An elliptically polarized monochromatic light is obtained from monochromatic light select unit and is reflected at an interface between an objective film and substrate. An extinction wavelength of the reflected light, when the amount thereof detected at photodetector becomes zero, is detected at control...http://www.google.es/patents/US4606641?utm_source=gb-gplus-sharePatente US4606641 - Apparatus for measuring film thickness
