14 August 2006 Chromatic confocal phase stepping interferometer
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Abstract
The idea which initiated this work is to adapt the principle of phase coding to a Confocal Chromatic type sensor in order to enhance the dynamic range and the axial resolution of height measuring systems. When using a classical Confocal chromatic type measurement, we obtain a "coarse" value of the distance. Then by the means of a spectrally adjustable light source and a chromatic objective, we choose the wave length in order to focus in the plane of the object. Next, by measuring the phase coding, we can precisely define the profile of the object. So, in the end, we obtain the subnanometric preciseness of the phase coding while clearing up the ambiguity that is inherent to interferometric methods thanks to the confocal chromatic measurement. The initial idea is to use a Linnik interferometer, with a dedicated chromatic objective in the object arm, and an achromatic objective in the reference arm. A change of the wave length leads to a variation of the focus plane in relation to the object. This equivalence between a variation in wave length and a variation in distance, caused by the axial chromatism, leads us to imagine a system of phase coding in which the usual movement of the reference mirror is replaced by a variation of the wave length. We then obtain an interferometer without any moving part, which produces the extensive depth of field that is peculiar to chromatic systems.
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Joseph Cohen-Sabban, "Chromatic confocal phase stepping interferometer", Proc. SPIE 6292, Interferometry XIII: Techniques and Analysis, 629216 (14 August 2006); doi: 10.1117/12.681677; https://doi.org/10.1117/12.681677
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