It was found that the analysis of harmonic components determining the time dependence the transmission coefficient of a bismuth silicate crystal during sinusoidal modulation of the probe radiation over the wavelength makes it possible to determine the spectral position of the maxima for intracenter transitions in defective centers contributing to impurity optical absorption. It was found that the analysis of harmonic components, which determine the time dependence transmission coefficientof an aluminum-doped bismuth titanate crystal with sinusoidal modulation of probe radiation over a wavelength, makes it possible to determine the spectral position of the maxima for intracenter transitions in defective centers that contribute to impurity optical absorption.