  The Deep Underground Neutrino Experiment (DUNE) is a dual-site experiment for
long-baseline neutrino oscillation studies, able to resolve the neutrino mass
hierarchy and measure $\delta_{CP}$. DUNE will also have sensitivity to
supernova neutrinos and to processes beyond the Standard Model, such as nucleon
decay searches. The Far Detector (FD) will consist of four liquid argon TPC
(17.5 kt total mass) with systems for the detection of charge and scintillation
light produced by an ionization event. The charge detection system permits both
calorimetry and position determination. In addition, the photon-detection
system (PDS) enhances the detector capabilities for all DUNE physics drivers.
The PDS of the first FD module consists of light collector modules placed in
the inactive space between the innermost wire planes of the TPC anode. The
light collectors, the so-called X-ARAPUCAS, are functionally a light trap that
captures wavelength-shifted photons inside boxes with highly reflective
internal surfaces where they are guided to Silicon Photo-multipliers (SiPM) by
wavelength-shifting (WLS) bars. Functionality and operational tests of the
X-ARAPUCAS to be installed in ProtoDUNE-SP phase II (FD DUNE prototype at the
scale 1:20), as well as the measurement of their absolute detection efficiency
is reported in this publication.
