  The internal luminescence quantum efficiency ($Q_\mathrm{i}^\mathrm{lum}$)
provides an excellent assessment of the optoelectronic quality of
semiconductors. To determine $Q_\mathrm{i}^\mathrm{lum}$ of perovskite films
from the experimentally accessible external luminescence quantum efficiency
($Q_\mathrm{e}^\mathrm{lum}$) it is essential to account for photon recycling,
and this requires knowledge of the photon escape probability ($p_\mathrm{e}$).
Here, we establish an analysis procedure based on a curve fitting model that
accurately determines $p_\mathrm{e}$ of perovskite films from photoluminescence
(PL) spectra measured with a confocal microscope and an integrating sphere
setup. We show that scattering-induced outcoupling of initially-trapped PL
explains commonly observed red-shifted and broadened PL spectral shapes and
leads to $p_\mathrm{e}$ being more than 10% higher in absolute terms compared
to earlier assumptions. Applying our model to CH$_3$NH$_3$PbI$_3$ films with
exceptionally high $Q_\mathrm{e}^\mathrm{lum}$ up to 47.4% sets a real
benchmark for $Q_\mathrm{i}^\mathrm{lum}$ at $78.0 \pm 0.5\%$, revealing there
is beyond a factor of two more scope for reducing non-radiative recombination
than previously thought.
