  We investigate the QED Compton process (QEDCS) in longitudinally polarized
lepton-proton scattering both in the elastic and inelastic channels and show
that the cross section can be expressed in terms of the polarized equivalent
photon distribution of the proton. We provide the necessary kinematical
constraints to extract the polarized photon content of the proton using this
process at HERMES, COMPASS and eRHIC. We also discuss the suppression of the
major background process coming from virtual Compton scattering. We point out
that such an experiment can give valuable information on $g_1(x_B, Q^2)$ in the
small $x_B$, broad $Q^2$ region at the future polarized collider eRHIC and
especially in the lower $Q^2$, medium $x_B$ region in fixed target experiments.
