A solid state lighting device includes a thermally conductive plate having a subassembly mounted to at least one side of the plate. Each subassembly includes a mounting frame with a back side and an open center, and a PCB supporting a plurality of LEDs with electrical leads extending therefrom. The PCB is supported by the frame so that the back of the PCB is flush with the back side of the frame. A cup-like light diffuser having a rim is secured to the front of the frame so that it surrounds the frame and covers the LEDs. Fastening devices fasten the subassembly against the one side of the plate so that the back of the PCB is in intimate thermal contact with the plate and the diffuser rim abuts the plate so that when the LEDs are energized via their leads, a maximum amount of light from the LEDs is directed through the diffuser and waste heat from the LEDs is efficiently conducted away by the plate which may also constitute a reflector.