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In chemistry orthochromatic refers to a dye or stain which does not change color on binding to a target as opposed to metachromatic stains which change color. In literal sense it comes from ortho ie straight or correct + chromatic ie colored. As for example Toluidine blue stains nucleic acids by its orthochromatic color (i.e., blue) but it stains mast cell granules by becoming metachromatic which in this case is red.
In spectral terms orthochromatic refers to maintaining the position of the spectral peaks and metachromatic in contrast refers to a shift in the position which can be either hypsochromic or bathochromic depending on its direction.
In photography Orthochromatic refers to any spectrum of light that is devoid of red light.
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 until that time 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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