The objective of the proposed SBIR Phase I program is to develop an instrument that provides real-time, in situ measurements of substrate temperature in optical coating reactors. The instrument will employ an inexpensive diode laser, fiber optic components, and established laser interferometry methods to determine substrate temperature at multiple locations with a replicate precision of better than 0.1 degrees C in a measurement time of less than 0.2 seconds. The precision may be improved with increasing measurement time, if desired. The instrument (an "optical thermometer") will be demonstrated on optical substrates made of a variety of materials in state-of-the-art industrial reactors specializing in UV, visible, near-IR and IR optical coatings. The fast response of the sensor will enable coaters to use, for the first time, precise measurements of bulk substrate temperature to identify temperature nonuniformities during the coating process, refine and improve coating processes in real time, and minimize batch-to-batch variations through closed-loop process control.