  DUNE is a dual-site experiment for long-baseline neutrino oscillation
studies, neutrino astrophysics and nucleon decay searches. ProtoDUNE Dual Phase
(DP) is a 6x6x6m3 liquid argon time-projection-chamber (LArTPC) that recorded
cosmic-muon data at the CERN Neutrino Platform in 2019-2020 as a prototype of
the DUNE Far Detector. Charged particles propagating through the LArTPC produce
ionization and scintillation light. The scintillation light signal in these
detectors can provide the trigger for non-beam events. In addition, it adds
precise timing capabilities and improves the calorimetry measurements. In
ProtoDUNE-DP, 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, the ProtoDUNE-DP photon detection
system performance is evaluated with a particular focus on the different
wavelength shifters, such as PEN and TPB, and the use of Xe-doped LAr,
considering its future use in giant LArTPCs. The scintillation light production
and propagation processes are analyzed and a comparison of simulation to data
is performed, improving understanding of the liquid argon properties
