[ 0043 ] fig1 illustrates a first embodiment of the planar light source according to the present invention , fig1 a representing a perspective view of the planar light source 10 , fig1 b representing a side view of the same , fig1 c representing an enlarged view of portion a in fig1 b . in fig1 reference number 1 represents leds as a light source , 2 a light guide plate , 3 a prism sheet and 7 a liquid crystal panel . the light guide plate 2 is rectangular in plan view and formed of a light - transmitting resin , such as polycarbonate . 2 a denotes a top surface of the light guide plate 2 , and 2 b a bottom surface . designated 2 c is a light receiving side surface 2 c facing the leds 1 . the light guide plate 2 has an auxiliary reflection surface 2 bh in its bottom surface 2 b near the light receiving side surface 2 c . the auxiliary reflection surface 2 bh has relatively small prism undulations or textured undulations . the prism or textured undulations are provided in a regular or irregular pattern . in the bottom surface 2 b , a portion adjoining the auxiliary reflection surface 2 bh is formed with a prism reflection surface having a plurality of asymmetric prisms 2 bp . the asymmetric prisms 2 bp each have a down slope 2 bp 1 and an up slope 2 bp 2 . the down slope 2 bp 1 is so configured that its distance to the top surface 2 a sharply increases as a point of interest on the slope moves away from the light receiving side surface 2 c . the up slope 2 bp 2 is so configured that its distance to the top surface 2 a moderately decreases . as shown in fig1 c , an inclination angle a of the up slope 2 bp 2 of the asymmetric prisms 2 bp is about 1 degree to several degrees . three leds 1 are held on a retainer member 1 b at positions facing the light receiving side surface 2 c . in this construction , when a predetermined amount of electricity is supplied from a power supply not shown to the leds 1 , the leds 1 illuminate in white or a predetermined color . of the rays of light emitted from the leds 1 , one that has entered the light guide plate 2 through the light receiving side surface 2 c , as shown by a dotted line in fig1 b , has a smaller refracted angle than the critical angle of the light guide plate 2 , so that , on the same principle as explained in the conventional case of fig5 when the ray first reaches the top surface 2 a , its incident angle on the surface ( θ 1 in fig5 ) is greater than the critical angle and thus totally reflected . then , the ray is repetitively reflected between the top surface 2 a and the bottom surface 2 b , as shown by the dotted line in fig1 b , with its incident angle decreasing by an inclination angle of the slope α upon each reflection . when the incident angle is less than the critical angle , the ray is refracted and emitted outside . the light ray that has followed a normal path and exited from the top surface 2 a of the light guide plate 2 enters the prism sheet 3 . rays of light that were refracted downward by the light receiving side surface 2 c and reached the auxiliary reflection surface 2 bh are scattered there . some of the scattered rays directly travel toward the top surface 2 a and some exit downward and are reflected by a reflector plate 6 to reenter the light guide plate 2 and travel toward the top surface 2 a , as indicated by solid lines in fig1 . the light that has reached the top surface 2 a , if its incidence angle on the top surface 2 a is less than the critical angle , exits the light guide plate 102 and enters the prism sheet 3 . so , there are light paths using the auxiliary reflection surface 2 bh in addition to the light paths using the asymmetric prisms 2 bp indicated by a dashed line in fig1 . as a result , light can be emitted upward from even an area of the top surface 2 a of the light guide plate 2 which is close to the light receiving side surface 2 c , thus expanding the illumination area to near the light receiving side surface 2 c . the light that has entered the prism sheet 3 is specular - reflected in the prism sheet 3 until it changes it propagation direction to a z direction . the light traveling in the z direction is now incident on the liquid crystal panel 7 . therefore , the light passes through the liquid crystal in an ideal direction , making a clear and vivid image display possible . in the light guide plate 2 of the planar light source 10 according to the first embodiment , as shown in the enlarged view of fig1 c , no step , such as observed in the conventional planar light source , is formed at a boundary between the auxiliary reflection surface 2 bh and the prism reflection surface of the asymmetric prisms 2 bp . the step discussed here refers to the one formed in the conventional planar light source 120 of fig6 ( a clearly distinguishable step with a right - angled edge , equivalent to 102 d in fig8 ). therefore , no secondary light source caused by the step is formed . this means that a dark - bright fringe pattern produced during illumination by the secondary light source - induced bright lines is not formed , thus preventing a possible degradation of liquid crystal display quality . [ 0048 ] fig2 is a plan view showing how the planar light source 10 of fig1 appears when illuminated . as shown in the figure , a whole surface area r of the light guide plate 2 is uniformly illuminated , with no such conspicuous bright lines as observed in the conventional planar light source . the reason that no step is formed is that , when the light guide plate 2 is molded from resin , inserts ( in - cavity molding pieces having recessed and raised transfer surfaces ) installed inside a mold frame are formed as a single - piece insert . that is , the two pieces that have conventionally been used in the prior art are formed into a single integral insert . since the undulation pattern is transferred from this one - piece insert , the step formed by combining a plurality of inserts is eliminated and is thus no longer transferred to the light guide plate . next , a method of manufacturing the planar light source 10 shown in fig1 will be explained , centering particularly on the manufacture of the light guide plate 2 . fig3 shows a method of forming the light guide plate 2 . in fig3 a reference number 21 denotes a mold for molding the light guide plate 2 which has a single insert 23 installed inside a mold frame 22 . the insert 23 is made by forming two components , which have conventionally been formed separate , into a single integral piece . the surface of the insert 23 is formed with an undulated auxiliary pattern portion 23 s corresponding to the auxiliary reflection surface 2 bh of the light guide plate 2 and with an undulated prism pattern portion 23 p corresponding to the reflection surface of the asymmetric prisms 2 bp . fig3 b is a cross section showing the undulations near a boundary on the surface of the insert 23 between the auxiliary pattern portion 23 s and the prism pattern portion 23 p . it is seen that the boundary between the two patterns is smooth , with no such a step as observed in the prior art ( see fig1 ). next , a process of forming the light guide plate 2 using the mold 21 will be explained . first , the insert 23 is set inside the mold frame 22 and then a melted resin is injected into a cavity . this causes the undulations of the auxiliary pattern portion 23 s and the prism pattern portion 23 p formed on the surface of the insert 23 to be transferred onto the bottom surface 2 b of the light guide plate 2 . since the insert 23 has no step on its surface between the two undulated patterns , as described above , the bottom surface 2 b of the light guide plate 2 is not formed with the above - described step found in the conventional light guide plate 2 . now , one example method of forming the prism pattern portion 23 p and the auxiliary pattern portion 23 s on the surface of the insert 23 will be explained by referring to fig3 c . ( 1 ) first , prism grooves are cut , as by mechanical machining , in the entire surface of the insert 23 to form the prism pattern portion 23 p . ( 2 ) then , with a part of the prism pattern portion 23 p left exposed , the remaining part of the prism pattern portion 23 p is covered with a mask m of resin or rubber . ( 3 ) in this state , the exposed part of the prism pattern portion 23 p is ground as by honing to form the auxiliary pattern portion 23 s consisting of an irregular pattern of fine , textured undulations . ( 4 ) as a last step , the mask m is removed . the insert 23 thus fabricated has the prism pattern portion 23 p and the auxiliary pattern portion 23 s formed on desired , allocated areas of its surface . furthermore , the insert 23 has no step on its surface at a boundary between the two pattern portions 23 p and 23 s . as described above , in the planar light source of this invention , since no step is formed on the reflection surface of the light guide plate when the insert pattern is transferred during molding , bright lines which in prior art are induced by the step become indistinguishable . as a result , the formation of a dark - bright fringe pattern caused by the bright lines is prevented , improving a liquid crystal display quality .