The refractive index of air has to be taken into account for interferometric length measurements. It is usually measured by a refractometer or is calculated from the data of a weather station. However, the refractive index in the interferometer path may be different from that of the refractometer or weather station sensors due to gradients and short term fluctuations. By using two laser sources with different wavelengths the refractive index within the interferometer path can be derived from the dispersion relation. As a result of the dispersion relation both laser wavelengths should differ as much as possible within the restrictions of the optical components. An investigation of the achievable resolution is shown where an existing HeNe laser is combined with an infrared diode laser at 780 nm wavelength to replace a weather station or refractometer.
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