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.

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