As a manufacturer of cutterheads, test records and phono cartridges, we have a need for an absolute reference in the actual modulation of the record groove walls. Measurements of recorded levels and channel separation on phonograph discs, using the method of optical interferometry, have previously been described in the literature. The technical drawbacks of this method are: tedious preparation of samples and the necessity for taking photomicrographs plus the sensitivity to mechanical vibrations. We have improved, together with a major manufacturer of optical equipment, the measurement of groove modulation such that above-mentioned deficiencies are eliminated. The measurements are characterized by simplicity and mechanical stability. The sample can be taken either from a lacquer or pressed record. No metal coating is necessary because of a high intensity monochromatic light source in conjunction with a special collector lens. Photographs are avoided because the information can be read directly off a calibrated micrometer drum and the stability is achieved because the reference and the object planes are congruent.
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