A set-up for the accurate measurement of aperture areas based on the superposition of Gaussian laser beams to generate a uniform irradiance has been developed at the Bureau International des Poids et Mesures (BIPM). The uncertainty budget for this method is presented. Results obtained for several apertures are compared with calibrations made at the National Physical Laboratory (NPL). Measurements made on different types of aperture are also compared with the results of an additional technique for determining area ratios. In this case the agreement depends systematically on the type of aperture, because of the imperfect structure of the aperture edges. This work highlights the importance of the determination of the "effective area" for the intended application of the aperture and the use of high-quality diamond-turned apertures.
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