The heat transfer to a wafer in a RTP furnace is simulated by an analytical/numerical model. The model includes radiant heat transfer to the wafer from the lamps, the heat conduction within the wafer, and the emission of radiation from the wafer. Geometric optics are used to predict the radiant heat flux distribution over the wafer. The predicted wafer surface temperature distribution is compared to measurements made in an RTP furnace for two different reflector geometries. Lamp configurations and the resulting heat flux that are required to produce a uniform wafer temperature are defined. Also investigated is the effect of patterns on the wafer surface temperature.