ENHANCEMENT OF WIRE DIAMETER MEASUREMENTS - COMPARISON BETWEEN FRAUNHOFER-DIFFRACTION AND LORENZ-MIE THEORY

The Fraunhofer diffraction is compared with a numerical simulation bas ed on the Lorenz-Mie theory to improve the performance of a laser diff raction instrument. This instrument, based on the analysis of diffract ion fringes at the focal plane of a lens, is especially suitable for i ndividual wire diameter measurements. We show that the error due to Fr aunhofer approximation increases with an increase of the reflectivity and a decrease of the object diameter. Comparison with Lorenz-Mie theo ry enables us to correct these errors. For a given object, the laser d iffraction instrument reproduces diameter measurements with a very sma ll dispersion. (C) 1996 Society of Photo-Optical instrumentation Engin eers.