A technique for measuring the thickness of silicon films grown epitaxially on sapphire is described. The method is nondestructive, is performedin situ, and can be used to monitor film thickness while the film is growing. The method utilizes the infrared emission from the sapphire substrate and from the growing film. The radiation from the sapphire substrate is partially transmitted through the silicon and partially reflected in the silicon, establishing an interference pattern which can be used to determine silicon film thickness. Using a detector with peak sensitivity at 2.4 &mgr;, silicon film thicknesses to the nearest ±0.1 &mgr; are readily measurable. The absolute accuracy of ±0.1 &mgr; is independent of film thicknesses. Films in the range from 0.1 to 15 &mgr; have been measured. Increased detector sensitivity can extend the maximum measurable film thickness.