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Short for Orthochromatic – a high contrast film that's sensitive to all colour other than red and deep orange. As a result these can be handled under red safelight conditions and are designed primarily for reproducing halftone images or artistic black and white shots that have few mid tones.
A filtered light thats used in a darkroom to allow you to see while handling light sensitive material. Orange is used for most black & white materials, red for ortho, brown for variable contrast papers and a very deep green for colour materials.