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.