Certain isotropic biopolymers, when aligned in regular arrays, exhibit a type of birefringence called Form Birefringence. The amount of birefringence of the array is sensitive to the refractive index of the surrounding medium and the spacing of the molecules of the array. An example is the birefringence of spindle microtubules (R) and starch grains (below).
Types of Birefringence
Intrinsic birefringence: Anisotropy exhibited by crystals. The regular array of atoms in the material is the source of the birefringence. Calcium oxylate crystals in plants (right) are visualizewd easily using PLM.
Strain Birefringence is a type of anisotropy that is induced by stresses on materials such as plastic (right) and glass. Machined plastic parts and ground glass lenses develop strain birefringence. Anealing glass removes the strain, and thus the source of birefringence. De-stresses objectives are necessary for high quality PLM. These objectives are labeled "POL".
Strain birefringence visualized using PLM. Image from the University of Cambridge Micrograph Library.
Inoué, S. 1989. Video Microscopy.