Polarised light microscopy: principles and practice for the rheumatologist.

Polarised light microscopy is used to detect and measure matter (in fluids, tissues, or cells) that has crystalline or crystalline-like properties. Because light has electromagnetic properties, it can be influenced by the electromagnetic properties of material through .which it passes. In many materials this influence is equal in whatever direction the light is vibrating. But crystalline material specifically influences the speed of light that passes either along or across it. Variation in the speed of light in one or other direction cannot be detected with normal illumination, in which the light is vibrating in all directions perpendicular to the light path, but only with plane-polarised light-that is, with light in which all the vibrations have been filtered out except for one direction.