refers to any spectrum of light that is devoid of red light.
Orthochromatic photography refers to an emulsion
that is sensitive to only blue and green light, and thus can be processed with a red safelight
. Using it, blue objects appear lighter and red ones darker because of increased blue sensitivity. Standard panchromatic film can be used with a Cyan-lens-filter (devoid of red light) to produce similar effect.
Orthochromatic films were first produced by Hermann Carl Vogel in 1873 by adding small amounts of certain aniline based dyes to photographic emulsions which had hitherto been sensitive to blue light only, work that was extended by others including J. M. Eder, who introduced the use of the red dye erythrosine in 1884.