Polarimeter

A polarimeter is an optical instrument used to measure the polarization state of light, particularly the angle of rotation of the plane of polarization caused by optically active substances. In rotation polarimetry, linearly polarized light passes through a sample in a tube; if the sample is optically active, the plane of polarization is rotated by an angle α. A second polarizer, the analyzer, is rotated to find a position of minimum transmitted intensity (the null position). From α, and knowing the path length l and the sample concentration c, the specific rotation [α] is defined as [α]λ = α/(l c), with λ the wavelength used. Common practice uses the sodium D line (589 nm) as the standard wavelength.

The instrument comprises a light source, a polarizer, the sample tube, an analyzer, and a detector. Some configurations include a compensator or retardation plate to measure birefringence or to resolve sign ambiguities. Modern polarimeters may be automatic, employing rotating analyzers, photoelastic modulators, or electro-optic components with digital readouts.

Types range from manual polarimeters used for chemical analysis, such as determining sugar concentration, to automatic polarimeters designed for routine quality control, as well as specialized instruments for optical materials characterization and astronomical polarimetry.

Applications span chemistry and biochemistry for assessing concentration and enantiomeric purity, the pharmaceutical and food industries for quality control, and materials science and astronomy for polarization measurements of light. The concept of optical rotation has origins in early 19th-century work on optically active substances.