JP2001343507A - Lens sheet, planar light source element using the same and liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a prism sheet excellent in abrasion resistance without incurring the considerable lowering of luminance as a planar light source element. SOLUTION: In the prism sheet with many prism arrays each having a nearly triangular section formed in parallel on at least one surface, the tops of the prism arrays are circular arc-shaped parts each having <=5 μm radius of curvature or flat parts each having <=5 μm width.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a notebook computer,
The present invention relates to a liquid crystal display device used as a display such as a portable liquid crystal television and a surface light source element and a prism sheet used for the liquid crystal display device.
The present invention relates to a prism sheet having improved scratch resistance, a surface light source element using the same, and a liquid crystal display device.

[0002]

2. Description of the Related Art In recent years, color liquid crystal display devices have been used in portable notebook personal computers, desktop personal computer liquid crystal monitors,
It has been widely used in various fields as a portable liquid crystal television or a video integrated liquid crystal television. This liquid crystal element does not emit light by itself, but serves as a shutter for transmitted light. In order to improve the visibility of a liquid crystal display device, light is emitted from the back of a liquid crystal element (liquid crystal panel) called a backlight. Are generally used.

[0003] Such a backlight is composed of a fluorescent tube, a light guide, a reflection sheet, a prism sheet and the like as described in, for example, JP-A-2-84618 and JP-A-3-69184. Is done. Among them, the prism sheet is disposed on the light exit surface of the light guide, improves the optical efficiency of the backlight, and improves the brightness. For example, the prism sheet is provided on one surface of the light-transmitting sheet. It is a lens sheet in which prism rows each having an isosceles triangular cross section having a length of 50 μm and a prism apex angle of 60 ° to 100 ° are formed in parallel.

[0004] The prism sheet is formed of a series of very fine prisms as described above. The sharp end of the prism array causes damage during handling and vibration of the prism array during use. There is a problem that an optical defect is generated due to a scratch on the tip. As described in Japanese Utility Model Application Laid-Open No. 3-69184, this prism row is not damaged even when the prism sheet is placed so that the prism row forming surface is in the opposite direction to the light exit surface of the light guide. Scratches are required, but when the prism sheet is placed such that the prism row forming surface is on the light emitting surface side of the light guide as described in JP-A-2-84618, Abrasion resistance to contact between the light guide and the prism array is strongly required.

[0005]

A method for improving the scratch resistance of such a prism sheet is disclosed in Japanese Patent Application Laid-Open No.
As described in JP-A-143153, a method for improving the scratch resistance of the resin forming the prism rows has been proposed, but the scratch resistance cannot be sufficiently improved due to the relationship with the optical characteristics of the prism sheet. Was something. Japanese Patent Application Laid-Open No. H11-30501 proposes a method of improving abrasion resistance by forming the top of a prism array into an arc shape having a radius of 10 to 25 μm. And the original effect of using the prism sheet was greatly impaired.

Therefore, an object of the present invention is to improve the scratch resistance of a prism sheet without causing a significant decrease in luminance as a surface light source element.

[0007]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present inventors have modified the shape of the top of the prisms of the prism array of the prism sheet into a very small specific shape, so that the luminance as the surface light source element can be improved. The present inventors have found that the scratch resistance of the prism sheet is improved without causing a significant decrease in the above, and the present invention has been achieved.

That is, the prism sheet of the present invention is a prism sheet in which a large number of substantially triangular cross-section prism rows are formed in parallel on at least one surface, and the tips of the prism rows have a radius of curvature of 5 μm or less. It is characterized by being arc-shaped. Further, the prism sheet of the present invention is a prism sheet in which a large number of prism rows having a substantially triangular cross section are formed in parallel on at least one surface, and a flat portion having a width of 5 μm or less is formed at the tip of the prism row. It is characterized by having been done. Further, the surface light source element of the present invention comprises a light source, a light guide having at least one light incident surface facing the light source and a light exit surface substantially orthogonal thereto, and a light exit surface side of the light guide. And a prism sheet as described above mounted thereon. The liquid crystal display device of the present invention is characterized in that a liquid crystal display element is arranged on a prism sheet of such a surface light source element.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The prism sheet of the present invention is shown in FIG.
Alternatively, as shown in FIG. 2, the prism sheet is a prism sheet in which a large number of prism rows 2 having a substantially triangular cross section are formed in parallel on at least one surface, and a top 3 of the prism row 2 (hereinafter, referred to as a prism top). Has a minute specific shape. In FIG. 1, the prism top 3 of the prism sheet is formed in an arc shape having a radius of curvature of 5 μm or less.
A flat portion having a width of 5 μm or less is formed on the prism top 3 of the prism sheet. In this way, by forming the top 3 of the prism array 2 of the prism sheet into a minute specific shape, it is possible to minimize the decrease in luminance as the surface light source element and to improve the scratch resistance of the prism sheet. be able to.

In the lens sheet of the present invention, as shown in FIG. 1, when the arc shape of the prism apex 3 is an arc shape having a curvature radius of more than 5 μm, the luminance of the surface light source element tends to be greatly reduced. Yes, preferably 4 μm
The thickness is more preferably 3 μm or less. The radius of curvature of the arc is preferably 1 μm or more, more preferably 1.5 μm or more, from the viewpoint of the scratch resistance of the prism sheet. On the other hand, as shown in FIG. 2, when the width of the flat portion formed on the prism top 3 exceeds 4 μm, the luminance of the surface light source element tends to decrease significantly, and is preferably 4 μm or less, and more preferably 4 μm or less. The range is 3 μm or less. In addition, the width of the flat portion is preferably 1 μm or more, more preferably 1.5 μm or more, from the viewpoint of the scratch resistance of the prism sheet.
As described above, by reducing the arc shape to a radius of curvature of 5 μm or less or the width of the flat portion to 5 μm or less, the reduction in luminance of the surface light source element falls within about 10% or less, and the prism does not cause a significant reduction in luminance. The abrasion resistance of the sheet can be significantly improved.

Such a shape of the prism top 3 can be given by a method such as cutting or polishing of the prism array 2 of the prism sheet, but is given in advance to a lens mold used for forming the prism sheet. It is efficient. That is, when forming a prism pattern on a lens mold, cutting is performed using a cutting tool that matches a target prism shape (a circular arc shape or a flat shape).

In the prism sheet of the present invention, the pitch of the prism rows 2 is preferably 20 to 100 μm, and more preferably 20 to 80 μm.
This is because if the pitch of the prism array 2 is less than 20 μm, the brightness as a surface light source element tends to decrease,
If the thickness exceeds μm, the occurrence of moire between the pixels of the liquid crystal element and the prism array 2 tends to be remarkable. The apex angle of the prism array 2 of the present invention is 50 to 15
It is preferable that the angle is in the range of 0 °. Generally, when a prism sheet is mounted and used as a surface light source element such that a prism row forming surface is in a direction opposite to a light emitting surface of a light guide, an apex angle is in a range of 80 to 100 °. And more preferably in the range of 90 to 100 °. On the other hand, when the prism sheet is mounted and used as the surface light source element such that the prism row forming surface is on the light emitting surface side of the light guide, it is preferable that the angle be in the range of about 50 to 75 °. Preferably it is in the range of 55-70 °. In the present invention, the apex angle of the prism array 2 refers to the angle of the intersection of two prism surfaces forming one prism array. Further, in the present invention, the refractive index of the prism array 2 is set such that the prism array 2 is used as a surface light source element by placing a prism sheet so that the prism array forming surface is in the opposite direction to the light emitting surface of the light guide. In order to improve the brightness as a surface light source element, it is preferable that the refractive index is relatively high, the refractive index is preferably 1.55 or more,
More preferably, it is 1.7 or more.

The prism sheet of the present invention can be made of a synthetic resin having a high light transmittance. Examples of such a synthetic resin include a methacrylic resin, an acrylic resin, a polycarbonate resin, and a vinyl chloride resin. In particular, methacrylic resin is optimal because of its high light transmittance, heat resistance, mechanical properties, and moldability. Such a methacrylic resin is a resin containing methyl methacrylate as a main component, and is preferably a resin having 80% by weight or more of methyl methacrylate. When forming the prism rows 2 on the prism sheet, the transparent synthetic resin plate may be formed by hot pressing using a mold member having a desired surface structure, or the prism sheet may be formed by extrusion molding, injection molding, or the like. At the time of manufacturing, the shape may be given at the same time. Further, the shape may be transferred by a lens mold using a heat or light curable resin or the like. Above all, since it is excellent in productivity, transferability of a prism pattern, and the like, as shown in FIGS. Is preferably formed.

The light-transmitting substrate 1 constituting the prism sheet of the present invention is not particularly limited as long as it is a sheet or a film-like material that transmits active energy rays such as ultraviolet rays and electron beams. However, a transparent resin sheet or film of a polyester resin, an acrylic resin, a polycarbonate resin, a vinyl chloride resin, a polymethacrylimide resin, or the like is preferable. In particular, it is made of polymethyl methacrylate having a lower refractive index than the refractive index of the prism array 2 and having a low surface reflectance, a mixture of polymethyl acrylate and polyvinylidene fluoride resin, a polycarbonate resin, and a polyester resin such as polyethylene terephthalate. Are preferred. The thickness of the light-transmissive substrate 1 varies depending on the application, and is in the range of 50 to 500 μm. However, it is preferably 70 μm or more for easy handling. Those having a size of 200 μm or less are preferred. The translucent substrate 1
In order to improve the adhesion between the prism array 2 composed of the active energy ray-curable composition and the translucent substrate 1, the surface thereof is subjected to an adhesion improving treatment such as an anchor coating treatment. Is preferred.

The active energy ray-curable composition for forming the prism rows 2 of the prism sheet is not particularly limited as long as it can be cured with an active energy ray such as an ultraviolet ray or an electron beam. ,
(Meth) acrylate, epoxy (meth) acrylate, urethane (meth) acrylate and the like. Among them, (meth) acrylate-based ones are particularly preferred from the viewpoint of their optical characteristics and the like. In particular, in terms of handleability, curability, etc., polyvalent acrylate and / or polyvalent methacrylate (hereinafter, referred to as poly (meth) acrylate), monoacrylate and / or monomethacrylate (hereinafter, mono (meth) acrylate) And those containing a photopolymerization initiator by active energy rays as a main component. Representative polyvalent (meth) acrylates include polyol poly (meth) acrylate, polyester poly (meth) acrylate, epoxy poly (meth) acrylate, urethane poly (meth) acrylate, and the like. These are used alone or as a mixture of two or more. Also, as the mono (meth) acrylate,
Mono (meth) acrylates of monoalcohols, mono (meth) acrylates of polyols and the like can be mentioned.

When the prism sheet of the present invention is manufactured using an active energy ray-curable composition, for example, the active energy ray-curable composition liquid is injected into a lens mold having a predetermined prism pattern. Then, the translucent substrate 1 is overlaid. Next, a method of irradiating an active energy ray such as an ultraviolet ray or an electron beam through the translucent substrate 1, polymerizing and curing the active energy ray-curable composition liquid, and peeling the liquid from the lens mold to obtain a lens sheet is used. .

Next, the surface light source device of the present invention will be described with reference to FIG. As shown in FIG. 3, the surface light source element of the present invention includes a light source 6 such as a fluorescent lamp disposed at one end of a light guide 5 made of a transparent resin or the like, and a light exit surface of the light guide 5. A reflection layer 7 such as a reflection film is disposed on the opposite surface, and the prism sheet 4 is placed on the light exit surface of the light guide 5. Further, if necessary, a diffusion sheet may be placed on the light emitting surface of the light guide 5 or on the prism sheet 4. At least one light guide 5 facing the light source 6 is used as the light guide 5.
As long as one side is a light incident surface and a surface substantially perpendicular to the light incident surface is a light emitting surface, various shapes such as a plate, a wedge, and a boat can be used. It is composed of high synthetic resin. In order to effectively introduce light from the light source 6 to the light guide 5, it is preferable that the light incident surfaces of the light source 6 and the light guide 5 be covered with a case or film coated with a reflective agent on the inside. Furthermore, the prism sheet 4
Alternatively, two or three or more lens sheets can be stacked and placed on the light emitting surface of the light guide 5. The light source 6 may be disposed at at least one end of the light guide 5, but a plurality of light sources may be disposed as necessary.

As the synthetic resin constituting the light guide 5, various kinds of synthetic resins having high transparency such as methacrylic resin, acrylic resin, polycarbonate resin and vinyl chloride resin are used, and extrusion molding and injection molding are used. It can be manufactured by an ordinary molding method such as In particular, methacrylic resin is excellent in light transmittance, heat resistance, mechanical properties, and moldability, and is most suitable as a light guide material. Such a methacrylic resin is a resin containing methyl methacrylate as a main component, and it is preferable that methyl methacrylate is 80% by weight or more.

In the surface light source element of the present invention, the prism sheet 4 mounted on the light exit surface of the light guide 5 has a prism row forming surface oriented in accordance with the emission light characteristics of the light guide 3 used. It can be selected as appropriate. For example, when a light guide 5 having a lens surface, a matte surface, or the like formed on a light emitting surface or a back surface is used, the emitted light emitted from the light guide 5 has a strong directivity that is greatly inclined from the normal direction. Therefore, it is preferable to arrange the prism sheet 4 so that the prism row forming surface is on the light emitting surface side of the light guide 5 as shown in FIG. The prism sheet 4 of the present invention is particularly suitable as the prism sheet 4 used for such a surface light source element.

The liquid crystal display device of the present invention uses the surface light source element having the above structure as a backlight, and mounts the liquid crystal display element on the lens sheet 4 of such a surface light source element. The liquid crystal display element is not particularly limited, and any of an active matrix driven TFT liquid crystal display element and a simple matrix driven STN type liquid crystal display element can be used. Also, TFT
In the liquid crystal display device, various active devices such as polysilicon, amorphous silicon, metal insulator and metal can be used.

[0021]

The present invention will be described below in detail with reference to examples. Example 1 A diamond tool (manufactured by Tokyo Diamond Tool Co., Ltd.) having an arc shape with a radius of curvature of 2 μm at the tip having an apex angle of 65 ° was prepared. Next, thickness 3 mm, 300 mm × 40
A prism array was cut at a pitch of 50 μm on the surface of a copper plate of three types of JIS brass having a size of 0 mm using the above-mentioned diamond tool to form a prism pattern, followed by ganigen plating to prepare a lens mold. An appropriate amount of an acrylic UV curable monomer mixture is injected into this lens-type prism pattern, and a 125 μm-thick polyethylene terephthalate film (PET film, Toyobo A430)
0) was gradually superposed from one end of the lens mold into which the UV-curable monomer mixture was injected, while being pressed with a roll, toward the other end, and the injected UV-curable monomer mixture was spread over the entire lens mold. Thereafter, ultraviolet rays were irradiated for 45 seconds by using three 6.4 kW ultraviolet lamps (manufactured by Western Quartz Co., Ltd.) having an irradiation intensity of 80 W / cm disposed above the PET film to polymerize and cure the ultraviolet curable monomer mixture. Apex having a radius of curvature of 2 μm and a vertex angle of 65 μm on one surface of the PET film peeled from the lens mold.
Thus, a prism sheet was obtained in which a number of prism rows having a refractive index of 1.59 were formed in parallel. The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance.

On the other hand, using glass beads (FGB-120 manufactured by Fuji Seisakusho) having a particle size of 125 to 149 μm,
m from a distance of m, blasting was performed at a spraying pressure of 4 kg / cm ² to make one surface rough and the other surface mirror-finished using an acrylic resin (Acrypet VH5 # 000 manufactured by Mitsubishi Rayon Co., Ltd.). ) Is injection molded to a thickness of 3 mm, 243
A light guide of mm × 194 mm was obtained.

A silver-evaporated PET film is adhered to the two short end faces and the other end face of the obtained light guide by adhesive processing, and silver-evaporated PET is applied to the rear face opposite to the roughened light emission face. The film was taped to form a reflective surface. On the other end face of the light guide, a straight-tube cold-cathode tube with a diameter of 2 mm (NMBSM2BWE253 manufactured by Harrison)
W) is wound around a PET film on which silver is deposited,
The prism sheet is placed on the light emitting surface of the light guide such that the prism rows are substantially parallel to the light incident surface of the light guide, and the prism row forming surface faces the light emitting surface side of the light guide, The surface light source element having the structure shown in FIG. 3 was assembled. The presence or absence of a defect or the like in a state where the obtained surface light source element and liquid crystal display element were mounted was visually checked, but no defect was particularly visually recognized.

A DC power supply was connected to the obtained cold cathode tube of the surface light source element via an inverter (CXA-M10L manufactured by TDK Corporation), and DC 12 V was applied to light the lamp, and the luminance in the normal direction was measured with a luminance meter. It was measured. Table 1 shows the measurement results as relative values when the luminance of Comparative Example 1 was set to 100. The decrease in luminance was about 3% as compared with Comparative Example 1. Using the obtained surface light source element, a sweep durability test at a frequency of 10 to 500 Hz was performed by a sine wave vibration test at an acceleration amplitude of 0.5 G and a sweep cycle of 10 cycles. After the test, the cold cathode fluorescent lamp was turned on to visually check the presence or absence of a defect of the surface light source element, but no defect was confirmed. The results of the vibration test are shown in Table 1 as "O" when no defect was confirmed and "X" when defect was confirmed.

Example 2 A prism sheet was obtained in the same manner as in Example 1, except that the arc shape at the tip of the diamond cutting tool was changed to a radius of 4 μm.
The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. The presence or absence of a defect or the like in a state where the obtained surface light source element and liquid crystal display element were mounted was visually checked, but no defect was particularly visually recognized.

Using the obtained surface light source element, the result of measuring the normal luminance in the same manner as in Example 1 is shown in Table 1 as a relative value when the luminance of Comparative Example 1 is set to 100. The decrease in luminance was 9% as compared with Comparative Example 1. Also, a vibration test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Example 3 A prism sheet was obtained in the same manner as in Example 1 except that a flat shape having a width of 2 μm was provided at the tip of a diamond tool. The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. Although the presence or absence of a defect or the like in a state where the obtained surface light source element and the liquid crystal display element were mounted was visually checked,
In particular, no defect was visually recognized.

Using the obtained surface light source element, the result of measuring the normal luminance in the same manner as in Example 1 is shown in Table 1 as a relative value when the luminance of Comparative Example 1 is set to 100. The decrease in luminance was 5% as compared with Comparative Example 1. Also, a vibration test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Example 4 A prism sheet was obtained in the same manner as in Example 1 except that a flat shape having a width of 4 μm was provided at the tip of the diamond cutting tool. The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. Although the presence or absence of a defect or the like in a state where the obtained surface light source element and the liquid crystal display element were mounted was visually checked,
In particular, no defect was visually recognized.

Using the obtained surface light source element, the result of measuring the normal luminance in the same manner as in Example 1 is shown in Table 1 as a relative value when the luminance of Comparative Example 1 is set to 100. The decrease in luminance was 10% as compared with Comparative Example 1. Also, a vibration test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 1 A prism sheet was obtained in the same manner as in Example 1 except that a diamond carbide having a sharp tip was used. The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching, but it was easily scratched and poor in scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. The presence or absence of a defect or the like in a state where the obtained surface light source element and liquid crystal display element were mounted was visually checked, but no defect was particularly visually recognized.

Using the obtained surface light source element, the normal line luminance was measured in the same manner as in Example 1, and this luminance was set to a reference value of 100.
And Further, a vibration test was performed in the same manner as in Example 1.
The results are shown in Table 1. After the end of the vibration test, the fluorescent tube was turned on to confirm the presence or absence of a defect in the surface light source element. As a result, a part of the surface light source element appeared white. Observation of the defective portion with an electron microscope revealed that the top of the prism row of the prism sheet was partially missing.

Comparative Example 2 A prism sheet was obtained in the same manner as in Example 1, except that the arc shape at the tip of the diamond cutting tool was changed to a radius of 8 μm.
The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. The presence or absence of a defect or the like in a state where the obtained surface light source element and liquid crystal display element were mounted was visually checked, but no defect was particularly visually recognized.

Using the obtained surface light source element, the result of measuring the normal luminance in the same manner as in Example 1 is shown in Table 1 as a relative value when the luminance of Comparative Example 1 is set to 100. The decrease in luminance was 25% as compared with Comparative Example 1. Also, a vibration test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 3 A prism sheet was obtained in the same manner as in Example 1 except that a flat shape having a width of 8 μm was provided at the tip of a diamond tool. The surface of the obtained prism sheet on which the prism rows were formed was scratched with a nail to check the degree of scratching. However, the sheet was not easily scratched and had excellent scratch resistance. A surface light source element was assembled in the same manner as in Example 1 using this prism sheet. Although the presence or absence of a defect or the like in a state where the obtained surface light source element and the liquid crystal display element were mounted was visually checked,
In particular, no defect was visually recognized.

Using the obtained surface light source element, the results of measuring the normal luminance in the same manner as in Example 1 are shown in Table 1 as relative values when the luminance of Comparative Example 1 is set to 100. The decrease in luminance was 21% as compared with Comparative Example 1. Also, a vibration test was performed in the same manner as in Example 1, and the results are shown in Table 1.

[0037]

[Table 1]

[0038]

According to the present invention, a prism sheet having excellent scratch resistance is provided by forming an apex having a radius of curvature of 5 μm or less or a flat portion having a width of 5 μm or less at the top of a prism row of the prism sheet. A surface light source element and a liquid crystal that can obtain a high-quality image without causing defects due to scratches when assembling or using the surface light source element and without causing a significant decrease in luminance as the surface light source element A display device can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic partial sectional view showing one embodiment of a prism sheet of the present invention.

FIG. 2 is a schematic partial sectional view showing one embodiment of a prism sheet of the present invention.

FIG. 3 is a schematic view showing a surface light source element of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Translucent base material 2 Prism row 3 Prism top part 4 Prism sheet 5 Light guide 6 Light source 7 Reflective layer

Claims (6)

[Claims] 1. A prism sheet having a large number of substantially triangular cross-section prism rows formed in parallel on at least one surface thereof, wherein the front ends of the prism rows are arc-shaped with a radius of curvature of 5 μm or less. Prism sheet. 2. A prism sheet in which a large number of substantially triangular cross-section prism rows are formed in parallel on at least one surface, and a flat portion having a width of 5 μm or less is formed at the end of the prism row. Prism sheet. 3. The prism sheet according to claim 1, wherein a prism row is formed on at least one surface of the translucent substrate by an active energy ray-curable composition. 4. A light guide having a light source, at least one light incident surface facing the light source, and a light exit surface substantially orthogonal to the light source, and a light guide mounted on the light exit surface of the light guide. Item 1
A surface light source element comprising the prism sheet according to any one of items 1 to 3. 5. The surface light source device according to claim 4, wherein the prism sheet is placed so that a prism row forming surface thereof is on a light emitting surface side of the light guide. 6. A liquid crystal display device comprising a liquid crystal display element disposed on a prism sheet of the surface light source element according to claim 4.