  We use QCD kinetic theory to compute photon production in the chemically
equilibrating Quark-Gluon Plasma created in the early stages of high-energy
heavy-ion collisions. We do a detailed comparison of pre-equilibrium photon
rates to the thermal photon production. We show that the photon spectrum
radiated from a hydrodynamic attractor evolution satisfies a simple scaling
form in terms of the specific shear viscosity $\eta/s$ and entropy density
$dS/d\zeta \sim {\scriptstyle \left(T\tau^{1/3}\right)^{3/2}}_\infty$. We
confirm the analytical predictions with numerical kinetic theory simulations.
We use the extracted scaling function to compute the pre-equilibrium photon
contribution in $\sqrt{s_{NN}}=2.76\,\text{TeV}$ 0-20\% PbPb collisions. We
demonstrate that our matching procedure allows for a smooth switching from
pre-equilibrium kinetic to thermal hydrodynamic photon production. Finally, our
publicly available implementation can be straightforwardly added to existing
heavy ion models.
