Dichroismus

Dichroism, sometimes written as dichroismus in German and related languages, is the property of a material to absorb light differently depending on the polarization or direction of the incident electromagnetic radiation. In anisotropic substances, the absorption coefficients for different polarization states can differ, leading to visible color differences when the material is viewed along different orientations. Dichroism is distinct from birefringence, which is a difference in refractive index rather than absorption, though both phenomena can occur simultaneously in the same material.

Linear dichroism (LD) refers to differential absorption of light polarized along two principal crystal or molecular axes. It is commonly observed in oriented crystals, stretched polymers, and some minerals. Pleochroism, the color change observed in crystals with orientation, is a form of dichroism caused by LD.

Circular dichroism (CD) is differential absorption of left- versus right-handed circularly polarized light. CD arises from optical activity or chiral asymmetry in molecules. CD spectroscopy is a standard tool in chemistry and biophysics for characterizing the structure of biomolecules: far-UV CD probes protein secondary structure, while near-UV CD reports tertiary structure; nucleic acids and other chiral systems also show characteristic CD spectra.

Measurements are made with spectropolarimeters or CD spectrometers that compare absorbance of different polarization states across wavelengths. Applications span mineralogy, materials science, and biochemistry, including assessment of protein folding, peptide conformation, polymer orientation, and chiral optical materials.