Polarimeter: A device used to measure the amount and direction that a substance rotates plane polarized light. A polarimeter consists of a light source, a monochromator (filters out all but a specific wavelength of light), a polarizer (converts the light beam to plane polarized light), a sample tube (holds the sample being measured), a second polarizer (to determine the degree of rotation), and a light detector. The process of measuring the degree of rotation is called polarimetry.