PHOTOGRAPHIC SBNSITOMETRY 201 



which, except for the necessary changes required by the fact that we are dealing with 

 reflected light rather than transmitted light, is identical with the definition of density 

 already given by Eq. (13). 



Just as the scattering of light in passing through a negative produces values of 

 density which depend somewhat on the conditions under which measurements are 

 made, so does the scattering of light from the surface of the paper cause similar 

 vagaries in the determination of the density of printing papers. The reflectance of 

 printing papers partakes of the characteristics of both diffuse and specular effects. 

 Therefore, the conditions under which density measurements are made, and especially 

 the angles at which the light strikes and is reflected from the paper, should be specified. 

 Furthermore, the surface texture of the particular grade of paper affects appreciably 

 the reflection coefficient R. The result is that, although equal amounts of silver 

 deposit per unit area should, theoretically, give the same density readings under ideal 

 conditions, this condition is not encountered in practice. 



There are two fundamental characteristics of printing materials (so far as concerns 

 their sensitometric characteristics) which are of major importance in the making of a 

 proper print. ^ These are: (1) the contrast or the density range and (2) the exposure 

 range of the material. 



The density range, by which is meant the range in density from the minimum (or 

 zero) to the maximum of which the paper is capable, is termed the contrast of the 

 paper. ^ Thus, if the maximum density of the paper is D2 and the minimum density is 

 Di, then the contrast (or density range) of the printing material is [as has been shown 

 in Eq. (18)] 



C = AD = D2 - Di (52) 



The density range, or contrast, of the printing material determines the maximum and 

 minimum blackness of the silver deposit on the print. For a given type of emulsion, 

 on a paper surface of specified texture, the density is a function of the exposure, as 

 given by the exposure-density characteristic of the printing material. But for a 

 given type of emulsion and a given exposure, the density obtained is affected by the 

 surface texture of the paper. The density range, or contrast, is important in that it 

 determines the range in tones, from black to white, which are to appear in the final 

 print. 



The second important characteristic of printing materials is the exposure range 

 Er of the printing material. It is defined in much the same way as latitude for nega- 

 tive materials, 



Er = logio E2 - logio El (logio E units) (53) 



E 

 — -pT (exposure units) (54) 



and defines the range of exposure (exposure is defined to be ^ = It) corresponding to 

 the density range for which we may expect to obtain a deposit of silver when the print 

 is exposed. 



The exposure range is of practical importance in that it determines the minimum 

 to maximum density range of the negative from which a satisfactory print may be 

 made. If we consider the negative as the original subject and if we consider that we 

 are "photographing" this negative (which we now consider as our original image), 



1 There are, of course, other factors which may be taken into account for aesthetic, psychological, 

 or artistic reasons, but we are not concerned with these factors at this point. 



* Perhaps it may be well to remind the reader that we are here concerned only with the sensitometric 

 aspects of printing materials. The subjective evaluation of contrast, from the artistic, aesthetic, or 

 psychological point of view may not always agree with the sensitometric definition. But we are not 

 concerned here with subjective evaluations. 



