Imaging birefringent interferometers are used to measure plasma flow in 2-D via the Doppler shift of a spectral emission line. Applications include plasma physics study in fusion energy experiments and in the Earth's upper atmosphere. We present a new, to the best of our knowledge, method for wavelength calibration that does not require measurement at the rest wavelength of the targeted spectral line, nor measurement using a tuneable laser source. This is useful when such light sources are not available. Fringes measured at known wavelengths from the emission lines of gas-discharge lamps are used to constrain an instrument model which can generate the required calibration data. In the process, optical path difference, dispersion and misalignments are characterized. The "2π ambiguity" of interferogram phase data is handled using circular statistics, allowing the wavelength span of the calibration lines to far exceed the unambiguous measurement range of the instrument. The technique is demonstrated to an accuracy of ±1 pm (±0.7 km/s flow-equivalent) over a 40 nm visible wavelength range.