Dichroism

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 Wavelength s ( Colour s), ''or'' one in which light rays having different Polarization s are absorbed by different amounts.

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 Mirror s and Filter s, usually treated with Optical Coating s, 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 Camcorder s, which uses several coatings to split light into red, green and blue components for recording on separate CCD Arrays . 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 Crystal s such as Tourmaline . In these Crystal s, 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 Mineral s. 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 Molecule s 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.

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Dichroism