An optical apparatus is disclosed, the apparatus comprising an optical element having a reflective surface for reflecting incident radiation in a beam path, and at least one sensor configured to sense radiation corresponding to a temperature of a respective portion of a backside surface of the optical element. Also disclosed is a method of controlling a temperature of a reflective surface of an optical element in a lithographic apparatus.

A thermal imaging system is provided. The thermal imaging system includes an explosion-proof housing with an optical window configured to contain an explosive pressure. The optical window allows electromagnetic thermal energy to pass. A thermal imaging sensor is disposed within the explosion-proof housing. Thermal imaging electronics are coupled to the thermal imaging sensor and configured to provide at least one thermal image based on a signal from the thermal imaging sensor. A lens assembly is disposed at least in front of the optical window external to the explosion-proof housing. A composite optical window for thermal imaging is also provided. In another embodiment, a thermal imaging system includes an explosion-proof housing having an optical window configured to contain an explosive pressure. An infrared (IR) camera is disposed within the explosion-proof housing. A reflector reflects electromagnetic thermal energy to the IR camera, but prevent an object from impacting the optical window.