Of
black-and-white photochemical film: which is sensitive only to the
green and
blue parts of the
spectrum, and not to
red colours. In earlier times, the orthochromatic film was often used for
photography, but after the
panchromatic film was invented in
1904, its use as
first-
generation originals has declined.
Orthochromatic emulsions are however much used today for making copies of black-and-white orignals. This is the reason why they use red lamps in dark rooms: since the paper is unaffected by red light, the lamps enable the person who is developing the paper copies to see what he or she is doing, without ruining the copies.