The Fraunhofer diffraction is compared with a numerical simulation based on the Lorenz-Mie theory to improve the performance of a laser diffraction instrument. This instrument, based on the analysis of diffraction fringes at the focal plane of a lens, is especially suitable for individual wire diameter measurements. We show that the error due to Fraunhofer approximation increases with an increase of the reflectivity and a decrease of the object diameter. Comparison with Lorenz-Mie theory enables us to correct these errors. For a given object, the laser diffraction instrument reproduces diameter measurements with a very small dispersion. (C) 1996 Society of Photo-Optical instrumentation Engineers.