Dichroism

Principle

Dichroism has two related but distinct meanings in optics. With the first one, a dichroic material causes visible light to be split up into distinct beams of different wavelengths (colors), not to be confused with dispersion as happens in a prism (see Light and Light: refraction). With the 2^nd one, light rays having different polarizations (see Light: polarization), are absorbed by different amounts.

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

Fig 1 Dichroic filters

Application

General

The most common example is the dichroic filter. Dichroic filters operate using the principle of interference (see Light: diffraction and Huygens’ principle). Alternating layers of an optical coating are built up upon a glass substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. By controlling the thickness and number of the layers, the wavelength of the bandpass can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb much energy during operation and so become much less warm as absorbance filters (see for absorbance Lambert-Beer law).

Other examples of the wavelength type of dichroism are the dichroic mirror and the dichroic prism. The latter is used in some camcorders (miniaturized video cameras), which uses several coatings to split light into red, green and blue components. This is also applied in the CCD camera.

Medicine and food industry

The dichroism of optically active molecules (see More Info) is used in the food industry to measure syrup concentration, and in medicine as a method to measure blood sugar (glucose) in diabetic people.

More Info

The original meaning of dichroic refers to any optical device, which can split a beam of light into two beams with differing wavelengths.

Basically, a dichroic filter has more than one transmission peak, with transmission frequencies harmonically related. However, in practice one needs nearly always a filter with one transmission peak. These are the filters, which are called interference filters. The higher harmonics, with have much lower transmission, are mostly attenuated by an absorbance filter. Side bands (similar of those of Fig. 1 in Light: diffraction) have very small transmission. They can strongly be diminished by stacking two identical filters, but on the cost of transmission of the principal peak.

The second meaning of dichroic refers to a material in which light in different polarization states, traveling through it, experience a varying absorption. The term comes from 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 colors when viewed with light having differing polarizations. This is more generally referred to as pleochroism, and the technique can be used to identify minerals.

Dichroism also occurs in optically active molecules, which rotate linearly polarized light (see Light: polarization). Depending on the 3-D molecular structure the rotation is left (levorotatory) or right (dextrorotatory). This is known as circular dichroism. Glucose is dextrorotatory and fructose strongly levorotatory. However, basically an optic active substance has a dextrorotatory and levorotatory version.

Dichroism occurs in liquid crystals (substances with properties between those of a conventional liquid, and those of a solid crystal) due to either the optical anisotropy of the molecular structure (resulting in more than one refractive index, see Light: Snell’s law) or the presence of impurities or the presence of dichroic dyes.