520 RADIATION HIOLOGY 



the spectral distribution of sunlight iiiidcr different conditions. Curve 

 describes sunlight outside the earth's atmosphere as determined by extra- 

 l)olation, showing a maximum at about 0.48 m- As the sunlight passes 

 through the atmosphere, its spectrum is modified by the absorption and 

 scattering of some wave lengths to greater extent than others, as indicated 

 in curves 1 and 2 of the figure. Curves R and C are, respectively, spectral 

 sensitivity curves for the rods and cones of the human eye. It may be 

 seen that the sensitivity of the eye becomes virtually nil before the long- 

 wave-length limit of sunburn is reached at 0.32 m- It is obvious, there- 

 fore, that the eye gives no direct information as to the amount of ery- 

 themal radiation in sunlight. Neither does it give much indirect infor- 

 mation, because some of the factors that greatly influence the intensity of 

 the erythemal portion of sunlight have little effect on the portion which 

 the eye sees. 



The amount of erythemal radiation in sunlight varies much more widely 

 with time of day, season, and other factors than does the visible portion of 

 the spectrum. This is because of differences in absorption and scattering 

 by the atmosphere. The intensity of the erythemal region of the spec- 

 trum of sunlight is greatly diminished by absorption by ozone, which cuts 

 off virtually all wave lengths shorter than 0.285 fx.^^ The wdde variation 

 in the intensity of the erythemal portion of the spectrum with season, lati- 

 tude, and time of day also depends on absorption by this gas. For 

 example, when the sun moves from zenith to G0° from zenith — the condi- 

 tions represented by curves 1 and 2 — the erythemal spectrum is greatly 

 reduced. But the change in intensity registered by the human eye is rela- 

 tively small because the gases of the atmosphere absorb the visible light 

 only slightly. Sixty degrees in zenith angle corresponds to 4 hr in time, 

 so even on a bright midsummer day one is not likely to be sunburned 

 before 8 o'clock in the morning, or after 4 o'clock in the afternoon, 

 although at both times the sun is shining brightly so far as the eye can 

 discern. Obviously, chance of sunburn should be judged in terms of time 

 of day rather than visual impression. Of course, it must be remembered 

 to correct for daylight saving and for the difference between the two 

 extremes of an official time zone, where the deviation from true time may 

 be over an hour. Variation in ultraviolet radiation with season is anothei- 

 matter that tends to be left out of account. In the north temperate zone 

 the maximum insolation occurs on June 21, the summer solstice. One 

 unfamiliar with the problem may be surprised to learn that the erythemal 

 radiation may be more intense at the beginning of May than it is at the 

 end of August. This is likely to lead to severe sunburn in the springtinn^ 

 when, moreover, the subject is not likely to have as thick a corneum as at 



" For a discussion of solar radiation and the factors dotorininin^ its intensity and 

 spectral di.stribution at the earth's surface see Chap. ,i of this volume by Sanderson 

 and Hulburt. 



