  The idea of implementing electroluminescence-based amplification through
transparent multi-hole structures (FAT-GEMs) has been entertained for some
time. Arguably, for such a technology to be attractive it should perform at
least at a level comparable to conventional alternatives based on wires or
meshes. We present now a detailed calorimetric study carried out for 5.9~keV
X-rays in xenon, for pressures ranging from 2 to 10~bar, resorting to different
geometries, production and post-processing techniques. At a reference voltage
5~times above the electroluminescence threshold ($E_{EL,th}\sim0.7$~kV/cm/bar),
the number of photoelectrons measured for the best structure was found to be
just 18\%~below that obtained for a double-mesh with the same thickness and at
the same distance. The energy resolution stayed within 10\% (relative) of the
double-mesh value.
  An innovative characteristic of the structure is that vacuum ultraviolet
(VUV) transparency of the polymethyl methacrylate (PMMA) substrate was
achieved, effectively, through tetraphenylbutadiene (TPB) coating of the
electroluminescence channels combined with indium tin oxide (ITO) coating of
the electrodes. This resulted in a $\times 2.25$-increased optical yield
(compared to the bare structure), that was found to be in good agreement with
simulations if assuming a TPB wavelength-shifting-efficiency at the level of
WLSE=0.74-1.28, compatible with expected values. This result, combined with the
stability demonstrated for the TPB coating under electric field (over 20~h of
continuous operation), shows great potential to revolutionize
electroluminescence-based instrumentation.
