Optical member, surface light source device, and liquid crystal display device

Abstract

This optical member includes a light guiding plate and an optical sheet which is overlapped with the light guiding plate in a first direction. The light guiding plate includes a light emission surface, a rear surface that faces the light emission surface, and a light incidence surface and a reverse surface that are positioned between the light emission surface and the rear surface. The light incidence surface and the reverse surface face each other in a second direction which is not parallel with the first direction. The optical sheet includes a first surface and a second surface that faces the first surface. The optical sheet includes a base material film and a plurality of unit prisms that are arranged in the second direction and that constitute the second surface. The rear surface, the light emission surface, the second surface, and the first surface are positioned in that order in the first direction. The retardation of the base material film is 800 nm or greater. In a projection onto a plane perpendicular to the first direction, the size of the angle between the slow axis of the base material film and the second direction is 5-85°.

Description

Optical member, surface light source device, and liquid crystal display device

[0001] The present disclosure relates to an optical member, a surface light source device, and a liquid crystal display device.

[0002] As disclosed in Patent Document 1, a liquid crystal display device is known. The liquid crystal display device includes a surface light source device and a liquid crystal display panel. The surface light source device illuminates the liquid crystal display panel from behind as a backlight. The liquid crystal display panel usually includes a first polarizing plate, a liquid crystal cell, and a second polarizing plate. The liquid crystal cell forms a large number of pixels. The liquid crystal cell can convert the linearly polarized light component transmitted through the first polarizing plate into a polarized light component that can be transmitted through the second polarizing plate for each pixel.

[0003] Since the first polarizing plate and the second polarizing plate are used in the liquid crystal display device, the light utilization efficiency in the liquid crystal display device is usually less than 50%. Therefore, improving the light utilization efficiency has become an issue in liquid crystal display devices and the components used in liquid crystal display devices.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-82109

[0005] The present disclosure aims to improve the light utilization efficiency.

[0006] The first optical member in the present embodiment includes a light guide plate and an optical sheet overlapped with the light guide plate in a first direction. The light guide plate includes a light emitting surface, a back surface facing the light emitting surface, an incident surface and an opposite surface located between the light emitting surface and the back surface. The incident surface and the opposite surface face each other in a second direction that is non-parallel to the first direction. The optical sheet includes a first surface and a second surface facing the first surface. The optical sheet includes a base material film and a plurality of unit prisms arranged in the second direction to form the second surface. The back surface, the light emitting surface, the second surface, and the first surface are located in this order in the first direction. The retardation of the base material film at a wavelength of 589 nm is 800 nm or more. In the projection onto a plane perpendicular to the first direction, the magnitude of the angle between the slow axis of the base material film and the second direction is 5° or more and 85° or less. [[ID=****]]

[0007] The second optical member in this embodiment comprises a light guide plate and an optical sheet superimposed on the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction, the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a base film and a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the retardation of the base film at a wavelength of 589 nm is 2000 nm or more, and the magnitude of the angle between the slow axis of the base film and the second direction in projection onto a surface perpendicular to the first direction is 5° or more and 85° or less.

[0008] The third optical member in this embodiment comprises a light guide plate and an optical sheet facing the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface facing the optical sheet, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction, the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the second ratio of the maximum brightness due to the first polarization component emitted from the first surface to the maximum brightness due to the second polarization component emitted from the first surface is 1.24 or less, The first ratio of the maximum brightness caused by the first polarization component incident on the second surface to the maximum brightness caused by the second polarization component incident on the second surface is 1.30 or more, the first polarization component is a linear polarization component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linear polarization component that vibrates in a direction perpendicular to the reference plane.

[0009] The surface light source device in this embodiment comprises one of the first to second optical members in this embodiment, and a light source that emits light incident on the light guide plate from the light-receiving surface.

[0010] The liquid crystal display device in this embodiment comprises a surface light source device and a liquid crystal display panel superimposed on the surface light source device in the first direction.

[0011] The present invention aims to improve the efficiency of light utilization.

[0012] Figure 1 is a schematic diagram illustrating an embodiment of the present disclosure, showing an example of a liquid crystal display device. Figure 2 is a schematic diagram showing an example of a liquid crystal display panel and a surface light source device that may be included in the liquid crystal display device shown in Figure 1. Figure 3 is a plan view showing the surface light source device shown in Figure 2. Figure 4 is a cross-sectional view showing the surface light source device shown in Figure 2. Figure 5 is a perspective view showing an example of a light guide plate that may be included in the surface light source device shown in Figure 2. Figure 6 is a partially enlarged cross-sectional view showing the light guide plate shown in Figure 5. Figure 7 is a perspective view showing an example of an optical sheet that may be included in the surface light source device shown in Figure 2. Figure 8 is a partially enlarged cross-sectional view showing the optical sheet shown in Figure 7. Figure 9 is a plan view showing an example of a substrate film included in the optical sheet shown in Figure 7. Figure 10A is a longitudinal cross-sectional view showing an example of a reflective sheet that may be included in the surface light source device shown in Figure 2. Figure 10B is a longitudinal cross-sectional view showing another example of a reflective sheet that may be included in the surface light source device shown in Figure 2. Figure 11 is a graph showing an example of the angular distribution of brightness caused by a first polarization component and a second polarization component incident from the light guide plate to the optical sheet. Figure 12 is a graph showing an example of the angular distribution of brightness caused by the first and second polarization components emitted from the optical sheet. Figure 13 is a graph showing another example of the angular distribution of brightness caused by the first and second polarization components emitted from the optical sheet. Figure 14 is a cross-sectional view corresponding to Figure 4, showing a modified example of the light guide plate. Figure 15 is a graph showing the reflectance of P-waves and S-waves in free-end reflection. Figure 16 is a graph showing the reflectance of P-waves and S-waves in fixed-end reflection.

[0013] This embodiment relates to the following <1> to <14>.

[0014] <1> An optical member comprising: a light guide plate; and an optical sheet superimposed on the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction; the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a base film and a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the retardation of the base film at a wavelength of 589 nm is 800 nm or more, and the magnitude of the angle between the slow axis of the base film and the second direction in projection onto a surface perpendicular to the first direction is 5° or more and 85° or less.

[0015] <2> An optical member comprising: a light guide plate; and an optical sheet superimposed on the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction; the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a base film and a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the retardation of the base film at a wavelength of 589 nm is 2000 nm or more, and the magnitude of the angle between the slow axis of the base film and the second direction in projection onto a surface perpendicular to the first direction is 5° or more and 85° or less.

[0016] <3> The optical member according to <1> or <2>, wherein, in projection onto a plane perpendicular to the first direction, the magnitude of the angle between the slow axis of the base film and the second direction is 5° or more and 30° or 60° or more and 85°.

[0017] <4> The optical member according to <1>, wherein the second ratio of the maximum brightness caused by the first polarization component emitted from the first surface to the maximum brightness caused by the second polarization component emitted from the first surface is 1.24 or less, the first ratio of the maximum brightness caused by the first polarization component incident on the second surface to the maximum brightness caused by the second polarization component incident on the second surface is 1.30 or more, the first polarization component is a linear polarization component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linear polarization component that vibrates in a direction perpendicular to the reference plane.

[0018] <5> The device comprises a light guide plate and an optical sheet facing the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface facing the optical sheet, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction, the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the second ratio of the maximum brightness due to the first polarization component emitted from the first surface to the maximum brightness due to the second polarization component emitted from the first surface is 1.24 or less, An optical member wherein the first ratio of the maximum brightness caused by the first polarization component incident on the second surface to the maximum brightness caused by the second polarization component incident on the second surface is 1.30 or more, the first polarization component is a linear polarization component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linear polarization component that vibrates in a direction perpendicular to the reference plane.

[0019] <6> The optical component according to <4> or <5>, wherein the second ratio is 1.20 or less.

[0020] <7> The optical member according to any one of <1> to <6>, comprising a reflective sheet facing the back surface, wherein the reflective sheet includes a metal layer, the back surface includes an inclined surface, and the inclined surface is inclined with respect to the second direction such that it approaches the light-emitting surface in the first direction from the light-receiving surface toward the opposite surface in the second direction.

[0021] <8> The optical member according to any one of <1> to <7>, wherein the retardation of the substrate film at a wavelength of 589 nm is 800 nm or more and 8200 nm or less.

[0022] <9> The optical member according to any one of <1> to <8>, wherein the retardation of the substrate film at a wavelength of 589 nm is 1900 nm or more and 8200 nm or less.

[0023] <10> The optical member according to any one of <1> to <9>, wherein the retardation of the substrate film at a wavelength of 589 nm is 2000 nm or more and 3200 nm or less.

[0024] <11> A surface light source device comprising an optical member described in any one of <1> to <10>, and a light source that emits light incident on the light guide plate from the light-receiving surface.

[0025] <12> A liquid crystal display device comprising a surface light source device as described in <11>, and a liquid crystal display panel superimposed on the surface light source device in the first direction.

[0026] <13> The liquid crystal display device according to <12>, wherein the liquid crystal display panel includes a first polarizing plate, a liquid crystal cell, and a second polarizing plate, the surface light source device, the first polarizing plate, the liquid crystal cell, and the second polarizing plate are positioned in this order in the first direction, the first polarizing plate transmits the second polarization component and shields the first polarization component, the first polarization component is a linearly polarized component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linearly polarized component that vibrates in a direction perpendicular to the reference plane.

[0027] <14> The liquid crystal display device according to <12> or <13>, wherein the second direction is perpendicular to the horizontal direction.

[0028] This embodiment will be described below with reference to the drawings. Note that, for ease of understanding, the scale and aspect ratios of the drawings attached to this specification have been appropriately altered and exaggerated from those of the actual object. Some components shown in some drawings may be omitted in others.

[0029] In this specification, terms such as "parallel," "perpendicular," and "identical," as well as values ​​of length and angle, which specify shapes, geometric conditions, and their degrees, are not limited to their strict meanings but are interpreted to include a range of values ​​to which similar functions can be expected.

[0030] In this specification, terms such as "sheet," "film," and "plate" are not distinguished from each other solely on the basis of name differences. For example, "optical sheet" is not distinguished solely from components such as optical films or optical plates solely on the basis of name differences. "Light guide plate" is not distinguished solely from components such as light guide sheets or light guide films solely on the basis of name differences. "Reflective sheet" is not distinguished solely from components such as reflective films or reflectors solely on the basis of name differences.

[0031] In this specification, the normal direction of a sheet-like (film-like, plate-like) member means the direction parallel to the normal or perpendicular to the sheet surface of the sheet-like (film-like, plate-like) member in question. The "sheet surface (film surface, plate surface)" means the surface that coincides with the sheet-like (film-like, plate-like) member in question when the sheet-like (film-like, plate-like) member in question is observed as a whole.

[0032] In this specification, when multiple candidate upper limits and multiple candidate lower limits are given for a certain parameter, the numerical range of that parameter may be constructed by combining any one candidate upper limit and any one candidate lower limit. As an example, consider the statement, "Parameter B may be A1 or greater, A2 or greater, A3 or greater. Parameter B may be A4 or less, A5 or less, or A6 or less." In this example, the numerical range of parameter B may be A1 or greater and A4 or less, A1 or greater and A5 or less, A1 or greater and A6 or less, A2 or greater and A4 or less, A2 or greater and A5 or less, A2 or greater and A6 or less, A3 or greater and A4 or less, A3 or greater and A5 or less, or A3 or greater and A6 or less.

[0033] To clarify the directional relationships between drawings, some drawings use arrows with common symbols to indicate the first direction D1, the second direction D2, and the third direction D3 as common directions. The tip of the arrow is the first side of each direction. The opposite side of the arrow is the second side of each direction. For example, as shown in Figure 1, the symbol of a dot inside a circle indicates an arrow pointing from the plane of the drawing toward the viewer, in a direction perpendicular to the plane of the drawing.

[0034] "Suppression" means to prevent or hinder the realization or occurrence of something. "Suppression" means not only completely preventing the realization or occurrence of something, but also reducing the possibility of it happening or making it less likely to occur.

[0035] Figures 1 to 16 are diagrams illustrating this embodiment. Figures 1 to 14 show specific examples of liquid crystal display devices and surface light source devices according to this embodiment. Figure 1 shows a schematic representation of the liquid crystal display device 10. Figure 2 is a configuration diagram showing a specific example of the liquid crystal display device 10 shown in Figure 1. Figure 3 is a plan view of the surface light source device 20 shown in Figure 2. Figure 4 is a cross-sectional view of the surface light source device 20 shown in Figure 2.

[0036] In the following, this embodiment will be described using an example in which the surface light source device 20 and the optical member 28 are applied to the liquid crystal display device 10. In this embodiment, in the liquid crystal display device 10 using the surface light source device 20 and the optical member 28, measures have been taken to improve the utilization efficiency of the light emitted from the light source 24.

[0037] As shown in Figure 1, the liquid crystal display device 10 includes a liquid crystal display panel 15 and a surface light source device 20. The surface light source device 20 includes an optical member 28 and a light source 24. The light source 24 emits light. The light emitted from the light source 24 is incident on the optical member 28. The direction of travel of the light is adjusted within the optical member 28. The liquid crystal display panel 15 forms an image by utilizing the polarization state of the light emitted from the surface light source device 20. In this embodiment, the polarization state of the light emitted from the light source 24 is also adjusted in the optical member 28. By controlling the polarization state of the light, the utilization efficiency of the light emitted from the light source 24 can be improved.

[0038] In the illustrated example, the liquid crystal display panel 15 and the surface light source device 20 are stacked in a first direction D1. The first direction D1 is the stacking direction of the liquid crystal display panel 15 and the surface light source device 20. As shown in Figure 1, each of the liquid crystal display panel 15 and the surface light source device 20 may extend in a direction perpendicular to the first direction D1, which is the stacking direction.

[0039] The illustrated liquid crystal display panel 15 and surface light source device 20 are both flat. As shown in Figure 1, the first direction D1 may be the thickness direction of the liquid crystal display panel 15 and the surface light source device 20, respectively. The first direction D1 may be parallel to the normal direction of the liquid crystal display panel 15. The first direction D1 may be perpendicular to the display surface 11 of the liquid crystal display panel 15. The first direction D1 may be parallel to the normal direction of the surface light source device 20. The first direction D1 may be perpendicular to the light-emitting surface 21 of the surface light source device 20.

[0040] The second direction D2 and the third direction D3 are perpendicular to each other. The first direction D1 and the second direction D2 are perpendicular to each other. The first direction D1 and the third direction D3 are perpendicular to each other.

[0041] As shown in FIG. 3, the liquid crystal display panel 15 and the surface light source device 20 may have a rectangular shape when observed from the first direction D1. The illustrated liquid crystal display panel 15 and surface light source device 20 include a pair of edges extending in the second direction D2 and the third direction D3, respectively, when observed from the first direction D1.

[0042] The third direction D3 may be parallel to the horizontal direction.

[0043] The second direction D2 may not be parallel to the horizontal direction and may be perpendicular to the horizontal direction. The second direction D2 may be parallel to the vertical direction. The second direction D2 may be inclined to the vertical direction by a slight angle. The first side in the second direction D2 may be the upper side in the vertical direction. The second side in the second direction D2 may be the lower side in the vertical direction.

[0044] The first direction D1 may be parallel to the horizontal direction. The first direction D1 may be inclined to the horizontal direction by a slight angle.

[0045] Each component of the liquid crystal display device 10 will be described in further detail.

[0046] The liquid crystal display panel 15 will be described in further detail. The liquid crystal display panel 15 may constitute the display surface 11. An image may be displayed on the display surface 11. In the example shown in FIG. 1, the surface light source device 20 illuminates the liquid crystal display panel 15 from behind as a backlight. The liquid crystal display panel 15 may be a transmissive liquid crystal display panel. The type of the liquid crystal display panel is not particularly limited. The liquid crystal display panel 15 may be a TN type liquid crystal display panel, a VA type liquid crystal display panel, or an IPS type liquid crystal display panel.

[0047] As shown in FIG. 2, the liquid crystal display panel 15 may include a first polarizing plate 16, a liquid crystal cell 17, and a second polarizing plate 18. The first polarizing plate 16, the liquid crystal cell 17, and the second polarizing plate 18 are positioned in this order from the second side to the first side in the first direction D1. The surface light source device 20, the first polarizing plate 16, the liquid crystal cell 17, and the second polarizing plate 18 are positioned in this order in the first direction D1. The first polarizing plate 16 is positioned between the liquid crystal cell 17 and the surface light source device 20 in the first direction D1. The liquid crystal cell 17 is positioned between the second polarizing plate 18 and the surface light source device 20 in the first direction D1.

[0048] The first side is the observer side. In the illustrated example, the second polarizing plate 18 constitutes the display surface 11. The first polarizing plate 16 constitutes the light incident side surface of the liquid crystal display panel 15.

[0049] The first polarizing plate 16 and the second polarizing plate 18 each transmit one linearly polarized light component and block the other linearly polarized light component. The vibration direction of one linearly polarized light component is perpendicular to the vibration direction of the other linearly polarized light component. The first polarizing plate 16 is also called a lower polarizing plate. The second polarizing plate 18 is also called an upper polarizing plate.

[0050] The first polarizing plate 16 and the second polarizing plate 18 may each include a polarizer. The polarizer transmits only the linearly polarized light component vibrating in a predetermined direction from natural light (unpolarized light), circularly polarized light, and elliptically polarized light. The polarizer may include a uniaxially stretched polyvinyl alcohol (PVA) film.

[0051] The liquid crystal cell 17 adjusts the polarization state of the light that passes through the first polarizing plate 16 and travels toward the second polarizing plate 18. The liquid crystal cell 17 may adjust the polarization state of the light by applying a voltage. The liquid crystal cell 17 may include electrodes, an alignment film, and a liquid crystal layer. The type of the liquid crystal cell 17 is not particularly limited. The type of the liquid crystal cell 17 may be various types. The liquid crystal cell 17 may be a TN type liquid crystal cell, a VA type liquid crystal cell, or an IPS type liquid crystal cell.

[0052] The liquid crystal cell 17 may contain multiple pixels. The liquid crystal cell 17 functions as a shutter that controls the transmission or shielding of light from the surface light source device 20 for each pixel. A pixel is the smallest unit that constitutes an image.

[0053] As shown in Figure 2, in many liquid crystal display panels 15, the first polarizing plate 16 and the second polarizing plate 18 are arranged in a crossed nicol state. In the crossed nicol state, the transmission axis of the first polarizing plate 16 is perpendicular to the transmission axis of the second polarizing plate 18.

[0054] The first polarizing plate 16 transmits linearly polarized light components that vibrate in a direction parallel to the transmission axis. The first polarizing plate 16 shields linearly polarized light components that vibrate in a direction perpendicular to the transmission axis. The first polarizing plate 16 may also shield light by absorption or reflection.

[0055] The second polarizing plate 18 transmits linearly polarized light components that vibrate in a direction parallel to the transmission axis. The second polarizing plate 18 shields linearly polarized light components that vibrate in a direction perpendicular to the transmission axis. The second polarizing plate 18 may also shield light by absorption or reflection.

[0056] As shown in the example in Figure 2, in many liquid crystal display panels 15, the second polarizing plate 18 transmits the first polarization component LP1 and blocks the second polarization component LP2. As shown in the example in Figure 2, in many liquid crystal display panels 15, the first polarizing plate 16 blocks the first polarization component LP1 and transmits the second polarization component LP2.

[0057] The first polarization component LP1 is a linearly polarized component that vibrates in a direction parallel to the reference plane. The second polarization component LP2 is a linearly polarized component that vibrates in a direction perpendicular to the reference plane. The reference plane is parallel to both the first direction D1 and the second direction D2. The reference plane is a plane parallel to the plane of paper in Figure 2. When the first polarization component LP1 travels parallel to the reference plane, it is a P wave incident on the first polarizer 16 and the second polarizer 18. When the second polarization component LP2 travels parallel to the reference plane, it is an S wave incident on the first polarizer 16 and the second polarizer 18.

[0058] In the illustrated example, the transmission axis of the second polarizer 18 is parallel to the second direction D2. The transmission axis of the second polarizer 18 is perpendicular to the third direction D3. In the illustrated example, the transmission axis of the first polarizer 16 is perpendicular to the second direction D2. The transmission axis of the first polarizer 16 is parallel to the third direction D3. In the illustrated example, the image light emitted from the display surface 11 to form an image is the first polarization component LP1.

[0059] Figures 15 and 16 show the reflectance of P-waves and S-waves. Figure 15 shows the reflectance when light is incident from a medium with a low refractive index to a medium with a high refractive index. Figure 15 shows the reflectance at the free end. Figure 16 shows the reflectance when light is incident from a medium with a high refractive index to a medium with a low refractive index. Figure 16 shows the reflectance at the fixed end. In the example shown in Figure 16, the reflectance becomes 100% when the angle of incidence is greater than or equal to the critical angle of total reflection.

[0060] As shown in Figures 15 and 16, the reflectivity of P-waves is independent of the angle of incidence and is less than or equal to the reflectivity of S-waves. When the angle of incidence is the Brewster angle, the reflectivity of P-waves is 0%. When the angle of incidence is the Brewster angle, the difference between the reflectivity of S-waves and P-waves is maximized. Light reflected from surfaces such as water, snow, and the ground contains more S-waves than P-waves.

[0061] The angle of incidence is the angle (°) between the direction normal to the plane of incidence and the direction of propagation of the incident light. The angle of incidence is between 0° and 90°.

[0062] To block S-waves reflected from surfaces such as water, snow, and the ground, the shielding axis (absorption axis, reflection axis, etc.) of the polarized sunglasses 5 is parallel to the horizontal direction when worn by the wearer. The transmission axis of the polarized sunglasses 5 is normally parallel to the vertical direction when worn by the wearer.

[0063] In the example shown in Figure 2, the image light emitted from the liquid crystal display device 10 is the first polarization component LP1. The first polarization component LP1 can pass through the polarized sunglasses 5. Therefore, the wearer of the polarized sunglasses 5 can observe the image displayed on the liquid crystal display device 10.

[0064] The surface light source device 20 will now be described.

[0065] As shown in Figure 4, the surface light source device 20 includes a light source 24 and an optical member 28. The optical member 28 adjusts the direction of propagation of light emitted from the light source 24. The optical member 28 may constitute the light-emitting surface 21. The optical member 28 includes a light guide plate 30 and an optical sheet 50. The light guide plate 30 and the optical sheet 50 are stacked in a first direction D1.

[0066] Light emitted from the light source 24 may be incident on the light guide plate 30. The light may also be incident on the light guide plate 30 from a direction nonparallel to the first direction D1, for example, from a direction perpendicular to the first direction D1. The light source 24 may be positioned to the side of the light guide plate 30. The light source 24 may face the light guide plate 30 from a direction nonparallel to the first direction D1, for example, from a direction perpendicular to the first direction D1. The surface light source device 20 may be configured as an edge light type.

[0067] As shown in Figure 4, the optical member 28 may further include a reflective sheet 70. The reflective sheet 70, the light guide plate 30, and the optical sheet 50 are positioned in this order in the first direction D1. The reflective sheet 70, the light guide plate 30, and the optical sheet 50 are positioned in this order from the second side to the first side in the first direction D1. The optical sheet 50 may constitute the light-emitting surface 21 of the surface light source device 20.

[0068] The first direction D1 is the stacking direction of the reflective sheet 70, the light guide plate 30, and the optical sheet 50. As shown in Figure 4, each of the reflective sheet 70, the light guide plate 30, and the optical sheet 50 may extend in a direction perpendicular to the first direction D1, which is the stacking direction. The reflective sheet 70, the light guide plate 30, and the optical sheet 50 may extend in the second direction D2 and the third direction D3, respectively. The reflective sheet 70, the light guide plate 30, and the optical sheet 50 may each be flat. As shown in Figure 4, the first direction D1 may also be the thickness direction of each of the reflective sheet 70, the light guide plate 30, and the optical sheet 50. The first direction D1 may also be parallel to the normal direction of each of the reflective sheet 70, the light guide plate 30, and the optical sheet 50.

[0069] As shown in Figure 3, the reflective sheet 70, the light guide plate 30, and the optical sheet 50 may have a rectangular shape when observed from the first direction D1. The reflective sheet 70, the light guide plate 30, and the optical sheet 50 may have a pair of edges extending in the second direction D2 and the third direction D3, respectively, when observed from the first direction D1. In the illustrated example, light emitted from the light source 24 is incident on the optical member 28 from the second side in the second direction D2.

[0070] The light source 24 will now be described. The light source 24 is not particularly limited. The light source 24 may be a fluorescent lamp such as a linear cold cathode tube. The light source 24 may also include light-emitting elements such as point-shaped LEDs (light-emitting diodes) or incandescent light bulbs. As shown in Figure 4, the light source 24 may include a plurality of point-shaped light-emitting elements 25 arranged in the third direction D3. The light-emitting elements 25 may be light-emitting diodes (LEDs). The light-emitting elements 25 may be positioned facing the light guide plate 30 from the side. In the illustrated example, the light-emitting elements 25 face the light guide plate 30 from the second side in the second direction D2. Unlike the illustrated example, the light emitted from the light-emitting elements 25 may be guided to the optical element 28 by a reflective member or the like.

[0071] The components of the optical member 28 will be described in the following order: the light guide plate 30, the optical sheet 50, and the reflective sheet 70.

[0072] As shown in Figure 5, the light guide plate 30 is plate-shaped. The light guide plate 30 may include a pair of main surfaces: a light-emitting surface 31 and a back surface 32. The light-emitting surface 31 may face the first side in the first direction D1. The light-emitting surface 31 may face the optical sheet 50. The back surface 32 may face the second side in the first direction D1. The back surface 32 may face the reflective sheet 70.

[0073] The light guide plate 30 may include a side surface located between the light-emitting surface 31 and the back surface 32. The side surface of the light guide plate 30 may face a direction perpendicular to the first direction D1.

[0074] The light guide plate 30 may include a light-receiving surface 33 and an opposite surface 34 as its sides. The light-receiving surface 33 and the opposite surface 34 may face the second direction D2. In the illustrated example, the light-receiving surface 33 faces the second side in the second direction D2. Light from the light source passes through the light-receiving surface 33 and enters the light guide plate 30. The light-emitting element 25 is positioned facing the light-receiving surface 33. The light-receiving surface 33 may be a surface parallel to both the first direction D1 and the third direction D3. The light-receiving surface 33 may also be perpendicular to the second direction D2. In Figure 5, the position of the light-receiving surface 33 facing the light-emitting element 25 that constitutes the light source 24 is shown.

[0075] In the illustrated example, the opposite surface 34 faces the first side in the second direction D2. Light entering the light guide plate 30 from the light-receiving surface 33 travels through the light guide plate 30 toward the opposite surface 34 facing the light-receiving surface 33 in the second direction D2, which is the light-guiding direction. The opposite surface 34 may be a surface parallel to both the first direction D1 and the third direction D3. The opposite surface 34 may also be perpendicular to the second direction D2.

[0076] In the illustrated example, the light guide plate 30 further includes a first side surface 35a and a second side surface 35b that extend between the light-receiving surface 33 and the opposite surface 34, as side surfaces located between the light-emitting surface 31 and the back surface 32. The first side surface 35a and the second side surface 35b face the third direction D3.

[0077] The light guide plate 30 primarily guides light incident from the light-receiving surface 33 in the second direction D2. The material constituting the light guide plate 30 may be a transparent material, such as a transparent resin material. The light guide plate 30 may also be transparent in the first direction D1. "Transparent" means that the total light transmittance is 50% or more, but may also be 80% or more, or 90% or more. The total light transmittance of the light guide plate 30 in the first direction D1 may be 50% or more, 80% or more, or 90% or more.

[0078] A light source that mimics the spectrum of the D65 standard light (hereinafter referred to as the D65 light source) is used to measure total light transmittance. Before measuring total light transmittance, the D65 light source is lit for 15 minutes to stabilize its output. The incident angle on the measurement sample when measuring total light transmittance is set to 0°. The test environment when measuring total light transmittance is set to a temperature of 23°C ± 2°C and a relative humidity of 50% ± 5%. The measurement sample is placed in the test environment for 16 hours before the start of the test. Other measurement conditions when measuring total light transmittance follow JIS K7361-1:1997.

[0079] The total light transmittance shall be the arithmetic mean of the five measured values. The five measured values ​​shall be taken at five different measurement locations on the optical sheet being evaluated. The five measurement locations shall be at least 10 mm apart from each other.

[0080] As shown in Figures 4 to 6, the back surface 32 may be configured as an uneven surface. The back surface 32 may include a plurality of inclined surfaces 37 and a plurality of connecting surfaces 39. As shown in Figures 5 and 6, the inclined surfaces 37 and connecting surfaces 39 may be arranged alternately in the second direction D2.

[0081] In the illustrated example, the inclined surface 37 is inclined with respect to the second direction D2 such that it approaches the light-emitting surface 31 on the first side in the second direction D2. That is, the distance between the light-emitting surface 31 and one of the inclined surfaces 37 along the first direction D1 gradually decreases toward the first side in the second direction D2.

[0082] Light guidance within the light guide plate 30 is due to total internal reflection at the pair of main surfaces of the light guide plate 30, namely the light-emitting surface 31 and the back surface 32. Reflection by the inclined surface 37 reduces the angle of incidence when the light enters the pair of main surfaces 31 and 32. When the angle of incidence to the pair of main surfaces 31 and 32 falls below the critical angle of total internal reflection, the light can exit the light guide plate 30. The inclined surface 37 functions as a light extraction element for extracting light from the light guide plate 30.

[0083] The connecting surface 39 may be inclined in the opposite direction to the inclined surface 37 with respect to the first direction D1, or it may extend along the third direction D3. By positioning the connecting surface 39 between two adjacent inclined surfaces 37, the thickness variation of the light guide plate 30 at each position in the second direction D2 can be reduced.

[0084] The back surface 32 may further include a flat surface 38 in addition to the inclined surface 37 and the connecting surface 39. The magnitude of the angle between the second direction D2 and the flat surface 38 may be smaller than the magnitude of the angle between the second direction D2 and the inclined surface 37. The magnitude of the angle between the second direction D2 and the flat surface 38 may be smaller than the magnitude of the angle between the first direction D1 and the connecting surface 39. As shown in Figures 4 and 6, the flat surface 38 may be parallel to the second direction D2. By providing the flat surface 38, the distribution of the inclined surface 37 along the second direction D2, which is the light guide direction, can be adjusted. By adjusting the distribution of the inclined surface 37, the distribution of the amount of light emitted from the light guide plate 30 along the second direction D2 can be adjusted.

[0085] The angle between the second direction D2 and the inclined surface 37 refers to the angle between the second direction D2 and the inclined surface 37 in a cross-section along both the first direction D1 and the second direction D2. The angle between the second direction D2 and the flat surface 38 refers to the angle between the second direction D2 and the flat surface 38 in a cross-section along both the first direction D1 and the second direction D2. The angle between the second direction D2 and the connecting surface 39 refers to the angle between the second direction D2 and the connecting surface 39 in a cross-section along both the first direction D1 and the second direction D2. These angles are between 0° and 90°. Figure 6 shows the angle θy between the second direction D2 and the inclined surface 37.

[0086] The angle θy between the second direction D2 and the inclined surface 37, that is, the inclined surface angle θy of the inclined surface 37 with respect to the second direction D2, may be 0.10° or more, 1.0° or more, or 1.5° or more. By setting a lower limit on the inclined surface angle θy, the efficiency of light extraction from the light guide plate 30 can be improved. The inclined surface angle θy of the inclined surface 37 with respect to the second direction D2 may be 5.0° or less, 3.0° or less, or 2.0° or less. By setting an upper limit on the inclined surface angle θy, the direction of propagation of light emitted from the light-emitting surface 31 of the light guide plate 30 can be restricted to a narrow angular range.

[0087] The angle θy between the second direction D2 and the inclined surface 37 may be 0.10° or more and 5.0° or less, 1.0° or more and 5.0° or less, or 1.5° or more and 5.0° or less. The angle θy between the second direction D2 and the inclined surface 37 may be 0.10° or more and 3.0° or less, 1.0° or more and 3.0° or less, or 1.5° or more and 3.0° or less. The angle θy between the second direction D2 and the inclined surface 37 may be 0.10° or more and 2.0° or less, 1.0° or more and 2.0° or less, or 1.5° or more and 2.0° or less.

[0088] The inclined surface 37 may also be a flat surface. The inclination angle θy of each inclined surface 37 may be constant within that inclined surface 37. The inclined surface 37 may include a curved surface. The inclined surface 37 may include a bent surface. The inclination angle θy of an inclined surface 37 that includes a curved surface or a bent surface is the inclination angle measured at the center position in the second direction D2.

[0089] In the example shown in Figure 4, the proportion of the inclined surface 37 on the back surface 32 increases as you move from the light-receiving surface 33 towards the opposite surface 34 along the second direction D2, that is, from the second side to the first side in the second direction D2. With this configuration, it is possible to promote the emission of light from the light guide plate 30 in the region spaced away from the light-receiving surface 33 along the light guide direction. It is possible to suppress the decrease in the amount of emitted light as you move away from the light-receiving surface 33. It is possible to make the illuminance distribution along the second direction D2 on the light-emitting surface 31 uniform.

[0090] As shown in Figure 5, the inclined surface 37 may extend in the third direction D3. The angle θy between the inclined surface 37 and the second direction D2 may be constant at each position in the third direction D3. The connecting surface 39 may extend in the third direction D3. The angle between the connecting surface 39 and the first direction D1 may be constant at each position in the second direction D2. The flat surface 38 may extend in the third direction D3. The angle between the flat surface 38 and the second direction D2 may be constant at each position in the third direction D3.

[0091] The light guide plate 30 is not limited to the configuration shown. The back surface 32 of the light guide plate 30 does not have to include a flat surface 38. The back surface 32 does not have to include a connecting surface 39.

[0092] The light guide plate 30 can be manufactured using extrusion molding, UV molding, injection molding, etc. Various materials can be used as the material for the light guide plate 30. The material for the light guide plate 30 may be a transparent resin mainly composed of one or more of the following: acrylic resin, polystyrene resin, polycarbonate resin, polyethylene terephthalate resin, polyacrylonitrile resin, etc. The light guide plate 30 may contain a base material portion of the resin material and a light-diffusing component dispersed in the base material portion. The light-diffusing component may be inorganic particles such as silica (silicon dioxide) or alumina (aluminum oxide). The light-diffusing component may also be organic particles such as acrylic resin, polycarbonate resin, or silicone resin.

[0093] As shown in Figure 7, the optical sheet 50 is plate-shaped. The optical sheet 50 may include a pair of main surfaces, a first surface 51 and a second surface 52. The first surface 51 may face the first side in the first direction D1. The second surface 52 may face the second side in the first direction D1. The second surface 52 may face the light guide plate 30. The first surface 51 may be the light-emitting side of the optical sheet 50. The second surface 52 may be the light-receiving side of the optical sheet 50. In the illustrated example, the second surface 52 is configured as a prism surface 53.

[0094] The optical sheet 50 may include a main body 55 and a unit prism 60. The main body 55 may be in the form of a sheet. The main body 55 may include a light-emitting side 55a and a light-receiving side 55b. As shown in the illustrated example, the light-emitting side 55a of the main body 55 may constitute the first surface 51 of the optical sheet 50. The first surface 51 may be a surface perpendicular to the first direction D1.

[0095] The first surface 51 may be a flat surface. The first surface 51 may be a matte surface. The first surface 51 may be an uneven surface. The first surface 51 may include linear protrusions and linear recesses extending in the second direction D2 for the purpose of adjusting the light intensity and brightness in the third direction D3. The linear protrusions and linear recesses extending in the second direction D2 may be arranged in the third direction D3.

[0096] Multiple unit prisms 60 may be provided on the light-receiving side 55b of the main body 55. Multiple unit prisms 60 may be provided without gaps on the light-receiving side 55b of the main body 55. Multiple unit prisms 60 may constitute the second surface 52 of the optical sheet 50. The unit prisms 60 may constitute a part of the second surface 52 of the optical sheet 50, or they may constitute the entire second surface 52 of the optical sheet 50. The prism surface 53 may be composed of multiple unit prisms 60.

[0097] A "unit prism" has the function of changing the direction of light by exerting optical effects such as refraction and reflection on the light. A "unit prism" is not distinguished from a "unit shape element," "unit optical element," and "unit lens" solely based on differences in name.

[0098] Multiple unit prisms 60 may be arranged in a second direction D2. Each unit prism 60 may extend linearly. Each unit prism 60 may extend in a straight line, as shown in the example in Figure 7. Each unit prism 60 may extend in a straight line in a third direction D3 perpendicular to the second direction D2, which is the arrangement direction, as shown in the example in Figure 7. Multiple unit prisms 60 may constitute a linear array prism, as shown in the example in Figure 7. Each unit prism 60 may be columnar.

[0099] The arrangement of the multiple unit prisms 60 is not limited to the example shown in Figure 7. The multiple unit prisms 60 may be arranged in two or more directions, including a second direction D2 and a direction nonparallel to the second direction D2. The multiple unit prisms 60 may constitute a two-dimensionally arranged microlens array.

[0100] Each unit prism 60 may include a first prism surface 61 and a second prism surface 62. The first prism surface 61 and the second prism surface 62 may face the second direction D2. The first prism surface 61 may be located on the first side in the second direction D2. The second prism surface 62 may be located on the second side in the second direction D2. The prism surface 53 may include the first prism surface 61 and the second prism surface 62. The prism surface 53 may be composed only of the first prism surface 61 and the second prism surface 62. The second prism surface 62 may function as an incident surface to which light emitted from the light guide plate 30 is incident. The first prism surface 61 may function as a reflecting surface that reflects light that has passed through the second prism surface 62 and is traveling through the unit prism 60.

[0101] As shown in the example in Figure 8, the first prism surface 61 and the second prism surface 62 may each extend from the main body 55. The first prism surface 61 and the second prism surface 62 may be connected to the main body 55 at one end. As shown in the example in Figure 8, the first prism surface 61 and the second prism surface 62 may be connected to each other at the other end. The first prism surface 61 and the second prism surface 62 may form the top 63 of the unit prism 60 furthest from the main body 55 in the third direction D3 at the other end.

[0102] In the illustrated example, in the main cross-section of the optical sheet 50 parallel to both the first direction D1 and the second direction D2, the first prism surface 61 and the second prism surface 62 approach each other as they move away from the main body 55. In the main cross-section of the optical sheet 50 shown in Figure 8, the unit prism 60 has a tapering cross-sectional shape.

[0103] In the example shown in Figure 7, the cross-sectional shape of the optical sheet 50 at the main cross-section is constant at each position in the third direction D3. The illustrated unit prism 60 has a constant cross-sectional shape and extends in the third direction D3. In the illustrated optical sheet 50, multiple unit prisms 60 have the same configuration. Not limited to the illustrated example, multiple unit prisms 60 may have different configurations. Not limited to the illustrated example, the cross-sectional shape of each unit prism 60 may differ at each position in the third direction D3.

[0104] As shown in Figure 8, the first prism surface 61 may be a bent surface. The first prism surface 61 may include a first element surface 66, a second element surface 67, and a third element surface 68. The first element surface 66, the second element surface 67, and the third element surface 68 may be connected in order. The first element surface 66, the second element surface 67, and the third element surface 68 may be arranged in this order from the apex 63 to the base end of the unit prism 60. The second element surface 67 may be located between the first element surface 66 and the third element surface 68. The first element surface 66 may constitute the apex 63 at one end. The first element surface 66 may be connected to one end of the second element surface 67 at the other end. The second element surface 67 may be connected to one end of the third element surface 68 at the other end. The third element surface 68 may be connected to the main body 55 at the other end.

[0105] In the example shown in Figure 8, the first prism surface 61 includes three elemental surfaces. The first prism surface 61 may include four or more elemental surfaces, or it may include only two elemental surfaces, or it may include only a single elemental surface. The first prism surface 61 may have a constant inclination angle throughout its entire surface.

[0106] The angle between the prism surface or element surface and the first direction D1 is defined as the inclination angle θx (°). The inclination angle θx (°) shall be between 0° and 90°. For prism surfaces or element surfaces that include a curved surface, the inclination angle shall be the angle measured at the center position of the prism surface or element surface in the second direction D2.

[0107] The inclination angle of the element surfaces included in the first prism surface 61 may decrease as the position of the element surface approaches the main body 55 from the top portion 63. In the illustrated example, the inclination angle θxe1 of the first element surface 66 may be greater than the inclination angle θxe2 of the second element surface 67. The inclination angle θxe2 of the second element surface 67 may be greater than the inclination angle θxe3 of the third element surface 68. With this configuration, the entire area of ​​the first prism surface 61 can be effectively utilized to perform optical path adjustment.

[0108] A lower limit may be set on the difference in inclination angle (°) between the first element surface inclination angle θxe1 and the third element surface inclination angle θxe3. By setting a lower limit on the difference in inclination angle (°), the light incident on the optical sheet 50 from the light guide plate 30 can be diffused by reflection at the first prism surface 61. By setting a lower limit on the difference in inclination angle (°), the viewing angle of the liquid crystal display device 10 can be effectively widened.

[0109] An upper limit may be set on the difference (°) between the tilt angle θxe1 of the first element surface and the tilt angle θxe3 of the third element surface. By setting an upper limit on the difference (°) of the tilt angle, excessive diffusion can be suppressed. By setting an upper limit on the difference (°) of the tilt angle, the light source can be used more efficiently for image formation in the liquid crystal display device 10.

[0110] The difference in inclination angle (°) between the first element surface 66 and the third element surface 68 may be 5° or more, 8° or more, 10° or more, or 12° or more. The difference in inclination angle (°) between the first element surface 66 and the third element surface 68 may be 20° or less, 18° or less, or 15° or less.

[0111] The difference in inclination angle (°) between the first element surface 66 and the third element surface 68 may be 5° or more and 20° or less, 8° or more and 20° or less, 10° or more and 20° or less, or 12° or more and 20° or less. The difference in inclination angle (°) between the first element surface 66 and the third element surface 68 may be 5° or more and 18° or less, 8° or more and 18° or less, 10° or more and 18° or less, or 12° or more and 18° or less. The difference in inclination angle (°) between the first element surface 66 and the third element surface 68 may be 5° or more and 15° or less, 8° or more and 15° or less, 10° or more and 15° or less, or 12° or more and 15° or less.

[0112] The inclination angle θxe1 of the first element surface may be 25° or more, 30° or more, or 35° or more. The inclination angle θxe1 of the first element surface may be 55° or less, 50° or less, or 45° or less.

[0113] The first element surface inclination angle θxe1 may be 25° or more and 55° or less, 30° or more and 55° or less, or 35° or more and 55° or less. The first element surface inclination angle θxe1 may be 25° or more and 50° or less, 30° or more and 50° or less, or 35° or more and 50° or less. The first element surface inclination angle θxe1 may be 25° or more and 45° or less, 30° or more and 45° or less, or 35° or more and 45° or less.

[0114] The inclination angle θxe2 of the second element surface may be 19° or greater, 24° or greater, or 29° or greater. The inclination angle θxe2 of the second element surface may be 49° or less, 44° or less, or 39° or less.

[0115] The second element surface inclination angle θxe2 may be 19° or more and 49° or less, 24° or more and 49° or less, or 29° or more and 49° or less. The second element surface inclination angle θxe2 may be 19° or more and 44° or less, 24° or more and 44° or less, or 29° or more and 44° or less. The second element surface inclination angle θxe2 may be 19° or more and 39° or less, 24° or more and 39° or less, or 29° or more and 39° or less.

[0116] The third element surface inclination angle θxe3 may be 12° or greater, 17° or greater, or 22° or greater. The third element surface inclination angle θxe3 may be 42° or less, 37° or less, or 32° or less.

[0117] The third element surface inclination angle θxe3 may be 12° or more and 42° or less, 17° or more and 42° or less, or 22° or more and 42° or less. The third element surface inclination angle θxe3 may be 12° or more and 37° or less, 17° or more and 37° or less, or 22° or more and 37° or less. The third element surface inclination angle θxe3 may be 12° or more and 32° or less, 17° or more and 32° or less, or 22° or more and 32° or less.

[0118] The inclination angle θx2 of the second prism surface 62 may be 20° or more, 25° or more, or 30° or more. The inclination angle θx2 of the second prism surface 62 may be 50° or less, 45° or less, or 40° or less.

[0119] The inclination angle θx2 of the second prism surface 62 may be 20° or more and 50° or less, 25° or more and 50° or less, or 30° or more and 50° or less. The inclination angle θx2 of the second prism surface 62 may be 20° or more and 45° or less, 25° or more and 45° or less, or 30° or more and 45° or less. The inclination angle θx2 of the second prism surface 62 may be 20° or more and 40° or less, 25° or more and 40° or less, or 30° or more and 40° or less.

[0120] The length Wx of the unit prism 60 in the second direction D2, that is, the width Wx of the unit prism 60, may be 12 μm or more, 14 μm or more, or 16 μm or more. The length Wx of the unit prism 60 in the second direction D2 may be 32 μm or less, 30 μm or less, or 28 μm or less.

[0121] The length Wx of the unit prism 60 in the second direction D2, i.e., the width Wx of the unit prism 60, may be 12 μm or more and 32 μm or less, 14 μm or more and 32 μm or less, or 16 μm or more and 32 μm or less. The length Wx of the unit prism 60 in the second direction D2, i.e., the width Wx of the unit prism 60, may be 12 μm or more and 30 μm or less, 14 μm or more and 30 μm or less, or 16 μm or more and 30 μm or less. The length Wx of the unit prism 60 in the second direction D2, i.e., the width Wx of the unit prism 60, may be 12 μm or more and 28 μm or less, 14 μm or more and 28 μm or less, or 16 μm or more and 28 μm or less.

[0122] The ratio of the length Wx1 of the first prism surface 61 in the second direction D2 to the length Wx of the unit prism 60 in the second direction D2 (Wx1 / Wx) may be 0.32 or greater, 0.37 or greater, or 0.42 or greater. The ratio of the length Wx1 of the first prism surface 61 in the second direction D2 to the length Wx of the unit prism 60 in the second direction D2 (Wx1 / Wx) may be 0.62 or less, 0.57 or less, or 0.52 or less.

[0123] The ratio of the length Wx1 of the first prism surface 61 in the second direction D2 to the length Wx of the unit prism 60 in the second direction D2 (Wx1 / Wx) may be 0.32 or more and 0.62 or less, 0.37 or more and 0.62 or less, or 0.42 or more and 0.62 or less. The ratio of the length Wx1 of the first prism surface 61 in the second direction D2 to the length Wx of the unit prism 60 in the second direction D2 (Wx1 / Wx) may be 0.32 or more and 0.57 or less, 0.37 or more and 0.57 or less, or 0.42 or more and 0.57 or less. The ratio of the length Wx1 of the first prism surface 61 in the second direction D2 to the length Wx of the unit prism 60 in the second direction D2 (Wx1 / Wx) may be 0.32 or more and 0.52 or less, 0.37 or more and 0.52 or less, or 0.42 or more and 0.52 or less.

[0124] The length Hx of the unit prism 60 in the first direction D1, that is, the height Hx of the unit prism 60, may be 8.0 μm or more, 10 μm or more, or 12 μm or more. The length Hx of the unit prism 60 in the first direction D1 may be 20 μm or less, 18 μm or less, or 16 μm or less.

[0125] The length Hx of the unit prism 60 in the first direction D1 may be 8.0 μm or more and 20 μm or less, 10 μm or more and 20 μm or less, or 12 μm or more and 20 μm or less. The length Hx of the unit prism 60 in the first direction D1 may be 8.0 μm or more and 18 μm or less, 10 μm or more and 18 μm or less, or 12 μm or more and 18 μm or less. The length Hx of the unit prism 60 in the first direction D1 may be 8.0 μm or more and 16 μm or less, 10 μm or more and 16 μm or less, or 12 μm or more and 16 μm or less.

[0126] The length of the optical sheet 50 in the first direction D1, that is, the thickness of the optical sheet 50, may be 115 μm or more, 125 μm or more, or 135 μm or more. The length of the optical sheet 50 in the first direction D1 may be 175 μm or less, 165 μm or less, or 155 μm or less.

[0127] The length of the optical sheet 50 in the first direction D1 may be 115 μm or more and 175 μm or less, 125 μm or more and 175 μm or less, or 135 μm or more and 175 μm or less. The length of the optical sheet 50 in the first direction D1 may be 115 μm or more and 165 μm or less, 125 μm or more and 165 μm or less, or 135 μm or more and 165 μm or less. The length of the optical sheet 50 in the first direction D1 may be 115 μm or more and 155 μm or less, 125 μm or more and 155 μm or less, or 135 μm or more and 155 μm or less.

[0128] The optical sheet 50 can be manufactured using extrusion molding, UV molding, injection molding, etc. Various materials can be used as the material for the optical sheet 50. The material for the optical sheet 50 may include a transparent resin mainly composed of one or more of the following: acrylic resin, polystyrene resin, polycarbonate resin, polyethylene terephthalate resin, polyacrylonitrile resin, etc. The material for the optical sheet 50 may also include a reactive resin (such as an ionizing radiation-curable resin) based on epoxy acrylate resin or urethane acrylate resin.

[0129] As shown in Figure 8, the optical sheet 50 may include a base film 57. The base film 57 functions as a substrate for supporting the unit prism 60.

[0130] The base film 57 may directly support the unit prism 60. The base film 57 may also be in contact with the unit prism 60.

[0131] As shown in Figure 8, the optical sheet 50 may include a land portion 59 between the base film 57 and the unit prism 60. The land portion 59 may be made of the same material as the unit prism 60. There may be no seam between the land portion 59 and the unit prism 60. The land portion 59 and the unit prism 60 may be integrally molded. The land portion 59 and the unit prism 60 may be integrally molded on the base film 57.

[0132] In the example shown in Figure 8, the base film 57 constitutes the first surface 51 of the optical sheet 50. The base film 57 constitutes the light-emitting side surface 55a of the main body portion 55. The land portion 59 constitutes the light-receiving side surface 55b of the main body portion 55.

[0133] The land portion 59 and the unit prism 60 may be formed by curing a coating applied to the base film 57. The land portion 59 and the unit prism 60 may also be cured products of an ionizing radiation-curable resin composition. The land portion 59 and the unit prism 60 may also be cured products of an ultraviolet-curable resin composition. The land portion 59 and the unit prism 60 may constitute a molded resin portion 58 connected to the base film 57.

[0134] The resin used in the base film 57 may be an olefin resin such as polyethylene or polypropylene. The resin used in the base film 57 may be a vinyl resin such as polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, ethylene-vinyl acetate copolymer, or ethylene-vinyl alcohol copolymer. The resin used in the base film 57 may be a polyester resin such as polyethylene terephthalate, polyethylene naphthalate, or polybutylene terephthalate. The resin used in the base film 57 may be an acrylic resin such as poly(meth)acrylate or poly(meth)acrylate ethyl. The resin used in the base film 57 may be a styrene resin such as polystyrene, a polyamide resin such as nylon 6 or nylon 66, or a cellulose resin such as triacetylcellulose. Further examples of resins used in the base film 57 include resins such as polycarbonate, polyimide resins, norbornene, and cycloolefin resins obtained from cycloolefins such as dicyclopentadiene. The base layer may contain only one of the above-mentioned resins, or it may contain two or more of the above-mentioned resins.

[0135] The base film 57 may have optical anisotropy. The base film 57 may have in-plane birefringence. The base film 57 may be a stretched resin film. The base film 57 may be a uniaxially oriented film, a simultaneously biaxially oriented film, or a sequentially biaxially oriented film. By having optical anisotropy in the base film 57, the polarization state of the light emitted from the light source 24 can be adjusted, as will be described later.

[0136] A lower limit may be set for the retardation (Re) of the base film 57. By setting a lower limit for the retardation (Re) of the base film 57, the polarization state of the light emitted from the light source 24 can be appropriately adjusted. The retardation of the base film 57 at a wavelength of 589 nm may be 800 nm or more, 810 nm or more, 1300 nm or more, 1900 nm or more, 2000 nm or more, 2100 nm or more, 2200 nm or more, 2500 nm or more, or 3200 nm or more.

[0137] An upper limit may be set for the retardation (Re) of the base film 57. By setting an upper limit for the retardation (Re) of the base film 57, the mechanical strength of the base film 57 can be appropriately ensured. By setting an upper limit for the retardation (Re) of the base film 57, tearing of the base film can be suppressed. The retardation of the base film 57 at a wavelength of 589 nm may be 12000 nm or less, 9000 nm or less, 8200 nm or less, 6000 nm or less, 4000 nm or less, or 3200 nm or less.

[0138] The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 12000 nm, 810 nm to 12000 nm, 1300 nm to 12000 nm, 1900 nm to 12000 nm, 2000 nm to 12000 nm, 2100 nm to 12000 nm, 2200 nm to 12000 nm, 2500 nm to 12000 nm, or 3200 nm to 12000 nm. The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 9000 nm, 810 nm to 9000 nm, 1300 nm to 9000 nm, 1900 nm to 9000 nm, 2000 nm to 9000 nm, 2100 nm to 9000 nm, 2200 nm to 9000 nm, 2500 nm to 9000 nm, or 3200 nm to 9000 nm. The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 8200 nm, 810 nm to 8200 nm, 1300 nm to 8200 nm, 1900 nm to 8200 nm, 2000 nm to 8200 nm, 2100 nm to 8200 nm, 2200 nm to 8200 nm, 2500 nm to 8200 nm, or 3200 nm to 8200 nm. The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 6000 nm, 810 nm to 6000 nm, 1300 nm to 6000 nm, 1900 nm to 6000 nm, 2000 nm to 6000 nm, 2100 nm to 6000 nm, 2200 nm to 6000 nm, 2500 nm to 6000 nm, or 3200 nm to 6000 nm.The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 4000 nm, 810 nm to 4000 nm, 1300 nm to 4000 nm, 1900 nm to 4000 nm, 2000 nm to 4000 nm, 2100 nm to 4000 nm, 2200 nm to 4000 nm, 2500 nm to 4000 nm, or 3200 nm to 4000 nm. The retardation of the substrate film 57 at a wavelength of 589 nm may be 800 nm to 3200 nm, 810 nm to 3200 nm, 1300 nm to 3200 nm, 1900 nm to 3200 nm, 2000 nm to 3200 nm, 2100 nm to 3200 nm, 2200 nm to 3200 nm, 2500 nm to 3200 nm, or 3200 nm or more and 3200 nm or less.

[0139] Retardation is also called "in-plane phase difference." Retardation is a value obtained by "(nx - ny) × T [nm]." The unit of retardation is "nm."

[0140] In the above formula for calculating retardation, "T" is the thickness [nm] of the base film 57.

[0141] In the above formula for calculating retardation, "nx" is the refractive index in the slow phase axis direction, which is the direction in which the refractive index is greatest within the plane of the base film 57. In the above formula for calculating retardation, "ny" is the refractive index in the fast phase axis direction, which is perpendicular to the slow phase axis direction within the plane of the base film 57.

[0142] Upper and lower limits may be set for the magnitude of the slow axis inclination angle θ57. As shown in Figure 9, the slow axis inclination angle θ57 is the smaller of the angles between the slow axis A57 of the base film 57 and the second direction D2 in the projection onto a plane perpendicular to the first direction D1. The slow axis inclination angle θ57 is a value between 0° and 90°. The magnitude of the slow axis inclination angle θ57 is the absolute value of the slow axis inclination angle θ57.

[0143] By setting upper and lower limits on the size of the slow axis tilt angle θ57, the polarization state of the light emitted from the light source 24 can be appropriately adjusted. By setting upper and lower limits on the size of the slow axis tilt angle θ57, retardation of the base film 57 can effectively affect the polarization state of the transmitted light. The size of the slow axis tilt angle θ57 may be 5° to 85°, 5° to 50°, 5° to 40° or 50° to 85°, 5° to 30° or 60° to 85°, 20° to 25° or 65° to 70°. The size of the slow axis tilt angle θ57 may be 7° to 83°, 7° to 40° or 50° to 83°, 7° to 30° or 60° to 83°, 20° to 25° or 65° to 70°.

[0144] The refractive indices nx and ny of the base film 57 are values ​​measured using Otsuka Electronics' retardation measuring device "RETS-100". Similarly, the slow axis of the base film 57 is determined using Otsuka Electronics' retardation measuring device "RETS-100".

[0145] The refractive index nx, refractive index ny, and the direction of the slow axis are determined by testing the base film 57 obtained by removing the land portion 59 and unit prism 60 from the optical sheet 50. Therefore, prior to determining the refractive index nx, refractive index ny, and the direction of the slow axis, the base film 57 is removed from the optical sheet 50.

[0146] The refractive index nx and refractive index ny, and the slow axis are determined using "RETS-100" in the following procedure (P1) to (P6).

[0147] (P1) Before starting the measurement, the light source is turned on for at least 60 minutes. Turning on the light source for 60 minutes stabilizes the output of the light source. After that, the rotational analyzer method is selected, and the θ mode (mode for angular phase difference measurement and Rth calculation) is selected. By selecting the θ mode, the stage becomes a tilting rotation stage.

[0148] (P2) Next, input the following measurement conditions into the measuring device. Retardation measurement range: Rotational analyzer measurement spot diameter: φ5 mm Tilt angle range: -40° to 40° Measurement wavelength range: 400 to 800 nm Also, input the "average refractive index N of the film" and the "film thickness". For the "average refractive index N of the film", input the average refractive index of the base film 57 to be measured. If the base film 57 is polyethylene terephthalate, input "1.617" as the "average refractive index N of the film". For the "film thickness", input the thickness of the base film 57 measured by the method described later.

[0149] (P3) Subsequently, background data is acquired with no sample placed in the measuring device. The measuring device is a closed system. Background data is acquired each time the light source is turned on.

[0150] (P4) Next, the samples are placed on the measuring device. When taking multiple samples from the base film 57 to be measured, the orientation of the multiple samples is aligned. Specifically, the alignment is done in the following procedure: An arrow indicating the second direction D2 and the first side in the second direction D2 of the base film 57 to be measured is attached to each of the multiple samples taken from the optical sheet 50. An arrow indicating the second direction D2 and the first side in the second direction D2 is attached to the sample stand on which the samples are sequentially placed. When placing multiple samples on the sample stand, the second direction D2 and the third direction D3 are made parallel to each other, and the first side in the second direction D2 and the first side in the third direction D3 are aligned to each other.

[0151] (P5) Start the measurement. First, the measuring device rotates the sample 360° to identify the slow axis. Next, the measuring device presents the options of a slow axis and a fast axis, so select the slow axis. With the slow axis selected, the measuring device rotates the sample stage around the slow axis and measures the retardation (Re). The measuring device measures the retardation (Re) at several wavelengths within the set measurement wavelength range (400 nm to 800 nm) at each 10° interval within the set tilt angle range (-40° to 40°). Of the obtained measurements, the measurement value at a tilt angle of 0° and a wavelength of 589 nm is taken as the retardation (Re) value of the sample being measured.

[0152] (P6) Next, select the command "Display three-dimensional refractive index". A data table will be displayed, but do not use this data table as the measurement result. Select the command "Settings button". Then, enter the "Average refractive index of the film N" and the "Film thickness".

[0153] For "Average refractive index N of the film," enter the average refractive index of the base film 57 to be measured. If the base film 57 is a polyethylene terephthalate film, enter "1.617" as the "Average refractive index N of the film." For "Film thickness," enter the thickness of the base film 57 as measured by the method described later.

[0154] Based on the above, the measurement results for refractive index nx and refractive index ny are obtained. The measuring device outputs measured values ​​for refractive index nx and refractive index ny at several wavelengths. Of the obtained measured values, the measured value at a wavelength of 589 nm is taken as the refractive index nx and refractive index ny values ​​for the sample being measured. In other words, retardation refers to the value at a wavelength of 589 nm.

[0155] The refractive index nx and refractive index ny are each measured at seven locations. The maximum and minimum values ​​are excluded from the seven measurements, and the average value of the remaining five measurements is calculated. The resulting average value is taken as the refractive index nx and refractive index ny.

[0156] Before starting measurements of refractive indices nx and ny, the sample to be measured is left in the measurement environment for 16 hours. The measurement environment is set to a temperature of 23°C ± 2°C and a relative humidity of 50% ± 5%.

[0157] The thickness of the base film 57, which does not have the unit prism 60 or the molded resin part 58 laminated on it, is determined from the measurements taken at 20 measurement points using a film thickness gauge. The largest measurement, the second largest measurement, the smallest measurement, and the second smallest measurement are excluded from the measurements at the 20 measurement points, and the arithmetic mean of the measurements at the remaining 16 measurement points is taken as the thickness of the base film 57.

[0158] Nikon's "DigiMicro" product may be used as the film thickness gauge. For Nikon's "DigiMicro," the "MS-5C" + "MH-15M" may be used as the "stand" + "main unit," and the "TC-101A" may be used as the counter.

[0159] The thickness of the substrate film 57 incorporated into the optical sheet 50 is determined using a scanning transmission electron microscope (STEM). A cross-section of the optical sheet 50 containing the substrate film 57 to be evaluated is observed with a scanning transmission electron microscope, and the thickness of the substrate film 57 is measured from the image observed with the scanning transmission electron microscope. To enable cross-sectional observation, a measurement sample with the cross-section of the substrate film 57 exposed is prepared. A microtome is used to prepare the sample. A microtome manufactured by Leica is given as an example. A scanning electron microscope, model number S4800 manufactured by Hitachi High-Tech Corporation, is given as an example.

[0160] The thickness of the object to be measured is determined at seven measurement points by cross-sectional observation using a scanning transmission electron microscope. The maximum and minimum measurement values ​​from the seven measurement points are excluded, and the average of the remaining five measurement points is taken as the thickness of the polyester film with the easy-adhesion layer. The seven measurement points are located in a straight line at a pitch of 10 μm.

[0161] Before starting the thickness measurement, the sample to be measured is placed in the measurement environment for 16 hours. The measurement environment is set to a temperature of 23°C ± 2°C and a relative humidity of 50% ± 5%.

[0162] The thickness of the base film 57 may be 10 μm or more, 20 μm or more, 30 μm or more, or 40 μm or more. The thickness of the base film 57 may be 350 μm or less, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, 130 μm or less, or 100 μm or less.

[0163] The thickness of the base film 57 may be 10 μm or more and 350 μm or less, 20 μm or more and 350 μm or less, 30 μm or more and 350 μm or less, or 40 μm or more and 350 μm or less. The thickness of the base film 57 may be 10 μm or more and 300 μm or less, 20 μm or more and 300 μm or less, 30 μm or more and 300 μm or less, or 40 μm or more and 300 μm or less. The thickness of the base film 57 may be 10 μm or more and 250 μm or less, 20 μm or more and 250 μm or less, 30 μm or more and 250 μm or less, or 40 μm or more and 250 μm or less. The thickness of the base film 57 may be 10 μm or more and 200 μm or less, 20 μm or more and 200 μm or less, 30 μm or more and 200 μm or less, or 40 μm or more and 200 μm or less. The thickness of the base film 57 may be 10 μm or more and 150 μm or less, 20 μm or more and 150 μm or less, 30 μm or more and 150 μm or less, or 40 μm or more and 150 μm or less. The thickness of the base film 57 may be 10 μm or more and 130 μm or less, 20 μm or more and 130 μm or less, 30 μm or more and 130 μm or less, or 40 μm or more and 130 μm or less. The thickness of the base film 57 may be 10 μm or more and 100 μm or less, 20 μm or more and 100 μm or less, 30 μm or more and 100 μm or less, or 40 μm or more and 100 μm or less.

[0164] The base film 57 may consist of only one layer or multiple layers. The base film 57 may also contain an acrylic resin film. The base film 57 may also contain a primer layer such as an easy-adhesion layer. One or more etching treatments such as sputtering, corona discharge, ultraviolet irradiation, electron beam irradiation, chemical conversion, oxidation, and undercoating treatments may be applied to the surface of the base film 57. These treatments can improve the adhesion between the base film 57 and layers adjacent to the base film 57 (for example, the molded resin portion 58 or the land portion 59). The surface of the base film 57 may be subjected to cleaning treatments such as solvent cleaning or ultrasonic cleaning.

[0165] Let's elaborate further on the reflective sheet 70.

[0166] As shown in Figures 2 to 5, the reflective sheet 70 is in the form of a sheet. The reflective sheet 70 reflects the light emitted from the back surface 32 of the light guide plate 30 and returns it to the light guide plate 30. The reflection by the reflective sheet 70 may be specular reflection, diffuse reflection, or anisotropic diffuse reflection.

[0167] A reflective sheet 70 having specular reflection function may include a metal layer 72. A reflective sheet 70 having diffuse reflection function may be a sheet containing inorganic particles such as titanium oxide or silicon oxide. A reflective sheet 70 having anisotropic diffuse reflection function may be a sheet containing light diffusion components having a longitudinal direction in an oriented state, or a sheet containing convex or concave portions having a longitudinal direction in an oriented state.

[0168] Light emitted from the light guide plate 30, including the inclined surface 37 described above, travels within a narrow angular range that is greatly inclined with respect to the first direction D1. In combination with this light guide plate 30, the reflective sheet 70 may include a metal layer 72, as shown in Figure 10A. The metal layer 72 can return light to the light guide plate 30 mainly by specular reflection. With a reflective sheet 70 including a metal layer 72, in combination with the light guide plate 30, including the inclined surface 37 described above, the direction of light emission from the light guide plate 30 can be made within a narrow angular range that is greatly inclined with respect to the first direction D1.

[0169] The metal layer 72 can specularly reflect incident light. The metal layer 72 may be formed, for example, by vapor deposition. The metal layer 72 may include a vapor-deposited film. The metal layer 72 may be a metal film having high reflectivity. The metal layer 72 may include a silver vapor-deposited film. The metal layer 72 may include an aluminum vapor-deposited film. The metal layer 72 contributes to improving the total light reflectivity of the reflective sheet 70.

[0170] As shown in Figure 10B, the reflective sheet 70 may include a plurality of metal layers 72. In the example shown in Figure 10B, the reflective sheet 70 includes a first metal layer 72A and a second metal layer 72B. The first metal layer 72A is located between the second metal layer 72B and the light guide plate 30 in a first direction D1. The first metal layer 72A may be a silver vapor-deposited film, and the second metal layer 72B may be an aluminum vapor-deposited film.

[0171] As shown in Figures 10A and 10B, the reflective sheet 70 may include a support 71 that supports the metal layer 72. The support 71 supports other layers included in the reflective sheet 70. The materials constituting the support 71 may be resin, metal, paper, etc. The support 71 may be composed of two or more materials. In the example shown in Figure 10A, the metal layer 72 is located between the support 71 and the light guide plate 30 in the first direction D1. In the example shown in Figure 10B, the first metal layer 72A and the second metal layer 72B are located between the support 71 and the light guide plate 30 in the first direction D1.

[0172] As shown in Figures 10A and 10B, the reflective sheet 70 may include a surface layer 73. The surface layer 73 forms the surface of the reflective sheet 70 that faces the light guide plate 30. For example, the surface layer 73 functions as an anti-adhesion layer. The surface layer 73 suppresses surface contact between the reflective sheet 70 and the light guide plate 30. The surface layer 73 suppresses the reflective sheet 70 from sticking to the light guide plate 30. The surface layer 73 may be a layer having irregularities on its surface. The surface layer 73 may have irregularities formed by embossing. The surface layer 73 may contain particles and a binder resin and have irregularities caused by the particles. In the example shown in Figure 10A, the surface layer 73 is located between the metal layer 72 and the light guide plate 30 in the first direction D1. In the example shown in Figure 10B, the surface layer 73 is located between the first metal layer 72A and the light guide plate 30 in the first direction D1.

[0173] In the example shown in Figure 10A, the reflective sheet 70 includes a support 71, a metal layer 72, and a surface layer 73 in that order. The surface layer 73 faces the light guide plate 30 in the first direction D1. The metal layer 72 is located between the support 71 and the surface layer 73 in the first direction D1.

[0174] In the example shown in Figure 10B, the reflective sheet 70 includes a support 71, a second metal layer 72B, a first metal layer 72A, and a surface layer 73 in that order. The surface layer 73 faces the light guide plate 30 in the first direction D1. The first metal layer 72A and the second metal layer 72B are located between the support 71 and the surface layer 73 in the first direction D1.

[0175] The operation of the illustrated liquid crystal display device 10, surface light source device 20, and optical element 28 will be explained.

[0176] As shown in Figure 4, light is emitted from the light-emitting element 25. Lights L41 and L42 pass through the light-receiving surface 33 and enter the light guide plate 30. As shown in Figure 4, lights L41 and L42 are reflected at the light-emitting surface 31 and the back surface 32 within the light guide plate 30. The reflection at the light-emitting surface 31 and the back surface 32 is total internal reflection due to the refractive index difference between the material making up the light guide plate 30 and air. Lights L41 and L42 repeatedly reflect at the light-emitting surface 41 and the back surface 42 and proceed in the second direction (light-guiding direction) D2 within the light guide plate 30.

[0177] In the illustrated example, the back surface 32 includes an inclined surface 37. The inclined surface 37 is inclined so as it moves from the light-receiving surface 33 toward the opposite surface 34, it approaches the light-emitting surface 31. The inclined surface 37, together with the connecting surface 39 and the flat surface 38, forms the back surface 32. In the illustrated example, the connecting surface 39 extends in the first direction D1. Most of the light L41 and L42 traveling through the light guide plate 30 toward the first side in the second direction D2 enters the inclined surface 37 or the flat surface 38 of the back surface 32 without entering the connecting surface 39.

[0178] As shown in Figure 6, the incident angle θa2 when light L61 is incident on the inclined surface 37 is smaller by the inclination angle θy of the inclined surface 37 than the incident angle θa1 when the light L61 was incident on the light-emitting surface 31 immediately before. Next, the incident angle θa3 when light L61 is re-incident on the light-emitting surface 31 is smaller by the inclination angle θy of the inclined surface 37 than the incident angle θa2 when the light L61 was incident on the inclined surface 37 immediately before. In other words, by reflection from the inclined surface 37, the incident angle of the light decreases by the inclination angle θy of the inclined surface 37 each time it is reflected from the light-emitting surface 31 and the back surface 32. As a result, the incident angle of light L41, L42, and L61 traveling through the light guide plate 30 to the light-emitting surface 31 decreases by twice the angle θy of the inclined surface each time it is reflected from the inclined surface 37.

[0179] As described above, the angle of incidence of light to the light-emitting surface 31 and the back surface 32 gradually decreases due to reflection from the inclined surface 37, and becomes less than the critical angle of total internal reflection. Light incident on the light-emitting surface 31 or the back surface 32 at an angle of incidence less than the critical angle of total internal reflection can be emitted from the light guide plate 30. Light L41 and L42 emitted from the light-emitting surface 31 are directed toward the optical sheet 50 located on the light-emitting side of the light guide plate 30. On the other hand, light L32 emitted from the back surface 32 is reflected by the reflective sheet 70 located on the back surface of the light guide plate 30 and can be re-incidentated into the light guide plate 30.

[0180] In the illustrated example, as you move from the light-receiving surface 33 towards the opposite surface 34 along the light-guiding direction, the proportion occupied by the inclined surface 37 of the back surface 32 increases. Therefore, in the region of the light guide plate 30 close to the light-receiving surface 33, where the amount of emitted light tends to be high, it is possible to suppress the amount of emitted light from the light-emitting surface 31 of the light guide plate 30 from becoming too high. In the region of the light guide plate 30 away from the light-receiving surface 33, where the amount of emitted light tends to be low, a sufficient amount of emitted light from the light-emitting surface 31 of the light guide plate 30 can be ensured. As a result, the amount of emitted light along the second direction D2, which is the light-guiding direction, can be made uniform.

[0181] The emission angle θk of the light emitted from the light guide plate 30 becomes large, as shown in Figure 6, because the light had previously been traveling mainly in the second direction D2 within the light guide plate 30. The emission angle θk falls within a narrow angular range where the angle is relatively large. The light emitted from the light guide plate 30 travels in a direction within a narrow angular range that is significantly inclined with respect to the first direction D1.

[0182] The emission angle is the angle (°) between the direction of the normal to the emission surface and the direction of propagation of the emitted light. The emission angle is between 0° and 90°.

[0183] Figure 11 shows the angular distribution of luminance. Figure 11 shows the angular distribution of luminance caused by the first polarization component LP1 traveling from the light guide plate 30 to the optical sheet 50, and the angular distribution of luminance caused by the second polarization component LP2 traveling from the light guide plate 30 to the optical sheet 50. The luminance caused by the first polarization component LP1 traveling from the light guide plate 30 to the optical sheet 50 is the luminance caused by the first polarization component LP1 incident on the second surface of the optical sheet 50. The luminance caused by the second polarization component LP2 traveling from the light guide plate 30 to the optical sheet 50 is the luminance caused by the second polarization component LP2 incident on the second surface of the optical sheet 50. As shown in Figure 2, the first polarization component LP1 is a linearly polarized component that vibrates in a direction parallel to the reference plane. The second polarization component LP1 is a linearly polarized component that vibrates in a direction perpendicular to the reference plane. The reference plane is a plane parallel to both the first direction D1 and the second direction D2.

[0184] The luminance in the luminance angle distribution shown in Figure 11 is measured after removing the optical sheet 50 in the surface light source device 20 and the components of the surface light source device 20 located between the optical sheet 50 and the light-emitting surface 21. In the example shown in Figure 2, the optical sheet 50 constitutes the light-emitting surface 21. In the example shown in Figure 2, only the optical sheet 50 is removed from the surface light source device 20.

[0185] The luminance due to the first polarization component LP1 is measured with a polarizer superimposed on the light-receiving part of the luminance measuring device. The luminance due to the first polarization component LP1 is measured via a flat polarizer. When measuring the luminance due to the first polarization component LP1, the polarizer is positioned so that its transmission axis is parallel to the reference plane.

[0186] The luminance due to the second polarization component LP2 is measured with a polarizer superimposed on the light-receiving part of the luminance measuring device. The luminance due to the second polarization component LP2 is measured via a flat polarizer. When measuring the luminance due to the second polarization component LP2, the polarizer is positioned so that its transmission axis is perpendicular to the reference plane.

[0187] The luminance caused by the first polarization component LP1 and the luminance caused by the second polarization component LP2 are measured under the same conditions, although the polarizer orientations are different. The polarizer used to measure the luminance caused by the first polarization component LP1 is the same as the polarizer used to measure the luminance caused by the second polarization component LP2.

[0188] The luminance angle distribution shown in Figure 11, which is caused by each polarization component, shows the luminance in each direction within a reference plane parallel to both the first direction D1 and the second direction D2. In the graph shown in Figure 11, the vertical axis represents luminance. The unit of luminance is (lm / Sr / m ２ ) or "nit". In the graph shown in Figure 11, the angle of the direction in which the brightness was measured, as indicated on the horizontal axis, is defined as the angle tilted towards the first side from the first direction D1 to the second direction D2, with the positive value being the positive value.

[0189] To obtain the luminance angle distribution shown in Figure 11, the luminance is measured in a direction inclined by k (°) with respect to the first direction D1. k is an integer between -80 and 80. That is, when obtaining the luminance angle distribution, the direction in which the luminance is measured is inclined with respect to the first direction D1 at angles of -80° to 80° in 1° increments. The luminance in a direction inclined by 0° with respect to the first direction D1 represents the luminance at the light-emitting surface 31 in the direction of the first direction D1. In the surface light source device 20 shown in Figure 2, the luminance at the light-emitting surface of the light guide plate 30 is measured and the first ratio is calculated.

[0190] For a specific example of a surface light source device employing the shapes and dimensions described above, the luminance angle distribution shown in Figure 11 was obtained.

[0191] With the light guide plate 30 including the inclined surface 37, the angle at which maximum brightness can be obtained for both the first polarization component LP1 and the second polarization component LP2 can be set to 60° to 85° and 70° to 80°.

[0192] With the light guide plate 30 including the inclined surface 37, the half width at half maximum in the luminance angle distribution can be made between 5° and 25°, and between 5° and 15°. The half width at half maximum is the magnitude of the angle at which the direction in which half the luminance of the maximum luminance is obtained, located between the first direction D1 and the direction in which the maximum luminance is obtained, is inclined from the direction in which the maximum luminance is obtained to the other side in the second direction D2.

[0193] Light emitted from the light guide plate 30 then enters the optical sheet 50. The optical sheet 50 includes a unit prism 60 whose top portion 63 protrudes toward the light guide plate 30. In the illustrated example, the longitudinal direction of the unit prism 60 is perpendicular to the second direction D2.

[0194] As shown in Figures 4 and 6, the light rays L41, L42, and L61 heading toward the optical sheet 50 pass through the second prism surface 62 of the unit prism 60 and enter the unit prism 60. As shown in Figures 4 and 6, the light rays L41, L42, and L61 are reflected by the first prism surface 61, which faces the second direction D2 and the second prism surface 62, thereby changing their direction of propagation. The reflection at the first prism surface 61 may be total internal reflection. The light rays L41, L42, and L61 are bent by the reflection at the first prism surface 61 so that the angle their direction of propagation makes with respect to the first direction D1 becomes smaller.

[0195] The optical sheet 50 raises the direction of light propagation, which is tilted significantly in the first direction D1 by reflection at the unit prism 60, so that it aligns with the first direction D1. In combination with the light guide plate 30 which includes the inclined surface 37, the optical sheet 50 can exhibit excellent optical path adjustment functionality.

[0196] Light whose optical path has been adjusted by the unit prism 60 is directed toward the first surface 51 of the optical sheet 50. In the illustrated example, the first surface 51 constitutes the light-emitting surface 21 of the surface light source device 20. As shown in Figure 2, light passes through the light-emitting surface 21 and is emitted from the optical member 28 and the optical sheet 50. The light emitted from the light-emitting surface 21 illuminates the liquid crystal display panel 15 from behind. The liquid crystal display panel 15 forms an image on the display surface 11 by selectively transmitting light from the surface light source device 20 to each pixel. An observer can observe the image on the display surface 11.

[0197] Incidentally, as shown in Figures 15 and 16 already referenced, the reflectivity of P-waves and S-waves are different. At any angle of incidence, the reflectivity of P-waves is less than or equal to that of S-waves. When light traveling through the light guide plate 30 is incident on the light-emitting surface 31 at an angle of incidence less than the critical angle of total internal reflection, the second polarization component LP2 is reflected at the light-emitting surface 31 with a higher reflectivity than the first polarization component LP1. The light emitted from the light guide plate 30 toward the optical sheet 50 contains more of the first polarization component LP1 than the second polarization component LP2, as can be understood from the brightness angle distribution in Figure 11.

[0198] Even when light emitted from the light guide plate 30 is incident on the optical sheet 50, the second polarization component LP2 is reflected at the second surface 52 with a higher reflectivity than the first polarization component LP1. Considering the utilization efficiency of the light emitted from the light source 24, it is preferable that the light traveling from the light guide plate 30 to the optical sheet 50 contains a larger amount of the first polarization component LP1.

[0199] A lower limit may be set for the first ratio of the first polarization component LP1 incident on the second surface 52 of the optical sheet 50 to the second polarization component LP2. By setting a lower limit for the first ratio, the reflectance on the second surface 52 of the optical sheet 50 can be reduced. By reducing the reflectance, reflection loss can be suppressed. By reducing the reflectance, the utilization efficiency of the light emitted from the light source 24 can be improved. The first ratio may be 1.30 or higher, 1.35 or higher, 1.36 or higher, or 1.40 or higher.

[0200] The first ratio may be 1.30 or more and 2.00 or less, 1.35 or more and 2.00 or less, 1.36 or more and 2.00 or less, or 1.40 or more and 2.00 or less.

[0201] The first ratio is the ratio of the first polarization component LP1 incident on the second surface 52 of the optical sheet 50 to the second polarization component LP2 incident on the second surface 52 of the optical sheet 50. The first ratio is obtained by dividing the maximum brightness attributable to the first polarization component LP1 by the maximum brightness attributable to the second polarization component LP2. The first ratio has no units.

[0202] The maximum luminance attributable to the first polarization component LP1 is identified as the maximum luminance in the luminance angle distribution attributable to the first polarization component LP1 extending from the light guide plate 30 to the optical sheet 50. The luminance angle distribution attributable to the first polarization component LP1 extending from the light guide plate 30 to the optical sheet 50 is measured by the method already described with reference to Figure 11. That is, the luminance angle distribution attributable to the first polarization component LP1 is the luminance angle distribution measured via a polarizer placed in superimposed on the light receiving section of the luminance measuring device. The luminance is measured with the optical sheet 50 and the components located on the light-emitting surface 21 side of the optical sheet 50 removed from the surface light source device to be evaluated. The polarizer is positioned such that its transmission axis is parallel to a reference plane parallel to both the first direction D1 and the second direction D2.

[0203] The maximum luminance attributable to the second polarization component LP2 is identified as the maximum luminance in the luminance angle distribution attributable to the second polarization component LP2 from the light guide plate 30 toward the optical sheet 50. The luminance angle distribution attributable to the second polarization component LP2 from the light guide plate 30 toward the optical sheet 50 is measured by the method already described with reference to Figure 11. That is, the luminance angle distribution attributable to the second polarization component LP2 is the luminance angle distribution measured via a polarizer placed in superimposed on the light receiving section of the luminance measuring device. The luminance is measured with the optical sheet 50 and the components located on the light-emitting surface 21 side of the optical sheet 50 removed from the surface light source device to be evaluated. The polarizer is positioned such that its transmission axis is perpendicular to both the first direction D1 and the second direction D2.

[0204] The reflectance and Brewster angle shown in Figures 15 and 16 can be adjusted by the refractive index of the light guide plate 30 and the shape of the light-emitting surface 31 and the back surface 32 of the light guide plate 30. Therefore, the first ratio can be adjusted by the refractive index of the light guide plate 30 and the shape of the light-emitting surface 31 and the back surface 32 of the light guide plate 30.

[0205] As shown in Figure 2, light transmitted through the optical sheet 50 is incident on the liquid crystal display panel 15. The liquid crystal display panel 15 includes a first polarizing plate 16, a liquid crystal cell 17, and a second polarizing plate 18, arranged from the incident side to the exit side in the first direction D1. As described above, in many liquid crystal display panels 15, the first polarizing plate 16 shields the first polarization component LP1 and transmits the second polarization component LP2. The second polarizing plate 18, which is arranged in a crossed nicol state with the first polarizing plate 16, usually transmits the first polarization component LP1 and shields the second polarization component LP2.

[0206] The liquid crystal cell 17 adjusts the polarization state of light that passes through the first polarizing plate 16 and heads toward the second polarizing plate 18 for each pixel. Light whose direction of vibration of linear polarization is maintained in the liquid crystal cell 17 is blocked by the second polarizing plate 18. Light whose direction of vibration of linear polarization is converted in the liquid crystal cell 17 can pass through the second polarizing plate 18. The light that passes through the second polarizing plate 18 becomes image light that forms an image observable by the observer. The liquid crystal cell 17 can form a desired image by controlling the polarization state for each pixel.

[0207] The light transmitted through the second polarizing plate 18 is the first polarization component LP1. The image light is mainly composed of the first polarization component LP1. As shown in Figure 2, the image light can pass through the polarizing sunglasses 5 worn by the observer. The observer wearing the polarizing sunglasses 5 can observe the image.

[0208] As described above, the light incident on the second surface (light-receiving side) 52 of the optical sheet 50 contains more of the first polarization component LP1 than the second polarization component LP2. However, the first polarization component LP1 is blocked by the first polarizing plate 16 of the liquid crystal display panel 15.

[0209] The ratio of the first polarization component LP1 emitted from the first surface (light-emitting side) 51 of the optical sheet 50 to the second polarization component LP2 is defined as the second ratio. The second ratio may be less than the first ratio of the first polarization component LP1 incident on the second surface 52 of the optical sheet 50 to the second polarization component LP2. By setting the second ratio to less than the first ratio, the utilization efficiency of the light emitted from the light source 24 can be improved.

[0210] From the viewpoint of improving the utilization efficiency of the light emitted from the light source 24, an upper limit may be set for the second ratio. By setting an upper limit for the second ratio, the transmittance of the first polarizing plate 16 can be increased. By setting an upper limit for the second ratio, the shielding rate of the first polarizing plate 16 can be reduced. By setting an upper limit for the second ratio, the utilization efficiency of the light emitted from the light source 24 can be improved. The second ratio may be 1.24 or less, 1.23 or less, 1.22 or less, 1.21 or less, 1.20 or less, 1.19 or less, 1.18 or less, 1.16 or less, 1.10 or less, or 1.08 or less.

[0211] The second ratio may be 1.00 or more and 1.24 or less, 1.00 or more and 1.23 or less, 1.00 or more and 1.22 or less, 1.00 or more and 1.21 or less, 1.00 or more and 1.20 or less, 1.00 or more and 1.19 or less, 1.00 or more and 1.18 or less, 1.00 or more and 1.16 or less, 1.00 or more and 1.10 or less, or 1.00 or more and 1.08 or less. The second ratio may be greater than 1.00 and 1.24 or less, greater than 1.00 and 1.23 or less, greater than 1.00 and 1.22 or less, greater than 1.00 and 1.21 or less, greater than 1.00 and 1.20 or less, greater than 1.00 and 1.19 or less, greater than 1.00 and 1.18 or less, greater than 1.00 and 1.16 or less, greater than 1.00 and 1.10 or less, and greater than 1.00 and 1.08 or less.

[0212] The second ratio is the ratio of the first polarization component LP1 emitted from the first surface 51 of the optical sheet 50 to the second polarization component LP2 emitted from the first surface 51 of the optical sheet 50. The second ratio is obtained by dividing the maximum brightness attributable to the first polarization component LP1 by the maximum brightness attributable to the second polarization component LP2. The second ratio has no units.

[0213] The maximum brightness attributable to the first polarization component LP1 is measured from the brightness angle distribution attributable to the first polarization component LP1 emitted from the optical sheet 50. The brightness angle distribution attributable to the first polarization component LP1 emitted from the optical sheet 50 is the brightness angle distribution measured via a polarizer placed in superimposed on the light-receiving section of the brightness measuring device. The brightness is measured with the components located on the light-emitting surface 21 side of the optical sheet 50 removed from the surface light source device under evaluation. The polarizer is positioned such that its transmission axis is parallel to a reference plane parallel to both the first direction D1 and the second direction D2.

[0214] The maximum brightness due to the second polarization component LP2 is measured from the brightness angle distribution due to the second polarization component LP2 emitted from the optical sheet 50. The brightness angle distribution due to the second polarization component LP2 emitted from the optical sheet 50 is the brightness angle distribution measured via a polarizer placed in superimposed on the light-receiving section of the brightness measuring device. The brightness is measured with the components located on the light-emitting surface 21 side of the optical sheet 50 removed from the surface light source device under evaluation. The polarizer is positioned such that its transmission axis is perpendicular to a reference plane parallel to both the first direction D1 and the second direction D2.

[0215] Figure 12 shows an example of the angular distribution of luminance, and Figure 13 shows another example of the angular distribution of luminance. Figures 12 and 13 show the angular distribution of luminance caused by the first polarization component LP1 emitted from the optical sheet 50, and the angular distribution of luminance caused by the second polarization component LP2 emitted from the optical sheet 50.

[0216] The luminance in the luminance angle distribution shown in Figures 12 and 13 is measured after removing the components of the surface light source device 20 located between the optical sheet 50 and the light-emitting surface 21. In the example shown in Figure 2, the optical sheet 50 constitutes the light-emitting surface 21. In the example shown in Figure 2, the luminance is measured without removing the components of the surface light source device 20.

[0217] The luminance due to the first polarization component LP1 is measured with a polarizer superimposed on the light-receiving part of the luminance measuring device. The luminance due to the first polarization component LP1 is measured via a flat polarizer. When measuring the luminance due to the first polarization component LP1, the polarizer is positioned so that its transmission axis is parallel to the reference plane.

[0218] The luminance due to the second polarization component LP2 is measured with a polarizer superimposed on the light-receiving part of the luminance measuring device. The luminance due to the second polarization component LP2 is measured via a flat polarizer. When measuring the luminance due to the second polarization component LP2, the polarizer is positioned so that its transmission axis is perpendicular to the reference plane.

[0219] The luminance caused by the first polarization component LP1 and the luminance caused by the second polarization component LP2 are measured under the same conditions, although the polarizer orientations are different. The polarizer used to measure the luminance caused by the first polarization component LP1 is the same as the polarizer used to measure the luminance caused by the second polarization component LP2.

[0220] The luminance angle distributions shown in Figures 12 and 13, which are caused by each polarization component, show the luminance in each direction within a reference plane parallel to both the first direction D1 and the second direction D2. In the graphs shown in Figures 12 and 13, the vertical axis represents luminance. The unit of luminance is (lm / Sr / m²). ２ ) or "nit". In the graphs shown in Figures 12 and 13, the angle of the direction in which the brightness was measured, as indicated on the horizontal axis, is defined as the angle tilted towards the first side from the first direction D1 to the second direction D2, with the positive value being the positive value.

[0221] To obtain the luminance angle distribution, the luminance at the light-emitting surface of the polarizer is measured in a direction tilted by k (°) with respect to the first direction D1. k is an integer between -80 and 80. That is, when obtaining the luminance angle distribution, the direction in which the luminance is measured is tilted with respect to the first direction D1 at angles between -80° and 80° in 1° increments. The luminance at the first surface 51 of the optical sheet 50 in a direction tilted 0° with respect to the first direction D1 represents the luminance at the light-emitting side surface 51 in the direction of the first direction D1.

[0222] For a surface light source device employing the shapes and dimensions described above, the luminance angle distributions shown in Figures 12 and 13 were obtained.

[0223] The inventors of this invention found that the polarization state of light transmitted through the optical sheet 50 could be adjusted by the base film 57 contained in the optical sheet 50. The second ratio can be adjusted by the retardation (Re) and the slow axis tilt angle θ57 of the base film 57. More specifically, by setting the above-mentioned lower limit for the retardation (Re) of the base film 57 and the above-mentioned upper and lower limits for the slow axis tilt angle θ57 of the base film 57, the second ratio can be made smaller than the first ratio. By setting the above-mentioned lower limit for the retardation (Re) of the base film 57 and the above-mentioned upper and lower limits for the slow axis tilt angle θ57 of the base film 57, the second ratio could be made 1.24 or less.

[0224] By setting the retardation (Re) of the base film 57 to 800 nm or more, the polarization state of linearly polarized light transmitted through the base film 57 can be disrupted. With retardation of 800 nm or more, the light of the first polarization component can be converted into the second polarization component. The base film 57 imparts a sufficiently large phase difference to the transmitted light compared to the wavelength of the transmitted light, so that the ratio of the first polarization component LP1 to the second polarization component LP2 approaches 1. In other words, by transmitting through the optical sheet 50, the ratio of the first polarization component LP1 to the second polarization component LP2 is reduced.

[0225] In addition, retardation of 800 nm or more, which is greater than the wavelength of transmitted light, causes the second polarization component LP2 emitted from the optical sheet 50 to contain various wavelengths. Therefore, the color tinting of the image light can be effectively suppressed, and the deterioration of the color reproducibility of the liquid crystal display device 10 can be suppressed.

[0226] When the slow axis tilt angle θ57 is between 5° and 85°, the slow axis A57 of the base film 57 is tilted with respect to the vibration direction of the linearly polarized component. By tilting the slow axis A57 of the base film 57 with respect to the vibration direction of the linearly polarized component, retardation of the base film 57 can effectively influence the polarization state of the linearly polarized component.

[0227] In the embodiment described above, the optical member 28 includes a light guide plate 30 and an optical sheet 50 superimposed on the light guide plate 30 in a first direction D1. The light guide plate 30 includes a light-emitting surface 31, a back surface 32 facing the light-emitting surface 31, and a light-receiving surface 33 and an opposite surface 34 located between the light-emitting surface 31 and the back surface 32. The light-receiving surface 33 and the opposite surface 34 face a second direction D2 which is non-parallel to the first direction D1. The optical sheet 50 includes a first surface 51 and a second surface 52 facing the first surface 51. The optical sheet 50 includes a plurality of unit prisms 60 arranged in the second direction D2 to constitute the second surface 52. The back surface 32, the light-emitting surface 31, the second surface 52, and the first surface 51 are located in this order in the first direction D1.

[0228] In the first optical member 28 according to this embodiment, the second ratio of the first polarization component LP1 emitted from the first surface 51 of the optical sheet 50 to the second polarization component LP2 is 1.24 or less. The first ratio of the first polarization component LP1 incident on the second surface 52 to the second polarization component LP2 is 1.30 or more. The first polarization component LP1 is a linearly polarized component that vibrates in a direction parallel to a reference plane that is parallel to both the first direction D1 and the second direction D2. The second polarization component LP2 is a linearly polarized component that vibrates in a direction perpendicular to the reference plane.

[0229] In the second optical member 28 according to this embodiment, the optical sheet 50 includes a base film 57. The retardation (Re) of the base film 57 is 800 nm or more. In projection onto a plane perpendicular to the first direction D1, the magnitude of the angle (lagging axis inclination angle) θ57 between the lagging axis A57 of the base film 57 and the second direction D2 is 5° or more and 85° or less.

[0230] According to the first and second optical members 28 of this embodiment, the reflectance of the optical sheet 50 can be reduced for light traveling from the light guide plate 30 to the optical sheet 50. In addition, the transmittance of the first polarizing plate 16 of the liquid crystal display device 10 can be increased for light incident from the optical member 28 to the liquid crystal display device 10. Therefore, the utilization efficiency of light emitted from the light source 24 can be increased.

[0231] According to experimental results conducted by the inventors of this invention, the second ratio could be significantly reduced relative to the first ratio by increasing the retardation (Re) of the base film 57, specifically by setting it to 800 nm or more. By increasing the retardation (Re) of the base film 57 to a certain extent, specifically by setting it to 3200 nm or more, the second ratio was more strongly influenced by the slow axis tilt angle than by the retardation (Re). From the viewpoint of reducing the second ratio, the retardation (Re) may be set to 2000 nm or more and 3200 nm or less, or to 2100 nm or more and 3200 nm or less.

[0232] Here, we will explain some of the experimental results obtained by the inventors of this case.

[0233] Liquid crystal display devices of Examples 1 to 15 and Comparative Examples 1 to 12 were prepared.

[0234] Comparative Example 1 was a liquid crystal display device incorporated into a commercially available notebook computer. The liquid crystal display device according to Comparative Example 1 had the configuration shown in Figure 2. That is, the liquid crystal display device included a liquid crystal display panel and a surface light source device. The surface light source device included a light source and optical components. The liquid crystal display panel included a first polarizing plate, a liquid crystal cell, and a second polarizing plate. The optical components included a reflective sheet, a light guide plate, and an optical sheet. The optical sheet included a main body and a unit prism. The main body of the optical sheet included a base film. For Comparative Examples 1 to 3, Comparative Examples 5 to 12, and Examples 1 to 15, the base film was a biaxially oriented polyethylene terephthalate film. For Comparative Example 4, the base film was polycarbonate.

[0235] The liquid crystal display devices according to Comparative Examples 2 to 12 and Examples 1 to 15 had the same configuration as the liquid crystal display device according to Comparative Example 1, except for the base film included in the optical sheet. In the liquid crystal display devices according to Comparative Examples 2 to 12 and Examples 1 to 15, the magnitude of the retardation and / or slow axis tilt angle θ57 of the base film was made different from that of Comparative Example 1.

[0236] The retardation for the substrate films of Examples 1 to 15 and Comparative Examples 1 to 12 is shown in the "Re" column of Tables 1 and 2. The slow axis tilt angle θ57 for the substrate films of Examples 1 to 15 and Comparative Examples 1 to 12 is shown in the "Slow Axis Tilt Angle (°)" column of Tables 1 and 2. The retardation and slow axis were measured using the method described above. The retardation and slow axis were measured using the retardation measuring device "RETS-100" from Otsuka Electronics Co., Ltd.

[0237] For the surface light source devices of Examples 1 to 15 and Comparative Examples 1 to 12, the first ratio and the second ratio were calculated. The first ratio and the second ratio were determined by measuring the luminance angle distribution caused by the first polarization component LP1 and the luminance angle distribution caused by the second polarization component LP2, respectively, using the method described above.

[0238] A uniaxially oriented polyvinyl alcohol film (PVA film) was used as the polarizer. The PVA film was coated on both sides with a pair of triacetylcellulose films (TAC).

[0239] For measuring luminance in the luminance angular distribution, an EZ Contrast XL80 manufactured by ELDIM Corporation was used. A polarizer was placed in the light-receiving section of the measuring device.

[0240] Figure 11 shows the angular distribution of luminance caused by the first polarization component from the light guide plate toward the optical sheet and the angular distribution of luminance caused by the second polarization component from the light guide plate toward the optical sheet for Example 1. The light guide plate, reflective sheet, and light source were common to all surface light source devices in Examples 1 to 15 and Comparative Examples 1 to 12. Therefore, the first ratio was the same value in Examples 1 to 15 and Comparative Examples 1 to 12.

[0241] Figure 12 shows the angular distribution of luminance caused by the first polarization component emitted from the optical sheet and the angular distribution of luminance caused by the second polarization component emitted from the optical sheet for Example 1. Figure 13 shows the angular distribution of luminance caused by the first polarization component emitted from the optical sheet and the angular distribution of luminance caused by the second polarization component emitted from the optical sheet for Example 2.

[0242] From the luminance angle distribution of Examples 1 to 15 and Comparative Examples 1 to 12, the maximum luminance caused by the first polarization component incident on the optical sheet and the maximum luminance caused by the second polarization component incident on the optical sheet were determined. The first ratio was calculated from the maximum luminance caused by the first polarization component incident on the optical sheet and the maximum luminance caused by the second polarization component incident on the optical sheet. The first ratios for Examples 1 to 15 and Comparative Examples 1 to 12 are shown in the "First Ratio" column of Tables 1 and 2.

[0243] From the luminance angle distribution of Examples 1 to 15 and Comparative Examples 1 to 12, the maximum luminance caused by the first polarization component emitted from the optical sheet and the maximum luminance caused by the second polarization component emitted from the optical sheet were determined. The second ratio was calculated from the maximum luminance caused by the first polarization component emitted from the optical sheet and the maximum luminance caused by the second polarization component emitted from the optical sheet. The second ratios for Examples 1 to 15 and Comparative Examples 1 to 12 are shown in the "Second Ratio" column of Tables 1 and 2.

[0244] For Examples 1 to 15 and Comparative Examples 1 to 12, the luminance in the first direction D1 (front-facing luminance) on the display surface of the liquid crystal display device was measured. The front-facing luminance for each example is shown as a ratio (%) to the front-facing luminance for Comparative Example 1 in the "Front-facing luminance ratio (%)" column of Tables 1 and 2. An EZ Contrast XL80 manufactured by ELDIM was used to measure the front-facing luminance.

[0245]

[0246]

[0247] Examples 1 to 15 showed increased brightness in the front direction compared to Comparative Examples 1 to 12. Examples 1 to 15 were able to improve the utilization efficiency of light emitted from the light source compared to Comparative Examples 1 to 12.

[0248] While one embodiment has been described with reference to specific examples, the above-mentioned example does not limit this embodiment. The above-described embodiment can be implemented in various other examples, and various omissions, substitutions, modifications, and additions can be made without departing from its essence.

[0249] An example of modification will be described below with reference to the drawings. In the following explanation and the drawings used therein, parts that can be configured in the same way as in the specific example described above will be given the same reference numerals as those used for the corresponding parts in the specific example described above, and redundant explanations will be omitted.

[0250] As shown in Figure 14, a reflective layer 46 may be superimposed on the opposite surface 34 of the light guide plate 30. The reflective layer 46 can reflect light emitted from the opposite surface 34 of the light guide plate 30. This improves the utilization efficiency of the light source.

[0251] Various modifications are possible to the light guide plate 30 described above. The light guide plate 30 described above includes an inclined surface 47 as a light extraction element. The light guide plate 30 may include other light extraction elements in place of or in addition to the inclined surface 47. The other light extraction element may be a light diffusion layer provided on the back surface 32. The light diffusion layer may include a binder component such as resin and a light diffusion component held in the binder component. The other light extraction element may be a light diffusion component embedded in the light guide plate. The light diffusion component constituting the light extraction element may be inorganic particles such as silica (silicon dioxide) or alumina (aluminum oxide). The light diffusion component constituting the extraction element may be organic particles such as acrylic resin, polycarbonate resin, or silicone resin.

[0252] Various modifications are possible to the optical sheet 50 described above. For example, the first prism surface 61 of the optical sheet 50 may be formed as a flat surface. The second prism surface 62 of the optical sheet 50 may be formed as a folded surface. At least one of the first prism surface 61 and the second prism surface 62 of the optical sheet 50 may be formed as a curved surface. The first prism surface 61 and the second prism surface 62 may be configured symmetrically with respect to a surface extending in the first direction D1.

[0253] The overall configuration of the surface light source device 20 and the liquid crystal display device 10 described above can also be modified in various ways. For example, the surface light source device 20 may include other components such as a light diffusion sheet.

[0254] 5: Polarized sunglasses, 10: Liquid crystal display device, 11: Display surface, 15: Liquid crystal display panel, 16: First polarizing plate, 17: Liquid crystal cell, 18: Second polarizing plate, 20: Surface light source device, 21: Light-emitting surface, 24: Light source, 25: Light-emitting body, 28: Optical component, 30: Light guide plate, 31: Light-emitting surface, 32: Back surface, 33: Light-receiving surface, 34: Opposite surface, 35a: First side surface, 35b: Second side surface, 37: Inclined surface, 38: Flat surface, 39: Connecting surface, 46: Reflective layer, 50: Optical sheet, 51: First surface, 52: Second surface, 53: Prism surface, 55: Main body, 55a: Light-emitting side surface, 55b: Light-receiving side surface, 5 7: Base film, 58: Molding resin part, 59: Land part, 60: Unit prism, 61: First prism surface, 62: Second prism surface, 63: Top, 66: First element surface, 67: Second element surface, 68: Third element surface, 70: Reflective sheet, 71: Support, 72: Metal layer, 73: Surface layer, θxe1: First element surface inclination angle, θxe2: Second element surface inclination angle, θxe3: Third element surface inclination angle, LP1: First polarization component, LP2: Second polarization component, D1: First direction, D2: Second direction, D3: Third direction, A57: Lagging axis, θ57: Lagging axis inclination angle, θy: Inclination surface angle

Claims

1. An optical member comprising: a light guide plate; and an optical sheet superimposed on the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction; the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a base film and a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, the retardation of the base film at a wavelength of 589 nm is 800 nm or more, and the magnitude of the angle between the slow axis of the base film and the second direction in projection onto a surface perpendicular to the first direction is 5° or more and 85° or less.

2. The optical member according to claim 1, wherein the retardation of the substrate film at a wavelength of 589 nm is 2000 nm or more.

3. The optical member according to claim 1, wherein, in projection onto a plane perpendicular to the first direction, the magnitude of the angle between the slow axis of the base film and the second direction is 5° or more and 30° or 60° or more and 85°.

4. The optical member according to claim 1, wherein the second ratio of the maximum brightness caused by the first polarization component emitted from the first surface to the maximum brightness caused by the second polarization component emitted from the first surface is 1.24 or less, the first ratio of the maximum brightness caused by the first polarization component incident on the second surface to the maximum brightness caused by the second polarization component incident on the second surface is 1.30 or more, the first polarization component is a linear polarization component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linear polarization component that vibrates in a direction perpendicular to the reference plane.

5. The device comprises a light guide plate and an optical sheet facing the light guide plate in a first direction, wherein the light guide plate includes a light-emitting surface facing the optical sheet, a back surface facing the light-emitting surface, and a light-receiving surface and an opposite surface located between the light-emitting surface and the back surface, the light-receiving surface and the opposite surface facing a second direction nonparallel to the first direction, the optical sheet includes a first surface and a second surface facing the first surface, the optical sheet includes a plurality of unit prisms arranged in the second direction to constitute the second surface, the back surface, the light-emitting surface, the second surface, and the first surface are located in this order in the first direction, and the second ratio of the maximum brightness due to the first polarization component emitted from the first surface to the maximum brightness due to the second polarization component emitted from the first surface is 1.24 or less. An optical member wherein the first ratio of the maximum brightness caused by the first polarization component incident on the second surface to the maximum brightness caused by the second polarization component incident on the second surface is 1.30 or more, the first polarization component is a linear polarization component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linear polarization component that vibrates in a direction perpendicular to the reference plane.

6. The optical member according to claim 4 or 5, wherein the second ratio is 1.20 or less.

7. The optical member according to any one of claims 1 to 5, comprising a reflective sheet facing the back surface, wherein the reflective sheet includes a metal layer, the back surface includes an inclined surface, and the inclined surface is inclined with respect to the second direction such that it approaches the light-emitting surface in the first direction from the light-receiving surface toward the opposite surface in the second direction.

8. A surface light source device comprising an optical member according to any one of claims 1 to 5, and a light source that emits light incident on the light guide plate from the light-receiving surface.

9. A liquid crystal display device comprising a surface light source device according to claim 8, and a liquid crystal display panel superimposed on the surface light source device in the first direction.

10. The liquid crystal display device according to claim 9, wherein the liquid crystal display panel includes a first polarizing plate, a liquid crystal cell, and a second polarizing plate, the surface light source device, the first polarizing plate, the liquid crystal cell, and the second polarizing plate are positioned in this order in the first direction, the first polarizing plate transmits the second polarization component and shields the first polarization component, the first polarization component is a linearly polarized component that vibrates in a direction parallel to a reference plane parallel to both the first and second directions, and the second polarization component is a linearly polarized component that vibrates in a direction perpendicular to the reference plane.

11. The liquid crystal display device according to claim 9, wherein the second direction is the horizontal direction and the vertical direction.

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