Photoreflectance (PR) spectroscopy system and method for accumulating separately a “pump on” light beam and a “pump off light beam reflecting off a sample. The system comprises: (a) a probe source for producing a probe beam, the probe beam is used for measuring spectral reflectivity of a sample, (b) a pump source for producing a pump beam, (c) at least one spectrometer, (d) a first modulation device to allow the pump beam to alternatingly modulate the spectral reflectivity of the sample, so that, a light beam reflecting from the sample is alternatingly a “pump on” light beam and a “pump off light beam, (e) a second modulation device in a path of the light beam reflecting off the sample to alternatingly direct the “pump on” light beam and the “pump off light beam to the at least one spectrometer, and (f) a computer.