Square lay-in luminaire made of sheet steel; optionally for modular ceilings 600 x 600 or 625 x 625 with visible grid system; surface powder coated; optionally two or three linear light insets made of plastic; high quality reflector with micro-faceted, aluminum-vaporised surface; precise radiation characteristic with symmetrical light distribution; exchangeable light insets; incl. converter, suitable for through-wiring; plug and play connection cables between light insets and converter with plug/socket system; energy-efficient LEDs with very good colour rendering