The present invention relates to a light guiding plate for a front light mounted on a reflective display unit.
To reduce the power consumption, some conventional display units have a front light. When the outside light is sufficient, this type of display unit uses outside light such as sunlight and room lightening. When the outside light is insufficient, the display unit uses the front light. Such display units include a reflective display unit with a front light.
FIG. 5(a) shows a prior art reflective liquid crystal display unit 51 having a front light 53. The display unit 51 includes a reflective liquid crystal panel 52. The front light 53 faces the liquid crystal panel 52. The front light 53 includes a light guiding plate 54, and a light source 55 facing one end (incidence plane) 54a of the light guiding plate 54. The thickness of the light guiding plate 54 is substantially uniform. An exit plane 54b of the light guiding plate 54 faces the liquid crystal panel 52 on a reflection-exit plane 54d, which is opposite to the exit plane 54b, grooves (not shown) are formed to reflect light from the light source 55 to the exit plane 54b. The light from the light source 55 into the light guiding plate 54 through the incidence plane 54a is guided through the light guiding plate 54 while being totally reflected. The light is then radiated onto the liquid crystal panel 52 through the exit plane 54b. 
FIG. 5(b) shows another prior art reflective liquid crystal display unit 151. The display unit 151 includes a wedge plate type light guiding plate 154. The thickness of the light guiding plate 154 gradually decreases from an incidence plane 154a to an opposite end plane 154c. Other than the light guiding plate 154, the structure of the display unit 151 is the same as the display unit 51 of FIG. 5(a).
To improve the visibility of the liquid crystal panel 52, the light guiding plates 54, 154 are designed to guarantee the brightness. A less amount of incident of incident light escapes from the wedge type light guiding plate 154 through the opposite end plane 54c, which is opposite to the incidence plane 54a, compared to the flat plate type light guiding plate 54 shown in FIG. 5(a). Compared to the flat plate type light guiding plate 54, the wedge type light guiding plate 154 has more uniform brightness (refer to Japanese Laid Open Patent Publication No. 11-242220).
The brightness of a flat type light guiding plate 54 is improved by deepening the grooves. However, excessive depth makes the grooves visible and therefore degrades the visibility of the display unit. The wedge type light guiding plate 154, on the other hand, easily improves the brightness compared to the flat plate type light guiding plate 54. However, if the thickness of the waveguide 154 at the incidence plane 54a and the thickness at the opposite end plane 54c are significantly different, that is, if the angle θ defined by the reflection-exit plane 54d and the exit plane 54b is large, parallax is produced. This results in a double image. A ray L1 escapes from the light guiding plate 154 through the exit plane 54b and is reflected by the liquid crystal panel 52. Then, the ray L1 reenters the light guiding plate 154 and exits through the reflection-exit plane 54d. Part of the ray L1 is reflected by the reflection-exit plane 54d. The reflected part of the ray L1 is reflected by the exit plane 54b and exits the light guiding plate 154 through the reflection-exit plane 54d. This portion of the ray L1 is expressed by L2 in FIG. 6. As a result, the image on the display 52 is doubled.
If the inclination angle θ is not appropriate, the brightness will be uneven. That is, a uniform brightness cannot be obtained. Japanese Laid Open Patent Publication No. 11-242220 discloses that wedge plate type light guiding plate improves the uniformity of the brightness. However, the publication does not disclose the relationship of the inclination angle 0 to the parallax and a uniform brightness distribution.