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HANDBOOK OF PHOTOGRAPHY 



Even ordinary spectrograms, however, are very useful, particularly in comparing 

 various emulsions, since they present graphically the differences in spectral sensitivity. 

 This is particularly true with emulsions used for certain scientific purposes where the 

 sensitivity over a limited wavelength range may be very important. 



Unfortunately, neither the spectrogram nor the equal-energy curve can be defined 

 by a few numerical constants. For most practical applications, it is sufficient to know 

 the integrated response over various spectral regions, and this can be expressed 

 numerically. 



In most practical applications, as for instance in color photography, this selection 

 of definite spectral regions is accomplished by the use of color filters which transmit 

 only the region desired. Therefore, the "filter factors," or the ratios between the 

 sensitivity to the unfiltered light and the sensitivity to the portions of that light passed 

 by the filters, often give the most useful quantitative information. 



Obviously, the filter factor will depend on the quality of the light as well as the 

 transmission of the filter and the color sensitivity of the emulsion. Thus the factors 



1.2 



0.6 



"t^ 

 U 



5 l4 



^ 2-8 

 o 



-^ 2.2 



5.6 

 3.0 



6.6 5.2 5.8 4.4 3.0 3.6 2.2 2.8 1.4 0.0 Q6 1.2 1.8 

 LogKKC.) 



Fig. 7. — Curves for several typical photographic materials illustrating failure of the 



reciprocity law. 



quoted by the manufacturer of an emulsion can be accurate for light of only one qualitjs 

 and it will be necessary to make a correction when light of any other quality is used. 



Reciprocity Failure. — In calculations of photographic exposures, validity is 

 implicity assumed for the reciprocity law of Bunsen and Roscoe, which states that the 

 product of a photochemical reaction is proportional to the total energy involved, i.e., 

 the product of intensity and time, and is independent of the absolute value of either 

 factor separately. It has long been known, however, that this rule does not hold for 

 photographic emulsions and that for each emulsion there is an optimum intensity for 

 which the exposure required to give a certain density is a minimum. This is shown 

 for several commercial plates in Fig. 7, where the curves show the exposures required 

 to produce a density of 0.6 for a wide range of intensities. 



In most normal photographic practice this reciprocity law failure is not of great 

 concern, since, as can be seen, most of the curves show very little slope over the range 

 of exposure times ordinarily encountered. However, in any photometric work and 

 in cases where the exposures are very long, as in astronomical photography, or 

 extremely short, as in sound recording, the reciprocity faikire must be taken into 

 account. 



Closely related to the reciprocity failure is the intermittency effect, ^ or failure of 

 an exposure given in several installments to give the same density as a continuous 



1 Webb, J. H., The Relationship between Reciprocity Law Failure and the Intermittency Effect in 

 Photographic Exposure, J. Optical Soc. Am., 23, 157 (1933). 



