed as a solution to the problem of measurement 

 of energy in the ultraviolet region for general biological application. 



The practical necessity of establishing some criterion for the compara- 

 tive evaluation of different types of ultraviolet sources for medical 

 purposes cannot be escaped. This has led to the adoption of certain 

 standard units. 



The Council on Physical Therapy [1934] has tentatively adopted the 

 Finsen unit (F.U.) as a beam of homogeneous radiation of wavelength 

 2967 A with flux density of 10 /iw/cm 2 . 



It has been observed that 2 F.U. for 15 minutes is a representative 

 requirement for minumum perceptible erythema. Therefore it has 

 been proposed that 1 erythemal unit be denned as 2 F.U., or 20 juw/cm 2 



o 



for 15 minutes of homogeneous radiation of wavelength 2967 A. 



In the use of such a unit it must be borne in mind that even small 

 changes in operating conditions of a source will change the energy-wave- 

 length output and hence the radiation erythemal equivalents. 



Absorption of Radiant Energy 



The transmission of radiant energy through a medium is always 

 accompanied by a certain amount of absorption, regardless of the wave- 

 length incident on the medium. Media which are commonly referred 

 to as transparent, if not employed in too great thickness, transmit with- 

 out appreciable absorption the range of wavelengths comprised within 

 the region of the visible spectrum. 



In general, however, thej^ show powerful absorption in the infra-red 

 and ultraviolet regions, and if a sufficiently great thickness is employed 

 absorption will be found present even in the range of visible radiations. 

 Pure water, which is one of the most transparent of the common sub- 

 stances, appears distinctly blue in long columns, showing that it absorbs 

 more or less completely the red end of the spectrum. 



It is conventional to distinguish between two types of absorption: 

 general, in which the absorbing power is very nearly the same for all 

 wavelengths ; and selective, in which the absorption is more or less limited 

 to a narrow spectral region. Lampblack, developed photographic films, 

 neutral filters, and some forms of close-meshed rocking wire screens 

 represent the first type. Analine dyes, inorganic colored salts, blood, 

 bile, and generally all colored media represent the second type, in which 



