Method and apparatus for measuring optical retardation in transparent

Abstract

A method and apparatus for measuring optical retardation of a transparent material, such as the protective transparent cover sheet or substrate which protects the recording layer of an optical or magneto-optical disk. The method of the invention involves the steps of producing a beam of plane-polarized radiation in which the plane of polarization rotates continuously, directing such beam through a sample of transparent material where optical retardation is to be measured, and continuously comparing the angle of polarization of the beam entering the sample with the angle of plane polarization of the beam exiting the sample. Preferably, the beam and sample are continuously moved relative to another, whereby the optical retardation is measured at different points on the sample. The beam-producing step may be effected by passing a beam of circularly polarized radiation through a plane-polarizing filter while continuously rotating the filter. In this case, the comparing step may be achieved by (a) redirecting the beam exiting the sample back through the rotating filter and (b) monitoring the intensity of the beam after passing back through such filter.