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HANDBOOK OF PHOTOGRAPHY 



able throughout the day, the apparent reddening of the spectrum near sunrise or sun- 

 set being caused by the absorption of the shorter wavelengths as the rays pass through 

 longer paths of the earth's atmosphere. Because of the wide ranges which take place 

 in the intensity and spectral distribution of the sun's rays as they reach the earth, it is 

 evident that it is impossible to speak of "sunlight" (for any given particular condi- 

 tion) as representing a suitable standard of luminous intensity. 



Although the radiant energy from the sun, at any given time, cannot be used as a 

 standard, it does provide, however, one of the most convenient and practical of lumin- 



FiG. 13. 



M U U 

 Month 



-Solar illumination for each month of the year, and for various periods of the day, 

 as determined from measurements made at Washington, D. C. 



ous light sources. Numerous measurements on the spectral distribution and intensity 

 of the sun's rays which reach the earth have been made, and the mean or average 

 value of these measurements are well known, do not vary, and consequently may be 

 used to specify "mean noon sunlight." Thus, mean noon sunlight as measured at 

 Washington, D. C, is often used as a standard and as a comparison for other light 

 sources. 



Mean noon sunlight illumination for various times of the year and day at the 

 surface of the earth at Washington, D. C, is given in Table I. This table represents 

 measurements made with the sun shining unobscured on a cloudless day. The solar 

 illumination is shown graphically in Fig. 13. The curves are plotted in terms of hours 

 from noon, instead of in terms of local standard time. Thus the curve marked 2 

 applies for conditions at 10 a.m. or 2 p.m. standard time while the curve represents 

 noon data. 



The manner in which the relative luminous energy from the sun varies throughout 

 the day is shown in Fig. 14. These curves are plotted on a relative basis, with noon 



