Orthochromatic Emulsion
A black and white emulsion sensitive to blue and green but not to red light.
Measured in
nm
In depth
Because it is blind to red, orthochromatic film renders skin blemishes and lips very dark and skies as blank white, which gives period photographs their distinctive look. Its practical advantage is that it can be handled under a red safelight, making it convenient for large-format work and for those learning to develop by inspection.
Examples in practice
A portrait on orthochromatic film shows freckles and reddened skin as almost black, an effect some photographers seek deliberately.