SUNBURN 



■497 



and Terns (1946a), thresholds were measured on a number of subjects 

 under these different conditions and calculated in terms of E(i. (13-5). 

 The results summarized in Table 13-1 show that the approximation is not 

 a very close one. The threshold values are of the same order of magni- 

 tude, but vary systematically according to the source, being higher when 

 only the longer wave lengths are present. 



Fig. 13-4. Diagrammatic representation of the epidermis. /, /c, and /„ are intensi- 

 ties at the levels indicated; Ic and \m are the thicknesses of the layers indicated; Oe 

 and am are the attenuation coefficients of the layers indicated. {AjUr Blum and 

 Terus, 19466.) 



A simplified model may help us in inquiring into the causes of such 

 deviation. Let us indicate by D the quantity of dilator substance elabo- 

 rated in the malpighian layer and resulting in grossly observed erythema. 

 Then for a given wave length X, we may write 



D = 



At 

 hc/X 



7 



(13-6) 



where A represents the amount of radiant energy of wave length X 

 absorbed per unit of time by the light absorber for the photochemical 

 reaction leading to the formation of dilator substance, and t is the dura- 

 tion of the exposure. The (juantity .4/, like Q\, is measured in units of 

 energy, whereas the amount of D formed must be a function of the number 

 of quanta absorbed ; this number is obtained by dividing by the energy of 

 the quantum, hc/X, where h is Planck's constant and c is the velocity of 

 light. The coefficient 7 is the ratio of molecules of dilator substance 

 formed to the number of quanta absorbed, corresponding to the efficiency 

 or quantum yield of any photochemical reaction ; it may or may not vary 

 with wave length. 



Let us assume for convenience that the corneum and the malpighian are 

 sharply separated homogeneous layers which may be represented as in 

 Fig. 13-4. The intensity at the bottom of the corneum when a beam of 



