Abstract: A method for controlling the temperature of a growing semitransparent layer (12) during a production deposition process on a major surface of a production wafer (14). During the production process heat is applied to the wafer by an intensity adjustable heat source (34) and the production wafer emits a broad spectrum of light. In the method a test deposition process is performed using a set of infrared wavelengths of the light emitted from a test wafer to determine a set of test parameter values including temperature values over time for the test growing layer. The production deposition process is then performed and the intensity of infrared light emissions from the wafer are measured to form a set of production infrared light intensity values at the same test run set of infrared wavelengths used in the test deposition.