A wave plate (retarder or phase shifter) is an optical element which changes the polarization of the light wave that passes through. Wave plates work by shifting the phase between two perpendicular polarization components of the light wave. A common configuration of a wave plate is a plane-parallel substrate of birefringent crystal, typically quartz, mica, or magnesium fluoride. Wave plates come in configured for different retardations such as a quarter, half or full wave. A quarter wave plate changes linear polarization to circular, while a half wave plate changes the handedness of the circular polarization. By using different combinations it is possible to isolate one or two wavelengths simultaneously, eliminate or compensate for ellipticity and even rotate one wavelength relative to another. The utility of wave plates depends upon a number of factors such as operating temperature range, wavelength bandwidth, damage threshold, etc.