Practical aspects of aperture-area measurements by superposition of Gaussian laser beams

A set-up for the accurate measurement of aperture areas based on the superp osition of Gaussian laser beams to generate a uniform irradiance has been d eveloped at the Bureau International des Poids et Mesures (BIPM). The uncer tainty budget for this method is presented. Results obtained for several ap ertures are compared with calibrations made at the National Physical Labora tory (NPL). Measurements made on different types of aperture are also compa red 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 highl ights the importance of the determination of the "effective area" for the i ntended application of the aperture and the use of high-quality diamond-tur ned apertures.