Orthochromatic Photography
Orthochromatic photography refers to a photographic emulsion that is sensitive to only blue and green light, and thus can be processed with a red safelight. The increased blue sensitivity causes blue objects to appear lighter and red ones darker. A cyan lens filter—which removes red light—can be used with standard panchromatic film to produce a similar effect.
Orthochromatic films were first produced by Hermann Wilhelm Vogel in 1873 by adding small amounts of certain aniline-based dyes to photographic emulsions which had thitherto been sensitive to blue light only. This work was extended by others including Josef Maria Eder, who introduced the use of the red dye erythrosine in 1884.
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