Birefringence can be observed under a polarizing microscope or a microscope with a polarized filter. In normal light (or other electromagnetic radiation) the waves are random directions about the axis of the light wave. In polarized light the waves are all in the same direction.
Picture from MicroscopyU website.
When polarized light is passed through an anisotropic* crystal the light ray is divided into 2 rays depending upon the polarization. The different paths these light rays take is called double refraction or birefingence. A polarizing filter on a microscope will isolate the polarized light and indicate that a birefringent material is present. The diagnosis of Gout is dependent on observing the birefringent crystals of Uric Acid in the sinovial fluid of the affected joint. The Unico H608 microscope includes a polarizer filter kit which aids doctors in the detection of Gout. A Uric Acid slide is also available which helps doctors setting their microscope for the diagnosis of Gout.
*Anisotropic – having a physical property which is directionally dependent. Wood is anisotropic in strength as it can be split easier along the grain than against the grain.