PHOTOGRAPHIC MATERIALS 



127 



BLUE , GREEN 



RED 



1 



3 



however, and the degree of sensitizing is apt to vary, not only from plate to plate 

 but even over the surface of one plate, therefore, with the wide variety of sensitizings 

 now available in commercial emulsions and the possibility of obtaining any of a large 

 number of other sensitizings on special order, ^ the need for sensitizing by bathing has 

 practically disappeared. 



Measurement and Specification of Spectral 

 Sensitivity. — The spectral sensitivity of an emul- 

 sion can be measured by the ordinary sensitomet- 

 ric procedures by confining the exposing light to 

 a single wavelength, as with a monochromator, or 

 to limited regions of the spectrum, as with color 

 filters. However, a quicker and more convenient 

 method, which at the same time gives more 

 readily comprehended pictorial presentation, is to 

 expose the material in a spectrograph equipped 

 with a neutral density wedge over the slit.^ 



In either case, the results show the spectral 

 response of the material to the particular light 

 employed. In order to obtain the absolute spec- 

 tral sensitivity, it is necessary to take into account 

 the spectral distribution of energy in the exposing 

 light. 



Spectrograms contain the more complete gen- 

 eral information but require considerable care in 

 interpretation. In particular, it is necessary to 

 consider the spectral quality of the light used in 

 the exposure. Spectrograms are usually made to 

 "day-light," which is supposed to correspond to 

 "mean noon sunlight," which is defined as having 

 a color temperature of 5400°K., or to "tungsten" 

 at a color temperature of 2360°K., which is com- 

 paratively poorer in blue and stronger in red than 

 the "daylight." In either case, the short wave- 

 length, or blue, end of the spectrogram is artifi- 

 cially restricted by absorption by the glass in the 

 apparatus, so that the spectrogram is apt to give a 

 false impression of the sensitivity to blue. 



The wedge over the slit generally has a uniform 

 density gradient, so that the exposure decreases 

 logarithmically with the height above the base line. 



Spectrograms for various types of negative 

 materials are shown in Fig. 6. 



A spectrogram is, essentially, a response curve 

 for light of one particular quality. Equal-energy 

 curves (Fig. 5) in which the absolute sensitivities are plotted against wavelengths 

 give more quantitative information but are also much more difficult to construct. 

 From the equal-energy curve, response curves can be obtained for any light whose 

 spectral distribution is known, merely by multiplying the sensitivity at each wave- 

 length by the relative energy in the light at that wavelength. 



' Mees, C. E. K., Photographic Plates for Use of Spectroscopy and Astronomy, J. Optical Soc. Am., 

 21, 754 (1931); 22, 204 (1932); 23, 229 (1933). 



2 Jones, L. A., Photographic Sensitometry, /. Soc. Motion Picture Engrs., 18, 324 (1932). 



700 



7 



BLUE ' GREEN ' RED 

 400 500 600 



Fig. 6. — Spectrograms, made 

 with tungsten illumination, and 

 various color sensitizings, as fol- 

 lows: 1, ordinary unsensitized 

 emulsion; 2, 3, and 4, various 

 degrees of orthochromatic sensi- 

 tizing; 5, type A panchromatic; 6, 

 type B panchromatic; and 7, type 

 C panchromatic. Curve 8 shows 

 the spectral sensitivity of the 

 human eye. 



