  Building upon the most recent CT18 global fit, we present a new calculation
of the photon content of the proton based on an application of the LUX
formalism. In this work, we explore two principal variations of the LUX ansatz.
In one approach, which we designate "CT18lux," the photon PDF is calculated
directly using the LUX formula for all scales, $\mu$. In an alternative
realization, "CT18qed," we instead initialize the photon PDF in terms of the
LUX formulation at a lower scale, $\mu\! \sim\! \mu_0$, and evolve to higher
scales with a combined QED+QCD kernel at
$\mathcal{O}(\alpha),~\mathcal{O}(\alpha\alpha_s)$ and $\mathcal{O}(\alpha^2)$.
While we find these two approaches generally agree, especially at intermediate
$x$ ($10^{-3}\lesssim x\lesssim0.3$), we discuss some moderate discrepancies
that can occur toward the end-point regions at very high or low $x$. We also
study effects that follow from variations of the inputs to the LUX calculation
originating outside the pure deeply-inelastic scattering (DIS) region,
including from elastic form factors and other contributions to the photon PDF.
Finally, we investigate the phenomenological implications of these photon PDFs
for the LHC, including high-mass Drell-Yan, vector-boson pair, top-quark pair,
and Higgs associated with vector-boson production.
