  The detection of VUV scintillation light, e.g. in (liquid) argon detectors,
commonly includes a reflector with a fluorescent coating, converting UV photons
to visible light. The light yield of these detectors depends directly on the
conversion efficiency. Several coating/reflector combinations were produced
using VM2000, a specular reflecting multi layer polymer, and Tetratex, a
diffuse reflecting PTFE fabric, as reflector foils. The light yield of these
coatings was optimised and has been measured in a dedicated liquid argon setup
built at the University of Zurich. It employs a small, 1.3 kg LAr cell viewed
by a 3-inch, low radioactivity PMT of type R11065-10 from Hamamatsu. The
cryogenic stability of these coatings was additionally studied. The optimum
reflector/coating combination was found to be Tetratex dip coated with
Tetraphenyl-butadiene with a thickness of 0.9 mg/cm$^2$ resulting in a 3.6
times higher light yield compared to uncoated VM2000. Its performance was
stable in long term measurements, ran up to 100 days, in liquid argon. This
coated reflector was further investigated concerning radioactive impurities
found to be suitable for current and upcoming low-background experiments.
Therefore it is used for the liquid argon veto in Phase II of the GERDA
neutrinoless double beta decay experiment.
