A substrate processing apparatus comprises a chamber 28 capable of processing a substrate 20. A radiation source 58 provides radiation that is at least partially reflected from the substrate in the chamber. A radiation detector 62 is provided to detect the reflected radiation and generate a signal. A controller 100 is adapted to receive the signal and determine a property of the substrate 20 in situ during processing, before an onset of during or after processing of a material on the substrate 20.