An orthochromatic ("correct colors") photographic emulsion is a black & white film emulsion, sensitive to wider range of colors than the earliest color-blind (blue sensitive) types based on unmodified silver halides. This name was used for the very first time in 1884, for dry photographic plates sensitized with Erythrosin.
Orthochromatic emulsions still have high sensitivity to blue, generally correct sensitivity to green and bright yellow, but low sensitivity to orange and are practically insensitive to red, as they do not register wavelengths longer than approximately 560-600 nm (medium yellow to orange). Furthermore, orthochromatic emulsions have decreased sensitivity in the 500 nm area (pale blue), as compared to color-blind emulsions, and so they reproduce blue colors more accurately.
Orthochromatic emulsions with equalized sensitivity to green and yellow were sometimes called isoorthochromatic emulsion, while typical orthochromatic emulsions had insufficient sensitivity to green as compared to yellow.
|Barnet Orthochromatic Plate Box|
image by John Rochon (Image rights)
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