Laser diffraction wire diameter measurements: correction of diffraction models by interferometric calibration

Juan Carlos Martinez-Anton; Ibrahim Serroukh; Eusebio Bernabeu

doi:10.1117/12.364275

21 September 1999 Laser diffraction wire diameter measurements: correction of diffraction models by interferometric calibration

Juan Carlos Martinez-Anton, Ibrahim Serroukh, Eusebio Bernabeu

Author Affiliations +

Proceedings Volume 3824, Optical Measurement Systems for Industrial Inspection; (1999) https://doi.org/10.1117/12.364275
Event: Industrial Lasers and Inspection (EUROPTO Series), 1999, Munich, Germany

Abstract

The laser diffraction is a robust and precise technique to monitor wire diameters in-line. However, classical Fraunhofer diffraction formulas are not appropriate for 3-dimensional object size determination. The Babinet's principle allow to use such formulas only for angles of diffraction that tend to zero. A real diffraction measurement necessarily takes a finite angular range (approximately 10 degrees) and therefore, an error will be introduced if using classical formulas. The exact electromagnetic formulation is not appropriate to deal with 3-D objects, basically because it does not provide explicit formulas to determine the wire diameter. We have worked a pseudo-empirical approach out to reach simple accurate and reliable diffraction formulas that use exclusively the fringe pattern. To validate the diffraction formulas we need a calibration of the wire diameter. To accomplish this, we have introduced a hybrid set-up which allows interferometric and diffraction measurements over the same area of the wire. Using a He-Ne laser and a plus or minus 10 degree measurement range we observe, typically, a diameter overestimation of approximately 0.5 microns for different metallic wires (approximately 30 - 300 micrometer). From this work, we can also extract a practical physical insight to diffraction phenomena in terms of the Geometrical Theory of Diffraction (GTD). Many optical techniques for metrology of high resolution must take into account diffracted light.

Citation Download Citation

Juan Carlos Martinez-Anton, Ibrahim Serroukh, and Eusebio Bernabeu "Laser diffraction wire diameter measurements: correction of diffraction models by interferometric calibration", Proc. SPIE 3824, Optical Measurement Systems for Industrial Inspection, (21 September 1999); https://doi.org/10.1117/12.364275

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