When blue-sensitive photographic materials are modified to respond to green, yellow, orange, or red light, the method used is called spectral or dye sensitization. The first practical work in this area was by H. W. Vogel in 1873. Vogel, applying earlier theories suggested by J. Waterhouse, proved that collodion plates could be made sensitive to yellow and greenish yellow wavelengths by tinting them with selected coal tar dyes, including corallin, aldehyde green, eosin, and cyanin.
In 1875 Abney theorized that the dyes combined with the silver to produce colored organic salts of silver that were sensitive to light. He also stated that the dyes themselves were reduced by light producing a nucleus onto which metallic silver would be deposited. Additional research by Amory, Abney, Eder, Ives, and others contributed to the techniques used for the commercial production of orthochromatic (greensensitive) and isochromatic (orange-sensitive) silver bromide gelatin plates. By using more than one dye sensitizer, it was eventually possible to produce panchromatic (full spectral sensitivity) gelatin plates just after the turn of the century.
The use of negative materials with complete spectral sensitivity was not met with great acceptance in the beginning, because the plates required loading and processing in total darkness. Many continued to use orthochromatic plates that were developed by inspection for much of their work. Panchromatic or color-specific emulsions were essential when color separations were required.