PHOTOGRAPHIC LIGHT SOURCES 



271 



spectral characteristics, all the light sources have been plotted with their intensity at 

 560 m/i taken as unity. 



Knowing the characteristics of the light source and the film, it is now possible to 

 determine the relative photographic effects of the light sources on a film of specified 



1.0 



0.8 



^0.6 



% 0.4 



<o 



0.2 



B 

 C 



300 



400 



700 



800 



500 600 



Wavelenglh in Millimicrons 



Fig. 9. — Relative spectral sensitivity of typical photographic materials. Curve A 

 represents the sensitivity of panchromatic material; curve B, orthochromatic materials; 

 and curve C, noncolor-sensitive materials. 



characteristics when a definite exposure is given. This photographic effect is known 

 as "photicity" and is analogous to luminosity in visual effects. The preceding equa- 

 tions indicate that we are to multiply, wavelength by wavelength, the sensitivity of 

 the film bj^ the spectral-energy distribution of the light source; the area under the 



180 



160 



140 



±120 



f 100 



QJ 80 

 > 



J 60 



IX 



40 

 20 



300 



400 



700 



800 



500 600 



Wavelength in mja. 



Fig. 10. — Spectral-intensity curves for common light sources used in photography plotted 

 relative to the energy at 560 millimicrons, which is assumed to be unity or 100 per cent. 



new curve is then proportional to the photographic intensity of the light source in 

 question when used with the film exposure time. In Fig. 11 are shown the resulting 

 curves for three types of photographic materials (orthonon, orthochromatic, and 

 panchromatic) when used with sunlight as a source of light. Since, for a given time 



