PHOTOGRAPHIC SENSITOMETRY 



183 



Certain development characteristics of any particular photographic material may 

 be deduced from the values of 7 and k. The following table is prepared from material 

 on this subject by L. A. Jones. 



Time of Development for Required Gamma. — Sometimes it is required to know the 

 development time to obtain a given value of 7. Provided the development constant is 

 known [or can be calculated from Eq. (33)], the development time can be determined 

 from the equation 



7 = 7oo(l — e-f") = 7„ — 7„e~*^« (34) 



(35) 



Taking the natural logarithm of both sides, dividing by k, and making all values posi- 

 tive, we obtain 



-l'-(^f^) 



(36) 



It is therefore seen that the time of development depends upon the value of 7 to 

 which the material is processed, as well as upon the maximum value of 7 of the mate- 

 rial. For purposes of preparing tables showing relative sensitometric characteristics 

 of photographic materials, a value of gamma equal to unity is arbitrarily chosen. 

 The time of development to attain unity gamma is expressed symbolically as Ty=i and 

 is given by 



-IM^) 



(37) 



If 7„ is less than unity, this equation has no meaning, as evidenced by the fact that 

 Ty^i will then be negative. 



Spectral Sensitivity of Photographic Materials. — No treatment of the subject of 

 photographic sensitometry would be complete without some mention of the spectral 

 sensitivity of photographic materials or the relative sensitivity to radiations of various 

 colors or wavelengths and the methods of determining spectral sensitivity. This is 

 especially true since panchromatic materials have come into extensive use. The 



