This investigation stemmed from an attempt to explain the nature of the dispersion occurring when a beam of light is reflected from a test record. Examination through color filters revealed the existence of two distinct patterns of lines which the author has termed theAandBlines.A‐lines are identified with the frequency andBlines with the amplitude of the recorded sound.B‐line patterns can be readily related to the modulation velocity and lead to an accurate determination of the recorded velocity characteristic of a test record. Characteristics by conventional light patterns are found to be in error owing to diffraction of light at the edges of the pattern. This error is estimated by subtracting the modulation velocity byB‐line patterns from that determined by conventional light pattern techniques, and it is found to be approximately equal to the calculated diffraction error from assumed triangular wavelets. Calibrations of representative test records byB‐line patterns are in much better agreement than those by conventional light patterns with the calibrations obtained by the variable‐speed turn‐table technique, especially when the latter are corrected by the estimated amount of error owing to the elastic deformation of the modulated groove by the pickup load.