Photographic emulsions spectral sensitivity

Sensitizers as well as desensitizers form a reversal oxidoreduction system with silver halides, according to both pH and pAg of the photographic emulsion. But besides the specific influence of the emulsion, the efficiency of a sensitizing dye depends on many other factors such as its adsorption, its spectral absorption, the energetic transfer yield, the dye aggregate to the silver halide, and finally on its desensitizing property in... 

Spectral Sensitization. Photographic silver haHde emulsions ate active with light only up to about 500 nm. However, theh sensitivity can be extended within the whole visible and neat-H spectral region up to about 1200—1300 nm. This is reached by the addition of deeply colored dyes that transfer excited electrons or excitation energy to the silver haHde. 

Fig. 13. Single-sheet diffusion transfer plate (a) stmcture (b) upon exposure to light (c) development and (d) washing off and finish. In (a) the plate is first coated with a receiver layer of small (<5 nm) catalytic sites. The photographic layer is a spectrally sensitized silver haUde emulsion. In (c) the exposed areas develop as silver metal. Unexposed areas diffuse down to the receiver layer and form the printing image. In (d) the emulsion is washed off, revealing...

B. H. Carroll and C. J. Staud Spectral sensitivity and light absorption of dye-sensitized photographic emulsions. J. chem. Physics S, 878 (1937)-... 

The dyes that perform this function are called spectral sensitisers because they are used to sensitise the photographic emulsion to light across the whole visible spectrum and beyond. Since silver halides are naturally blue sensitive, the green and red spectral sensitiser dyes must also be able to reduce the natural sensitivity of the silver halide emulsion to blue light. The most important class of compounds used in this... 

Dyes having similar constitutions (e.g., 625) are claimed to improve spectral sensitivity in classical photography (88JAP63/110444). Other cyanine dyes (509, 512, 607) have already been described (Sections IV,A,5,e IV,A,5,f IV,B,3,c). It has also been reported that fused tropylium salts such as 224-type compounds stabilize photographic emulsions (90JAP02/ 156240). 

Polymethine dyes derived from benzothiazoles are employed for the spectral sensitization of photographic emulsions. 

The contrast (y) of a photographic emulsion also is dependent on wavelength, where the contrast generally increases as the wavelength increases. A calibration curve for an emulsion, therefore, is only correct at the particular wavelength at which it was obtained. Spectral sensitivity curves for some commonly used spectral plates and films are shown in Figure 6-8. 

Spectroscopic detection limits differ for different elements many elements can be detected at very low concentration levels, some as low as 10 g. The spectroscopist should become familiar with detection limits of elements of most concern in his particular field and under his excitation conditions. The sensitivity of qualitative spectral analysis is dependent on the type and size of the sample, the excitation conditions, and the sensitivity of the photographic emulsion and the optical system used with the spectrograph. For best results excitation conditions should be maintained as uniform as possible. 

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