An optical module includes a transparent substrate and a refractive optical element mounted on the substrate. A conductive heating trace is deposited on the substrate around the refractive optical element. A temperature sensor senses a temperature of the substrate. Control circuitry is coupled to the temperature sensor so as to measure a difference between the temperature of the substrate and a target operating temperature of the module, and to drive a current through the conductive heating trace, responsively to the difference, so as to heat the substrate to the target operating temperature.