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.