PHOTOGRAPHIC LIGHT SOURCES 273 



small extent is the increased photicity due to increased sensitivity of the peaks of their 

 spectral curves. 



Figure 12 shows curves plotted for the orthochromatic material whose character- 

 istics are given in Fig. 9, but when used with three different light sources. The curve 

 of smallest area is that obtained when a tungsten lamp at a temperature of 2360°K. is 

 used to expose the film. The intermediate curve is that obtained with a photoflood 

 tungsten-filament lamp, whereas the curve with the highest peak is that for sunlight. 

 These curves show that, for a photographic material whose spectral sensitivity is that 

 given by S\ of Fig. 9, the sun produces the greatest photicity; while photoflood lamps 

 are next. Tungsten-filament lamps at 2360°K. (a temperature commonly used in 

 photographic sensitometric work) produces the least photographic effect for the photo- 

 graphic material selected. The resultant photicity curves would be different for 

 photographic materials having other spectral-sensitivity curves. From an examina- 

 tion of the film and light-source curves, it would appear that the photographic effect of 

 the sun and of the photoflood lamps would be about equal for the panchromatic mate- 

 rials, although a curve would have to be plotted to test the accuracy of this statement. 

 It should be pointed out that Figs. 9 and 10 are plotted in terms of relative spectral 

 sensitivity, and relative spectral-energy distribution, respectively. Consequently 

 the photicity curves are relative and are to be used only for comparisons between two 

 or more materials plotted in the same figure. The curves could, of course, be plotted 

 in some definitely established units, but this is not necessary for illustrative purposes. 



Classification of Light Sotirces. — ^Light sources for photographic purposes may be 

 divided into two convenient classifications: (1) natural light sources whose inherent 

 characteristics of intensity, time of occurrence, time of duration, and spectral char- 

 acteristics are entirely out of the control of human beings; and (2) artificial light 

 sources whose characteristics are very largely under human control and are, conse- 

 quently, much more convenient and flexible than natural light sources. 



Natural Light Sources. — Natural light sources may be divided into the following 

 groups : 



1. Astronomical: 



a. The sun 



6. The moon 



c. The stars and planets 



2. Meteorological: 



a. Daylight, resulting from sun shining on the earth's atmosphere 



b. Lightning 



c. Aurora borealis 



3. Luminescent materials 



Of these natural light sources, only sunlight and daylight (which results from the 

 sun) are of sufficient importance to be generally used for ordinary photographic pur- 

 poses. For special purposes, especially scientific work, some of the other sources may 

 be used, but their use is relatively infrequent. 



The Sun. — The spectrum of the sun is continuous. Light from the sun is closely 

 approached by that of an incandescent source operating at about 5600°K. The spec- 

 trum of the sun as received at the earth's surface, is modified by the scattering and 

 absorption of the earth's atmosphere. Absorption and scattering produced by 

 particles in the atmosphere are the principle causes for this modification. Scattering 

 and absorption increase as the sun approaches the horizon, since for this condition, 

 the rays reaching the earth travel through a much longer atmospheric path than when 

 the sun is at the zenith. The intensity of the sun's rays reaching the earth depends 

 not only upon the time of day but also upon the time of year and the latitude of the 

 observer. Changes in the spectral distribution of the light of the sun are easily observ- 



