Orthochromatic ("correct colors") photographic emulsion is early black and white film emulsion, sensitive to wider range of colors than original color-blind (blue sensitive) emulsion. This name was used for the very first time in 1884, for dry photographic plates sensibilized with Erythrosin.
Orthochromatic emulsion still has too high sensitivity to blue, generally correct sensitivity to green and bright yellow, but has too low sensitivity to orange and is practically insensitive to red, as it does not register wavelengths longer than approximately 560-600 nm (medium yellow to orange). Furthermore, orthochromatic emulsion has decreased sensitivity in the 500 nm area (pale blue), as compared to color-blind emulsions, and so it reproduces brightness of blue colors more accurately.
Orthochromatic emulsion with equalized sensitivity to green and yellow was also sometimes called isoorthochromatic emulsion, while "real" orthochromatic emulsion had then too low sensitivity to green as compared to yellow.
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