A noncontact temperature measurement technique, diffraction-order analysis, based on monitoring the change in diffraction angle from a grating as a result of thermal expansion, is described and results for both Si and GaAs are presented. Two incident beams are used to provide a differential measurement, independent of wafer tilt. Image processing techniques are used to calculate the relative temperature in near real time from the optical signals. Good agreement between optical and the thermocouple temperature measurements is obtained, with an accuracy and precision of +/-0.3 degrees C demonstrated over a 20-600 degrees C temperature range for a GaAs sample. Analysis of the effects of all six rigid-body motions of the wafer on the measurement is presented. The measurement is independent of all translational motions; rotational motions (pitch, roll, and yaw) can all be monitored with the same measurement scheme and the temperature measurement corrected for their effects. In many applications in semiconductor manufacturing, wafers are rotated to ensure uniformity. The diffraction-order analysis technique is demonstrated for a rotating wafer with a 3 sigma precision of 1.95 degrees C.