Diffuse reflectance spectroscopy can be used as a noninvasive probe for measurement of temperature in real time. We have measured the precision of this technique to determine the temperature of Si and GaAs substrates during semiconductor processing. Our results show that the standard deviation of the noninvasive optical technique is less than 1.5 °C for GaAs and less than 2.0 °C for Si over the temperature range 25 °C<T<600 °C. This standard deviation compares favorably to that for a type‐Kthermocouple used in the same measurements: s.d.<1.5 °C. These results support the notion that noninvasive optical temperature measurement can be used in semiconductor processing with a precision approaching that of a thermocouple. ©1995 American Institute of Physics.