512 



(limiit it i(\s l)y 



UADIATION BIOLOGY 



Dhc 





Q,n [e-»i>'pe-»'''(l — e-"'-''")/3]7\ 



Dhc 



(13-17) 



[g-ac/c(l _ e-«".'".)|3]'yX 



Since for monochromatic- radiation the vahie.s of 1) at the threshold should 

 be the same with or without the preventive, we may cancel out to obtain 



l\ = ('--"'-. (13-18) 



That is, with monochromatic radiation the ratio P should measure, 

 within the limits of accuracy, the attenuation by the preventive, and 



8 12 16 20 24 28 32 36 40 



ERYTHEMAL EFFECTIVE ENERGY THRESHOLD fQJ, 

 ergs cm'2 x lO"* 



Fig. 13-10. Variation of protection P with erythcnuU threshold in 22 suhjeets. All the 

 points were ol)tainP(l with layers of the same sunburn preventive 25 m thick spread on 

 skin of the abdomen. (After Bhnn et ai, 1946.) 



should be independent of the threshold of the individual. P\ would, of 

 course, vary with wave length acc^ording to the spectral characteristics 

 of the preventive. 



With polychromatic radiation — e.g., sunlight — the problem is not so 

 simple, and in actual measurements it was found that the ratio P varied 

 systematically with the threshold as illustrated in Fig. 13-10. Let us 

 examine the possible reasons for such an unexpected variation. 



We may write E(\. (13-13) in a more general form, such as might be used 



