Apparatus and methods for measuring the temperature of a substrate are disclosed. The apparatus includes a source of temperature-indicating radiation, a detector for the temperature-indicating radiation, and a decorrelator disposed in an optical path between the source of temperature-indicating radiation and the detector for the temperature-indicating radiation. The decorrelator may be a broadband amplifier and/or a mode scrambler. A broadband amplifier may be a broadband laser, Bragg grating, a fiber Bragg grating, a Raman amplifier, a Brillouin amplifier, or combinations thereof. The decorrelator is selected to emit radiation that is transmitted, at least in part, by the substrate being monitored. The source is matched to the decorrelator such that the emission spectrum of the source is within the gain bandwidth of the decorrelator, if the decorrelator is a gain-driven device.