Abstract: The invention provides optical measuring apparatus including a laser which is arranged to have an output of electromagnetic radiation including at least two different simultaneously-generated frequencies. This output is fed to a detector, such as an optical diode, which produces a corresponding electric signal at the difference (beat) frequency. The beat frequency, any change in the beat frequency, or any rate of change of the beat frequency can now be examined electrically to the degree of accuracy generally available in that art, usually to a much higher degree than is available in the optical or mechanical arts. The parameter to be examined may be that of an element causing the production of the beat frequency such as a birefringent material, or may be that of an element which changes the frequency in some way, such as an angular orientation of two quarter-wave plates.