A backlight unit comprises: (1) a light source; (2) a light guide plate (LGP) having an incident face toward the light source and having a refractive index n.sub.LGP; (3) a reflective polarizer having formed birefringence in optical contact with the LGP including a layer having parameters of the reflective polarizer selected to provide R0>0.8 for one polarization state and T90>0.8 for the orthogonal polarization state, for light of wavelength of 550 nm, for least one reflective polarizer angle of light incidence .theta..sub.a.