  We present a space and time resolved interferometric plasma diagnostic for
use on plasmas where neutral-bound electron contribution to the refractive
index cannot be neglected. By recording simultaneously the plasma optical index
at 532 and 1064 nm, we are able to extract independently the neutral and free
electron density profiles. We report a phase resolution of 30 mrad,
corresponding to a maximum resolution on the order of
$4\times10^{23}~\mathrm{m}^{-3}$ for the electron density, and of $10^{24} ~
\mathrm{m}^{-3}$ for the neutral density. The interferometer is demonstrated on
centimeter-scale sparks triggered by laser filamentation in air with typical
currents of a few tens of A.
