White light interferometry

As described here, white light interferometry is a non-contact optical method for surface height measurement on 3D structures with surface profiles varying between tens of nanometers and a few centimeters. It is often used as an alternative name for coherence scanning interferometry in the context of areal surface topography instrumentation that relies on spectrally-broadband, visible-wavelength light (white light). Interferometry makes use of the wave superposition principle to combine waves in a way that will cause the result of their combination to extract information from those instantaneous wave fronts. This works because when two waves combine, the resulting pattern is determined by the phase difference between the two waves—waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference. While white light interferometry is not new, combining old interferometry techniques with modern electronics, computers, and software has produced extremely powerful measurement tools. Yuri Denisyuk and Emmett Leith, have done much in the area of white light holography and interferometry. Even though there are a number of different interferometer techniques, three are most prevalent: diffraction grating interferometers. vertical scanning or coherence probe interferometers. white light scatter-plate interferometers. While all three of these interferometers work with a white light source, only the first, the diffraction grating interferometer, is truly achromatic. Here the vertical scanning or coherence probe interferometers are discussed in detail due to their extensive use for surface metrology in today’s high-precision industrial applications. A CCD image sensor like those used for digital photography is placed at the point where the two images are superimposed. A broadband “white light” source is used to illuminate the test and reference surfaces. A condenser lens collimates the light from the broadband light source.