  Optical readout of Gas Electron Multipliers (GEM) provides very interesting
performances and has been proposed for different applications in particle
physics. In particular, thanks to its good efficiency in the keV energy range,
it is being developed for low-energy and rare event studies, such as Dark
Matter search. So far, the optical approach exploits the light produced during
the avalanche processes in GEM channels. Further luminescence in the gas can be
induced by electrons accelerated by a suitable electric field. The CYGNO
collaboration studied this process with a combined use of a triple-GEM
structure and a grid in an He/CF$_4$ (60/40) gas mixture at atmospheric
pressure. Results reported in this paper allow to conclude that with an
electric field of about 11~kV/cm a photon production mean free path of about
1.0~cm was found.
