A database interpolation method is used to rapidly calculate a predicted optical response characteristic of a diffractive microstructure as part of a real-time optical measurement process. Each cabinet comprises at least one casing 4, 5, which casings are arranged to connect fiber cables with equipment in the cabinets via fan-out fiber cables 6, 7. Prior to applying pressure to the rivet to fix the rivet to the piston plate, the rivet head is clamped between the piston plate and the cover.