  Direct photometric measurements of the cosmic optical background (COB)
provide an important point of comparison to both other measurement
methodologies and models of cosmic structure formation, and permit a cosmic
consistency test with the potential to reveal additional diffuse sources of
emission. The COB has been challenging to measure from Earth due to the
difficulty of isolating it from the diffuse light scattered from interplanetary
dust in our solar system. We present a measurement of the COB using data taken
by the Long-Range Reconnaissance Imager (LORRI) on NASA's New Horizons mission,
considering all data acquired to 47 AU. We employ a blind methodology where our
analysis choices are developed against a subset of the full data set, which is
then unblinded. Dark current and other instrumental systematics are accounted
for, including a number of sources of scattered light. We fully characterize
and remove structured and diffuse astrophysical foregrounds including bright
stars, the integrated starlight from faint unresolved sources, and diffuse
galactic light. For the full data set, we find the surface brightness of the
COB to be $\lambda I_{\lambda}^{\mathrm{COB}}$ $=$ 21.98 $\pm$ 1.23 (stat.)
$\pm$ 1.36 (cal.) nW m$^{-2}$ sr$^{-1}$. This result supports recent
determinations that find a factor of 2 ${-}$ 3 $\times$ more light than
expected from the integrated light from galaxies and motivate new diffuse
intensity measurements with more capable instruments that can support spectral
measurements over the optical and near-IR.
