PHOTOGRAPHIC LIGHT SOURCES 



277 



Meterological Light Sources. — Of the meterological sources of light, skylight or 

 daylight is by far the most important for photographic purposes. Approximately 

 one-fifth of the total illumination on a clear day is due to light from the sky (which, 

 of course, results from the scattering of sunlight in the earth's atmosphere), rather 

 than direct sunlight itself. For a cloudy, overcast sky in which the sun is not visible, 

 the illumination on a vertical surface is almost independent of the direction of this 

 surface and that of the sun. For a clear day, however, the illumination varies con- 

 siderably with the angle between the sun and the vertical surface, for in this case direct 

 sunlight enters into the consideration of the total or net illumination. 



Table II shows the illumination from daylight for various conditions, whereas 

 Table I shows the illumination due to direct sunlight. The total illumination may be 

 estimated from these tables. For a clear day the net illumination is the sum of these 

 two illuminations. On cloudy days with the sun obscured, the net illumination is that 

 due to daylight and is given in Table II. For other conditions, some suitable propor- 

 tion of the two components of the net illumination should be used. 



Artificial Light Sources. — Artificial light sources may be classified for convenience 

 according to the type of spectra they produce, or the purity of the emitted spectra. 

 For special types of photographic work (especially in scientific photography) such 

 classifications may be useful. However, for ordinary photography a somewhat 

 arbitrary classification, based largely upon the apparatus employed to produce the 

 light, appears to be generally more useful and will be employed in this section. 



Table II. — Illumination from Sky Light, Lumens per Sq. Ft.'- 



Solar 

 altitude, 



On hori- 

 zontal 

 surface 



Zenith 

 luminosity 



Clear Sky, Winter 



1 From H. H. Kimball, Trans. Ilium. Eng. Soc. (N. Y.), 18, 457 (1923). 



