Displacement measurements in air for the calculable capacitor

The NIST calculable capacitor relies on precision interferometric measurements of displacement to determine absolute capacitance values. These displacement measurements, when made in vacuum, can have uncertainties of δL = 10 pm. Measurements made in air, on the other hand, are more difficult owing to the index of refraction. Here we evaluate two approaches to measure the displacement in air, a brute force one-wavelength method and a more refined two-wavelength method.