Dichroism

The term can be used for optical elements which somehow act on light with a substantial dependence on the optical wavelength. In particular, the transmissivity can be wavelength-dependent. The term is particularly used in conjunction with dielectric mirrors; see the article on dichroic mirrors.

Dichroism can also refer to the polarization-dependent absorption of light in certain materials — an effect also known as diattenuation when it involves linear polarization. At first, this use of the term dichroism may seem counterintuitive, since the term literally suggests a dependence on wavelength (i.e., color) rather than polarization. However, some crystalline materials — such as tourmaline, where the effect was first observed — appear in different colors depending on the polarization of the incident light. This occurs because their optical absorption, and consequently their apparent color or reflectance, depends on both polarization and wavelength. The wavelength dependence essentially results from the anisotropic band structure. Of course, there are not just two distinct colors, as the name might suggest, but rather a continuous range of color mixtures for intermediate polarization orientations.

Note that there is also the phenomenon of trichroism, observed on biaxial crystals. That means that the observed colors are mixtures of three rather than two different colors.

Polarization-dependent absorption is used for some kinds of optical polarizers, namely for sheet polarizers based on polymer films. For such applications, the films may be optimized for minimum wavelength dependence, and the term dichroism may then be regarded as somewhat inappropriate.

Dichroism is in most cases related to linear polarization directions, but there is also circular dichroism, where the difference in optical properties occurs for different circular polarization directions.