Lightpipe radiation thermometers (LPRTs) are used as temperature monitoring sensors in most rapid thermal processing (RTP) tools for semiconductor fabrication. These tools are used for dopant anneal, gate oxide formation, and other high temperature processing. In order to assure uniform wafer temperatures during processing these RTP tools generally have highly reflecting chamber walls to promote a uniform heat flux on the wafer. Therefore, only minimal disturbances in the chamber reflectivity are permitted for the sensors, and the small 2 mm diameter sapphire lightpipe is generally the temperature sensor of choice. This study was undertaken to measure and model the effect of LPRT proximity on the wafer temperature. Our experiments were performed in the NIST RTP test bed using a NIST thin‐film thermocouple (TFTC) calibration wafer. We measured the spectral radiance temperature with the center lightpipe and compared these with the TFTC junctions and with the three LPRTs at the mid‐radius of the wafer. We measured LPRT outputs from a position flush with the reflecting plate to within 2 mm of the stationary wafer under steady‐state conditions with wafer‐to‐cold plate separation distances of 6 mm, 10 mm and 12.5 mm. Depressions in the wafer temperature up to 25 °C were observed. A finite‐element radiation model of the wafer‐chamber‐lightpipe was developed to predict the temperature depression as a function of proximity distance and separation distance. The experimental results were compared with those from a model that accounts for lightpipe geometry and radiative properties, wafer emissivity and chamber cold plate reflectivity. © 2003 American Institute of Physics