Inoptics, adichroic material is either one which causesvisible light to be split up into distinct beams of differentwavelengths (colours) (not to be confused withdispersion), or one in which light rays having differentpolarizations are absorbed by different amounts.[1]
The original meaning ofdichroic, from theGreekdikhroos, two-coloured, refers to any optical device which can split a beam of light into two beams with differing wavelengths. Such devices includemirrors andfilters, usually treated withoptical coatings, which are designed to reflect light over a certain range of wavelengths and transmit light which is outside that range. An example is thedichroic prism, used in somecamcorders, which uses several coatings to split light into red, green and blue components for recording on separateCCD arrays, however it is now more common to have aBayer filter to filter individual pixels on a single CCD array. This kind of dichroic device does not usually depend on the polarization of the light. The termdichromatic is also used in this sense.
The second meaning ofdichroic refers to the property of a material, in which light in different polarization states traveling through it experiences a differentabsorption coefficient; this is also known asdiattenuation. When the polarization states in question are right and left-handedcircular polarization, it is then known ascircular dichroism (CD). Most materials exhibiting CD arechiral,[2] although non-chiral materials showing CD have been recently observed.[3] Since the left- and right-handed circular polarizations represent twospin angular momentum (SAM) states, in this case for a photon, this dichroism can also be thought of as spin angular momentum dichroism and could be modelled usingquantum mechanics.
In somecrystals,[which?], such astourmaline, 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.[dubious –discuss] This is more generally referred to aspleochroism,[4] and the technique can be used inmineralogy to identifyminerals. In some materials, such asherapathite (iodoquinine sulfate) orPolaroid sheets, the effect is not strongly dependent on wavelength.
Dichroism, in the second meaning above, occurs inliquid crystals due to either theoptical anisotropy of the molecular structure or the presence of impurities or the presence ofdichroic dyes. The latter is also called aguest–host effect.[5]
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