PHOTOGRAPHIC SENSITOMETRY 155 



chronological order: (1) sunlight, (2) the British Parliamentary candle, specifications 

 for which were given as early as 1860; (3) a phosphorescent plate in which the radia- 

 tion was provided by a layer of calcium sulphide activated by burning magnesium 

 ribbon; (4) the Harcourt pentane lamp which burns a mixture of pentane vapor and 

 air; (5) the Hefner lamp which burns pure amyl acetate (and which is still sometimes 

 used as a standard light source); (6) acetylene flames of various types with completely 

 specified burners; and (7) incandescent electric lamps. Unfortunately none of these 

 standards is ideally suited to photographic sensitometry, principally because of lack 

 of precision and reproducibility, but also in some cases because of unsuitable spectral 

 distribution of energy. 



The characteristics necessary in a light source for photographic sensitometry are: 



1. Accurately known (preferably standardized) and appropriate spectral-energy 

 distribution. 



2. Adequate intensity of output so that exposures need not be too long. 



3. Stability of characteristics throughout the useful life of the light source. 



4. Reproducibility of characteristics between successive uses of the light source. 



5. Convenience in use. 



These requirements are fulfilled in the most desirable manner, for purposes of 

 photographic sensitometry, by incandescent lamps which have been carefully stand- 

 ardized and which are operated under carefully controlled conditions. 



Modern practical sources of light for photographic sensitometry may be made 

 from : 



1. Cylindrical acetylene flame devised by Fery and Sheppard and Mees, fitted 

 with suitable filter (Wratten No. 79). 



2. Tungsten lamps operating at a color temperature of 2360°K. fitted with a Davis 

 and Gibson liquid filter. 



This latter type of light source was standardized as a satisfactory light source for 

 photographic purposes at the seventh International Congress of Photography which 

 met in London in 1928. 



Both the spectral-energy distribution of the radiation emitted from the luminous 

 source as well as the intensity or magnitude of these radiations affect the density of 

 the exposed and developed photographic material. Consequently these two charac- 

 teristics of the luminous source must be accurately known if precise sensitometric 

 results are to be obtained. 



The spectral composition of a luminous source specifies the relative amounts of 

 energy emitted at various wavelengths. Usually this relation is represented by 

 means of a spectral-energy curve in which the abscissas are wavelength and the 

 ordinates are relative energy. Figure 1 shows spectral-energy curves for one type of 

 luminous source. 



Effect of Spectral Distribution of Light Source. — It is important that the spectral- 

 energy distribution or color composition of the radiations emitted by any source 

 which is to be used as a standard of intensity in photographic sensitometry shall be 

 known and appropriate because photographic materials do not have the same spectral 

 sensitivity as does the human eye, which is used in the subjective evaluation of the 

 characteristics of any light source. Furthermore, with respect to their spectral 

 sensitivities, photographic materials differ enormously among themselves. Photo- 

 graphic materials are more sensitive at the blue and ultraviolet end of the visible 

 spectrum than elsewhere. Consequently a denser silver deposit will result if the 

 luminous source of given absolute intensity has most of its energy in the blue end of 

 the spectrum rather than in some other region. In fact, the use of certain types of 

 safe lights for photographic darkrooms depends upon so choosing the light source with 

 respect to the film characteristics that the visible light occurs at that portion of the 



