A back light system (2) includes a light guide plate (50). The light guide plate includes an array of prisms (55) formed thereon, and a nano-particle layer (80) formed on the prisms for diffusing light beams propagating therethrough. The nano-particle layer contains a mass of transparent nano-particles with diameters preferably in the range from 30.about.100 nm. A material of the particles may be TiO.sub.2 (titanium oxide), SiO.sub.2 (silicon dioxide), or Al.sub.2O.sub.3 (aluminum oxide). Each light beam passing through the nano-particle layer is scattered, and most of the scattering occurs over a limited range of angles. The scattered light beams exit the nano-particle layer mostly over the limited range of angles, with a medial direction of propagation in the range of angles being substantially parallel to a viewing direction. The back light system can yield high brightness and uniform luminescence, and is relatively simple in construction and inexpensive.