Abstract: Rays of light from three light-emitting diodes 5 incident at a thicker side end edge 2c of a light guide plate 2 made of a transparent plate. A ratio of grating part width/non-grating part width in a unit width of a diffraction grating 3 provided on a rear surface 2b of the light guide plate 2 is varied, and grating constant of a diffraction grating 4 of a front surface 2a provided perpendicularly to the diffraction grating 3, is set to a fixed value smaller than a mean grating constant of the diffraction grating of the rear surface 2b, so that uniform, high brightness at the front surface 2a of the light guide plate 2 can be obtained. Therefore, the light guide plate can perform high, uniform brightness over a whole illuminating surface even with a point light source, and yet can prolong life of batteries by virtue of reduction in power consumption of the light source.

Abstract: A light guide plate is provided which utilizes the phenomenon of diffraction based on wave optics of light, and which can provide much higher and more uniform brightness over an entire illuminated surface than the brightness level achievable by the prior art and can assure longer battery life through reduced power consumption for a light source. The light guide plate consists of a transparent plate, at least on one end of which light rays from the light source fall. The light guide plate has a diffraction grating printed or worked on a bottom surface thereof such that at least one of a grating part width/non-grating part width ratio in unit-width or a sectional configuration of the diffraction grating is varied so as to enhance and uniform light intensity on a top surface of the light guide plate.