order go out practically vertically from the surface 2a of the light guide plate 2 as shown by arrows that is the diffraction grating 3 in which numerous minutely ruled grooves of 1 100 order of conventional prisms synergistically cooperate to function as such can provide outgoing light of far much higher intensity than conventional triangular pyramid prisms 33 see fig6 a 6c can provide wherein sides of the prism are dimensionally large on the order of 0 16 mm each and each prism individually effects total reflection of light simply in a photogeometrical way and without cooperation with adjacent prism since the ratio of grating part width non grating part width in unit width of diffraction grating 3 that is diffraction efficiency of the grating ratio of diffracted light intensity to incident light intensity becomes greater in a direction away from the fluorescent tube 4 side end 2c the quantity of diffracted light will increase in proportion to any decrease in light quantity which occurs in a direction away from the light source as a result the surface 2a of the light guide plate 2 is illuminated with light of high intensity and uniform brightness white light emitted from the fluorescent tube has a spectral distribution with.