Dichroic

Dichroic describes a property of certain materials and coatings that exhibit two different colors or levels of transmission depending on the polarization of light or the viewing angle. The term comes from the Greek di- meaning two and chrōma meaning color. In optics, dichroism typically refers to polarization-dependent absorption, where light polarized along one crystallographic axis is absorbed differently than light polarized along a perpendicular axis. When examined with polarized light, a dichroic material may reveal two distinct colors or intensities; under unpolarized light it may appear single-colored.

There are two practical forms of dichroic elements. Intrinsic dichroism arises from anisotropic absorption within a crystal or compound, producing color changes with orientation or polarization. Dichroic coatings, by contrast, use engineered thin-film interference to reflect one spectral range while transmitting another; these coatings can be tuned for specific wavelength bands and are often sensitive to the angle of incidence. Dichroic filters and mirrors are widely used to separate or combine wavelengths, polarize light, or produce color effects in photography, microscopy, astronomy, and stage lighting.

Applications include camera and scientific filters, polarized sunglasses, and decorative dichroic glass. In laboratory optics, dichroic beam splitters route light by wavelength while maintaining high transmission in the desired channel. While related concepts such as pleochroism describe minerals that change color with direction under polarized light, dichroism emphasizes two-color or two-channel behavior that is particularly relevant to coatings and optical filters.

See also: polarization, birefringence, pleochroism, optical filters.