High-resolution single-shot measurement of semi-transparent films by Chromatic Confocal Coherence Tomography (CCCT)

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In everyday production measurement tasks often occur where micro topographies are optically measured through contaminations (like oil films) or semitransparent films, or the actual properties of the film (e.g. paint layers) itself are of interest. For single surfaces, there are a number of methods which allow for a topography measurement with sub-micron precision. Some of them can be modified to sample a plurality of superposed interfaces as for the examples above. However, the measurement of the actual thickness of the films is disturbed by the unknown refractive index of the material. To achieve a correct measurement of the real thickness of the film, these modified topography methods depend on knowledge of this refractive index and/or intensive modelling. No method is known that is able to measure film thickness and refractive index simultaneously with a high axial and lateral resolution in a single-shot manner. To solve this problem, the new method of Chromatic Confocal Coherence Tomography (CCCT) is proposed. CCCT is based on the proven topography method of Chromatic Confocal Spectral Interferometry (CCSI), from which it shall evolve mainly by improved signal evaluation. The information of the surface under test is coded in two independent channels of the CCSI/CCCT signal: the confocal envelope as well as the interferometric wavelet. These two channels are affected by the refractive index in different ways: while the confocal channel assumes a too low film thickness, the interferometric channel measures optical pathlength and therefore a thickness higher than the real thickness. However, by suitable combination of both channels, a simultaneous measurement of both, refractive index and real film thickness, is possible.