A three-dimensional printing system is configured to selectively solidify a build material at a build plane in a layer-by-layer manner. The three-dimensional printing system includes a laser module, a scan module, and a controller. The laser module is for emitting a light beam along a main optical path from the laser module to the build plane. The scan module includes a motorized mirror and a sensor. The motorized mirror includes a substrate having an optical coating that reflects at least 90% of incoming beam power such that the mirror transmits no more than 10% of the incoming beam power. The sensor is positioned to receive transmitted light from the mirror. The controller is configured to operate the laser module to emit the light beam along the main optical path, analyze a signal from the sensor, and based upon the analysis, to estimate a calibration error for the laser module.