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511 



better understanding of the various factors involved. On first consider- 

 ation the problem appears essentially a physical one. The desired end is 

 to obtain a protective layer of some kind that will absorb the erythemal 

 spectrum but preferably not absorb much of the visible. Obviously, the 

 film cannot be too thick, must be pliable, and should meet certain "cos- 

 metic " requirements. The corneum of the skin, which may be taken as a 

 model for it, seems to be a much better protective agent than any artificial 

 one that has been devised. The corneum is a scattering and absorbing 

 agent with a very high attenuation coefficient for the wave lengths of the 

 erythemal spectrum. The problem becomes more complex when another 

 selectively absorbing layer is put on top of the corneum. 



Fig. 13-9. Diagrammatic representation of the optical conditions of epidermis with 

 a layer of preventive superposed. /, Ip, I^ and /„ are intensities at the levels 

 indicated; Ip, U, Im are thicknesses of the layers indicated; Up, Uc, and am are attenu- 

 ation coefficients of the layers indicated. (After Blum et al., 1946.) 



We may extend our previous analysis to cover this situation. Figure 

 13-9, in which the preventive is indicated as having the thickness Ip and 

 the attenuation coefficient a^, illustrates this situation. The intensity Ip 

 of a monochromatic beam at the bottom of this layer is given by 



Ip = /e-""'". (13-15) 



Following the same reasoning as in developing Eqs. (13-9) to (13-12) 

 we may write 



Dhc 



Qp\ — 



^g-aplpQ-adc^l _ e-«".''")^]^X 



(13-16) 



to describe the situation when the preventive is in place. An index of the 

 protection afforded by the prev^entive should be given by the ratio of the 

 thresholds with (Qpr) and without {Qr) the pre\'entive; this ratio we will 

 designate for monochromatic radiation as P\, which is related to the other 



