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Proceedings Paper

Roughness parameters and surface deformation measured by coherence radar
Author(s): Peter Ettl; Berthold E. Schmidt; M. Schenk; Ildiko Laszlo; Gerd Haeusler
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Paper Abstract

The 'coherence radar' was introduced as a method to measure the topology of optically rough surfaces. The basic principle is white light interferometry in individual speckles. We will discuss the potentials and limitations of the coherence radar to measure the microtopology, the roughness parameters, and the out of plane deformation of smooth and rough object surfaces. We have to distinguish objects with optically smooth (polished) surfaces and with optically rough surfaces. Measurements at polished surfaces with simple shapes (flats, spheres) are the domain of classical interferometry. We demonstrate new methods to evaluate white light interferograms and compare them to the standard Fourier evaluation. We achieve standard deviations of the measured signals of a few nanometers. We further demonstrate that we can determine the roughness parameters of a surface by the coherence radar. We use principally two approaches: with very high aperture the surface topology is laterally resolved. From the data we determine the roughness parameters according to standardized evaluation procedures, and compare them with mechanically acquired data. The second approach is by low aperture observation (unresolved topology). Here the coherence radar supplies a statistical distance signal from which we can determine the standard deviation of the surface height variations. We will further discuss a new method to measure the deformation of optically rough surfaces, based on the coherence radar. Unless than with standard speckle interferometry, the new method displays absolute deformation. For small out-of-plane deformation (correlated speckle), the potential sensitivity is in the nanometer regime. Large deformations (uncorrelated speckle) can be measured with an uncertainty equal to the surface roughness.

Paper Details

Date Published: 29 September 1998
PDF: 8 pages
Proc. SPIE 3407, International Conference on Applied Optical Metrology, (29 September 1998); doi: 10.1117/12.323304
Show Author Affiliations
Peter Ettl, Univ. of Erlangen-Nuernberg (Germany)
Berthold E. Schmidt, Univ. of Erlangen-Nuernberg (Switzerland)
M. Schenk, Univ. of Erlangen-Nuernberg (Germany)
Ildiko Laszlo, Univ. of Erlangen-Nuernberg (Germany)
Gerd Haeusler, Univ. of Erlangen-Nuernberg (Germany)

Published in SPIE Proceedings Vol. 3407:
International Conference on Applied Optical Metrology
Pramod Kumar Rastogi; Ferenc Gyimesi, Editor(s)

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