TANNING 



137 



Mercury 

 arc 



Tungsten 

 filament 



Corex D glass 



Tungsten 

 electrode 



Argon gas 

 Pool of mercury 



Fig. IV-3. "Sunlight Lamp" 

 (Type S-l) General Electric Company. 

 (By courtesy of Forsythe, Barnes, and 

 Easley [1931].) 



that the minimum perceptible degree is very definite. The second 

 degree is described as " vivid," but a moderate tan results from it and 

 the relative time of exposure is 

 about 2.5 times that of degree 1. 



Tanning 



The color of normal human skin 

 depends upon its pigment content 

 and the back-scatter of that part 

 of the incident energy which has 

 succeeded in penetrating without 

 absorption (Edwards and Duntley 

 [1939]). Below the external horny 

 layer (corneum) is found the basal 

 cell layer in which the principal 

 concentration of photoactive pig- 

 ments is located. A network of 

 blood vessels is found in the layers 

 below these basal cells. The next deeper layer is called the derma, 

 below which lie the subcutaneous tissues. 



It has been suggested that the tanning mechanism is probably a 

 photodynamic action in the presence of oxygen and is due to the oxida- 

 tion of pigments already present in a colorless reduced state. The 

 bleaching of tanned skin shows it to be a reversible process quite inde- 

 pendent of the formation of new pigment. 



The major pigments are found to be melanin and an allied diffuse 

 substance melanoid. Carotene was also found in the subcutaneous 

 tissues. In different races the melanin content of the skin is found to 

 increase in the following order: Japanese, Hindu, mulatto, and negro. 

 Albino skins apparently do not have the ability to produce pigmentation 

 even with the aid of long-wave ultraviolet light. 



Irradiation of the normal skin with long-wave ultraviolet light from 



o 



the spectral region extending from 3000 to 4500 A produces a darken- 

 ing of the pigment found in the skin. The maximal effect is obtained 



o 



from the narrow band of energy lying near 3400 A, according to Henschke 

 and Schulze [1939], or at 3850 A according to Hausser [1938]. 



It is doubtful that an erythema accompanies the tanning when this 

 spectral range is used conservatively. Owing to the filter action of the 

 cumulative darkening pigment, it seems reasonable to expect that the 

 erythema threshold accompanying tanning is very much greater. It 



o 



was found, for instance, that if a normal skin was irradiated with 3850 A 

 it took 18 X 10 7 ergs/cm 2 or 3.4 X 10 19 photons/cm 2 of skin to produce 



