A luminaire has a light source carrier around a central light output window, with light sources facing a first, at least partially upward, direction. An upper reflector cover reflects light to a range of light output directions, at least partially downwardly, through the central window opening. A lens structure is formed over the set of light sources having a light diffusing output area and reflecting side walls extending between the light sources and the light diffusing output area. This design enables the luminaire to be very slim. The design may also avoid the need for a light diffusing output window.