The temperature dependence of refractive indices of optical materials is characterized in this work by what we call their normalized thermo-optic coefficients. These are determined experimentally through interferometric measurements of thermal expansion and of changes in optical thickness at a few laser wavelengths as function of temperature. A suitable vectorial formalism applied to these data allows predicting the thermal evolution of the refractive index all over the useful range of transparency. The validity and reliability of our methodology is demonstrated through temperature tuning of a mid-IR $\mathrm{Hg}{\mathrm{Ga}}_2{\mathrm{S}}_4$ optical parametric oscillator (OPO) pumped at $1.0642\mu \mathrm{m}$ by a Nd:YAG laser. Measured thermal variation of both signal and idler wavelengths agrees very well with the predicted one.