Orthrochromatic

Orthrochromatic, commonly spelled orthochromatic, is a term used in photography and optics to describe materials or processes whose spectral sensitivity is largely confined to the blue-green portion of the light spectrum, with little sensitivity to red wavelengths. In practice, orthochromatic media form images mainly from blue and green light and render red tones as dark or muted, since red light does not contribute significantly to the image.

Spectral sensitivity and imaging effects of orthochromatic media differ from those of panchromatic media, which are sensitive to the full visible spectrum including red. As a result, red objects and scenes containing red tones can appear notably lighter or darker on orthochromatic imagery depending on the specific emulsion and lighting. This characteristic was a defining feature of early black-and-white photography.

Historically, orthochromatic emulsions were developed in the late 19th century as an alternative to earlier blue-sensitive materials. They were widely used in the first decades of black-and-white photography and in some newspaper and portrait work before the advent of panchromatic emulsions in the 1920s and 1930s, which offered more natural grayscale rendering across all colors. The transition to panchromatic film reduced the practical appeal of orthochromatic media for general photography.

In contemporary practice, orthochromatic emulsions are largely obsolete for new work, though they remain of interest to historians, archival specialists, and enthusiasts working with vintage equipment. The term may also appear in discussions of historical color-reproduction techniques and spectral sensitivity in early photographic research.