The method 500 can further include directing the interference radiation at the second surface of the substrate and directing the control radiation at the control surface, wherein the radiation at least partially reflects from the respective surfaces, as in 508. The control surface can have a highly polished reflective surface to reflect a large proportion of the radiation it receives. Further embodiments can include a control surface which reflects a known portion or a measured proportion of the radiation it receives. The control radiation reflected from the control surface can be directed back at the radiation sensor through the partially reflective mirror. The interference radiation can be directed at the second surface of the substrate.

As the surface of the substrate melts due to the heat source, the interface between the melted surface and the non-melted underside moves lower in the substrate. The change in depth may be less than a micron over a set period of time. However, the interface between the melted surface and the non-melted substrate creates a mirror-like surface for the interference radiation. As the interface is moving with the increase in melted surface, the interface can be used to determine the overall depth of the melt in comparison to a known control distance.

The method 500 can further include measuring the interference between the reflected interference radiation and the reflected control radiation, as in 510. The control radiation and the interference radiation are reflected back at the radiation sensor, which can be used to detect changes in the power and the spatial shape of the interference radiation and the control radiation.