The partially reflective mirror 30 can be positioned at an angle, such as a 45 degree angle, to create control radiation 22 and interference radiation 24. The control radiation 22 can be redirected toward the control surface 32, which can be a highly polished reflecting mirror positioned perpendicular to the control radiation 22. The control radiation 22 can then be reflected back to the partially reflective mirror 30 which can redirect the control radiation 22 to the radiation sensor 20. Simultaneously, the interference radiation 24 can pass through the port 8 of the substrate support 6 toward the second side of the substrate 14. The port 8 may be a hole or it may have a lens (not shown) that is transparent to IR light. The lens could allow for both support of the substrate support 6 and functionality such as cooling, gas flow or vacuum through the substrate support 6. A first side of the substrate 14 can be melted creating a melted surface 26. The interference radiation 24 can then be reflected from the back of the melted surface 26 toward the radiation sensor 20. The interference radiation 24 can be reflected along the same path as the control radiation 22 creating a combined radiation 28. The power and the spatial shape of the combined radiation 28 along with the known thickness of the substrate 14 can then be used to determine the depth of the melted surface 26. In further embodiments, the lens can focus coherent light at the melted surface 26.