BACK LIGHT DEVICE

PURPOSE:To increase screen luminance from the front and improve the visibility by providing directivity characteristics which have a luminance peak in the front direction almost according to the visual angle of a liquid crystal panel so as to effectively utilize diffused light in the up-down direction of the back light device. CONSTITUTION:Light from a fluorescent lamp 6 is made incident on a light guide plate 4 from its flank incidence part 4a. In the light guide plate 4, the light is reflected repeatedly on a bottom surface 4c and a front surface 4b and emitted upward as uniform back light from the front surface 4b. A prism- shaped light converging plate 2 which is sufficiently narrower than the pixel pitch of the liquid crystal panel 1 and almost accords with the visual angle characteristics of liquid crystal is arranged between a diffusion plate 3 and the liquid crystal panel 1 to accord the directivity characteristics of the back light with the visual angle characteristics of the liquid crystal panel 1. Consequently, the back light in a direction other than the visual angle of a liquid crystal display is converged on the front and the luminance when the display screen is viewed from the front is greatly increased.