Within the EURAMET joint research project 'Absolute Long-distance Measurement in Air' an absolute measuring heterodyne interferometer was developed for a measuring range of 1 km with a targeted accuracy of 0.1 mm. Since one dominating uncertainty source for interfere-ometric length measurement is the refractive index of air, especially under outdoor conditions, a two wavelength compensation method is integrated in the set-up. Measurements were performed on a 50 m comparator equipped with a HeNe laser interferometer.
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