Orthochromatic Materials

black-and-white light-sensitive photographic materials that have been sensitized (that is, given additional sensitivity) to green and yellow rays.

Photographic emulsions containing silver halide have intrinsic light-sensitivity in the blue-violet region of the visible spectrum (wavelengths of 400–500 nanometers [nm]) and are almost entirely insensitive to green, yellow, and red rays. To supplement the sensitivity in additional spectral regions, special additives called sensitizing pigments are introduced into the emulsions. The light-sensitivity caused by these additives is called the supplementary light-sensitivity. Orthochromatic materials have supplementary sensitivity to visible light with a wavelength of 500–600 nm (in conrast to orthochromatic materials, panchromatic materials are also sensitized to red rays with a wavelength of 600–700 nm). For daylight the supplementary light-sensitivity of orthochromatic materials is about 25–30 percent of the total light-sensitivity; for the light of incandescent lamps, 50–60 percent.

Two kinds of orthochromatic materials are manufactured. The orthochromatic materials proper have lower sensitivity in the blue-green region. The iso-orthochromatic materials have uniform sensitivity to all rays with wavelengths of 400–580 nm. Since orthochromatic materials are not sensitive to red rays, they are usually used for photographing objects without any red parts, for photomicrography, for reproduction of black-and-white images, and for X-ray photographs taken from a green fluorescent screen. Photographs taken on orthochromatic materials can be observed during development if only red lighting is used.