In a surface lighting unit comprising a footprint portion, a sidewall portion, and an optical reflector with an opening spaced apart by a predetermined distance from the footprint portion. The opening substantially satisfies a relation A=bx2+c, assuming A is the open area ratio, i.e. the ratio of the opening area in a preset predetermined region to the area of the preset predetermined region, b and c are constants, and x is the distance from the center of the optical reflector. The opening has a circular non-through hole concentric to the center of the optical reflector, a concentric arcuate hole on the outside thereof, and a plurality of round holes farther on the outside thereof.