An optical method of measuring the transparent ice thickness, based on the effect of total internal reflection by the transparent ice — gas interface, is presented. The light circle, whose diameter is proportional to the ice thickness and eccentricity is proportional to the slope gradient of the ice surface in the measurement region, is formed around a small-size light source in the plane of diffuser surface. Using several light sources, the method allows measurement of the transparent ice thickness field. This method can be applied for transparent ice thickness measurements on a rotor blade of wind turbine.