Title: Optical quality assurance of GEM foils,
Abstract: An analysis software was developed for the high aspect ratio optical scanning
system in the Detec- tor Laboratory of the University of Helsinki and the
Helsinki Institute of Physics. The system is used e.g. in the quality assurance
of the GEM-TPC detectors being developed for the beam diagnostics system of the
SuperFRS at future FAIR facility. The software was tested by analyzing five
CERN standard GEM foils scanned with the optical scanning system. The
measurement uncertainty of the diameter of the GEM holes and the pitch of the
hole pattern was found to be 0.5 {\mu}m and 0.3 {\mu}m, respectively. The
software design and the performance are discussed. The correlation between the
GEM hole size distribution and the corresponding gain variation was studied by
comparing them against a detailed gain mapping of a foil and a set of six lower
precision control measurements. It can be seen that a qualitative estimation of
the behavior of the local variation in gain across the GEM foil can be made
based on the measured sizes of the outer and inner holes.