Performing modulation spectroscopy by directing a probe beam and a pump beam at a strained semiconductor sample, modulating the pump beam, and reflecting the probe beam into a detector. The detector produces a direct current signal proportional to reflectance R of the probe beam and an alternating current signal proportional to the modulation of the reflectance .DELTA.R of the probe beam. Both R and .DELTA.R are measured at a multiplicity of probe beam photon energies, to provide a spectrum having at least one line shape. The spectrum is analyzed to measure energy differences between interband electronic transitions of the sample, and the strain of the sample is determined from the energy differences.