504 



RADIATION BIOLOGY 



/atioM of tlu* cpidciinis. Alxmt a yeai" earlier 1. Ilausscr (1988) liad 

 ii'portctl that a dark hiowii t-oloration i)f normal skin is brought about by 

 wave lengths longer than those which produce erythema. Ilenschke and 

 Schultze studied this phenomenon extensively, coming to the conclusion 

 that it represents the darkening of preformed melanin: it will be referred 

 to here as "pigment darkening." This differs in many respects from the 

 primary melanization that is brought about by the erythemal spectrum, 

 i.e., by wave lengths shorter than about 0.32 n. The action spectrum for 

 pigment darkening extends from about 0.8 ^ to about 0.42 n with a l)road 

 maximum near 0.34 ^i, as is represented in Fig. 13-8. In contradistinction 

 to erythema and pigment formation, pigment darkening is readily brought 

 about by sunlight passing through window glass, which by removing the 

 wave lengths shorter than 0.32 /j., prevents both erythema and primary 



024 026 028 030 032 34 36 38 040 042 

 WAVE LENGTH, _/^ 



Fig. 13-8. PD, action spectrum for pigment darkening. E, erythemal spectrum: S, 

 sunlight. All ordinates are arbitrarily chosen and do not indicate relative magnitudes. 

 (After Henschke and Schultze, 1939a.) 



melanization of the epidermis.'^ Pigment darkening may appear within 

 the first few minutes of exposure to sunlight and usually reaches its maxi- 

 mum within an hour, whereas pigment migration does not begin for at 

 least 24 hr, and new pigment formation only after a few days. Several- 

 hundredfold greater dosages of radiant energy are recjuired to bring about 

 pigment darkening than are necessary for erythema and primary melani- 

 zation. Pigment darkening is most pronounced in skin that has been 

 previously sunburned and still retains traces of suntan, whereas melani- 

 zation is greatest in skin not previously sunburned. Pigment darkening 

 does not occur when oxygen has been removed by blanching the skin by 

 pressing a quartz plate against it, whereas erythema and melanization are 

 not afTected by this treatment. Histological examination shows that 

 these longer wave lengths do not cause pigment migration or the forma- 



*" Common window glass, whicli is a fairly standard product, cuts out almost all the 

 wave lengths shorter than 0.32 ^l. \ person therefore does not ordinarily become sim- 

 burned by .sunlight passing through a window pane, although it is possible to elicit an 

 erythema in this way witli intense niidsuinmer sunlight. There have been window 

 glasses on the market in recent years which transmit somewhat more of the erythemal 

 radiation. 



