A highly efficient, lightweight solid state lighting panel is disclosed that has multiple LED sources, and emits a substantially uniform light intensity between said sources. The light from these sources is directed towards a highly reflective, diffusive, backing. These LED sources are placed between the reflector and a partially transmissive, partially reflective output coupler to form a cavity. The LEDs, which are mounted to printed circuit boards to form strips, can be either attached to the inner surface of the diffuser with adhesive, or suspended on a thermally dissipative structure within the cavity. By optimizing the reflector to have as high of a reflectance value as possible (>95%) along with a output diffuser with about 50% transmission and 50% reflection, one can obtain cavity transmissions higher than 90%. The output from this disclosed design is more pleasant to look at than those with LEDs that directly illuminate the diffuser, causing hotspots.