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A novel method for diameter estimation of small opaque objects using Fraunhofer diffraction

Vyas, Khushi and Lolla, Kameswara Rao (2010) A novel method for diameter estimation of small opaque objects using Fraunhofer diffraction. In: Conference on Reflection, Scattering, and Diffraction from Surfaces II, AUG 02-04, 2010, San Diego, CA.

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A novel optical method is proposed and demonstrated, for real-time dimension estimation of thin opaque cylindrical objects. The methodology relies on free-space Fraunhofer diffraction principle. The central region, of such tailored diffraction pattern obtained under suitable choice of illumination conditions, comprises of a pair of `equal intensity maxima', whose separation remains constant and independent of the diameter of the diffracting object. An analysis of `the intensity distribution in this region' reveals the following. At a point symmetrically located between the said maxima, the light intensity varies characteristically with diameter of the diffracting object, exhibiting a relatively stronger intensity modulation under spherical wave illumination than under a plane wave illumination. The analysis reveals further, that the said intensity variation with diameter is controllable by the illumination conditions. Exploiting these `hitherto unexplored' features, the present communication reports for the first time, a reliable method of estimating diameter of thin opaque cylindrical objects in real-time, with nanometer resolution from single point intensity measurement. Based on the proposed methodology, results of few simulation and experimental investigations carried-out on metallic wires with diameters spanning the range of 5 to 50 mu m, are presented. The results show that proposed method is well-suited for high resolution on-line monitoring of ultrathin wire diameters, extensively used in micro-mechanics and semiconductor industries, where the conventional diffraction-based methods fail to produce accurate results.

Item Type: Conference Paper
Series.: Proceedings of SPIE-The International Society for Optical Engineering
Publisher: The International Society for Optical Engineering.
Additional Information: Copyright of this article belongs to The International Society for Optical Engineering.
Keywords: Dimension estimation; optical diffraction; small opaque object; diameter measurement; metallic cylinders
Department/Centre: Division of Physical & Mathematical Sciences > Instrumentation Appiled Physics
Date Deposited: 31 Mar 2011 06:40
Last Modified: 01 Apr 2011 06:16
URI: http://eprints.iisc.ac.in/id/eprint/36356

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