Abstract: Semiconductor processing apparatus, including a chamber, into which a semiconductor wafer is introduced for processing thereof and a heater, which heats the wafer in the chamber. A radiation guide collects thermal radiation from a selected region of the wafer. A wafer support assembly supports the wafer and shields the radiation guide from radiation other than radiation from the region. A pyrometer, coupled to receive the radiation from the guide, analyzes the radiation to determine a temperature of the region, for use in controlling the processing.