The radiation sensitivity of silver halides ends for all practical purposes at about 525 mm . In Figure 3 Curve A illustrates the spectral sensitivity of a typical silver bromo-iodide emulsion and B illustrates the average human visual response curve. As the curves show, the maximum response of the eye is in the yellow-green near 550 mm which lies beyond the sensitivity range of the emulsion, which is much more sensitive to the violet and blue than the eye.

Figure 3: Curves Approximating theLight Sensitivity of Typical SilverHalide Crystals (blue) and                                                                                 the Human Eye (yellow)

The sensitivity of the silver halides may be extended to radiation of longer wavelengths by the addition of dyes or “color sensitizers.“Although referred to as dyes, color sensitizers are not ordinary dyes in that they are not used to color cloth or other materials.

Emulsion sensitizing results from the absorption of radiant energy by the dye at a wavelength that would not affect the silver halide, and the transfer of that “exposure” energy to the silver halide to form a latent image and make the affected grain developable. If a dye is a sensitizer, then its action depends upon the absorption characteristics of the silver halide-adsorbed color sensitizer complex, which may be quite different from the absorption of the dye itself. Since the sensitivity of such dyes varies greatly, it is often necessary to use a combination of materials to obtain a specific result. Some combinations, however, do not work well together, so that the system balance must be carefully studied before the final emulsion composition is determined. There are substances that may or may not be sensitizers themselves but greatly increase the sensitizing action of other dyes. These are known as super-sensitizersand are of considerable importance in facilitating the use of conventional sensitizing dyes. Whatever the dye used, the quantity required is always quite small.