  The Deep Underground Neutrino Experiment (DUNE) is a leading-edge experiment
for long-baseline neutrino oscillation studies, neutrino astrophysics and
nucleon decay searches. ProtoDUNE-Dual Phase (DP) is a 6x6x6 m$^3$ liquid argon
time-projection-chamber (LArTPC) operated at the CERN Neutrino Platform in
2019-2020 as a prototype of the DUNE Far Detector. In ProtoDUNE-DP, the
scintillation and electroluminescence light produced by cosmic muons in the
LArTPC is collected by photomultiplier tubes placed up to 7 m away from the
ionizing track. In this paper, we present the performance of the ProtoDUNE-DP
photon detection system, comparing different wavelength-shifting techniques and
the use of xenon-doped LAr as a promising option for future large LArTPCs. The
scintillation light production and propagation processes are analyzed and
compared to simulations, improving understanding of the liquid argon
properties.
