The Deep Underground Neutrino Experiment (DUNE) Far Detector (FD) will be formed by four 10-kton Liquid Argon (LAr) Time Projection Chambers (TPC) using both single and dual-phase technology. The dual-phase technology foreseen the charge amplification in the gas phase before the signal collection and is following a staged approach to demonstrate its feasibility at the DUNE FD scale. In 2017, a 4-ton demonstrator of 3x1x1 m3 volume was exposed to cosmic muons and demonstrated expected performance in terms of charge extraction and light collection. A bigger prototype (ProtoDUNE-DP), with an active volume of 6x6x6 m3, is currently under commissioning at CERN.
The photon detection system in these detectors is crucial to provide the trigger signal giving an absolute time reference for the charge acquisition system of rare non-beam events and to provide complementary calorimetry. An overview of the analysis of the light collected in the 4-ton demonstrator has been presented. These prototypes confirmed the performance of the light detection system to provide trigger based on the scintillation light signal, to characterize the LAr response to the crossing muons and to monitor the LAr purity. The analyzed data are compared with MC simulations to improve the values of less understood LAr optical parameters such as the Rayleigh scattering length.