Embodiments of the present invention provide apparatus and method for reducing noises in temperature measurement during thermal processing. One embodiment of the present invention provides a chamber for processing a substrate comprising a chamber enclosure defining a processing volume, an energy source configured to direct radiant energy toward the processing volume, a spectral device configured to treat the radiant energy directed from the energy source towards the processing volume, a substrate support disposed in the processing volume and configured to support the substrate during processing, and a sensor assembly configured to measure temperature of the substrate being processed by sensing radiation from the substrate within a selected spectrum.