A technique for determining the temperature of a sample including a semiconductor film 20 having a measurable optical absorption edge deposited on a transparent substrate 22 of material having no measurable optical absorption edge, such as a GaN film 20 deposited on an Al2O3 substrate 22 for blue and white LEDs. The temperature is determined in realtime as the film 20 grows and increases in thickness. A spectra based on the diffusely scattered light from the film 20 is produced at each incremental thickness. A reference division is performed on each spectra to correct for equipment artifacts. The thickness of the film 20 and an optical absorption edge wavelength value are determined from the spectra. The temperature of the film 20 is determined as a function of the optical absorption edge wavelength and the thickness of the film 20 using the spectra, a thickness calibration table, and a temperature calibration table.