The second side of the substrate 14 acts as a partially reflective mirror for the radiant interface detector 10, which will split the coherent radiation 12 into control radiation 22 which is reflected back to the radiation sensor 20 and interference radiation 24. The interference radiation 24 can pass through substrates that are transparent to the selected wavelength of light, such as silicon, quartz or sapphire substrates when using infrared light. A first side of the substrate 14 or a portion thereof can be melted creating a melted surface 26. The melted surface 26 is reflective to the coherent light, such as that from the interference radiation 24. 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, for example the pattern of intensity, 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.

The radiant interface detector 10 can also include a light selective barrier 25. The light selective barrier 25 can prevent coherent radiation from passing while allowing another coherent radiation to pass freely based on physical characteristics of the radiation, for example a bandpass or long-wavelength pass filter. In one embodiment, the light selective barrier 25 can block the wavelength of coherent radiation, such as a front side laser, without affecting the transmission of interference radiation through.