Car headlight reflector protective coating thickness measurement - using the polarization change in a linearly polarized light beam directed at the reflector

摘要

The method used is to direct at the headlight a linearly polarized beam produced by a laser, along a path parallel to but offset from the central axis of the parabolic mirror reflector. This beam is reflected through the focus (F) of the reflector and thence reflected back along a patch parallel to the original one, but now elliptically polarized to an extent dependent upon the thickness of the protective coating. The degree of ellipticity is measured in an analyser arrangement using a window (12) polarizer (16), transducer (15) and feedback amplification network (17, 21). The ratio of ellipticity is measured and from this the coating thickness can be inferred from a characteristic which is also provided.