Optical coupling by absorption and reflection of a wafer during rapid thermal anneals (RTA) determines the temperature transients during heating, and the behavior of thermally activated processes. It is shown experimentally that the heating rate and temperatures during the early phases of a RTA cycle depend on the doping of the wafer being heated. High doping is accompanied by high free carrier absorption which results in a relatively rapid increase in temperature. Differences of heating rates of 50 °C/sec and temperatures several hundred degrees centigrade are obtained in the first 5 sec of a RTA cycle for N+and N−wafers and significant effects on the heating rate of heavily implanted wafers are also seen. Experiments were carried out using thermocouples, optical pyrometer and also by observing the lateral solid‐phase growth of silicon on sapphire (SOS). A simple theoretical description is given for the enhanced heating rate for N+and heavily implanted wafers.