Abstract:
The planar light source of this invention comprises: a light guide plate made of a platelike, light-transmitting material formed with a reflection surface on a bottom side thereof, the reflection surface being formed by combining a plurality of kinds of reflection means; and an illuminating light source arranged opposite a light receiving side surface of the light guide plate; wherein the light guide plate has formed smooth a boundary on the reflection surface between the plurality of kinds of reflection means so as not to form any step between the adjoining reflection means. This construction prevents the formation of a dark-bright fringe pattern between the plurality of reflections means when the light source is illuminated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a planar light source for illuminating a liquid crystal panel and the like and a method of manufacture of the same.  
           [0003]    2. Description of the Prior Art  
           [0004]    Small liquid crystal displays have been used in recent years in cellular phones and other portable information terminals and, as a means for illuminating such liquid crystal displays, a planar light source is used. The planar light source is constructed of a plate-like light guide plate and light emitting diodes (LEDs) arranged to face a side surface of the light guide plate. Thanks to its ability to be reduced easily in size and thickness, the planar light source has found wide uses.  
           [0005]    In the planar light source, light emitted from the LEDs enters into the light guide plate and propagates as it is repetitively reflected within the light guide plate. The light is reflected or refracted by grooves or a textured pattern formed in an underside of the light guide plate until it goes out of the plate. The light emitted from the top surface of the light guide plate travels toward and illuminates the liquid crystal display.  
           [0006]    An example of such a conventional planar light source is shown in FIG. 4 (e.g., Japanese Patent Application No. 2002-093383, FIG. 7). FIG. 4 illustrates a construction of a planar light source  110 , FIG. 4A and FIG. 4B representing a perspective view and a side view thereof respectively.  
           [0007]    In FIG. 4, the planar light source  110  has LEDs  101  as a light source, a light guide plate  102 , a prism sheet  103 , a reflector plate  106  and a liquid crystal panel  107 . The light guide plate  102  is rectangular and made of a light-transmitting glass or resin. Denoted  102   a  is a top surface of the light guide plate  102 . Designated  102   c  is a light receiving side surface facing the LEDs  101 . Designated  102   b  is a bottom surface of the light guide plate  102 . The bottom surface  102   b  is formed with a plurality of asymmetric prisms  102   b   1  facing the top surface  102   a.  The asymmetric prisms  102   b   1  each comprise a down slope  102   b   11  whose distance to the top surface  102   a  sharply increases as it moves away from the light receiving side surface  102   c  and an up slope  102   b   12  whose distance to the top surface  102   a  moderately decreases. Arranged opposite the light receiving side surface  102   c  are three LEDs  101  supported on a retainer member  101   b.    
           [0008]    When a predetermined amount of electricity is supplied from a power supply not shown to the LEDs  101 , the LEDs  101  illuminate in white or a predetermined color. The light emitted from the LEDs  101  is refracted by the light receiving side surface  102   c  as it enters the light guide plate  102 . The light that has entered the light guide plate  102  is repetitively reflected between the top surface  102   a  and the bottom surface  102   b  of the light guide plate  102  before it is refracted by the top surface  102   a  and leaves the light guide plate  102 . The light then enters the prism sheet  103 , in which it is specular-reflected until its propagation direction is changed to a Z direction. The light traveling in the Z direction is now incident on the liquid crystal panel  107 . Therefore, the light passes through the liquid crystal in an ideal direction, making a clear and vivid image display possible.  
           [0009]    [0009]FIG. 5 is a side view showing a path of light emitted from the LEDs  101  that has entered the light guide plate  102 . In the figure, a light ray emitted from the LEDs  101  at an output or emittance angle of θi enters the light receiving side surface  102   c  of the light guide plate  102  at an incidence angle of θi. At this time, the ray is refracted on this plane and a relationship between the incidence angle θi and a refracted angle θ is, according to Snell&#39;s law, n·sinθ=sinθi assuming that a refractive index of air is 1 and a refractive index of the light guide plate  102  (made of polycarbonate and the like) is n. From this we obtain  
           θ=sin −1 ((1 /n )sin θ i )   (1)  
           [0010]    If, for example, the light guide plate  102  has a refractive index of n=1.58 and θi=90°, calculating the equation (1) results in θ=sin −1 (1/1.58)=39.3° and thus the critical angle θc is θc=39.3°.  
           [0011]    It should be noted, however, that since the incidence angle in reality is less than 90° at maximum, the refracted angle θ even at its maximum is less than the critical angle θc. The critical angle θc of the light guide plate  102  is generally around 40°, so the refracted angle θ even at its maximum does not exceed 40°. The light ray that has passed through the light receiving side surface  102   c  at the refracted angle θ is incident on the top surface  102   a  of the light guide plate  102  at an incidence angle θ 1 . At this time, as can be seen from FIG. 5, since a relation of (θ+θ 1 =90°) holds and the refracted angle θ is equal to or less than 40°, as described above, the incidence angle θ 1  is equal to or more than 50°, which is larger than the critical angle θc of around 40°. Thus, the ray incident on the top surface  102   a  is totally reflected at a reflection angle θ 1 .  
           [0012]    The reflected light then strikes, at an incidence angle of θ 2 =θ 1 −α, the up slope  102   b   12  of the bottom surface which has an inclination angle of α. Here the inclination angle α is about 1° to several degrees.  
           [0013]    The ray that has struck the up slope  102   b   12  at an incidence angle θ 2  is reflected by this surface at a reflection angle θ 2  and then strikes the top surface  102   a  at an incidence angle of θ 3 =θ 2 −α=θ 1 −2α. The ray is then reflected by the top surface  102   a  at a reflection angle θ 3 to hit the up slope  102   b   12  at an incidence angle of θ 4 =θ 3 −α=θ 1 −3α. Each time the light ray, that was first reflected by the top surface  102   a  at a reflection angle θ 1 , strikes the up slope  102   b   12  or the top surface  102   a,  its incidence angle decreases by an amount equal to the inclination angle α. That is, when the ray, that was first reflected at a reflection angle of θ 1 , strikes the up slope  102   b   12  or top surface  102   a  for an Nth time after repetitive reflections, its incidence angle θ N  is given by  
           θ N =θ 1 −( N −1)α  (2)  
           [0014]    In this light guide plate, the light incidence or reflection on its boundary surface, shown at θ 1 , is counted as the first incidence/reflection (i.e., N=1).  
           [0015]    When the decreasing incidence angle θ N  has the following relation with the critical angle θc:  
           θ N =θ 1 −( N −1)α&lt;θ c    (3)  
           [0016]    then, the ray passes through the top surface  102   a  or the up slope  102   b   12  of the bottom surface  102   b  and gets out of the light guide plate  102 . For example, if θ 1 =52°, α=1° and θc=40°, the condition of equation (3) is met when N is more than 13. This means that the light ray must strike the top or bottom surface of the light guide plate  102  fourteen times or more. Therefore, near the light receiving side surface  102   c  the ray does not escape to the outside. For example, if the light guide plate  102  has a thickness of 1 mm, the ray does not normally exit the light guide plate  102  from within about 3 mm of the light receiving side surface  102   c.  As a result, of the area of the light guide plate  102  the region that can be used as a light generation region decreases, reducing a space efficiency, which is detrimental to a size reduction of the device.  
           [0017]    Thus, the planar light source  110  of FIG. 4, rather than being used as is, is often improved as shown in FIG. 6. FIG. 6A is an overall side view and FIG. 6B is an enlarged view of a portion C of FIG. 6A. In FIG. 6, denoted  120  is an improved planar light source in which a reference number  102   bh  represents a textured reflection surface provided on the bottom surface  102   b  of the light guide plate  102  near the light receiving side surface  102   c.  The textured reflection surface  102   bh  has an irregular pattern of fine, recessed and raised portions. The planar light source  120  therefore has on the underside of the housing  102  a reflection means provided by the textured reflection surface  102   bh  in addition to a reflection means provided by the asymmetric prisms  102   b   1 . Designated  102 D is a step formed at a boundary between the reflection means of the asymmetric prisms  102   b   1  and the reflection means of the textured reflection surface  102   bh.  In other respects, the symbols and construction are similar to those of the planar light source  110  shown in FIG. 4. The step  102 D either rises or falls, and its edge has an angle of nearly 90°.  
           [0018]    As shown in FIG. 6, of the light rays that have entered from the LEDs  101  through the light receiving side surface  102   c  into the light guide plate  102 , some rays strike, and are scattered by, the textured reflection surface  102   bh  before traveling directly toward the top surface  102   a  and some rays go out of the light guide plate  102  and are reflected by the reflector plate  106  to reenter the light guide plate  102  and travel toward the top surface  102   a,  as indicated by solid lines in FIG. 6A. So, there are light paths involving the textured reflection surface  102   bh  in addition to the light paths using the asymmetric prisms  102   b   1 , as indicated by a dashed line. As a result, light can be emitted upward from even an area of the top surface  102   a  of the light guide plate  102  which is close to the light receiving side surface  102   c,  thus expanding the illumination area to near the light receiving side surface  2   c.    
           [0019]    However, even the improved light guide plate  102  often experiences the following problems. As shown in a plan view of FIG. 7, in an area S 1  within 3-4 mm of the light receiving side surface  102   c  of the light guide plate  102  several bright lines  14  show up (in FIG. 7 the bright lines are shown hatched with thick lines). S 2  represents an area where bright lines do not show. The conspicuous bright lines  14  are considered to be caused as follows. As shown in FIG. 6B, light rays from the LEDs  101  that entered the light receiving side surface  102   c  reach an edge portion of the step  102 D in the bottom surface  102   b  of the light guide plate  102 .  
           [0020]    If the edge portion has a rough surface, rather than a mirror surface, the light rays from the LEDs  101  enter the edge portion not through normal refraction but through scattering. That is, from the edge portion, a plurality of rays travel through the light guide plate  102  in different directions, making the edge portion look as if it were illuminating. Thus, the edge portion can be regarded as a secondary light source. Since the edge portion is formed at right angles, its transferability in a molding process is bad, rendering its surface rough, which in turn results in a secondary light source being easily formed.  
           [0021]    Next, as shown in FIG. 8 the secondary light source at the step  102 D emits rays of light in various directions. Of these rays, those that are incident on the top surface  102   a  of the light guide plate  102  at incidence angles less than the critical angle θc pass through this surface by refraction and get out of the light guide plate  102 , as indicated by rays s 21 , s 22 . This direct outward transmission of these rays occurs continuously in a wide range of area and therefore no bright lines are produced. When the incidence angles exceed the critical angle θc, the light rays are reflected by the top surface  102   a  and directed toward the bottom surface  102   b,  as indicated by rays S 31 , S 32 , S 33 . Then, after one to several reflections these rays pass through the top surface  102   a  and go out as illuminating light. The number of times that these rays are reflected before being emitted outside increases as the first incidence angle on the top surface  102   a  becomes larger, according to the principle already explained (see Equation (3)).  
           [0022]    That is, the number of reflections increases, from 1 to 2 to 3, according to the light paths S 31 , S 32 , S 33 . Hence, the positions on the top surface  102   a  of the light guide plate  102  from which the light rays go out are separated from each other, resulting in discrete bright lines as shown in FIG. 7. As for a width of light flux, let us turn to FIG. 9 and compare light fluxes φ 1  and φ 3  which correspond to the light paths s 31  and s 33  of FIG. 8. Let a width of light flux φ 1  as it exits the top surface  102   a  of the light guide plate  102  be b 1  and a width of light flux φ 3  be b 3 . It is seen that the light width b 3  is significantly larger than the width b 1 . This is considered due to the fact that as the number of reflections for each light path increases, a length of the light path also increases and, almost in proportion to the path length, the width of the light flux increases. Thus, as shown in FIG. 7, the width of the bright lines progressively widens away from the light receiving side surface  102   c  of the light guide plate  102 . As the number of reflections for each light path increases further and the width of each flux exiting the top surface  102   a  of the light guide plate  102  widens, a light quantity per unit area, i.e., brightness lowers, with the result that the bright lines become indistinguishable in an area more than a certain distance away from the light receiving side surface  102   c,  as shown in FIG. 7.  
           [0023]    As described above, the marked bright lines are caused by the step  102 D (see FIG. 6 and FIG. 8) in the bottom surface  102   b  of the light guide plate  102 . This step  102 D is formed by a plurality of inserts (in-cavity molding pieces having recessed and raised transfer surfaces) during the process of molding the light guide plate  102 . FIG. 10 illustrates essential parts of a conventional mold used in molding the light guide plate  102 . In FIG. 10, denoted  121  is a mold frame,  122  an insert for textured pattern, and  123  an insert for prism pattern. The insert  122  for the textured pattern has its surface formed with a pattern of undulations corresponding to the textured reflection surface  102   bh  of FIG. 6, and the insert  123  for the prism pattern has its surface formed with a pattern of recesses and projections corresponding to the asymmetric prisms  102   b   1  provided on the bottom surface  102   b  of the light guide plate  102 . The light guide plate  102  is formed in the following process. First, as shown in FIG. 10A, the insert  122  for the textured pattern and the insert  123  for the prism pattern are placed inside the mold frame  121 . Next, as shown in FIG. 10B, both of these inserts  122 ,  123  are set close together. Then, a melted resin is injected into the mold frame  121  to mold the light guide plate  102  and transfer the surface patterns of these inserts  122 ,  123  to the light guide plate  102 .  
           [0024]    In this process, as shown in a cross section of FIG. 10C, a step D is often formed between the textured pattern insert  122  and the prism pattern insert  123  and is transferred onto the light guide plate  102  as the step  102 D of FIG. 6. The edge of the step D is almost at right angles. It is noted that the step D, which is formed by a difference in thickness between the textured pattern insert  122  and the prism pattern insert  123 , is very difficult to eliminate by equalizing the thickness of these inserts because the thicknesses of the inserts  122 ,  123  change when forming their recessed/raised surface patterns.  
           [0025]    As described above, the step D formed between the inserts  122  and  123  is transferred to the conventional light guide plate  102  as the step  102 D in the reflection surface. So, when light is illuminated from the light guide plate  102  of the above construction, bright lines produced by this step show up. The bright lines in turn form bright and dark fringes, marring the appearance of the planar light source.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention has been accomplished in light of the above drawbacks experienced with prior art, and it is an object of the present invention to provide a planar light source which has a light guide plate and an illuminating light source such as LED arranged by the side of the light guide plate and which can prevent the formation of a bright line-induced, dark-bright fringe pattern by suppressing the bright lines produced by a step on a reflection surface of the light guide plate. It is also an object of this invention to provide a method of manufacturing the planar light source.  
           [0027]    To achieve the above objective, the planar light source of this invention comprises: a light guide plate made of a platelike, light-transmitting material formed with a reflection surface on a bottom side thereof, the reflection surface being formed by combining a plurality of kinds of reflection means; and an illuminating light source arranged opposite a light receiving side surface of the light guide plate; wherein the light guide plate has formed smooth a boundary on the reflection surface between the plurality of kinds of reflection means so as not to form any step between the adjoining reflection means.  
           [0028]    In one embodiment of the planar light source of this invention, the reflection surface is formed by using a single in-cavity molding piece which has transfer surfaces corresponding to the plurality of kinds of reflection means.  
           [0029]    As the plurality of kinds of reflection means, the reflection surface has a reflection means formed of prisms and a reflection means formed of textured undulations of a predetermined shape.  
           [0030]    To achieve the above objective, the method of manufacturing a planar light source according to this invention, which planar light source has on a bottom side of a light guide plate a reflection surface having a plurality of kinds of reflection means, comprises the steps of: installing inside a mold for the light guide plate a single in-cavity molding piece having transfer surfaces corresponding to the plurality of kinds of reflection means; and injecting a melted resin into the mold to form the reflection surface in such a manner as will not form a step between the plurality of kinds of reflection means.  
           [0031]    In one embodiment of the manufacturing method of this invention, the transfer surfaces of the single in-cavity molding piece are formed with an undulated transfer surface corresponding to a reflection means formed of prisms and with an undulated transfer surface corresponding to a reflection means formed of textured undulations of a predetermined shape.  
           [0032]    As the method of forming the transfer surfaces of the single in-cavity molding piece, this invention performs the steps of: forming, over the entire transfer surface corresponding to the reflection surface of the light guide plate, an undulated transfer surface corresponding to the reflection means formed of prisms; masking a part of the undulated transfer surface; and forming through honing, over the remaining unmasked part of the transfer surface, an undulated transfer surface corresponding to the reflection means formed of textured undulations of a predetermined shape so that the prism-based undulated transfer surface and the texture-based undulated transfer surface are formed on the same transfer surface of the single in-cavity molding piece. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 is an explanatory view showing a planar light source according to a first embodiment of the present invention.  
         [0034]    [0034]FIG. 2 is a plan view showing how the planar light source of FIG. 1 appears when illuminated.  
         [0035]    [0035]FIG. 3 is an explanatory diagram showing a process of manufacturing the planar light source of FIG. 1 and represents a second embodiment of the planar light source of this invention.  
         [0036]    [0036]FIG. 4 is an explanatory view showing a conventional planar light source.  
         [0037]    [0037]FIG. 5 is an explanatory diagram showing a normal reflection path in the planar light source of FIG. 4.  
         [0038]    [0038]FIG. 6 is an explanatory view showing an improved planar light source over FIG. 4.  
         [0039]    [0039]FIG. 7 is a plan view showing how the improved planar light source of FIG. 6 appears when illuminated.  
         [0040]    [0040]FIG. 8 is an explanatory diagram showing normal reflection paths in the improved planar light source of FIG. 6.  
         [0041]    [0041]FIG. 9 is an explanatory diagram showing abnormal reflection paths in the improved planar light source of FIG. 6.  
         [0042]    [0042]FIG. 10 is an explanatory diagram showing a process of manufacturing the planar light source of FIG. 6. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0043]    [0043]FIG. 1 illustrates a first embodiment of the planar light source according to the present invention, FIG. 1A representing a perspective view of the planar light source  10 , FIG. 1B representing a side view of the same, FIG. 1C representing an enlarged view of portion A in FIG. 1B.  
         [0044]    In FIG. 1, 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 FIG. 1C, 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.    
         [0045]    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 FIG.  1 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 FIG. 5, when the ray first reaches the top surface  2   a,  its incident angle on the surface (θ 1  in FIG. 5) 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 FIG. 1B, 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 .  
         [0046]    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 FIG. 1. 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 FIG. 1. 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.  
         [0047]    In the light guide plate  2  of the planar light source  10  according to the first embodiment, as shown in the enlarged view of FIG. 1C, 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 FIG. 6 (a clearly distinguishable step with a right-angled edge, equivalent to  102 D in FIG. 8). 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]    [0048]FIG. 2 is a plan view showing how the planar light source  10  of FIG. 1 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.  
         [0049]    Next, a method of manufacturing the planar light source  10  shown in FIG. 1 will be explained, centering particularly on the manufacture of the light guide plate  2 . FIG. 3 shows a method of forming the light guide plate  2 . In FIG. 3A 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.  FIG. 3B 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 FIG. 10).  
         [0050]    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 .  
         [0051]    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 FIG. 3C.  
         [0052]    (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.    
         [0053]    (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.  
         [0054]    (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.  
         [0055]    (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.    
         [0056]    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.