A planar surface illuminator (20) for installation below a liquid crystal display panel includes a light guide plate (22) and a plurality of light emitting diodes (21). The light guide plate has a bottom surface (223), an end surface (221) and a number of dots (23) formed on the bottom surface. The light emitting diodes are positioned adjacent the end surface to irradiate the light guide plate. Shortfall areas (223a) in the light guide plate between each two adjacent light emitting diodes are lightened by placing special dots (23a) therein, which special dots are made of melamine-based fluorescent particles. The melamine-based fluorescent particles function as small light sources, thus lightening the shortfall areas. The brightness of the planar surface illuminator is thereby balanced.