This reflectometer measures the principal angle of incidence θ¯ and the principal azimuth ψ¯ of a reflecting surface. To determine θ¯, the polarizer and analyzer angles are modulated at different frequencies. The difference frequency appears in a component of the detector output, and the amplitude of this component is directly proportional to cosδ. Consequently, θ¨ is located by searching for a null in this component. Modulation is achieved by mechanically oscillating the polarizing elements. A simple alignment procedure is entirely adequate in making the effect of alignment errors negligible with respect to the sensitivity of the instrument (which is better than 10 sec of arc for θ¯). The method for observing ψ¯ involves the measuring of the polarizer and analyzer rotations required to maintain the transmitted irradiance at a constant value. This eliminates the need for linearity in the photodetector. The value of tanψ¯ is accurate to 1 part in 1000. No corrections are necessary for source and detector polarizatio. Deliberately large polarizer and analyzer misaligmnents of 0.02 rad have no significant effect on the accuracy of the determination of tanψ. Although operation has been confined to visible wavelengths, the techniques should be capable of operating over a wider range of wavelengths.
W. Swindell, "A Precision Reflectometer," Appl. Opt. 7, 1455-1459 (1968)