Non-destructive quality control of microstructures at the manufacturing sta
ge is an important issue in the foreseen use of huge numbers of such gaseou
s detectors in the future high luminosity colliders. In this work we report
on the use of the scintillation light emitted by the avalanches in GEM cha
nnels for checking defects in the foils. The test system is described and d
ata on the relative efficiency of several gaseous mixtures are presented. T
he foil images obtained with a low-noise CCD system are analysed and compar
ed with the optical images obtained with an industrial inspection system of
high magnification. The validity of this test method is established and po
ssible extensions of its use are discussed. (C) 2000 Elsevier Science B.V.
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