Biaxial materials

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

In the case of optically biaxial materials, a magnetic field may alter the distribution of optical orientations at a temperature which is known to be too low to enable molecular reorientation (191. 

Figure 4. A practical distinction between optically uniaxial and optically biaxial drawn (or extruded) material. For optically uniaxial material, the area fraction exhibiting extinction between crossed circular polars is greatest when the normal to the plane of the thin section is parallel to the draw axis. For optically biaxial material, the greatest area fraction is observed in a section cut so that the angle between its normal and the draw axis is equal to half the optic axial angle of a monodomain.

Biaxial materials display two melatopes (Figure 5.13c) and a far more complex pattern of interference rings. ... 

Conoscopic images under white light illumination, (a and b) Uniaxial material an interference pattern consisting of two intersecting black bars (termed isogyres) forming Maltese cross-Kke pattern and circular distributions of interference colors (c) biaxial material. 

We note that for biaxial materials, a S + i>- D, where D is the biaxiaUty order parameter. 

A uniaxial material has one optic axis and a biaxial material has two. A birefringent material appears to be isotropic when a plane light wave passes through it along an optic axis. In terms of the indicatrix, the cross section in the... 

Figure 3.13. (a) The indicatrix of a biaxial birefrin-gent material. The principle refractive indices are A > B > C. The heavy dots in (b) outline the circular cross section. An optic axis is shown perpendicular to this plane. The two optic axes of the biaxial material are shown in (c). They lie in the plane A-C. 

An ellipsoid of revolution must have circular cross sections perpendicular to the rotational symmetry axis, and so this is the optic axis. Biaxial materials are not so simple. Consider the elliptical cross section that contains the maximum and minimum values. At some (four) points it must have the same radial distance as the third intermediate principal value. The two... 

Photoalignment can be applied to the fabrication of retardation films as well. Retardation films are used widely in LCDs for viewing angle enhancement as well as dispersion compensation. They can be made from uniaxial or biaxial materials. Within the class of uniaxial materials, many types of films can be made and they serve different functions. For example, for uniform films where the optical axis is fixed throughout the entire film, one can have A-plates, C-plates and O-plates as shown in Figure 5.8. 

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