An improved optical method for calibrating test records

Test records are often calibrated by measuring the width of the optical pattern formed when a sharp beam of light is reflected from the Modulated grooves. An error has been found in this measurement owing to diffraction of light at the edges of the pattern, which results in a fuzzy ending and a general enlargement of the pattern width, especially at high frequency. A new optical method has been devised for calibration of test records by the use of interference-line patterns. Two sets of interference lines have been identified: (a) Uniformly spaced lines which are related to the recorded frequency, the angular velocity of the record, and the wavelength of light (these have been called the A lines); (b) lines with variable spacing which are related to the amplitude of modulation and to the wavelength of light (these have been called the B lines). B-line patterns may be readily related to the theoretical width of the optical pattern free of diffraction error, with the resultant improvement in the accuracy of test-record calibration.