GEM-based detectors had a noticeable development in last years and have successfully
been employed in different fields from High Energy Physics to imaging applications.
Light production associated to the electron multiplication allows to perform an optical
readout of these devices. The big progress achieved in CMOS-based photosensors makes
possible to develop a high sensitivity, high granularity and low noise readout.
In this paper we present the results obtained by reading out the light produced by
a triple-GEM structure by means of a 4 mega-pixel CMOS sensor having a
noise level less than two photons per pixel.
The choice of a CF$_4$ rich gas mixture (He/CF$_4$ 60/40) and a detailed optimisation
of the electric fields allowed to reach a light-yield high enough to obtain
very visible signals from minimum ionizing particles.
In a test performed with 450 MeV electron beam, 800 photons per
millimeter were collected and a space resolution of about 75 $\mu$ m was obtained.
Micro-Pattern Gas Detectors, CMOS-Sensors, High-Granularity Tracker