orthochromatic

Ortho chromatic, or orthochromatic, refers to photographic emulsions that are insensitive to red light but are sensitive to blue and green light. The term derives from Greek roots meaning straight or correct in color, indicating a restricted spectral response.

Orthochromatic emulsions were developed in the late 19th century and were widely used through the early 20th century, before the advent of panchromatic films. Because red light does not affect these emulsions, red-toned areas in a scene do not register in the final image, while blue skies and green foliage tend to render relatively bright. This produced distinctive tonal renditions and was practical for certain kinds of photography and printing at the time.

Handling and workflow with orthochromatic materials differed from later panchromatic processes. Photographers often used safelights and exposure strategies suited to blue-green sensitivity, and printing methods were adjusted to account for the lack of response to red wavelengths. As panchromatic emulsions—sensitive to red as well as blue and green—became standard in the 1920s and 1930s, orthochromatic materials declined in general use for photography.

Today, orthochromatic materials remain of interest mainly in historical contexts and among specialists. The term is still used to describe certain legacy printing papers or film stocks that retain sensitivity to blue and green light while remaining insensitive to red.