190 HANDBOOK OF PHOTOGRAPHY 



Here, as with the case of the spectral sensitivity determined by means of the wedge 

 spectrograph, the results obtained are the spectral photicity of the material, i.e., the 

 relative sensitivity evaluated according to the light source used in the measurements, 

 rather than the absolute sensitivity of the photographic material. However, it is 

 ordinarily the spectral photicity, rather than the absolute spectral sensitivity of the 

 material, which is of greater practical importance. 



The method of determining the tricolor sensitivity of a photographic material 

 through the use of three filters which divide the spectrum into three approximately 

 equal broad bands may be extended, if desired, so that the spectral response at more 

 than three points may be determined. The procedure is the same as that already 

 outlined, except that more filters (usually "monochromatic" filters) are used to 

 determine the relative response at various wavelengths. Comparison with the test 

 strip for which no filter was used will enable the relative sensitivity to be determined 

 for the wavelength band which the filter transmits. The results obtained in this 

 manner can be expressed numerically, but the method becomes laborious and time 

 consuming and is not very precise. 



A similar method of determining the relative spectral-response characteristic of 

 photographic materials is available through the use of progressive cut filters, which 

 may be used to isolate various regions of the spectrum for the determination of the 

 relative spectral response in the transmission range. The disadvantages of this sj^stem 

 are similar to those enumerated above. This method has been used where a mono- 

 chromatic spectrophotometer or a wedge spectrograph is not available but otherwise 

 has little practical use. 



Speed or Sensitivity. — By speed or sensitivity of a photosensitive material is meant 

 the amount of radiant energy required to produce a stipulated density or density 

 difference. It has already been mentioned that the characteristics of the Z)-logio E 

 curve depend to a marked extent upon the processing to which the photographic mate- 

 rial is subjected. For the complete specification of the H and D characteristics of a 

 photographic material, a family of Z)-logio E curves is required. Therefore it is 

 evident that no single numeric or "figure of merit" can completely and adequately 

 specify the characteristics of the material. Nevertheless, several difi^erent methods of 

 expressing sensitivity or speed by means of a single quantity have been developed and 

 are more or less widely used in this country as well as in Europe. Because of the 

 fact that the true speed and sensitivity of the material depends upon the processing and 

 because the processing conditions in actual use frequently differ widely from those for 

 which the speed determinations were made, the absolute values of film speeds have 

 little significance. Relative speeds, as determined by the same manufacturer for 

 different types of emulsions of his manufacture, have somewhat greater significance. 

 Speed ratings of one manufacturer may not be comparable with those of another. 



It should be pointed out that the comparison of various types of photographic 

 sensitive materials on a basis of speed or sensitivity is a problem which not only is 

 difficult but which has not yet been completely and satisfactorilj^ solved. Several 

 methods of determining speed or sensiti^aty of photosensitive materials are available, 

 and each s.ystem has its advantages and shortcomings. In most of the systems the 

 speed rating is affected by conditions of processing, and this introduces complications 

 of a practical nature, tending to mitigate against too wide dissemination of speed 

 ratings by manufacturers who find themselves in a competitive field. Because of the 

 lack of any generally accepted and recognized standard of speed, manufacturers are 

 sometimes reluctant to give speed ratings of their photographic materials. Moreover, 

 the various methods of rating speed do not give comparable results, and for this 

 reason, the tables of film speeds which are published should be regarded merely as 

 qualitative and approximate. 



