Influence of the beam shape on aperture measurements with the laser beam scanning technique

The realization of a number of radiometric and photometric units requires one to measure accurately the area of precision apertures. The BIPM employs the laser-beam scanning technique which is based on the generation of a uniform irradiance by a two-dimensional superposition of many regularly spaced laser beams. The existing literature describes the formation of the uniform irradiance as a superposition of Gaussian laser beams and derives a condition for the largest allowable lattice spacing to obtain the necessary uniformity. In this paper we analyse the effect of the use of real laser beams which are almost, but not completely, Gaussian by applying Fourier analysis. Experimental results of aperture measurements with different spacings of the laser beams are also shown. The results of the calculations and the measurements show a characteristic dependence of the measured area on the grid spacing, even for spacings sufficiently small according to the condition based on Gaussian theory. This can be explained by very small deviations of the actual beam shape from a perfect Gaussian profile.