Where does dichroism collide?

In optical mineralogy and gemology, dichroism (more generally pleochroism) is the way an anisotropic crystal absorbs light differently depending on the vibration direction of the light relative to the crystal axes, so the stone shows two (or, if trichroic, three) distinct body colours as it is viewed or rotated. Nesse's Introduction to optical mineralogy explains the directional absorption behind it, and Read's Gemmology describes the dichroscope used to split and compare the colours for identification. The referent is direction-dependent selective absorption in a crystal, used to tell gems apart, and it is a different construct from a chemist's circular dichroism or an optician's dichroic mirror.[1][2]

Unit

The two or three body colours absorbed along different crystal directions

Recovery means

Not applicable

Measured by

Observed with a dichroscope or polariser, comparing the colours transmitted for light vibrating along different crystal directions

In biophysical spectroscopy, circular dichroism is the difference in how a chiral molecule absorbs left- versus right-circularly polarised light; because this difference depends on three-dimensional shape, the spectrum reports on a protein's secondary structure or a molecule's handedness. Kelly, Jess, and Price's How to study proteins by circular dichroism lays out how the measurement is made and interpreted. The referent is differential absorption of circularly polarised light by chiral matter, a distinct construct from a crystal's directional colour, sharing only the 'two ways of absorbing' root.[3]

Unit

The difference in absorbance between left- and right-circularly polarised light across wavelengths

Recovery means

Not applicable

Measured by

Measured by a CD spectropolarimeter as ellipticity or difference-absorbance versus wavelength, then fitted to estimate structure

In optics, a dichroic filter or mirror is a thin-film interference coating that sharply transmits one band of wavelengths while reflecting another, so it splits light by colour with very little absorption; it is the heart of beam-splitters, colour-separation systems, and many projector and stage optics. Hecht's Optics treats the multilayer interference that produces the effect. The referent here is wavelength-selective reflection by engineered layers, which, despite the shared name, works by interference rather than the selective absorption behind crystal pleochroism or circular dichroism.[4]

Unit

The transmitted and reflected wavelength bands set by the coating

Recovery means

Not applicable

Measured by

Characterised by the filter's transmission and reflection spectra and the cut-on/cut-off wavelengths of its passband