A green or yellow-green filter used with black-and-white film, it compensates for the difference between the color sensitivity of film and the relative luminous efficiency of the human eye for more natural-looking pictures.nikongrossary
(1) Representing correctly the relations of colors as found in a photographic subject. (2) A photographic emulsion sensitive to all visible colors except red - i.e. an ortho film is sensitive to blue and green light.
Film that is sensitive to only blue and green light.