A fixed-source array test station provides a compact cost-effective high-throughput test bed for testing optical sensors that require stimulus at fixed angular positions. At least some of the contact points are associated with mating contact points on a second surface of the MEA, which surface is opposite the first surface of the MEA, with a surface of a second bipolar plate making contact with said mating contact points, and that the contact points and the associated mating contact points are positioned one above the other in the stacking direction. A linking element connects two outer pistons so that they move in tandem. The way of attaching, or introducing, a particle is bringing a particle-containing gas to make contact to the surface, which surface has been made to be free of the oxidized film and other film.