The upper side of a GEM electrode can be coated with a photosensitive material; with proper voltages applied, a photoelectron extracted from the photocathode is sucked into the holes, and multiplies in avalanche. A cascade of GEM foils ca be used to obtain large gains and allow deetction and localization of single photoelectrons:

Photon detection with GEM: a photosensitive material is deposited on the upper side of the foil.	Single photoelectron deetcted charge spectrum; triple-GEM with CsI photocathode

Schematics of the detection setup, using a pulsed UV lamp.	The laboratory setup with a small size GEM detector.

	Center of gravity of recorded charge on anode strips for two positions of collimated beams 200 µm apart. The single photoelectron localization accuracy is ~50 µm rms.

For multihit events, such as those in RICH applications, a powerful yet simple readout scheme is the Hexaboard, a matrix of hexagonal pads interconnected in rows along three directions at 120^O:
 

Hexaboard: hexagonal pads, 500 µm in diameter, interconnected along three directions.	A double photon event recorded with the Hexaboard readout. The two-photon resolution is around one mm.

To know more: see T. Meinschad, L. Ropelewski and F. Sauli, Nucl. Instr. and Meth. A535(2004)324 PREPRINT