  Next generation aperture arrays are expected to consist of hundreds to
thousands of antenna elements with substantial digital signal processing to
handle large operating bandwidths of a few tens to hundreds of MHz.
Conventionally, FX~correlators are used as the primary signal processing unit
of the interferometer. These correlators have computational costs that scale as
$\mathcal{O}(N^2)$ for large arrays. An alternative imaging approach is
implemented in the E-field Parallel Imaging Correlator (EPIC) that was recently
deployed on the Long Wavelength Array station at the Sevilleta National
Wildlife Refuge (LWA-SV) in New Mexico. EPIC uses a novel architecture that
produces electric field or intensity images of the sky at the angular
resolution of the array with full or partial polarization and the full spectral
resolution of the channelizer. By eliminating the intermediate
cross-correlation data products, the computational costs can be significantly
lowered in comparison to a conventional FX~or XF~correlator from
$\mathcal{O}(N^2)$ to $\mathcal{O}(N \log N)$ for dense (but otherwise
arbitrary) array layouts. EPIC can also lower the output data rates by directly
yielding polarimetric image products for science analysis. We have optimized
EPIC and have now commissioned it at LWA-SV as a commensal all-sky imaging
back-end that can potentially detect and localize sources of impulsive radio
emission on millisecond timescales. In this article, we review the architecture
of EPIC, describe code optimizations that improve performance, and present
initial validations from commissioning observations. Comparisons between EPIC
measurements and simultaneous beam-formed observations of bright sources show
spectral-temporal structures in good agreement.
