Thermal imaging of heat sources in thermal processing systems for determination of workpiece temperature are provided. In one example, a thermal processing apparatus can include a processing chamber, a workpiece support, a plurality of heat sources configured to heat a workpiece, and at least one camera. The at least one camera can capture one or more images of thermal radiation of the plurality of heat sources during thermal treatment of the workpiece. In one example, a method for calibrating the camera can include obtaining the one or more images of thermal radiation of at least one heat source, obtaining one or more reference signals indicative of irradiation of the at least one heat source, and calibrating the camera based at least in part on a comparison between the one or more images of thermal radiation and the one or more reference signals indicative of irradiation of the heat source.