Abstract: A gradation pattern is formed by sandblasting a surface of a transparent substrate. The gradation pattern comprises countless recesses formed by eroding the surface structure. The distribution density of the recesses is low in the vicinity of sides on which primary illuminating light is incident, and high in the center of the pattern. As the original flat and smooth aspect of the plate is maintained between the recesses, the light guiding performance of the plate is maintained. By adjusting the gradation pattern, any desired bright/dark pattern can be obtained. The cost of manufacturing a light guiding plate and flat optical source device is reduced. A large illumination area having a high uniform brightness is made possible by sandblasting. And many applications and modes of use which were impossible with a conventional light guiding plate having a printed pattern, are now possible.