A spectrum analysis method for the linear, direct, and self-consistent measurement of dynamic modulation depth of polarization modulators is presented. This method utilizes the Bessel recurrence relation to determine the modulation depth from the photodetector voltage amplitudes at the fundamental frequency and its next three harmonics. Based on the existing J1-J4 method of dynamic phase-shift measurement in homodyne interferometry, this method is useful for calibration of polarization modulated ellipsometers. The method is demonstrated through the use of a highly birefringent transparent thin film of piezoelectric polyvinylidene fluoride with indium tin oxide electrodes. The theoretical analysis of the measured noise factor for the particular system configuration predicted a minimum detectable polarization modulation depth of 0.2 rad, and was experimentally verified.