The temperature dependence of the principal values of the refraction of light (specific optical retardation) in Sr1-xCaxTiO3 with x = 0.014, SrTi(16O0.0318O0.97)3 and KTa1–xNbxO3 with x = 0.018 single crystals have been measured and the magnitudes and temperature variations of polar contributions to the refraction were derived. From polar contributions to the refraction on the basis of the method developed by the authors the magnitude of the root mean square polarization fluctuations characterizing the short-range polar order in the low temperature ferroelectric phase were determined in the crystals under study.