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HANDBOOK OF PHOTOGRAPHY 



available, the sensitizing was improved to the extent that the present supersensitive 

 type of panchromatic emulsions have very high red and green sensitivitj^ 



The use of sensitizing dyes is not limited to the visible regions of the spectrum but 

 has enabled the sensitivity to be pushed far into the infrareu; hence now the photo- 

 graphic emulsion is much more versatile than the eye, and it is possible to take pictures 

 with the completely invisible radiations on either side of the visible spectrum. 



Types of Dyes. — In order to exert a sensitizing action, a dye must stain the silver 

 halide; beyond this, however, the various sensitizers apparently have no common 

 characteristics. Sensitizers are found in many chemical classes, but the majority of 

 the most useful dyes seem to belong to either the cyanines, the xanthenes, the styryls, 

 or the flavines.^ As would be expected, the dyes, or rather the dyed silver halide, 

 must absorb the wavelengths to which sensitivity is conferred. It has been found 

 that this holds quantitatively and that for any single dye the spectral sensitivity curve 



0.5 7 



1.5 



2.5 



2.0 



120 



100 



m'-^ 



40" 



20 



300 



400 



700 



500 600 



Wavelength in mjs 



Fig. 5. — Spectral characteristic of eye sensitivity (curve -D) compared with sensitivity 

 of various types of photographic materials. Curve A is the characteristic for a typical 

 noncolor sensitive film, curve B that for typical orthochromatic material, and curve C that 

 for a representative panchromatic emulsion. 



is practically exactlj^ the same shape as the spectral absorption curve of the dj^ed 

 grains.^ 



The amount of dye needed is very small, and in fact, if the concentration is 

 increased above a certain optimum value, the sensitivity decreases sharply. In a 

 few cases where measurements are available, this optimum seems to correspond 

 approximately to a monomoleeular laj^er of dye on the surface of the grains.^ 



Methods of Use. — Commercial emulsions are usually sensitized by adding the 

 sensitizing dye directh' to the emulsion before coating. Coated plates may also be 

 sensitized by bathing in solutions of the dj^es. The time of treatment, dye con- 

 centration, etc., vary with the different dyes.^ 



Bathing plates in sensitizing solutions was formerly practiced rather widely In- 

 experimenters in color photography and by scientific workers needing materials 

 sensitized to special regions of the spectrum. The manipulation is rather difficult, 



1 Staud, C. .T., J. a. Leermakers, and B. H. Carroll, Optical Sensitizing of Photographic Emul- 

 sion.s (paper presented at fall 1937 meeting of Am. Chem. Soc). 



- Leermakers, .J. A., Qviantitative Relationships Between Light Absorption and Spectral Sensitivity 

 of Dye-sensitized Photographic Emulsions, J. Chem. Phys., 5, 889 (1937). 



' Leermakers, J. A., B. H. Carroll, and C. J. Staud, Photographic Emulsions, ■/. Chem. Phys., 5, 

 893 (1937). 



■1 DtJNDON, M. L., Color Sensitizing Photographic Plates by Bathing, Am. Phot., 20, 670 (1926). 



