  It is well known and has been shown that the gain performance of Gaseous
Electron Multipliers (GEM) depends on the size of the holes. With an optical
scanner it is possible to measure the dimensions of the holes, and to predict
the performance of GEMs. However, the gain prediction of GEMs that are
manufactured with a double mask etching technique is not straightforward. With
the hole size information alone, it is not possible to make precise prediction
of the gain. We show that the alignment of the photo-masks between the two
sides of the GEM foils plays a crucial role. A misalignment of a few microns
can lower the gain substantially. The study is performed by using the Helsinki
high definition optical scanner for quality control of GEM foils, and this will
show its true potential.
