Photo-reflectance is an optical technique for investigating the material and electronic properties of thin films. Photo-reflectance measures the change in reflectivity of a sample in response to the application of an amplitude modulated light beam. In general, a photo-reflectometer consists of an intensity modulated "pump" light beam used to modulate the reflectivity of the sample, a second "probe" light beam used to measure the reflectance of the sample, an optical system for directing the pump and probe beams to the sample, and for directing the reflected probe light onto a photodetector, and a signal processor to record the differential reflectance.  The pump light is typically modulated at a known frequency so that a lock-in amplifier may be used to suppress unwanted noise, resulting in the ability to detect reflectance changes at the ppm level.
The utility of photo-reflectance for characterization of semiconductor samples has been recognized since the late 1960s. In particular, 
conventional photo-reflectance is closely related to electroreflectance in that the sample's internal electric field is modulated by the photo-injection of electron-hole pairs. The electro-reflectance response is sharply peaked near semiconductor interband transitions, which accounts for its usefulness in semiconductor characterization.  Photo-reflectance spectroscopy has been used to determine semiconductor bandstructures, internal electric fields, and other material properties such as crystallinity, composition, physical strain, and doping concentration.