Polarimeters
Polarimeters are instruments used to measure the rotation of the plane of vibration in polarized light. Light polarization can be measured using a Stokes vector polarimeter. The polarization properties of a particular material can be measured with a Mueller matrix polarimeter. Typically, Mueller matrix polarimeters are used to determine the purity of materials such as sugar, vitamins, steroids, and antibiotics. Polarimetry of thin films and surfaces is called ellipsometry, which is used to provide information about layers that are thinner than the wavelength of the light itself, down to a single atomic layer or less. An interferometer uses the interference pattern of two beams of light to make very precise measurements. Interferometers can measure minute surface irregularities in optical devices such as mirrors. In astronomy, interferometers can measure the distances between celestial objects and their diameters. The most sensitive polarimeters are based on interferometers, while more conventional polarimeters use polarizing filters, wave plates or other devices.
Choices for polarimeters include: complete vs. incomplete, sequential vs. simultaneous, monochromatic vs. polychromatic, non-imaging polarimeter vs. imaging polarimeter, and manual (visual) vs. electronic (automatic). For pharmaceutical testing, most work is done using 589 nm light (sodium D line). For electronic (automatic) polarimeters, total accuracies of 0.01° are possible. The light source in a polarimeter can be a spectral arc lamp (sodium or mercury), or an incandescent lamp (tungsten). Spectral arc lamps can be designed for visible, ultraviolet (UV) and infrared (IR) wavelengths, and are suitable for stability of wavelength and long-term calibration. Incandescent lamps are less expensive than spectral arc lamps, but limited to visible and IR radiation.
ASTM International (formerly called the American Society for Testing and Materials (ASTM), maintains ASTM C1426 regarding the standard practices for verification and calibration of polarimeters. Other standards describe how to use polarimeters in standard tests (i.e. ASTM F218 for analyzing stress in glass). The Society of Manufacturing Engineers (SME) maintains SME EM930116 for the rapid contour measurement of composite structures utilizing a three-dimensional (3D) tracking interferometer.
