PHOTOGRAPHIC LIGHT SOURCES 



275 



sunlight for each month taken as 100 per cent, but, as 100 per cent for the various curves 

 represents different values of absolute radiant energy, Fig. 14 should not be used in 

 such a manner that data from one curve is transferred to or compared with data of 

 another curve. These sets of curves show, for example, that at 9 a.m. (or 3 p.m.) the 

 light from the sun in October (or February) is about 82 per cent of its value at noon 

 for the same months, and therefore gives some indication of the increase in exposure 

 which should be allowed if photographs are made several hours before or after noon. 

 It is not correct to interpret these charts to indicate, for example, that the light from 

 the sun at 4 p.m. (or 8 a.m.) in June will be equal to that at 9 a.m. in October because 



100 



90 



80 



70 



:p60 



D 



§ 50 



i_ 

 _g 



to 40 



•^30 



20 



1 2 3 4 5 6 7 



Hour from Meridian 

 Fig. 14. — Diagram illustrating the manner in which the relative solar illumination 

 varies throughout the day. The curves are plotted with noon illumination for each par- 

 ticular month as 100 per cent. 



both happen to have values which are 82 per cent of their maximum value at noon, 

 for the noon sunlight in June is greater than that in October. It should be noted that 

 these curves are plotted for Washington, D. C. The use of this table at latitudes much 

 different than that of Washington would require correction. 



The spectral-energy distribution of the sun is an important consideration when 

 photographic work is done by daylight. Early in the morning and late in the after- 

 noon, the radiation of the sun contains relatively much more red and orange than it 

 does at noon. For panchromatic materials little or no increase in exposure may be 

 required in consequence of this fact. However, for orthochromatic materials, it may 

 be necessary to increase the exposure late in the afternoon or early in the morning 

 several times that required for panchromatic materials, simplj^ because there is rela- 

 tively less blue radiation in the sunlight (to which the orthochromatic material is 

 sensitive) and relatively much more red (to which the orthochromatic materials are 



