Today is the last day of data acquisition of a one-month measurement campaign at CERN building 182. We monitored the longterm stability of the light yield of a large-scale PEN-ESR wavelength shifter reflector cage. This is a collaborative project of members of the DarkSide, LEGEND, DUNE and DEAP collaborations from AstroCeNT Warsaw, the University of Zurich, the Technical University Munich, the University of Edinburgh, from Nikhef Amsterdam and the University of Hawai’i at Mānoa.