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HANDBOOK OF PHOTOGRAPHY 



the negative without diffusion, the value of density obtained from such measurements 

 will be higher than the diffused densit5^ The reason for this is that in this case not all 

 the transmitted light is used to actuate the indicating device; the diffuse component 

 does not affect the indicating device at all, or only slightly. Density determinations 

 based on such measurements are known as specular-density measurements. The 

 magnitude of the difference in determinations of diffuse and specular density will 

 depend upon the size and shape of the grains in the emulsion, grain-size frequency 

 distribution, thickness of the layer, number of developed grains per unit area of the 

 layer, and similar characteristics of the film. 



If the negative has some intermediate position from those mentioned above, some 

 intermediate value of density will be determined. Density determinations made in 



some such manner as shown in Fig. 10, 

 where the normal rays as well as some of 

 the diffused rays are effective in actuating 

 , the indicating device, are known as inter- 

 mediate densities. Obviously an indefinite 

 number of intermediate densities may be 

 determined, depending upon the relation of 

 Fig. 10. — Intermediate density may the film and the aperture of the integrating 

 be determined from measurements which sphere 



make use of the specular as well as some r^^^ re\2.tion between diffuse and specular 



oi the diffused rays of light passing ... , . 



through the negative. density gives a measure of the discrepancies 



of the various methods of density determina- 

 tion. The ratio of the diffuse density, sometimes designated by D\\ to the specular 

 density D-[}, has been designated as Q by A. Callier who first studied this subject. 

 The quantity Q, which is defined to be 



D\\ 



(15) 



is known as Callier's factor coefficient or Q factor. 



In general, Q may be expected to vary from 1.0 for grainless plates to 1.9 or more 

 for fast plates with appreciable grain. The value of Q is not a constant for a given 

 type of film or plate, however, but depends upon the density of the film or plate as 



Table III. — Measurements Showing Relation between Q and Density^ 



iFrom L. A. Jones, "Photographic Sensitometry," Eastman Kodak Co. (1934). 



