The first silver halide emulsions were sensitive only to blue and ultraviolet radiation. In 1873 Hermann Wilhelm Vogel discovered that the addition of dyes such as erythrosine to an emulsion extended its sensitivity well into the green region: he called this improved response orthochromatic. By 1890 other dyes had been found which extended the sensitivity throughout the visible spectrum: these emulsions were termed panchromatic. Orthochromatic emulsions remained popular for a long time, particularly among amateurs, as they could be processed under a red safe light. They are still used in the printing industry.
— Graham Saxby
