(or optical sensitizer, spectral sensitizer). An organic dye that, when adsorbed to the surface of the silver halide crystals of a photographic emulsion, makes the dyed crystals sensitive to light in the green, red, or infrared regions of the spectrum. A nonsensitized silver halide is not sensitive to such light.
Most sensitizing dyes are polymethine dyes. The molecules of the dye typically contain heterocyclic rings and a conjugated polymethine chain:
where n = 0, 1, 2, 3. The chemical nature of the heterocyclic rings and the length of the polymethine chain determine the dye’s region of spectral absorption, the dye’s region of sensitization, and the efficiency of the sensitizing action.