Methods and apparatus for measuring the melt depth of a substrate during pulsed laser melting are provided. The apparatus can include a heat source, a substrate support with an opening formed therein, and an interferometer positioned to direct coherent radiation toward the toward the substrate support. The method can include positioning the substrate with a first surface in a thermal processing chamber, heating a portion of the first surface with a heat source, directing infrared spectrum radiation at a partially reflective mirror creating control radiation and interference radiation, directing the interference radiation to a melted surface and directing the control radiation to a control surface, and measuring the interference between the reflected radiation. The interference fringe pattern can be used to determine the precise melt depth during the melt process.