Ellipsometry is an optical technique for characterization and surface analysis, based on the change in the polarization state of light when it is reflected onto a flat surface. Its applications, which are varied, range from photoelasticimetry in mechanics, crystalline optics, and saccharimetry in chemistry, to the measurement of the Faraday effect in magnetism or Kerr effects in electricity. It was developed in a laboratory at the École Polytechnique by Bernard Drevillon and Ramdane Benferhat, who then joined Jobin Yvon.