  The detection of photons produced during the avalanche development in gas
chambers has been the subject of detailed studies in the past. The great
progresses achieved in last years in the performance of micro-pattern gas
detectors on one side and of photo-sensors on the other provide the possibility
of making high granularity and very sensitive particle trackers. In this paper,
the results obtained with a triple-GEM structure read-out by a CMOS based
sensor are described. The use of an He/CF$_4$ (60/40) gas mixture and a
detailed optimization of the electric fields made possible to obtain and very
high GEM light yield. About 80 photons per primary electron were detected by
the sensor resulting in a very good capability of tracking both muons from
cosmic rays and electrons from natural radioactivity.
