  Non destructive quality control of microstructures at the manufacturing stage
is an important issue in the foreseen use of huge numbers of such gaseous
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 channels
for checking defects in the foils. The test system is described and data on the
relative efficiency of several gaseous mixtures are presented. The foil images
obtained with a low-noise CCD system are analysed and compared with the optical
images obtained with an industrial inspection system of high magnification. The
validity of this test method is established and possible extensions of its use
are discussed.
