A laser tuned to a wavelength close to the bandgap energy of the sample is directed onto the sample. This may be held in a cryostat to facilitate measurements being taken at low temperatures. A rapidly pulsing laser can be used as a quasi constant beam laser since measurements are taken continuously. When the laser beam is incident on the sample, photoluminescence occurs and light is emitted from the sample at wavelengths dependent on the sample composition. The sample is oriented such that the reflected laser beam and the PL emission propagate in different directions.