The tube reflector (20) is positioned at right angles beneath the roof reflector (11) and has parabolically curved inner wall surfaces (22). At least one horizontal fluorescent lamp is between the roof reflector and the tube reflector. The tube reflector is positioned in a plate-shaped grid screen (13) which has rectangular cells (14) formed by intersecting bridge=pieces and possessing parabolically curved reflection surfaces. The tube reflector is positioned in a central recess (18) in the middle of the grid screen.
