Temperature measurement by means of a pyrometer is affected by changes in the background illumination. Physical modeling is a very effective method to discern the origin of radiation contributions and separate the thermal radiation emitted by the object of interest from parasitic radiation. An observer algorithm making use of physical models was successfully applied to infrared pyrometry for rapid thermal processing. Rapid thermal processing is characterized by fast temperature changes in the range of several hundred degree per second. The heating source typically emits light within a broad wavelength band ranging from visible to infrared. Especially in rapid thermal processors that apply heat to both sides of a silicon wafer, this light is partially picked up by the pyrometer sensor. As a consequence these types of systems require methods to handle the fast changing radiation contribution of the heating source to the pyrometer signal.