Thermal stress induced by elevated temperature causes the refractive index of transparent materials to become non-uniform, influencing the light deflection and thus performance of imaging systems containing such materials. Here, a digital gradient sensing (DGS) method is developed to measure the full-field non-uniform thermal stress in a material with air disturbance. This real-time optical technique can provide the light deflection and distribution of the principal stress gradients in a transparent medium. The light deflection through the transparent medium caused by the thermal stress is obtained using the elasto-optical effect. Air convection at elevated temperatures also affects the light deflection in optical systems, so the DGS method is extended to eliminate air convection. The light deflection resulting from the heated air is separately identified and calculated from the total deflection. The validity of this method is demonstrated using a bilayer transparent film containing layers with known refractive index and different thermal expansion coefficients that is bent by the thermal stress. Application of the DGS method to a zinc sulfide specimen shows that the thermal stress in it is temperature dependent and can be used to characterize refractive index non-uniformity.