Dichroism

Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are absorbed by different amounts.

The original meaning of dichroic, from the Greek dikhroos, two-coloured, refers to any optical device which can split a beam of light into two beams with differing wavelengths. Such devices include mirrors and filters, usually treated with optical coatings, which are designed to reflect light over a certain range of wavelengths, and transmit light which is outside that range. An example is the dichroic prism, used in some camcorders, which uses several coatings to split light into red, green and blue components for recording on separate CCD arrays. This kind of dichroic device does not usually depend on the polarization of the light. The term dichromatic is also used in this sense.

The second meaning of dichroic refers to a material in which light in different polarization states travelling through it experience a varying absorption. The term came about because of early observations of the effect in crystals such as tourmaline. In these crystals, the strength of the dichroic effect varies strongly with the wavelength of the light, making them appear to have different colours when viewed with light having differing polarizations. This is more generally referred to as pleochroism, and the technique can be used in mineralogy to identify minerals. In some materials, such as herapathite (iodoquinine sulfate) or Polaroid sheets, the effect is not strongly dependent on wavelength, and so the term dichroic is something of a misnomer, but still used.

Optically active molecules exhibit differing absorption to light of opposite circular polarization. This is known as circular dichroism.

Which meaning of dichroic is intended can usually be inferred from the context. A mirror, filter, or beam splitter is referred to as dichroic in the colour-separating first sense; a dichroic crystal or material refers to the polarization-absorbing second sense.