Robust signal evaluation for Chromatic Confocal Spectral Interferometry

The hybrid measurement principle Chromatic Confocal Spectral Interferometry combines Spectral Interferometry with Chromatic Confocal Microscopy and therefore benefits from their respective advantages. Our actual demonstrator setup enables an axial measurement range up to 100 μm with resolution up to 5 nm depending on the employed evaluation method and the characteristics of the object’s surface. On structured surfaces, lateral features down to 1 μm can be measured. As the sensor raw signal consists of a Spectral Interferometry type wavelet modulated by a confocal envelope, two classes of evaluation methods working on the phasing or the position of the envelope are employed. Even though both of these information channels are subject to their respective problems, we show that a proper combination of the individual methods leads to a robust signal evaluation. In particular, we show that typical artifacts on curved surfaces, that are known from Chromatic Confocal Microscopy, are minimized or completely removed by taking the phasing of the Spectral Interferometry wavelet into consideration. At the same time the problem of determining the right fringe order of the Spectral Interferometry signal at surface discontinuities can be solved by evaluation of the confocal envelope. We present here a first approach using a contrast threshold on the signal and a median referencing for trusted sections of the analysed topography, which yields a reduction of artifacts in a submicron range on steep gradients, discontinuous specimen or curved mirror-like surfaces.