Orthochromatic

Part of speech: adjective

Pronunciation: /ˈɔː.θə.kɹəˈmæt.ɪk/

Definitions

1. Pertaining to the discernment of colors precisely, relating to substances or films that respond to the full spectrum of light minus red, and referring to photographic techniques that exclude red from color capture
2. Associated with the accurate perception of colors, indicating materials that react to all wavelengths of light except for red, and describing photographic methods that omit red tones
3. Describes materials or photographic processes that register the visible spectrum but exclude red wavelengths

Etymology: Orthochromatic is a technical coinage, built from two Greek roots. "Ortho-" comes from Ancient Greek "orthós", meaning straight, upright or correct, and "chromatic" comes from "chrōma", colour. Put together, the word describes a plate or film that gives a "correct" rendering of colour, which in practice meant responding to blue, green and yellow light. Early photographic emulsions were sensitive only to blue and ultraviolet light, so they recorded skies as white and faces as dark. Adding dyes that extended sensitivity to green and yellow produced orthochromatic materials, which handled those colours far more faithfully, yet still left red wavelengths almost unrecorded. Reds therefore appeared black, which is why these films were so useful for portraits of people with red lips and for work where a red safelight could be used in the darkroom.