A thermal processing system includes a source of laser radiation having an array of lasers emitting light at a laser wavelength, a substrate support, optics disposed between said source and said substrate support for forming a line beam in a substrate plane of the substrate support from the light emitted by the source of laser radiation, and scanning apparatus for effecting movement of said line beam relative to said substrate support in a direction transverse to the longitudinal axis of said line beam. The system further includes a housing encompassing said optics, a light detector disposed inside said housing for sensing an ambient light level, a power supply coupled to the source of laser radiation, and a controller governing said power supply and responsive to said light detector for interrupting said power supply upon an increase in the output of said light detector above a threshold ambient level.