dikroism

Dichroism is the differential absorption of light by a material that depends on the polarization state or the direction of light's propagation. It is observed in anisotropic substances whose absorbing transitions are oriented along specific molecular or crystallographic axes. In practice, dichroism is described by linear dichroism and circular dichroism.

Linear dichroism (LD) refers to different absorption for light polarized along two orthogonal linear directions. LD requires an oriented sample (such as a stretched polymer film, a liquid crystal, or a single crystal). The measured LD spectrum, often reported as the difference A_parallel minus A_perpendicular, provides information about the orientation of absorbing groups or transition dipoles and can be summarized by a dichroic ratio or an order parameter.

Circular dichroism (CD) is the differential absorption of left- and right-circularly polarized light. CD arises when chiral molecules or chiral environments interact differently with the two circular polarizations. It is widely used to study biomolecules: far-UV CD provides secondary structure content of proteins; nucleic acids show characteristic CD signatures; measured with a spectropolarimeter and expressed as delta A as a function of wavelength or as ellipticity.

Measurement and interpretation: The phenomena require polarized light sources and analyzers. LD measurements involve sample alignment; CD requires careful baseline correction and concentration control. Both provide qualitative and quantitative information about structure and orientation.

In minerals, the term pleochroism is often used for color changes with crystal orientation, a form of dichroism. Dichroism is distinct from birefringence, which is a difference in refractive index and does not involve differential absorption.

Note that the standard term is dichroism; some sources use the variant spelling dikroism.