The ICARUS liquid argon TPC was moved to Fermilab after a successful three-year long run at INFN-LNGS. Currently, ICARUS-T600 is collecting data exposed to Booster Neutrino beam and NuMI off-axis beam to clarify within SBN program the existence of sterile neutrinos. A light detection system, based on PMTs deployed behind the TPC wire chambers, is in place to detect vacuum ultraviolet photons produced by ionizing particles in liquid argon. This system is fundamental for the detector operation, providing an efficient trigger and contributing to the 3D reconstruction of events. Moreover, since the TPC is exposed to a huge flux of cosmic rays due to its shallow depths operations, the light detection system allows for the time reconstruction of events, contributing to the identification and to the selection of neutrino interactions within the beam spill gates. In this contribution preliminary comparison of MC simulation with DATA are presented for the light signal of cosmic muon.