We report the application of the technique of infrared transmission to measure the temperature of silicon wafers during the growth of silicon‐germanium alloy heteroepitaxial layers in a rapid thermal processing system. The silicon‐germanium alloy layers have negligible absorption at 1.3 and 1.55 &mgr;m over wide ranges of thickness, composition, and strain condition. The substantial improvement of the uniformity of layers grown using the technique to measure the temperature for feedback control of the lamp power has also been demonstrated.