The gauge is designed to measure the diameter of round materials on hot processing line based on the optical telecentric projection principle. The material is illuminated from one side by a homogeneous light source (emitter). From the opposite side, an optoelectronic system (receiver) detects the position of light/dark transition produced by the material edges. Since the optical system is made to provide an optical parallel projection, the measure is stable even in presence of a certain material displacement from nominal pass-line.