136 



BIOPHYSICALLY ACTIVE LIGHT 



TABLE IV-2 



Minutes Required for the Production op Erythema prom Artificial Sources 



By courtesy of M. Luckiesh. 



It should be kept in mind that a value of erythemal effectiveness 

 (Table IV-1) expressed in watts per square centimeter remains un- 

 changed only as long as the physical characteristics of the radiation do 

 not change. For example, such a factor established for a bare source 

 of radiation ceases to hold when the source is placed in a reflector which 

 does not reflect the ultraviolet rays as well as the visible rays. In Table 

 IV-2 is indicated the exposure time required to produce two degrees 

 of erythema, with common commercial artificial sources placed at a 

 distance of 76 cm (30 in.) from average untanned skin. 



Erythemal measurements are semiquantitative and uncertain, since 

 the untanned skins of individuals vary over a wide range, and it is essen- 

 tial to recognize a skin of approximately average sensibility. An 

 approach to a quantitative method of detecting " degree of erythema " 

 is attained by means of a strip of tape in which is punched a series of 

 holes \ to ^ in. in diameter. This is attached to the untanned skin of 

 the back, chest, or abdomen. The source is placed at a measured dis- 

 tance from the skin and then the effect of the incident energy is observed 

 at a given skin distance. As the time passes, the holes are successively 

 covered; the result is a series of exposures of various durations. For 

 accurate work a second series of holes should be used adjacent to the 

 first as a check. The intensity of illumination is varied according to 

 the inverse square law, and the minimum perceptible erythema is fairly 

 easy to establish. 



Luckiesh, during an extensive study of the reciprocity law over a 

 range of 25 to 400 ft-candles with the " Sunlight (Type S-l) Lamp " 

 (Fig. IV-3), found that the equality of products of exposure time and 

 ft-candles holds very well for this type of mercury-arc radiation, and 



