SUNJiUliN 



495 



Factors Affecting the Erythemal Threshold. There are various instances 

 ill wliirh it would bo coiu'enient to have a standard erythemal threshold 

 that would ai)pl.v to lii(> "average" person. This would be jiarticularly 

 us(>ful in coinp.Miiii^; the effects of differeiil polychi-omatic sources, e.g., 



0.23 



24 



25 



26 



27 0.28 



(a) 



29 



30 



031 



32 



UJ r 



75 



50 



2 5 - 



UJ 



I 



I I 



.J=^ 



--3Z; 



..^r^ri^ 



I 



s n 



m 



-i '--- 1 1 — -"-^v^^ u-r 



23 24 25 26 27 28 29 30 



WAVE LENGTH, JU 



31 



1 



32 



ib) 



Fig. 1.3-.3. Spectrum of a mercury arc (diagrammatic); (a) absolute intensities; (6) 

 "erythemal effective intensities," i.e., the absolute intensities corrected for the 

 erythemal spectrum shown at I in (a). The height of each vertical line represents 

 the intensity of radiant flux of this wave length (intermediate-pressure mercury arc). 

 The points where curve II cuts the vertical lines represent the intensities when a 

 Corex D filter was present. The points where curve III cuts the vertical lines repre- 

 sent the intensities when a pj'rex filter was present. Point V on the line at wave 

 length 0.2537 m represents the intensity of a low-pressure mercury arc, which may be 

 regarded as emitting only this wave length. {After Blum and Tents, 1946a.) 



natural sunlight and radiation from the mercury arc. On first consider- 

 ation, it might be expected that a reasonable approximation could be 

 made in terms of the erythemal spectrum, and there have been various 

 attempts to set such a standard. 



Let us represent the total amount of incident radiation within the 

 erythemal spectrum by the symbol Q and the threshold (luantity of such 

 radiation by Qr. Correspondingly, for monochromatic radiation we may 

 use the symbols Qx and Qxt- In Fig. 13-3a is diagramed the spectrum of a 

 mercury arc in the erythemal region. The spectrum is compo>sed of dis- 



