14 September 2007 New interferometric technique to measure the length (thickness) of opaque objects using a commercial interferometer
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Wavelength scanning interferometry offers many advantages over traditional phase shifting interferometry, most significantly the elimination of mechanical movement of the part/s for phase modulation by implementing a tunable light source. Further, Fourier analysis on the interference time history enables this technique to accurately measure distances, treating the distance between two optical surfaces as an interferometric cavity. We propose to use a newly acquired wavelength scanning Fizeau interferometer from Zygo Corporation, the MST (Multiple Surface Transform) to explore the limits of absolute thickness metrology to measure an opaque cavity, such as a gauge block. While transparent cavities can be measured with ease in a Fizeau setup, opaque cavities need additional optics. A two mirror Sagnac configuration in conjunction with the interferometer from Zygo Corporation is used to measure the length (thickness) of a 1 inch gauge block. Current gauge block measurements rely on comparison methods to determine the fractional length with respect to a reference or master gauge block or use techniques which require the absolute length of the gauge block to be previously known. By using wavelength scanning interferometry, the absolute length of the gauge block can be determined directly within limits of the repeatability of the instrument. While other techniques implement a point by point approach for measurement or use interpolation methods, we simply use a large aperture to provide the thickness variation over the sample. Experimental results of a 1 inch gauge block along with an uncertainty estimate are discussed.
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Amit R. Suratkar, Angela D. Davies, Faramarz Farahi, "New interferometric technique to measure the length (thickness) of opaque objects using a commercial interferometer", Proc. SPIE 6671, Optical Manufacturing and Testing VII, 66710N (14 September 2007); doi: 10.1117/12.733934; https://doi.org/10.1117/12.733934

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