184 HANDBOOK OF PHOTOGRAPHY 



recent introduction of practical forms of color films also makes desirable a treatment 

 of spectral sensitivity of photographic materials. 



A knowledge of the way in which sensitivity of photographic materials is dis- 

 tributed throughout the spectrum is of importance from the practical as well as the 

 theoretical points of view. In monochrome photography the spectral sensitivity of 

 the negative material completely governs the monochrome brightness of the colors 

 in the original image, although filters may be used to modify spectral characteristics 

 of the negative. The rendition of color in color photographic processes is determined 

 largely by the spectral sensitivity of the negative material. It is well known, for 

 example, that in black-and-white photography fairly pure red, orange and yellow 

 are rendered in about the same tone value as black with ordinary blue-sensitive mate- 

 rials. In the case of some panchromatic materials which have been rendered very 

 sensitive to the longer wavelengths of visible radiations, these same colors may be 

 rendered as almost white. The correct rendering of colored objects on the black-to- 

 white tone scale, which represents the entire discrimination gamut of the photographic 

 process, is conditioned almost entirely by the spectral sensitivity of the material. 

 It is evident, therefore, that a knowledge of the spectral sensitivity characteristic 

 of photographic materials which determines responses such as are mentioned above, 

 is of great importance wherever monochrome or color rendering of colored objects 

 is to be considered. 



A brief outline of the historical development of the determination of spectral 

 sensitivity is given by Jones. ^ 



Methods of Determining Color Response. — All the available methods used 

 for determining the spectral sensitivity or color response of photographic materials 

 involve the isolation of more or less narrow spectral bands. These bands are then 

 impressed on the photographic material under specified exposure conditions. The 

 response produced when the photosensitive material is exposed to these more or less 

 homogeneous radiations is then observed either qualitatively or quantitatively. For 

 this observation a wide variety of spectral instruments has been devised. Suitable 

 instruments include monochromatic sensitometers, spectrographs, tricolor tablets, 

 ratiometers, color charts, and filter assemblies. The methods for producing the 

 required spectral bands may be grouped into (1) dispersion radiation methods and 

 (2) methods using filters and white light. The dispersion methods make use of some 

 suitable dispersion element such as a prism or grating and may be roughly classified 

 into two types, depending upon what proportion of the spectrum acts on the photo- 

 graphic material at a given time. In one classification, only a narrow band of the 

 spectrum is exposed to the photosensitive material, and the type of instrument used to 

 provide the necessary narrow band of radiant energy is a monochromatic sensitometer. 

 In the other classification, which may make use of a spectrograph, the entire visible 

 spectrum — or a fairly large part of the spectrum — may be used in exposing the photo- 

 graphic material. Since the method of operation is different and since the results in 

 these two cases are expressible in different ways, it seems desirable to give some 

 brief consideration to these two dispersion methods. 



Dispersion ilethod Using Monochromatic Sensitometer. — In the dispersion method 

 using a monochromatic sensitometer, radiation of high spectral purity — as obtained 

 from a grating, prism, or double prism — is permitted to expose the photographic mate- 

 rial for varying lengths of time, corresponding to varying exposvires. The time of 

 exposure is varied by some suitable mechanism, a satisfactory method being that in 

 which a slotted sector disk provides varying exposures when it is rotated. A mono- 

 chromatic sensitometer operating on this principle and described by Jones and Sand- 



1 JoNiBS, L. A., "Photographic Sensitometry," Eastman Kodak Co. (1934). 



