To enable a light guiding plate to be reduced in thickness while suppressing the degradation in luminance and to efficiently achieve the reduction in the thickness of a back light device by providing the device with a light source for making light incident on the inside of a light guiding member from the end on the divergent side of the light guiding member.
A bar-shaped optical element 2 is disposed in a position facing the light incident surface 1a of the light guiding plate 1 and the spot light source 3 is disposed in the position facing the light incident surface 2a of this bar-shaped optical element 2. The bar-shaped optical element 2 is formed in a wedge shape having the light incident surface 2a from the spot light source 3, a surface 2c parallel with the incident surface 1a of the light guiding plate 1 and a surface 2b oblique with this surface 2c. Plural grooves 21 are formed on the surface 2b so as to be approximately perpendicular to the longitudinal direction. The light rays 31a and 31b incident on the bar-shaped optical element 2 from the light source 3 are lifted by the grooves 21 in such a manner that the reflection direction heads to the surface 2c. These light rays are then emitted from the surface 2c. If, on the other hand, the light is made incident on the flat parts between the grooves and the grooves, the reflected light is again reflected by the surface 2c.
HIBINO KATSUTOSHI
TSUCHIYA YOICHI
