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Capability of commercial trackers as compensators for the absolute refractive index of air



Patrick Egan


A procedure is presented which calibrates a commercial wavelength/refractive-index tracker, so that it can compensate for the absolute refractive index of air within 3e-8*n. The procedure employs ultrahigh-purity helium and argon as reference gases of known n(p, T) to deduce the two unknown parameters in the working equation of the tracker: gas pathlength and pressure-induced distortion error. The performance of the gas-calibration procedure is evaluated by comparing the corrected tracker against a master refractometer based on a Fabry--Perot cavity in nitrogen, a third reference gas of known n(p, T). In nitrogen, the calibrated trackers demonstrate accuracy at the level of 4e-9*n. Testing in a fourth reference gas---water vapor---reveals that the working equation of the trackers must include a third unknown parameter: an end-effect caused by a moisture-dependence of the reflection phase-shift. Correcting for this moisture-related error represents the largest contribution to measurement uncertainty, and explains why performance of the calibrated trackers is an order-of-magnitude worse in moist air than in pure gas. In air, the Fabry--Perot cavity-based refractometer performs within 5e-9*n, but is not a commercially-available device.
Precision Engineering


refractometry, precision measurement, air-wavelength, gas metrology


Egan, P. (2022), Capability of commercial trackers as compensators for the absolute refractive index of air, Precision Engineering, [online],, (Accessed May 24, 2024)


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Created May 26, 2022, Updated November 29, 2022