JP5820609B2 - Surface light source device and liquid crystal display device - Google Patents

Description

  The present invention relates to a prism sheet that can adjust the direction of light of a backlight light source in a front direction, a surface light source device including the prism sheet, and a liquid crystal display device.

  In recent years, in displays such as liquid crystal display devices, prism sheets having a function of condensing, diffusing, converging, deflecting, or reflecting light by refraction, reflection, diffraction, interference, or dispersion have been used. . The prism sheet usually has a prism row having a fine uneven shape by arranging a plurality of unit prisms having a predetermined refractive index on the surface. The prism sheet expresses a desired function by modulating light in this prism row by geometric optical action such as refraction or reflection or wave optical action such as diffraction. The shape of the resin material and the unit prism constituting the uneven shape of the row is determined.

  FIG. 8 is a schematic cross-sectional view showing an example of the configuration of a conventional liquid crystal display device. The liquid crystal display device 310 shown in FIG. 8 includes prisms on one surface side of the liquid crystal panel 10 in order from the liquid crystal panel 10 side. A sheet 110 and an edge light type surface light source 9 are provided. The prism sheet 110 includes a prism row 4 ′ on the liquid crystal panel 10 side, and the surface light source 9 has a light source 7 disposed on a side end surface of the light guide 6, and a light exit surface 6 a (on the liquid crystal panel side) of the light guide 6. On the side opposite to the surface), a reflecting plate 8 for preventing light leakage is provided. Although not shown, a conventional liquid crystal display device usually includes a diffusion sheet for diffusing light on the upper and lower surfaces or one of the surfaces of the prism sheet.

  In such a liquid crystal display device 310, the prism sheet 110 reflects or refracts the light emitted from the surface light source at the prism row 4 ′ so as to hide or make the light source image inconspicuous. The amount of light emitted to the side is increased, and the luminance in the direction perpendicular to the surface of the liquid crystal panel 10 (hereinafter, sometimes referred to as “front direction” or “normal direction”) is increased.

However, in the conventional liquid crystal display device as shown in FIG. 8, since the prism row of the prism sheet faces the liquid crystal panel side, most of the light emitted from the light guide is retroreflected in the prism sheet to guide the light. Of the light that is returned to the body side and emitted from the prism sheet, there is much light that is not collected in the normal direction, and there is a problem that the light use efficiency is poor.
Therefore, in recent years, a liquid crystal display device in which the prism rows of the prism sheet are arranged facing the light guide side has been developed for the purpose of improving the light utilization efficiency (see, for example, Patent Document 1). By arranging the prism sheet in such an orientation, the light emitted from the light guide is prevented from retroreflecting in the prism and the loss of light is reduced, so that the light use efficiency is improved and the liquid crystal display device The screen brightness increases.

  Also, as a prism sheet for further improving the brightness level in the front direction, Patent Document 2 has prism rows on both front and back surfaces, and the inner (light guide body side) prism rows and the outer side (liquid crystal panel). Side prism array is orthogonal to each surface, and the inner prism array is a light incident surface side inclined surface having a relatively large inclination angle and a light incident surface having a relatively small inclination angle. An asymmetric prism sheet is disclosed in which the inclined surface on the side and the opposite side are alternately repeated.

However, small liquid crystal display devices such as mobile phones, notebook computers, and portable game machines, which have been rapidly developed in recent years, have a small display screen and are used outdoors, so the brightness of the screen viewed from the front direction is It is required to be higher than conventional ones, and even with a surface light source device using a prism sheet disclosed in Patent Document 2, it is difficult to achieve the luminance required for a small liquid crystal display device. .
In addition, when the prism rows of the prism sheet are arranged facing the light guide body, using a conventional prism sheet reduces the contrast of the image of the liquid crystal display device, causes light leakage in black display, There are also problems such as a sense of incongruity. This is because the light emitted from the prism sheet causes side lobes. The side lobe is a portion of the directional light in which the reduced luminance once increases as the viewing angle increases from the direction of the luminance where the luminance is the maximum intensity.

JP 2007-48688 A International Publication No. 97/29398

  The present invention has been made to solve the above problems, and an object thereof is to provide a prism sheet that improves the luminance in the front direction, a surface light source device including the prism sheet, and a liquid crystal display device.

A surface light source device according to the present invention includes a light guide, a light source disposed on at least one edge of the light guide, and a prism sheet disposed on a light emission surface of the light guide. ,
The light guide has a maximum intensity at which the angle between the light incident from the light source and the normal to the light emission surface is greater than 60 degrees and 80 degrees or less, and the angle at which the half value width of the intensity distribution is ±± It is emitted as first directional light that is 5 degrees to ± 10 degrees,
The prism sheet has, on the light incident surface, a prism array in which a plurality of triangular prism unit prisms are arranged in parallel.
Each of the unit prisms has a substantially triangular prism shape defined by a first inclined surface and a plurality of flat surfaces having different inclination angles and defined by a second inclined surface facing the first inclined surface, and adjacent units. The first slope of one of the prisms and the second slope of the other are arranged to face each other,
Said first inclined surface has the first angle of inclination can be directional light is incident, a normal line angle is 30 to 37 degrees with respect to the light exit surface of the prism sheet,
The plurality of flat surfaces included in the second inclined surface are configured such that the first directional light incident from the first inclined surface is normal to the light emitting surface of the prism sheet for each component that has reached each flat surface. Each flat surface has an inclination angle for internal reflection so as to be the second directional light having the maximum intensity in the direction within ± 10 degrees from the direction, and the flat surface closer to the apex of the prism, The angle formed with the normal to the light exit surface of the prism sheet is large, and the angle formed with the normal to the light exit surface of the prism sheet is 30 to 37 degrees.
When the height from the base surface to the apex of the unit prism is 100%, the inclination angle of the flat surface changes to a position where the height from the base surface of the unit prism is 20 to 80%. Or a position where the height from the base surface of the unit prism is 10 to 50%, and the height from the base surface is closer to the apex of the unit prism than the position. A boundary where the inclination angle of the flat surface changes exists at a position of ˜90%.

  In the prism sheet according to the present invention, an angle that is a half width of the intensity distribution of the emitted light from the light emitting surface of the prism sheet can be ± 20 degrees or less.

  In the prism sheet according to the present invention, the first inclined surface may be composed of a plurality of flat surfaces having different inclination angles.

  The prism sheet concerning this invention can be equipped with the mat | matte layer which has a fine uneven | corrugated shape on the surface by the side of the said light-projection surface.

  The liquid crystal display device according to the present invention is characterized in that a liquid crystal panel is provided on the light output surface side of the surface light source device according to the present invention.

According to the present invention, the light emitted with directivity from the light guide is directed to the front direction by a plurality of flat surfaces having different inclination angles of the unit prism, and at the same time, the light distribution is narrowed and reflected. Thereby, the prism sheet which improves the brightness | luminance in a front direction can be provided.
In addition, according to the present invention, it is possible to provide a surface light source device equipped with a prism sheet for improving luminance in the front direction and a liquid crystal display device including the surface light source device.

It is typical sectional drawing which shows an example of the prism sheet which concerns on this invention. It is typical sectional drawing which shows an example of the unit prism which the prism sheet concerning this invention has. It is typical sectional drawing which shows another example of the unit prism which the prism sheet concerning this invention has. It is typical sectional drawing which shows another example of the unit prism which the prism sheet concerning this invention has. FIG. 5A is a graph showing an example of the intensity distribution of the luminance in the first directional light emitted from the light guide, and FIG. 5B shows the emitted light from the light emitting surface of the prism sheet. It is a graph showing an example of the intensity distribution of the brightness | luminance in. It is typical sectional drawing which shows an example of the surface light source device which concerns on this invention. It is typical sectional drawing which shows an example of the liquid crystal display device which concerns on this invention. It is typical sectional drawing which shows an example of the conventional liquid crystal display device.

  Next, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the spirit thereof.

In the present invention, (meth) acrylic resin represents acrylic resin and / or methacrylic resin, and (meth) acrylate represents acrylate and / or methacrylate.
In the present invention, the resin is a concept including a polymer in addition to a monomer and an oligomer.
In the definition of film and sheet in JIS-K6900, a sheet is thin and generally refers to a flat product whose thickness is small for the length and width, and the film is extremely thick compared to the length and width. A thin, flat product that is small and has an arbitrarily limited maximum thickness, typically supplied in the form of a roll. Therefore, it can be said that a film having a particularly thin thickness among the sheets is a film, but the boundary between the sheet and the film is not clear and is difficult to distinguish clearly. Therefore, in the present invention, the meaning of both a thick sheet and a thin sheet is meant. Is defined as “sheet”.
In the present invention, the molecular weight means a weight average molecular weight which is a polystyrene equivalent value measured by gel permeation chromatography (GPC) in a THF solvent when having a molecular weight distribution, and when having no molecular weight distribution, It means the molecular weight of the compound itself.

(Prism sheet)
The prism sheet of the present invention is a prism sheet having, on the light incident surface, a prism row in which a plurality of triangular prism unit prisms are arranged in parallel.
Each of the unit prisms has a substantially triangular prism shape defined by a first inclined surface and a plurality of flat surfaces having different inclination angles and defined by a second inclined surface facing the first inclined surface, and adjacent units. The first slope of one of the prisms and the second slope of the other are arranged to face each other,
The first inclined surface has an inclination angle at which first directional light having a maximum intensity in a predetermined direction can be incident,
Each flat surface included in the second inclined surface has the first directional light incident from the first inclined surface and having a light intensity distribution (not completely parallel light) on each flat surface. Each of the flat surfaces has an inclination angle for internal reflection so as to be second directional light having the maximum intensity in the normal direction with respect to the light exit surface of the prism sheet for each component that has reached.
The unit prism of the prism sheet of the present invention is substantially triangular in cross section, but the second side corresponding to the second slope is not strictly a straight side but consists of two or more straight lines. It has a polygonal line shape. That is, the shape of the cross section of the unit prism is strictly a polygon in which the second side includes two or more straight lines. In the present invention, the “transverse section” is a section in a direction orthogonal to the extending direction of the prism rows.
In the present invention, “first directional light having maximum intensity in a predetermined direction” means light having a first intensity distribution in which the peak of the maximum intensity of the luminance intensity distribution is in the predetermined direction.
In the present invention, the “normal direction (front direction)” does not have to be strictly perpendicular to the light exit surface of the prism sheet, and means a substantially normal direction within a range of ± 10 degrees.

The prism sheet according to the present invention enables the first directional light having a maximum intensity in a predetermined direction emitted from the light source to be incident on the prism sheet by adjusting an inclination angle of the first slope. By adjusting the inclination angle of each flat surface included in the second inclined surface, each of the flat surfaces is strong in the direction normal to the light exit surface of the prism sheet. It can be reflected as light having directivity. Therefore, the light in which the components of each light reflected from the respective flat surfaces are combined (the light emitted from the light exit surface of the prism sheet) has a strong directivity in the normal direction, thus improving the luminance in the front direction. can do.
Furthermore, the prism sheet of the present invention is easy to ensure the accuracy of the shape because the first inclined surface and the second inclined surface are flat surfaces, so that quality control is easy and mass productivity is possible. There is also a high feature.

Hereinafter, the behavior of the prism sheet and light according to the present invention will be described with reference to the drawings. In each figure, for convenience of explanation, as the behavior of light, representative rays corresponding to each light component are indicated by arrows, and the vertical / horizontal dimension ratio and the dimension ratio between the layers are appropriately exaggerated by changing from the actual dimensions. It is shown. In the following detailed description of the present invention, the angle of the first or second directional light means the angle at which the first or second directional light has the maximum intensity.
FIG. 1 shows a schematic cross-sectional view of an example of a prism sheet according to the present invention. A prism sheet 100 shown in FIG. 1 includes a plurality of unit prisms 3 each having a substantially triangular prism shape formed in parallel by a first inclined surface 1 and a second inclined surface 2 composed of two flat surfaces 2a and 2b having different inclination angles. It has prism rows 4 arranged. When the prism sheet 100 of the present invention is used in a surface light source device, the prism row 4 is disposed so as to face the light guide 6 side, and the extending direction of the prism row 4 (direction in which the prism ridge line runs) The light source is preferably arranged so as to be in a direction orthogonal to the light supply direction from the light source to the light guide 6 (parallel to the light incident surface). The first directional lights L1a and L1b having the maximum intensity in a predetermined direction emitted from the light guide 6 travel straight through the air layer (refractive index n0 = about 1.0) and then enter the first slope 1 Then, in the normal direction with respect to the light exit surface 5 of the prism sheet, each component that travels substantially straight in the prism and is reflected by the flat surfaces 2a and 2b included in the second inclined surface 2 and reaches each flat surface. The second directional light L2a and L2b having the maximum intensity is generated.
When the first directional light is incident on the first inclined surface 1 and reaches each flat surface included in the second inclined surface 2, the plurality of flat surfaces included in the second inclined surface 2 are unit prisms. Since the first directional light is blocked by the adjacent unit prism, the first directional light having a small angle with respect to the normal reaches the closer to the base portion. For this reason, the first directional light is separately illustrated as L1a and L1b for each component that reaches each flat surface included in the second inclined surface (the same applies to FIGS. 2, 4, and 6). However, in the present invention, the first directional light is light obtained by synthesizing components (L1a and L1b in FIG. 1) of each light emitted from the light guide, and the first directional light is legal. The angle formed by the line means the angle formed by the angle having the maximum intensity of the first directional light obtained by combining the components of the respective lights with the normal line.

1. FIG. 2 shows a schematic cross-sectional view of an example of a unit prism included in the prism sheet of the present invention. The unit prism 3 shown in FIG. 2 has a substantially triangular prism shape defined by the first inclined surface 1 and the second inclined surface 2 composed of two flat surfaces 2a and 2b having different inclination angles. In FIG. 2, the angle (incident surface angle) formed by the first inclined surface 1 and the normal to the light emission surface of the prism sheet is φ1, and the flat surface 2a and the normal to the light emission surface of the prism sheet are formed. The angle (first reflection surface angle) is denoted by φ2, and the angle (second reflection surface angle) formed by the flat surface 2b and the normal to the light exit surface of the prism sheet is denoted by φ3. In FIG. 2, the normal to the light exit surface of the prism sheet is indicated by a dotted line. The second directional lights L2a and L2b are in the normal direction with respect to the light exit surface of the prism sheet.

In the unit prism included in the prism sheet of the present invention, the second inclined surface 2 may be composed of three or more flat surfaces having different inclination angles. FIG. 3 shows a unit prism 3 ′ having a second inclined surface 2 composed of three flat surfaces 2a, 2b and 2c having different inclination angles. In FIG. 3, the angle (incident surface angle) formed between the first inclined surface 1 and the normal to the light exit surface of the prism sheet is φ1, and the angle formed between the flat surface 2a and the normal to the light exit surface of the prism sheet ( The angle formed by the flat surface 2b and the normal to the light output surface of the prism sheet (second reflection surface angle) is φ3, and the normal to the light output surface of the flat sheet 2c and the prism sheet The angle formed by (third reflection surface angle) is indicated as φ4. In FIG. 3, the normal to the light exit surface of the prism sheet is indicated by a dotted line.
Although the behavior of light is not shown in FIG. 3, in the unit prism 3 ′, the first directional light emitted from the light guide is incident from the first inclined surface 1 and transmitted through the first inclined surface 1. The first directional light is reflected by the flat surfaces 2a, 2b and 2c included in the second inclined surface 2, and has the maximum intensity in the normal direction with respect to the light exit surface of the prism sheet for each component reaching each flat surface. The second directional light having

  The first directional light emitted from the light guide (light synthesized from L1a and L1b in FIGS. 1 and 2) has a first intensity distribution in which the peak of the maximum intensity is in a predetermined direction. The direction of the peak of the maximum intensity is not particularly limited as long as the first directional light can be incident on the prism sheet, but the first directional light is usually normal to the light emitting surface of the light guide. The maximum angle is in the direction of the angle between 60 and 80 degrees. The first directional light is preferably directed to the majority of light at the angle formed with the normal line of the range, but light outside the range may exist.

The first directional light is not particularly limited, but the angle at which the half-value width of the intensity distribution can be set to ± 5 degrees or more, and is usually ± 10 to 20 degrees.
In the present invention, the full width at half maximum is from the angle at which the maximum value is 100% to the angle at which the luminance intensity is 50% of the maximum value at the peak of the maximum intensity of luminance. The directivity becomes weaker as the full width at half maximum is larger. FIG. 5A shows a graph showing an example of luminance intensity distribution in the first directional light emitted from the light guide. FIG. 5A shows a graph when the maximum intensity of the luminance of the first directional light is 70 degrees. The half-value width is obtained based on the peak of the maximum intensity by measuring the intensity distribution of the intensity of light emitted from the light guide at room temperature in the atmosphere using, for example, a luminance meter equipped with a goniometer. be able to.

  The inclination angle of the first slope 1 is not particularly limited as long as the first directional light can be incident thereon, and is appropriately adjusted depending on the direction in which the first directional light has the maximum intensity. Usually, the angle (φ1 (incident surface angle in FIG. 2)) formed by the first inclined surface 1 and the normal to the light exit surface of the prism sheet is 30 to 37 degrees.

  The plurality of flat surfaces included in the second inclined surface 2 internally reflect the first directional light incident from the first inclined surface 1 for each component that has reached each flat surface. The inclination angle of the plurality of flat surfaces included in the second inclined surface 2 is such that the reflected light obtained by internally reflecting the first directional light has the maximum intensity in the normal direction with respect to the light emitting surface of the prism sheet. Each flat surface is individually adjusted so as to be directional light. In this manner, the first directional light can be guided in the normal direction by appropriately adjusting the angle of each flat surface serving as the light reflecting surface of the prism sheet.

  The inclination angle of each flat surface included in the second inclined surface 2 is not particularly limited as long as the first directional light can be reflected as directional light having the maximum intensity in the normal direction. However, the first directional light is appropriately adjusted according to the direction having the maximum intensity, but it is preferable that the flat surface closer to the apex of the prism has a larger angle with the normal to the light exit surface of the prism sheet. That is, in the case of the unit prism 3 shown in FIG. 2, it is preferable that φ2> φ3, and in the case of the unit prism 3 ′ shown in FIG. 3, it is preferable that φ2> φ3> φ4. Thereby, the peak of the maximum intensity in the normal direction of the second directional light can be made narrower, the directivity of the second directional light can be improved, and the luminance in the front direction can be improved. .

  Furthermore, the angles (φ2 and φ3 in FIG. 2, φ2, φ3, and φ4 in FIG. 3) formed by the respective flat surfaces included in the second inclined surface and the normal to the light exit surface of the prism sheet are the first The directional light is appropriately adjusted according to a predetermined direction having the maximum intensity, and is not particularly limited, but is usually 30 to 37 degrees.

  As shown in FIG. 2, when the second inclined surface of the unit prism is composed of two flat surfaces, the position where the boundary where the inclination angle of the flat surface changes is appropriately determined depending on the directivity direction of the first directional light. Although not particularly limited, the height from the base surface of the unit prism to the top (H in FIG. 2) is 100%, and the height from the base surface of the unit prism is 20 to 80%. There may be a boundary at which the inclination angle of the flat surface changes.

  As shown in FIG. 3, when the second inclined surface of the unit prism is composed of three flat surfaces, the position where the boundary where the inclination angle of the flat surface changes is appropriately adjusted according to the directivity direction of the first directional light. Although not particularly limited, when the height from the base surface to the apex of the unit prism (H in FIG. 3) is 100%, the height from the base surface of the unit prism is 10 to 50%. And a boundary where the inclination angle of the flat surface changes at a position nearer to the apex of the unit prism than the position and at a height of 50 to 90% from the base surface.

  Further, the height from the base surface to the apex of the unit prism of the prism sheet of the present invention (H shown in FIG. 2) varies depending on the pitch of the prism rows (P shown in FIG. 2), and is not particularly limited. In the case of 50 μm, it is usually 30 to 45 μm. The apex of the unit prism of the prism sheet of the present invention may be a sharp shape as shown in FIG. 2, or may be a blunt apex where the vicinity of the top of the cross section in the thickness direction is chamfered, although not shown. You may cut so that it may become a flat surface. When the tip of the vertex of the unit prism is cut, the height from the base surface to the vertex is the height from the base surface to the flat surface of the tip.

  The pitch (P shown in FIG. 2) of the unit prism of the prism sheet of the present invention is not particularly limited, but is usually 10 to 100 μm.

  The light emitted from the light exit surface of the prism sheet of the present invention (the light synthesized from each second directional light) has the maximum intensity in the normal direction, and the half-value width of the light is the first directional light. It can be made smaller than the half width of. FIG. 5B is a graph showing an example of the intensity distribution of luminance in the light emitted from the light exit surface of the prism sheet. The full width at half maximum of the light emitted from the light exit surface of the prism sheet can be ± 20 degrees or less in a preferred embodiment of the present invention, and can be ± 10 degrees or less in a more preferred embodiment. . As the half-value width of the light emitted from the light exit surface of the prism sheet is smaller, the luminance in the front direction is improved.

In addition, the prism sheet of the present invention can be configured such that light from the light guide can be incident not only from the first slope but also from the second slope. In this way, not only in the so-called single-lamp surface light source in which the light source is disposed only on one edge of the light guide, but also in the so-called dual-lamp surface light source in which the light source is disposed on both edges of the light guide. The prism sheet of the present invention can be used. In the prism sheet of the present invention, when both the first slope and the second slope enter the light from the light guide, the light incident from the first slope is reflected by the second slope and emitted. Light incident from the second slope is reflected by the first slope and emitted. In the prism sheet of the present invention, when light is incident only from the first slope, an asymmetric prism (the shape of the unit prism is non-symmetric) as shown in FIGS. When light is incident from both the first slope and the second slope, the present invention has a unit prism in which both the first slope and the second slope have a plurality of flat surfaces as shown in FIG. The prism sheet can be preferably used, and the prism sheet can be a symmetric prism (the shape of the unit prism is line symmetric).
That is, in the prism sheet of the present invention in which light enters from both the first slope and the second slope, and both the first slope and the second slope have unit prisms composed of a plurality of flat surfaces, The inclination angle of one slope is an angle at which light from the light guide can be incident as described above, and the directional light having the maximum intensity in the normal line direction from the light incident from the second slope. It is also an angle that can be reflected as. Furthermore, the inclination angle of the second inclined surface is an angle at which the light incident from the first inclined surface can be reflected as directional light having the maximum intensity in the normal direction as described above, and the light guide It is also an angle at which light from can enter.
When the first slope is composed of a plurality of flat surfaces having different inclination angles, the preferable conditions for the inclination angles of the respective flat surfaces are the same as the preferable conditions for the respective flat surfaces included in the second inclined surface. It is.

  The unit prism 3 '' shown in FIG. 4 includes a first inclined surface 1 composed of two flat surfaces 1a and 1b having different inclination angles and a second inclined surface 2 composed of two flat surfaces 2a and 2b having different inclination angles. It has a defined substantially triangular prism shape. In the unit prism 3 '' shown in FIG. 4, the flat surface 1a, 1b and / or the second inclined surface 2 including the first inclined surface 1 includes the first directional light emitted from the light guide. It can enter from 2a, 2b. The first directional lights L1a and L1b incident from the flat surfaces 1a and 1b included in the first inclined surface 1 are reflected by the flat surfaces 2a and 2b included in the second inclined surface 2, and are reflected on the individual flat surfaces 2a and 2b. For each component that arrives, second directional light L2a and L2b having the maximum intensity in the direction normal to the light exit surface of the prism sheet is generated. The first directional lights L1′a and L1′b incident from the flat surfaces 2a and 2b included in the second inclined surface 2 are reflected by the flat surfaces 1a and 1b included in the first inclined surface 1, and are individually flat surfaces. Directional light L2′a and L2′b having maximum intensity in the normal direction with respect to the light exit surface of the prism sheet are generated for each component reaching 1a and 1b.

  In the prism sheet of the present invention, when only the second inclined surface is composed of a plurality of flat surfaces, or when both the first inclined surface and the second inclined surface are composed of a plurality of flat surfaces, each of the inclined surfaces includes a flat surface. The number of surfaces is not limited to that shown in the figure, and may be composed of three or more flat surfaces.

2. Layer structure of prism sheet The prism sheet of the present invention usually has a base layer and a prism layer in which a prism row is provided on one surface side of a light-transmitting base material from the viewpoint of excellent shape accuracy and mass productivity. However, a single layer structure formed by injection molding of a single material may be used. Prism row forming material when producing a prism sheet by providing a prism row on one side of a light transmissive substrate, and prism sheet formation when producing a single layer prism sheet by injection molding a single material The same material can be used as the material for use. Hereinafter, the prism row forming material and the single-layer prism sheet forming material are collectively referred to as a prism material.
The prism material functions as long as it is a material having a property of transmitting light efficiently, and is not particularly limited. For example, a known ultraviolet or electron beam curable resin, polycarbonate (PC), polyethylene terephthalate (PET), etc. Polyester resins, acrylic resins such as polymethylmethacrylate (PMMA), and light transmissive members such as glass. The ultraviolet or electron beam curable resin is typically a polymer having a polymerizable group such as (meth) acryloyl group, (meth) acryloyloxy group, epoxy group, thiol group and / or a single amount in the molecule. In particular, polyfunctional prepolymers such as urethane (meth) acrylate, epoxy (meth) acrylate, and silicon (meth) acrylate, trityrolpropane tri (meth) acrylate, dipentaerythritol hexa The main component is a polyfunctional monomer such as (meth) acrylate, but a highly cross-linked monomer can be used.

When the prism sheet of the present invention has a layer structure in which a prism row is provided on one side of the light transmissive substrate, the light transmissive substrate serves as a carrier for the prism row. As the light-transmitting substrate, a film used for a backlight mechanism of a normal optical display or a liquid crystal display, and at least an ionizing radiation-permeable material can be used as appropriate. Among them, polyethylene terephthalate (PET) ) And other polyester resins, polycarbonate (PC), acrylic resin such as polymethyl methacrylate (PMMA), stretched film made of cellulose triacetate (TAC), etc. are preferred, especially biaxially stretched from mechanical, chemical and optical performance The PET film is particularly desirable. A general biaxially stretched PET film is stretched in the flow direction and the width direction of the machine at the time of roll production, and the orientation direction of the molecules is also oriented obliquely from side to side with respect to the center of the machine. By being oriented in this way, the refractive index in the width direction is different from that in the flow direction, and the polarization direction of the emitted light can be adjusted by passing through such a film.
In addition, the thickness of a light-transmitting base material is 25-300 micrometers normally.

Necessary for preventing interference due to close contact with other members or concealing defects in appearance such as scratches on the surface of the prism sheet of the present invention where the prism row is not uneven (light emitting surface side). Accordingly, a mat layer having a fine uneven shape may be formed.
As a means for forming the mat layer, a mat agent coating or embossing can be used. When the mat layer is formed by coating the mat agent, the balance between expansion and contraction of the prism layer having the prism row and the mat layer can be adjusted, and the warp and the deflection of the prism sheet can be suppressed.
The matting agent is coated, for example, by coating a matting agent paint containing a matting agent in a binder made of a resin and a solvent on the surface of the prism layer opposite to the prism row.
Examples of the resin used for the binder of the mat layer include the same resins as those used for the light transmissive substrate.
Examples of the matting agent include fine particles such as polymethyl methacrylate (acrylic) beads, polybutyl methacrylate beads, polycarbonate beads, polyurethane beads, calcium carbonate beads, silicone beads, and silica beads. .
The fine particles are not particularly limited, but generally those having an average particle diameter of 1 to 20 μm can be used.
As a coating method of the matting agent paint, various known coating methods such as die coating, gravure coating, and slit reverse coating can be selected.
The uneven shape of the mat layer is not particularly limited, but the hill-like projections having a 10-point average roughness Rz = 0.1 to 10 μm in JIS B 0601 are two-dimensionally random with an average interval d = 1 to 100 μm. Can be formed in an array.
The thickness of the mat layer is usually 1 to 20 μm.

  In addition, the prism sheet according to the present invention may be provided with a light diffusion layer in order to provide a light diffusion function as necessary. The light diffusion layer is an arbitrary layer that is preferably provided, and may have any function of diffusing light, and a layer formed on a general light diffusion sheet can be used. For example, a layer in which light diffusing fine particles are dispersed in a translucent resin can be applied. The light diffusing layer can be provided at an arbitrary position of the prism sheet, and may be provided on a surface opposite to the prism row, or may be provided on a surface with the prism row. In the case where the prism sheet is composed of a light transmissive base material and a prism row, the prism sheet may be provided between the light transmissive base material and the prism row.

As translucent resin which comprises a light-diffusion layer, resin similar to resin used for the said translucent base material can be mentioned.
As the light diffusing fine particles, light diffusing fine particles generally used for light diffusing sheets are used. For example, polymethyl methacrylate (acrylic) beads, polybutyl methacrylate beads, polycarbonate beads, polyurethane type Examples include beads, calcium carbonate beads, and silica beads.
In addition, the thickness of a light-diffusion layer is 1-20 micrometers normally.

3. Manufacturing method of prism sheet The manufacturing method of the prism sheet of this invention is not specifically limited, A conventionally well-known method can be used. For example, a prism sheet mold having a desired unit prism shape is put into the mold of the prism sheet, and the prism sheet is batch-molded by injection molding, or a prism array mold having a desired unit prism shape. Put a prism row forming material such as UV curable resin, put a light transmissive base material on it, press the light transmissive base material to the prism row forming material using a laminator, etc. Examples include a method of curing the row forming material and peeling or removing the prism row mold (see FIG. 2 of JP-A-2009-37204).
In addition, in order to form a prism shape with a material for forming a prism array such as an ultraviolet curable resin on a light-transmitting substrate, a technique disclosed in Japanese Patent Application Laid-Open No. 5-169015 may be used. That is, a rotating roll intaglio having a concave and convex shape opposite to the prism shape is coated and filled with a prism row forming material liquid, and then a light-transmitting substrate is supplied to the plate surface to form a prism row forming material liquid. After pressing the roll intaglio from above and curing the prism row forming material liquid by ultraviolet irradiation or the like, the solidified prism row forming material is peeled off from the roll intaglio rotating with the light-transmitting substrate. Then, the prism sheet can be continuously manufactured.

(Surface light source device)
The surface light source device of the present invention includes a light source, a light guide that emits light incident from the light source as first directional light having a maximum intensity in a predetermined direction, and the prism sheet of the present invention, The prism sheet is arranged such that the surface having the prism row is on the light incident surface side. According to the surface light source device according to the present invention, the luminance in the front direction of the emitted light can be improved.

In the surface light source device according to the present invention, components such as a light source other than the prism sheet and a light guide are not particularly limited, and various specifications (forms) can be used. The prism sheet of the present invention is placed on the light emission surface side of a surface light source of the EL (electroluminescence) type or the like so that the surface having the prism row is on the light incident surface side. A light source device is configured. The edge-light type surface light source is usually configured such that light source light is incident from one end face or both end faces of a transparent light guide such as acrylic resin, and light is emitted from a light exit face on the liquid crystal panel side. On the other hand, a direct type surface light source has a liquid crystal panel and a reflector arranged in a manner sandwiching the light source, and is usually configured to reflect light from the light source to the liquid crystal panel side by the reflector.
In the present invention, an edge light type surface light source can be preferably used, and is preferably a surface light source in which the light source is disposed on at least one edge of the light guide.

Further, in the surface light source device according to the present invention, the prism sheet is arranged so that the prism row faces the light guide, and the extending direction of the prism row is a light supply direction to the light guide. It is preferable that they are arranged so as to be orthogonal (parallel to the light incident surface).
Furthermore, a shallow prism shape is provided on the surface of the light guide so as to be orthogonal to the incident direction of light, and light propagating through the light guide hits the prism shape, so that the light angle is gradually changed and guided. It is also possible to use a combination of emitting light from the light body.

  Hereinafter, an example of an edge light type surface light source device including the prism sheet of the present invention will be described in detail with reference to FIG. FIG. 6 is a schematic cross-sectional view showing an example of a surface light source device including the optical sheet according to the present invention. A surface light source device 200 shown in FIG. 6 includes a surface light source 9 and a prism sheet 100, and the surface light source 9 includes a light guide 6, a light source 7, and a reflection plate 8. In the surface light source device 200, the prism sheet 100 according to the present invention is provided on the light emitting surface 6a side of the light guide 6, and is arranged so that the prism row 4 (uneven surface) faces the light emitting surface 6a side. In the surface light source 9, the light emitted from the light source 7 provided on the side end surface of the light guide 6 enters the light guide 6 through the light incident surface 6b and has the maximum intensity in a predetermined direction. The light is emitted from the light emitting surface 6a as the first directional light. In FIG. 6, components of light constituting the first directional light are indicated as L1a and L1b. The emitted first directional lights L1a and L1b are incident on the prism sheet 100 from the first inclined surface 1, are internally reflected by the flat surfaces 2a and 2b constituting the second inclined surface 2, and are normal. The light is emitted from the light exit surface 5 of the prism sheet 100 as second directivities L2a and L2b having directivity in the direction.

The light guide is a plate-like body made of a translucent material, and light sent from the light source into the light guide is guided toward the thin end face while being subjected to scattering and reflection in the light guide. In the process of being illuminated, it is emitted from the light emitting surface gradually.
In the surface light source device of the present invention, the light emitted from the light emitting surface of the light guide is the first directional light described above. Therefore, a conventionally known light guide can be used as the light guide used in the present invention, but the light emitted from the light emitting surface is the first directional light having the first intensity distribution described above. Are appropriately selected and used.

  The material of the light guide is not particularly limited as long as it has a property of transmitting light efficiently, and examples thereof include acrylic resins such as polymethyl methacrylate (PMMA), polycarbonate resins, and glass.

Furthermore, it is preferable that a light scattering function is added to the inside or the surface of the light guide so that the light emitted from the light guide has the maximum intensity in a predetermined direction.
In order to add a light scattering function to the light guide, for example, a method of uniformly mixing and dispersing a different refractive index substance in a matrix made of resin or the like used in the light guide described above can be used. In the present invention, the “different refractive index substance” means a substance having a refractive index substantially different from the refractive index of the matrix. Depending on the particle size of the different refractive index particles by mixing and dispersing different refractive index particles having a predetermined particle size (generally, a correlation distance related to a refractive index inhomogeneous structure) as a different refractive index substance. Thus, the light emitted from the light emitting surface can have a preferential propagation direction. In other words, a strong or weakly collimated light beam is emitted from the light emitting surface.
The larger the particle size of the different refractive index particles to be mixed and dispersed in the light guide (in general, the larger the correlation distance related to the refractive index nonuniform structure), the more clearly the light emitted from the light emitting surface becomes a parallel light beam. .

  The mixing ratio of the different refractive index particles in the light guide is not particularly limited, and is adjusted so that the light guide has an appropriate light scattering function. In general, the mixing ratio of the different refractive index particles is selected to be smaller as the size of the light guide in the longitudinal direction increases. The “longitudinal direction” refers to a direction parallel to the light supply direction to the light guide (perpendicular to the extending direction of the light incident surface). This is because when the light scattering function is excessively imparted to the light guide, the propagation of light to a portion far from the light incident surface is hindered, and a brightness gradient may occur on the bright surface.

  On the other hand, the particle size of the different refractive index particles is a factor that determines the strength of the forward scattering property of each scattering process inside the light guide. In general, the larger the particle size, the stronger the forward scattering property. Due to this, if the particle size is relatively large, the preferential propagation direction of the light beam emitted from the light emitting surface of the light guide becomes clear. That is, a beam close to a parallel light beam can be obtained. Conversely, if the particle size is relatively small, the clarity of the preferential propagation direction of the light beam emitted from the light emitting surface of the light guide is lost. In the present invention, the size of the particle size is adjusted according to the strength of directivity required for the first directivity.

The thickness of the light guide is usually 1 to 10 mm, and the thickness may be constant over the entire range. As shown in FIG. 6, when a light source is provided on one end side, a side end surface on which the light source is provided. The taper shape may be such that the side is thickest and gradually becomes thinner toward the side opposite to the side end face. The reason why the thickness of the light guide is reduced as the distance from the side end surface is increased is to improve the light utilization efficiency and the uniformity of the brightness by the repeated slope reflection effect that occurs in the light guide.
In addition, as a light source arranged on both end faces of the light guide (so-called two-lamp type surface light source), a light guide using a flat light guide or a light guide having a cross-sectional shape that matches a straight wedge shape is used. What was used, what made the back side of the light guide an arch shape, etc. may be used.

  The light guide used in the present invention can be obtained by a known production method. As a method of mixing and dispersing a particulate material of a different refractive index substance in the matrix in order to add a light scattering function to the light guide, there is a method called a suspension polymerization method, for example. That is, when the particulate material is mixed in the monomer and the polymerization reaction is performed in a state of being suspended in hot water, a polymer material in which the particulate material is uniformly mixed can be obtained. If molding is performed using this as a raw material, a light guide of a desired shape having a light scattering function is manufactured. Suspension polymerization is performed for various particulate materials and monomer combinations (combination of particle concentration, particle size, refractive index, etc.), multiple types of materials are prepared, and these are selectively blended to form. If this is done, light guides with various characteristics can be manufactured. Moreover, if the polymer which does not contain a particulate material is blended, particle concentration can be easily controlled. Another method that can be used for uniform mixing of the particulate material is to knead the polymer material and the particulate material. Also in this case, kneading and molding (pelletizing) is performed with a combination of various particulate materials and polymers (combination of particle concentration, particle size, refractive index, etc.), and these are selectively blended to guide the light. It is possible to obtain light guides with various characteristics by molding and manufacturing.

  A double-sided prism light guide can also be used as the light guide of the surface light source device of the present invention. A double-sided prism light guide is a light guide obtained by forming prisms orthogonally on both sides of a flat plate made of a translucent material.

In the surface light source device of the present invention, in the case of the edge light type surface light source device, the light source is for causing light to enter from the at least one side end surface thereof, and is disposed along the side end surface of the light guide.
As the light source, for example, a linear light source such as a cold cathode tube is preferably used, but a point light source such as an incandescent bulb, LED (light emitting diode) may be arranged in a line along the side end surface, A plurality of small flat fluorescent lamps may be arranged along the side end surface.

The surface light source device of the present invention guides light emitted from a surface opposite to the light emitting surface of the light guide or a surface other than the light emitting surface from the side opposite to the light emitting surface of the light guide. A reflector for returning to the body and emitting from the light emitting surface may be provided.
The reflective plate is not particularly limited, and for example, a regular reflective silver foil sheet, a thin metal plate deposited with aluminum or the like, a diffuse reflective white foamed PET (polyethylene terephthalate) sheet, or the like is used.

(Liquid crystal display device)
The liquid crystal display device according to the present invention includes a liquid crystal panel on the light exit surface side of the surface light source device according to the present invention.
According to the liquid crystal display device of the present invention, the above-described surface light source device of the present invention is disposed as a backlight behind the liquid crystal panel according to a well-known arrangement method, whereby a liquid crystal display with high brightness in the front direction of the screen is obtained. Provided. This is because the light emitted from the surface light source device provided with the prism sheet of the present invention has strong directivity in the front direction.

FIG. 7 is a schematic cross-sectional view showing an example of a liquid crystal display device including the edge light type surface light source device shown in FIG. A liquid crystal display device 300 shown in FIG. 7 includes a surface light source device 200 including the prism sheet 100 according to the present invention on the liquid crystal panel 10 and one surface thereof (the surface opposite to the surface on which image display is observed in the liquid crystal display device). It has.
Note that the liquid crystal display device of the present invention may be a transmissive liquid crystal display device having a surface light source device (backlight) on the back side of the liquid crystal panel, or may display a back light on the back side together with the display of reflected light from outside light. It may be a transflective liquid crystal display device capable of both light display.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

  Hereinafter, the present invention will be described more specifically with reference to examples. These descriptions do not limit the present invention.

(Prism sheet)
As prism sheets A to H, a diamond tool is used so that the cross-sectional shape in the axial direction of the roll mold becomes the prism shape shown in Table 1 on the circumference of the roll mold surface, and a substantially triangular shape with a pitch of 50 μm. The groove to be shaped is cut with an NC lathe, and the mold is filled with an ultraviolet curable resin mainly composed of a prepolymer of a UV curable urethane acrylate resin. A UV curable resin was cured in the mold by UV irradiation from the back side of the material, and then peeled off together with the PET substrate.
The prism sheets A, B, C, F, G, and H have the unit prism shape shown in FIG. 2, and the incident surface angle, the first reflecting surface angle, and the second reflecting surface angle shown in Table 1 are shown in FIG. The angles correspond to φ1, φ2, and φ3. The prism sheet D has the unit prism shape shown in FIG. 3, and the incident surface angle, the first reflecting surface angle, the second reflecting surface angle, and the third reflecting surface angle shown in Table 1 are φ1 in FIG. The angles correspond to φ2, φ3, and φ4.
Table 1 also shows the position of the boundary where the inclination angle of the flat surface included in the second inclined surface of each prism sheet changes as the first boundary and the second boundary. This is indicated by the height (%) from the base surface to the boundary when the thickness is 100%. In the prism sheets A, B, C, F, G, and H, the boundary where the inclination angle of the flat surface changes is only one place indicated as the first boundary, and the prism sheet D has an inclination angle of the flat surface. The changing boundaries are the two places shown as the first boundary and the second boundary, the first boundary indicates the position of the boundary closer to the vertex of the unit prism, and the second boundary is close to the base surface Indicates the position of the boundary.
In the prism sheet E, the second inclined surface of the unit prism is composed of one flat surface, and the incident surface angle shown in Table 1 is an angle formed by the first inclined surface and the normal direction. Is the angle between the second slope and the normal direction.
The prism sheet H is a prism sheet with a back surface mat layer, and a mat layer was formed on a PET base material on the side surface opposite to the prism row by the following procedure. That is, a matting agent paint containing a binder made of crosslinked acrylic resin fine particles (n = 1.49) having an average particle size of 5 μm and a polyester resin (n = 1.55) and a solvent as a matting agent on a PET substrate. Was applied by slit reverse coating, and the solvent was dried to cure the matte layer.
In the dried coating film, hill-like projections having a 10-point average roughness Rz = 3 μm according to JIS B 0601 were formed in a two-dimensional random array with an average interval d = 30 μm.

(Light guide)
Next, creation of a light guide having a high direction of outgoing light will be described. First, a first roll mold in which a triangular shape having a vertical cross section of 90 degrees and a pitch of 50 μm is cut in the circumferential direction with a diamond tool and an NC lathe, and a cross section in the circumferential direction has a vertical angle of 178. A second roll mold in which a triangular shape with a pitch of 200 μm was cut in the axial direction was prepared.
A thermoplastic acrylic resin in the form of pellets is filled into an extruder and melted by heat, and is placed between the first and second roll molds in the form of a film from the die of the extruder under the die. Resin is extruded from a die, rolled between roll dies, cooled by a third roll, and molded into a light guide (double-sided prism light guide) in which prisms are formed on both sides of a flat plate. did. The cap of the roll, the extrusion temperature, and the extrusion speed were adjusted so that the acrylic resin formed on the plate-shaped light guide had a thickness of 0.8 mm.
Also, for comparison with the double-sided prism light guide, white ink is printed in dots by screen printing on a 0.8 mm thick acrylic plate as a light guide with low directivity of outgoing light and a small outgoing angle. The prepared light guide (dot pattern light guide) was prepared.

Example 1
The LED light source is disposed on the end surface orthogonal to the ridge line direction of the surface prism of the double-sided prism light guide obtained above, and a white PET reflector is placed under the light guide back prism, and the surface prism of the light guide The surface light source device was manufactured by arranging the prism sheet A in a direction perpendicular to the ridge line direction of the surface prism in a direction in which the prism surface is in contact with the surface prism of the light guide.

(Examples 2-5, Comparative Examples 1-4)
A surface light source device was manufactured in the same manner as in Example 1 with the combination of the prism sheet and the light guide shown in Table 2. In Comparative Example 4, the LED light source is disposed on the end face of the dot pattern light guide, the white PET reflector is disposed on the back surface of the light guide, the prism sheet A is arranged, and the ridge line direction of the prism is the light guide. The surface light source device was manufactured by arranging on the surface of the light guide so that it was parallel to the light incident surface and the prism surface was on the light guide.

(Measurement of intensity distribution of luminance of light emitted from the light guide)
As a result of measuring the intensity distribution of the luminance of the emitted light from the light guide with a goniometer, the angle of the emitted light from the double-sided prism light guide has a maximum intensity of 70 degrees and the half-value width is ± 10 degrees. It was. The outgoing light from the dot pattern light guide had an angle with the maximum intensity of 60 degrees and a half width of ± 20 degrees.

(Measurement of intensity distribution of luminance of light emitted from the light exit surface of the prism sheet)
As a result of measuring the intensity distribution of the luminance of the emitted light from the light emitting surface of the prism sheet of the surface light source device obtained in each example and each comparative example with a goniometer, in all the examples and comparative examples, the emitted light The angle at which has the maximum intensity was substantially in the normal direction (within ± 10 degrees from the strict normal direction). Table 2 shows the measurement results of the half-value width of the intensity distribution of the emitted light measured in each example and each comparative example.

(Backlight front luminance ratio evaluation)
Using a luminance meter (Topcon BM-7), the luminance in the front direction of the surface light source device obtained in each Example and each Comparative Example was measured, and the luminance value measured by the surface light source device of Comparative Example 1 The luminance ratio was calculated with 100%. The results are shown in Table 2.

(Appearance evaluation on LCD panel)
A liquid crystal panel was placed on the backlight, and interference fringes were visually confirmed. The evaluation results are shown in Table 2 when the interference fringes are at a level where they are not visually recognized on the liquid crystal panel, and when the interference fringes are visible on the liquid crystal panel but at a level where there is no problem with the images.

In the prism sheets used in Examples 1 to 5, the first slope has the first directional light (maximum intensity at 70 degrees and the half width is ± 10 degrees) emitted from the light guide. Each flat surface included in the second inclined surface has an incident angle that allows incidence of the first directional light incident from the first inclined surface on a prism basis for each component that has reached each flat surface. In the surface light source device provided with the prism sheet, since each flat surface has an inclination angle for internal reflection so as to be the second directional light having the maximum intensity in the normal direction with respect to the light emission surface of the sheet, The emitted light from the prism sheet has strong directivity in the normal direction, and the half-value width of the emitted light is ± 7.0 degrees to ± 8.2 degrees, which is a narrower intensity distribution than the first directional light. And the luminance was high in the evaluation of the front luminance ratio of the backlight.
Moreover, since the prism sheet H used in Example 5 has a matte layer, generation of interference fringes was prevented in the appearance evaluation of the liquid crystal panel.
Since the prism sheet E used in Comparative Example 1 has a second inclined surface (reflecting surface) formed of a single flat surface, the half-value width of the light emitted from the prism sheet of the surface light source device including the prism sheet E has a half width. The intensity distribution was ± 10.0 degrees, which was wider than that of the example, and as a result, the front luminance ratio of the backlight was inferior.
Since the prism sheet F used in Comparative Example 2 has a small incident surface angle and a small second reflection surface angle, the light emitted from the prism sheet of the surface light source device including the prism sheet F has a half-value width of ±. 10.5 degrees and a broad intensity distribution compared to the example, and as a result, the backlight was inferior to the front luminance ratio.
Since the prism sheet G used in Comparative Example 3 has a large incident surface angle and a large first reflection surface angle, the light emitted from the prism sheet of the surface light source device including the prism sheet G has a half width. The intensity distribution was ± 10.5 degrees, which was wider than that of the example, and as a result, the backlight front luminance ratio was inferior.
In Comparative Example 4, instead of the double-sided prism light guide, a dot pattern light guide (having a maximum intensity at 60 degrees and a half width of ± 20 degrees) was used. Even if is used, the light condensing property in the normal direction of the first directional light by the prism sheet is low, and the first directional light has a relatively wide intensity distribution with a half-value width of ± 20 degrees. In other words, the light emitted from the prism sheet of the surface light source device has a wide intensity distribution with a half-value width of ± 23 degrees, resulting in a small front luminance ratio of the backlight.

DESCRIPTION OF SYMBOLS 1 1st slope 1a, 1b Flat surface 2 2nd slope 2a, 2b, 2c Flat surface 3, 3 ', 3''Unit prism 4, 4' Prism row 5 Light-emitting surface 6 Light guide 6a Light emission surface 6b Light incident surface 7 Light source 8 Reflector 9 Surface light source 10 Liquid crystal panels L1a, L1b Light components L2a, L2b constituting the first directional light Second directional light 100, 110 Prism sheet 200 Surface light source device 300, 310 Liquid crystal display device

Claims (5)

A light guide, a light source disposed on at least one edge of the light guide, and a prism sheet disposed on a light emission surface of the light guide,
The light guide has a maximum intensity at which the angle between the light incident from the light source and the normal to the light emission surface is greater than 60 degrees and 80 degrees or less, and the angle at which the half value width of the intensity distribution is ±± It is emitted as first directional light that is 5 degrees to ± 10 degrees,
The prism sheet has, on the light incident surface, a prism array in which a plurality of triangular prism unit prisms are arranged in parallel.
Each of the unit prisms has a substantially triangular prism shape defined by a first inclined surface and a plurality of flat surfaces having different inclination angles and defined by a second inclined surface facing the first inclined surface, and adjacent units. The first slope of one of the prisms and the second slope of the other are arranged to face each other,
Said first inclined surface has the first angle of inclination can be directional light is incident, a normal line angle is 30 to 37 degrees with respect to the light exit surface of the prism sheet,
The plurality of flat surfaces included in the second inclined surface are configured such that the first directional light incident from the first inclined surface is normal to the light emitting surface of the prism sheet for each component that has reached each flat surface. Each flat surface has an inclination angle for internal reflection so as to be the second directional light having the maximum intensity in the direction within ± 10 degrees from the direction, and the flat surface closer to the apex of the prism, The angle formed with the normal to the light exit surface of the prism sheet is large, and the angle formed with the normal to the light exit surface of the prism sheet is 30 to 37 degrees.
When the height from the base surface to the apex of the unit prism is 100%, the inclination angle of the flat surface changes to a position where the height from the base surface of the unit prism is 20 to 80%. Or a position where the height from the base surface of the unit prism is 10 to 50%, and the height from the base surface is closer to the apex of the unit prism than the position. A surface light source device characterized in that a boundary where the inclination angle of the flat surface changes exists at a position of ˜90%.   2. The surface light source device according to claim 1, wherein an angle that is a half value width of an intensity distribution of emitted light from the light emitting surface of the prism sheet is ± 20 degrees or less.   The surface light source device according to claim 1, wherein the first slope is composed of a plurality of flat surfaces having different inclination angles.   The surface light source device according to any one of claims 1 to 3, wherein the prism sheet includes a mat layer having a fine uneven shape on a surface on the light emitting surface side.   A liquid crystal display device comprising a liquid crystal panel on a light exit surface side of the surface light source device according to any one of claims 1 to 4.

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