Dichroism

Dichroism
Dichroic redirects here. For the filter, see dichroic filter. For the glass, see dichroic glass.
Lampworked dichroic glass bead

Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are absorbed by different amounts.[1]

The original meaning of dichroic, from the Greek dikhroos, two-coloured, refers to any optical device which can split a beam of light into two beams with differing wavelengths. Such devices include mirrors and filters, usually treated with optical coatings, which are designed to reflect light over a certain range of wavelengths, and transmit light which is outside that range. An example is the dichroic prism, used in some camcorders, which uses several coatings to split light into red, green and blue components for recording on separate CCD arrays, however it is now more common to have a Bayer 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 term dichromatic is also used in this sense.

The second meaning of dichroic refers to a material in which light in different polarization states travelling through it experience a varying absorption. The term came about because of early observations of the effect in crystals such as tourmaline. In these crystals, 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. This is more generally referred to as pleochroism, and the technique can be used in mineralogy to identify minerals. In some materials, such as herapathite (iodoquinine sulfate) or Polaroid sheets, the effect is not strongly dependent on wavelength, and so the term dichroic is something of a misnomer, but still used.

Optically active molecules exhibit differing absorption to light of opposite circular polarization. This is known as circular dichroism.

Which meaning of dichroic is intended can usually be inferred from the context. A mirror, filter, or beam splitter is referred to as dichroic in the colour-separating first sense; a dichroic crystal or material refers to the polarization-absorbing second sense.

Dichroism in liquid crystals

Dichroism occurs in liquid crystals due to either the optical anisotropy of the molecular structure or the presence of impurities or the presence of dichroic dyes. The latter is also called a guest–host effect.[2] [3]

References

  1. ^ University Physics 6th Ed. F.W. Sears, M.W. Zemansky, H.D. Young ISBN 02101071991
  2. ^ Stephen M. Kelly (2000). Flat Panel Displays: Advanced Organic Materials. Royal Society of Chemistry. p. 110. ISBN 0854045678. http://books.google.com/books?visbn=0854045678&id=LMW0gofBH7sC&pg=PA110&lpg=PA110&dq=%22guest+host+effect%22#PPA109,M1. 
  3. ^ dichroic glass resources

See also


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Look at other dictionaries:

  • Dichroism — Di chro*ism, n. [Gr. ? two colored; di = di s twice + ? color.] (Opt.) The property of presenting different colors by transmitted light, when viewed in two different directions, the colors being unlike in the direction of unlike or unequal axes.… …   The Collaborative International Dictionary of English

  • dichroism — [dī′krō iz΄əm] n. [< Gr dichroos, of two colors < di , two + chrōs, skin, complexion, color (for IE base see CHROMA) + ISM] 1. the property that doubly refracting crystals have of transmitting light of different colors when looked at from… …   English World dictionary

  • dichroism — noun Date: 1819 the property of some crystals and solutions of absorbing one of two plane polarized components of transmitted light more strongly than the other; also the property of exhibiting different colors by reflected or transmitted light… …   New Collegiate Dictionary

  • dichroism — See circular dichroism …   Dictionary of molecular biology

  • dichroism — noun a) the property of some crystals of transmitting different colours of light in different directions b) the property of some anisotropic materials of having different absorption coefficients for light polarized in different directions;… …   Wiktionary

  • dichroism — The property of seeming to be differently colored when viewed from emitted light and from transmitted light. [G. di , two, + chroa, color] circular d. (CD) the change from circular polarization to elliptical polarization of monochromatic,… …   Medical dictionary

  • dichroism — dichroizmas statusas T sritis chemija apibrėžtis Medžiagos gebėjimas nevienodai sugerti skirtingomis kryptimis poliarizuotą šviesą. atitikmenys: angl. dichroism rus. дихроизм …   Chemijos terminų aiškinamasis žodynas

  • dichroism — dichroizmas statusas T sritis fizika atitikmenys: angl. dichroism vok. Dichroismus, m rus. дихроизм, m pranc. dichroïsme, m …   Fizikos terminų žodynas

  • dichroism — n. [Gr. dis, twice; chros, color] The property of showing two very different colors, one by transmitted light and the other by reflected light, or as some dyes staining different tissues different colors …   Dictionary of invertebrate zoology

  • dichroism — /duy kroh iz euhm/, n. 1. Crystall. pleochroism of a uniaxial crystal such that it exhibits two different colors when viewed from two different directions under transmitted light. 2. Chem. the exhibition of essentially different colors by certain …   Universalium

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