268 



HANDBOOK OF PHOTOGRAPHY 



The radiometric curves for various types of light sources are of importance in the 

 scientific and technical aspects of photography since the density of the image on the 

 negative depends not only upon the characteristics of the film or plate, together with 

 a filter, if this is used, but also to a very marked extent upon the characteristics of the 

 light source employed in making the exposure. This subject will be treated at greater 

 length in following sections in this chapter. 



Photographic Effects of Light. — The fundamental action of light impinging upon 

 a photographically sensitized material is to produce, after the necessary processing, a 

 deposit of finely divided metalhc silver which appears black by either transmitted or 

 reflected light. The blackness or opaqueness of this silver deposit is termed the 

 density of the photographic material. The density of the silver deposit depends, in a 



25 



20 



cl5 

 c 



10 



0.5 



0.6 

 A. 



0.7/1 



Fig. 7. — Line spectrum showing the relative intensity of the various Hnes in the spectrum 

 for a neon lamp. Practically all of the energy is in the red and orange part of the visible 

 band. 



rather complicated manner, on several factors, among the most important of which 

 may be mentioned: 



1. The intensity of the light source to which the photographic material is exposed. 



2. The spectral-energy distribution of the light source. 



3. The sensitivity or speed of the photographic material exposed to white light. 



4. The spectral sensitivity of the photographic material. 



5. The manner in which the exposure is made, i.e., whether the exposure is made 

 continuously or intermittently. 



6. The total duration of the time of exposure. 



7. The processing conditions which the photographic material undergoes subse- 

 quent to exposure. 



The effects of most of these factors upon the density of the photographic material 

 have been discussed in the chapter on Photographic Sensitometry. In this chapter 

 it is proposed to discuss, briefly, the first four factors listed above, since they are inti- 

 mately associated with light sources, and a knowledge of their effects will facilitate a 

 further discussion of the characteristics of light sources suitable for photography. 



