Abstract
The most common technique for diameter measurement of thin metallic cylinders is optical diffractometry. It consists in
illuminating the cylinder with a collimated monochromatic light beam, determining the diameter from the location of
the minima of the far field diffraction pattern. Babinet principle is normally assumed, being the diffraction pattern of the
cylinder equivalent to that of a strip whose width is equal to the cylinder diameter. Due to the three dimensional nature
of the cylinder, this model is not valid for accurate measurements. It has been experimentally shown that, when
compared to interferometry, Fraunhoffer model presents a systematic overestimation in the cylinder diameter. Rigorous
models which assume that the wire presents an infinite conductance have been developed. However, the refraction index
of the material has also appeared important for an accurate estimation since it produces a phase shift of the reflected
wave by the wire surface, modifying the state of polarization of the incident light beam and, as a consequence, the
location of the diffraction minima. In this work we propose a model based on the Geometrical Theory of Diffraction that
assumes both the three-dimensional nature and a finite conductance of the wire. Results for several materials are
presented, showing that the overestimation of the wire diameter depends on the state of polarization and wavelength of
the incident light beam, as well as the diameter and refraction index of the metallic wire.
