4D2 RADIATION HIOLOGY 



(Blum and Torus, 1940h), and thoro are regular changes with season and 

 other factors (see Ellinger, 1941). All these factors must he taken into 

 account in (luantitative studies, hut there are more serious difficulties 

 in the interpictation of measurements which will need discussion. 



The Ertjlhonal Spectrum. The \-alue of action spectra in characterizing 

 photohiological responses — aiding in fav'orahle cases in detecting the light- 

 absorbing sulistance in the primary photochemical process — has been dis- 

 cussed elsewhere in this book (see also Blum, 1950). The action spectrum 

 for sunburn as measured in terms of the erythemal threshold is generally 

 spoken of as the "erythemal spectrum." This has been determined by 

 several workers whose results are in close agreement as far as the general 

 character of the spectrum is concerned (Hausser, 1928; Luckiesh et a/., 

 1930; Coblentz et al., 1932). All these investigators found a maximum at 

 about 0.295 m and a minimum at about 0.28 m- It is customary to plot the 

 reciprocal of the threshold against wave length, as in Figs. 13-2, 3, and 7.'-' 



The shape of the erythemal spectrum must be affected by the corneum, 

 which overlies the malpighiaii layer and acts as a semi-opacjue filter 

 absorbing the wave lengths of the erythemal spectrum selectively. The 

 corneum is composed of flakelike elements and, owing to the reflection 

 and refraction of the light at the boundaries of these, the incident beam is 

 scattered very effectively. '^ This scattering greatly enhances the attenu- 

 ation of the radiation and renders measurements of the true absorption 

 difficult.''* Absorption spectra for human epidermis treated in various 

 ways to reduce scattering are shown in Fig. 13-2. Curves III and IV dis- 

 play the least scattering and may be taken as giving the most reliable 

 picture of the true absorption. Proceeding toward shorter wave lengths, 

 we see that absorption begins to increase rapidly in the neighborhood of 

 0.3 n, reaching a maximum at about 0.28 /x, then falling to a minimum 



'2 Hamperl et al. (1939b) state that they have found maxhna in the erj'theinal 

 spectrum at 0.295 /n and 0.26 fx with a minimum at 0.275 fx, and, for some skins with a 

 very thin corneum, another maximum at 0.23 n. These findings are described in a 

 brief note with no further detail and I have found no more complete description. 



1' Hen.schke (1948) has shown that the erythemal threshold for radiation striking 

 the skin at different angles follows the cosine law, which is characteristic of completely 

 diffusing surfaces. 



" In a layer which both ab.sorbs and scatters, as does the corneum, the attenuation 

 of a monochromatic parallel beam in passing through matter may be expressed as 



/, = / exp [-A-(/-x,sx)/| (13-1) 



where / and /; represent, respectively, the incident intensity and the intensity after 

 passing through thickness /; k is a constant. The absorption function r\ and the 

 scattering function sx are mutuall}* dependent l)ut may vary to different extents with 

 the characteristics of the absorbing layer and with wave length. In such a layer the 

 counterpart of the Bouger-Lambert absorption law is 



1 1 = /e--' (13-2) 



where the attenuation coefficient a = A-(rx,sx). 



