Abstract: A method and apparatus for measuring the emission coefficient of a semiconductor material for light of wavelength .lambda. having photon energy less than the semiconductor bandgap energy is introduced. The reflection coefficient for the light of wavelength .lambda. is measured while the semiconductor material is being irradiated with sufficient light having photon energy greater than the bandgap energy that the semiconductor material transmits little light of wavelength .lambda., and the emission coefficient is calculated from the measured reflection coefficient. The temperature of the semiconductor material can be calculated from the emission coefficient and the measured intensity of the thermally emitted radiation of wavelength .lambda..