JP2010062037A - Back light unit, and liquid crystal display - Google Patents

Abstract

Provided are a backlight unit in which a color filter can be omitted without using a separate member such as a condenser lens array, and a liquid crystal display device on which the backlight unit is mounted.
Three grating pieces groups 13gr.Gr.B, 13G, and 13R alternately arranged in the J direction on the top surface 11U of the light guide plate correspond to light of different wavelength ranges, and each grating The pieces 13gr.Gr.B, 13G, and 13R are diffracted and reflected so as to return only the incident light with an incident angle in a specific range to the side where the light travels in the corresponding specific wavelength region. In addition, a prism 15 is formed on the bottom surface 11B of the light guide plate 11 to reflect the light that is diffracted and reflected back toward the top surface 11U.
[Selection] Figure 1

Description

  The present invention relates to a backlight unit that supplies light to a liquid crystal display panel or the like, and a liquid crystal display device that includes the backlight unit.

  Conventionally, liquid crystal display devices capable of color display often have a color filter including a plurality of color patches mounted on a liquid crystal display panel. In this case, when white light passes through the color filter, only a part of the white light (for example, red light, green light, and blue light) is placed at a desired position on the liquid crystal display panel. And color display becomes possible. However, in the case of such color display, the color component of the light transmitted through the color filter is one color. For this reason, light of other color components is wasted.

  In order to prevent such waste (loss) of light, the liquid crystal display device of Patent Document 1 includes a liquid crystal display panel 159 and a backlight unit that supplies light to the liquid crystal display panel 159 as shown in FIG. A diffraction grating (volume hologram) 171 and a condenser lens array 172 are interposed between the light guide plate 111 of 149 (note that R, G, and B in the figure mean red, green, and blue). More specifically, the diffraction grating 171 is positioned on the light emitting surface (top surface) 111U of the light guide plate 111, and the condenser lens array 172 is positioned so as to cover the diffraction grating 171.

In such a liquid crystal display device 169, the diffraction grating 171 separates the white light reaching itself into red light, green light, and blue light, and diffracts and transmits them in different directions. The light traveling in such a manner is condensed by the condenser lens array 172 and reaches a desired position of the liquid crystal display panel 159. Therefore, the liquid crystal display device 169 can perform color display without including a color filter.
JP-A-10-253955

  However, when each light separated by the diffraction grating 171 is condensed by the condenser lens array 172 and reaches the liquid crystal display panel 159, various problems occur for the following reasons.

  Usually, since the distance between the liquid crystal display panel 159 and the light guide plate 111 is relatively short, the focal length that will exist from the condenser lens array 172 to the liquid crystal display panel 159 must also be short. For this purpose, the condenser lens array 172 has high power (high refractive power) or has to be increased in thickness. Then, when the condensing lens array 172 is formed of a resin capable of exhibiting high power, the cost is increased. When the condensing lens array 172 having an increased thickness is used, the backlight unit 149 and, consequently, the liquid crystal display device 169 are provided. Thickness increases.

  In addition, it is extremely difficult to condense each light separated by the diffraction grating 171 at a desired position of the liquid crystal display panel 159 with the condenser lens array 172. If the degree of light collection is not sufficient, the liquid crystal display panel 159 The quality of the image displayed on the screen also deteriorates.

  The present invention has been made to solve the above problems. And the objective provides the backlight unit which can abbreviate | omit a color filter, without using another member like a condensing lens array, and the liquid crystal display device which mounts it.

  The backlight unit includes a light source, a light receiving plate that receives light from the light source, an opposite surface that faces the light receiving surface, and a light guide plate that has two opposing surfaces that sandwich the light receiving surface and the opposite surface. Then, if the direction intersecting the direction from the light receiving surface to the opposite surface is the intersecting direction, in the backlight unit, the top surface which is one of the opposing surfaces of the light guide plate has different periods. A diffraction grating including at least three groups of grating pieces arranged in (1) is formed, and the three grating piece groups are alternately arranged along the crossing direction and correspond to light in different wavelength ranges.

  Furthermore, each grating piece group is diffracted and reflected so as to return only the incident light with an incident angle within a specific range to the side where the light travels, in the corresponding specific wavelength region, A refractive optical element that reflects the light diffracted and reflected back to the top surface is formed on the bottom surface that is the other surface of the opposing surfaces.

  In this case, each of the three lattice pieces is a part of light that is not totally reflected on the top surface, and is a light having a specific wavelength range corresponding to itself and having an incident angle within a specific range. Is diffracted and reflected in a specific direction (returning to the light traveling side). Then, the light for each specific wavelength range that is diffracted and reflected travels with relatively high directivity.

  However, since the three lattice piece groups are alternately positioned along the intersecting direction that intersects the direction from the light receiving surface to the opposite surface of the light guide plate, the light that is diffracted and reflected is color-coded in the intersecting direction. When the color-separated light is reflected by the refractive optical element so as to be perpendicular to the top surface, for example, the light reaching the top surface is emitted as it is perpendicular to the top surface.

  Then, as for the light emitted from the top surface, the light having a high directivity is emitted. That is, the backlight unit is allowed to proceed by color-coding white light without requiring separate members such as a color filter and a condenser lens array. As a result, the backlight unit becomes thinner by the thickness of another member, and the cost can be reduced.

  In the above three lattice piece groups, one of the blue light corresponding lattice piece groups corresponding to the wavelength range of blue light and one of the green color corresponding to the wavelength range of green light are matched to the three primary colors of light. It is desirable that one of the light-corresponding lattice piece groups is a red light-corresponding lattice piece group corresponding to the wavelength region of red light.

  In addition, it is desirable that the blue light corresponding lattice piece group, the green light corresponding lattice piece group, and the red light corresponding lattice piece group satisfy the following relational expression (M1).

d = λ / (2 · nd · sinθ) ... Relational expression (M1)
However,
nd: refractive index with respect to d line of material forming diffraction grating d: arrangement period of grating pieces for diffracting light in each grating piece group λ: wavelength of light θ: incident angle of light incident on diffraction grating The angle when the diffraction reflection angle by the incident light coincides.

  In addition, it is desirable that the overall length of the lattice piece is 50 nm or more and 1000 nm or less.

  In addition, it is desirable that the following relational expressions (C1) and (C2) are satisfied.

γ = θ ± Δ Relational expression (C1)
γ + 2 · δA + 2 · δB = 180 ° ... Relational expression (C2)
However,
Δ (°): Has a diffraction efficiency of 0.5 times or more than the diffraction efficiency of diffracted reflected light at θ.
Is an angle that generates diffracted reflected light, and is an angle γ (°) in the range of 0 ° <Δ <10 °: the sum of θ and Δ, and 0 for the diffraction efficiency of diffracted reflected light at θ. .5
Reflection angle of diffracted and reflected light having diffraction efficiency more than double δA (°): A triangular prism in which the refractive optical element is raised with respect to the bottom surface.
Out of the three corners of the prism, the two corners that touch the bottom
Angle δB (°) of the distant corner is a triangular prism in which the refractive optical element is raised with respect to the bottom surface, and its triangle
Out of the three corners of the prism, the two corners that touch the bottom
This is the angle that the nearest corner has.

Note that it is desirable that the backlight unit satisfy the following condition (C3) in order to ensure as much as possible the amount of emitted light perpendicular to the top surface.
δA <5 ° Condition (C3)

  By the way, the light guide plate may be formed of a single material having a single refractive index. The light guide plate is formed of two light guide layers having different refractive indexes, and the first light guide layer on the bottom side has a higher refractive index than the second light guide layer on the top side. May be.

  However, in the two-layer type light guide plate, total reflection may occur at the boundary between the first light guide layer and the second light guide layer, and using this, light reception from the top surface of the light guide plate is received. The distance from the surface can be easily extended. When such an extension occurs, an excessive amount of light is not emitted from the top surface of the light guide plate in the vicinity of the light source, and light amount unevenness does not occur in the light from the backlight unit.

  In order to suppress such unevenness in the amount of light, total reflection is required at the boundary between the first light guide layer and the second light guide layer. For this purpose, the intersection angle between the light receiving surface and the top surface is The light source that is 90 ° and supplies light to the light guide plate may satisfy ε1 in the following relational expression (F1).

ε1 <90 ° -CA ... Relational expression (F1)
However,
ε1: The outgoing angle of light traveling from the light receiving surface of the light guide plate with respect to the light receiving surface CA: the critical angle at the boundary surface between the first light guide layer and the second light guide layer.

  Note that a liquid crystal display device including the backlight unit as described above and a liquid crystal display panel having a color filter that transmits light from the backlight unit can be said to be the present invention.

  Furthermore, in such a liquid crystal display device, the color filter of the liquid crystal display panel that overlaps the top surface of the light guide plate includes three types of color patches that transmit light in different wavelength ranges. It is desirable to overlap the lattice piece group corresponding to the same wavelength range as the wavelength range of the light to be transmitted. This is because the color patch does not need to absorb unnecessary light, and the light from the backlight unit is effectively used.

  According to the backlight unit of the present invention, the light emitted from the top surface of the light guide plate is colored while having a relatively high directivity by diffracted reflected light from the diffraction grating on the top surface guided by the refractive optical element on the bottom surface. Divide. Therefore, this backlight unit does not require a separate member such as a color filter for color-coding white light and a condensing lens array for enhancing light condensing performance. As a result, the thickness and cost for another member are reduced.

[Embodiment 1]
The following describes one embodiment with reference to the drawings. For convenience, hatching, member codes, and the like may be omitted, but in such a case, other drawings are referred to. Moreover, the black circle on the drawing means a direction perpendicular to the paper surface. Further, the lines indicating the optical paths are illustrated so as not to overlap for convenience.

  FIG. 2 is an exploded perspective view of the liquid crystal display device 69. As shown in this figure, the liquid crystal display device 69 includes a liquid crystal display panel 59 and a backlight unit 49.

  In the liquid crystal display panel 59, an active matrix substrate 51 including a switching element such as a TFT (Thin Film Transistor) and an opposing substrate 52 facing the active matrix substrate 51 are bonded together with a sealant (not shown). Then, liquid crystal (not shown) is injected into the gap between the substrates 51 and 52 (note that the polarizing films 53 and 53 are attached so as to sandwich the active matrix substrate 51 and the counter substrate 52).

  Since the liquid crystal display panel 59 is a non-light-emitting display panel, the display function is exhibited by receiving light (backlight light) from the backlight unit 49. Therefore, if the light from the backlight unit 49 can uniformly irradiate the entire surface of the liquid crystal display panel 59, the display quality of the liquid crystal display panel 59 is improved.

  The backlight unit 49 includes an LED module (light source module) MJ, the light guide plate 11, and the reflection sheet 42.

  The LED module MJ is a module that emits light. An LED (Light Emitting Diode) 22 that emits light by being supplied with a current by being mounted on an electrode formed on the mounting surface of the mounting substrate 21 and the mounting substrate 21. ,including.

  Further, the LED module MJ preferably includes a plurality of LEDs (point light sources) 22 that are light emitting elements in order to secure the amount of light, and more preferably, the LEDs 22 are arranged in a line. However, for the sake of convenience, only a part of the LEDs 22 is shown in the drawing (hereinafter, the direction in which the LEDs 22 are arranged is also referred to as a J direction).

  The light guide plate 11 is a plate-like member having a side surface 11S and a top surface 11U (one surface of the facing surface) and a bottom surface 11B (the other surface of the facing surface) that are positioned so as to sandwich the side surface 11S. . And one surface (light-receiving surface 11Sa) of the side surface 11S receives the light from the LED 22 by facing the light emitting end of the LED 22. The received light is multiple-reflected inside the light guide plate 11 and is emitted outward from the top surface (exit surface) 11U as planar light.

  In the following, the side surface 11S facing the light receiving surface 11Sa will be referred to as the opposite surface 11Sb, and the direction from the light receiving surface 11Sa to the opposite surface 11Sb will be referred to as the K direction {particularly, this K direction is the J direction (crossing direction). Intersect (eg, orthogonal)}. Details of the light guide plate 11 will be described later.

  The reflection sheet 42 is positioned so as to be covered by the light guide plate 11. Then, one surface of the reflection sheet 42 facing the bottom surface 11B of the light guide plate 11 becomes a reflection surface. Therefore, the reflection surface reflects the light from the LED 22 and the light propagating through the light guide plate 11 back to the light guide plate 11 (more specifically, through the bottom surface 11B of the light guide plate 11) without leaking.

  In the backlight unit 49 as described above, the reflection sheet 42 and the light guide plate 11 are stacked in this order (the stacking direction is referred to as the L direction. The J direction, the K direction, and the L direction are mutually connected. It is desirable that the relationship be orthogonal. The light from the LED 22 is emitted as planar light (backlight) by the light guide plate 11, and the planar light reaches the liquid crystal display panel 59, and the liquid crystal display panel 59 is imaged by the planar light. Is displayed.

  Here, the light guide plate 11 of the backlight unit 49 will be described in detail with reference to FIGS. 1A to 1C, FIG. 2, and FIGS. 3A and 3B. 1A, FIG. 1B, and FIG. 1C are cross-sectional views of the backlight unit 49 shown in FIG. 2 along the line AA ′, the cross-section along the line BB ′, and the cross-section along the line CC ′. FIG. 3 is a diagram (three cross-sectional lines are also shown in FIG. 3A). 1A to 1C, white arrows indicate white light, broken arrows indicate -1st-order diffracted reflected light, and alternate long and short dash arrows indicate total reflected light (note that reference symbols W, B, G, R means white light, blue light, green light, red light).

  3A is a plan view showing the top surface 11U of the light guide plate 11 and the LED 22 of the backlight unit 49 shown in FIG. 2, and FIG. 3B is a bottom view of the light guide plate 11 of the backlight unit 49 shown in FIG. It is a top view which shows 11B and LED22.

  The light guide plate 11 is made of, for example, polycarbonate (refractive index nd: 1.59), and includes a diffraction grating DG in which grating pieces 13 are densely arranged on the top surface 11U as shown in FIGS. 1A to 1C. This diffraction grating DG is designed based on the known RCWA method (strict coupling wave theory) and the following relational expression (M0), and generates diffracted reflected light (-1st order diffracted reflected light) with relatively high light intensity. Let

n2 · sinθ2 = n1 · sinθ1 + m · λ / d (M0)
However,
n1: Refractive index of the medium on the incident side with respect to the top surface 11U θ1 (°): Angle (incident angle) of light incident on the top surface 11U with respect to the top surface 11U
n2: Refractive index of medium on the exit side with respect to the top surface 11U θ2 (°): Angle (reflection angle) of light reflected by the top surface 11U with respect to the top surface 11U
d (nm): Period interval of the diffraction grating DG
X: diffraction order λ (nm): wavelength of light (note that θ1, θ2 are easy to understand when considered as angles measured in the KL in-plane direction defined by the K direction and the L direction) .

When the incident side and the emission side with respect to the top surface 11U are the light guide plate 11, the relational expression (M0) can be expressed as the following relational expression (M0 ′).
n1 · sinθ2 = n1 · sinθ1 + m · λ / d (M0 ')

  A detailed description of the diffraction grating DG is as follows. The designed diffraction grating DG includes a plurality of rectangular parallelepiped (block-shaped) grating pieces 13, and these grating pieces 13 are located on the top surface 11 U of the light guide plate 11. The lattice pieces 13 are arranged at various periods d (pitch, arrangement period).

  For example, as shown in FIG. 1A, lattice pieces 13 densely packed with a period dB of 170 nm, as shown in FIG. 1B, lattice pieces 13 densely packed with a period dG of 200 nm, and with a period dR of 230 nm as shown in FIG. 1C. A dense lattice piece 13 can be mentioned. That is, on the top surface 11U of the light guide plate 11, the lattice pieces 13 are arranged with three types of periods d (dB, dG, dR).

  Further, since the lattice pieces 13 arranged at every period d (dB, dG, dR) are densely packed, they can be referred to as a lattice piece group 13gr (13gr.B, 13gr.G, 13gr.R), and are arranged at different periods. One patch PH is formed by gathering the lattice piece groups 13gr.B, 13gr.G, and 13gr.R (see FIG. 2 and FIG. 3A, and the square patch size is about 10 μm × 10 μm). The grid piece groups 13gr.B, 13gr.G, and 13gr.R are alternately arranged along the J direction, and the total length of each grid piece 13 (the distance from the root to the tip of the grid piece 13) is 300 nm.

  The light of blue light (wavelength of about 470 nm), green light (wavelength of about 550 nm), and red light (wavelength of about 620 nm) is about 60 ° with respect to the top surface 11U where the patches PH of the diffraction grating DG gather. When the light is incident at the incident angle (θ1), the light is diffracted and reflected by the diffraction grating DG to become diffracted reflected light, and has a reflection angle (θ2) of about 60 ° which is the same as the incident angle. However, the diffracted and reflected light travels back to the side traveling toward the diffraction grating DG. That is, the diffraction grating DG diffracts and reflects part of the light that reaches itself (light that enters with a specific range of incident angles) so as to return to the side where the light travels.

  FIG. 4A to FIG. 6C show the results of such diffraction reflection. In these figures, the polar coordinate center means the incident point of light to the diffraction grating DG located on the top surface 11U, and the angle means the reflection angle of the light reflected from the incident point with respect to the top surface 11U. . For the sake of convenience, the reflection angle of light that travels (forwards forward) away from the LED 22 is indicated by “+”, and the reflection angle of light that travels (rearward travels) closer to the LED 22 is indicated by “−”. . The dot represents the totally reflected light, and the dot represents the −1st order diffracted reflected light.

  4A to FIG. 6C show the behavior of light that occurs when blue light (wavelength 470 nm) reaches the diffraction grating DG, and FIGS. 5A to 5C show green light (wavelength 550 nm) shows the behavior of light that occurs when it reaches the diffraction grating DG, and FIGS. 6A to 6C show the behavior of light that occurs when red light (wavelength 620 nm) reaches the diffraction grating DG.

  4A, FIG. 5A, and FIG. 6A show the behavior of light that occurs when the lattice piece group 13gr.B arranged at a period (arrangement period dB: 170 nm) is reached, and FIG. 4B, FIG. 5B, and FIG. Shows the behavior of light that occurs when the lattice piece group 13gr.G arranged at a period (arrangement period dG: 200 nm) is reached. FIGS. 4C, 5C, and 6C show the period (arrangement period dR: 230 nm). The behavior of the light that occurs when the lattice piece group 13gr.R arranged in FIG.

  4A to 4C, especially FIG. 4A, the blue light reaches the grating piece group 13gr.B arranged at the arrangement period dB: 170 nm with an incident angle (θ1≈60 °) of about 60 °. In this case, totally reflected light and −1st order diffracted reflected light are generated. The minus first-order diffracted reflected light has a reflection angle (θ2≈60 °) of about −60 °. On the other hand, referring to FIG. 4B and FIG. 4C, the blue light is almost totally reflected when it reaches the grating pieces 13gr.G, 13gr.R arranged at a period other than 170 nm.

  Next, referring to FIGS. 5A to 5C, particularly FIG. 5B, when the green light reaches the grating piece group 13gr.G arranged at the arrangement period dG: 200 nm with an incident angle of about 60 °, Reflected light and -1st order diffracted reflected light are generated. The −1st order diffracted reflected light has a reflection angle of about −60 °. On the other hand, referring to FIGS. 5A and 5C, the green light is almost totally reflected when it reaches the grating pieces 13gr.B and 13gr.R arranged at a period other than 200 nm.

  Subsequently, referring to FIG. 6A to FIG. 6C, particularly FIG. 6C, when the red light reaches the grating piece group 13gr.R arranged at the arrangement period dR: 230 nm with an incident angle of about 60 °, Reflected light and -1st order diffracted reflected light are generated. The −1st order diffracted reflected light has a reflection angle of about −60 °. On the other hand, referring to FIG. 6A and FIG. 6B, the blue light is almost totally reflected when it reaches the grating pieces 13gr.B and 13gr.G arranged at a period other than 230 nm.

  Referring to the results of FIGS. 4A to 6C so far, the following conditions (A1) to (A5) are satisfied. That is, as shown in FIGS. 1A to 1C, when white light traveling from the LED 22 enters the diffraction grating DG (specifically, the top surface 11U including the diffraction grating DG) at about 60 ° (θ1≈60 °). ), When blue light, green light, and red light included in white light are incident on a group of grating pieces 13gr.B, 13gr.G, and 13gr.R corresponding to the wavelength range, It proceeds so as to return to the side proceeding toward the groups 13gr.B, 13gr.G, and 13gr.R {travels at a reflection angle θ2 (≈60 °)}. These lights are color-separated and have a relatively high directivity in order to correspond to the grating pieces 13gr.B, 13gr.G, and 13gr.R.

nd = 1.59 Condition (A1)
dB = 170 nm Condition (A2)
dG = 200 nm Condition (A3)
dR = 230 nm Condition (A4)
H = 300 nm Condition (A5)
However,
nd: refractive index with respect to d-line of material forming diffraction grating DG dB: arrangement period of grating piece 13 of grating piece group 13gr.B that diffracts blue light dG: grating piece group 13gr.G of diffracting green light Arrangement period of the grating pieces dR: Arrangement period of the grating pieces 13 of the grating piece group 13gr.R that diffracts red light H: Distance from the root to the tip of the grating piece 13 (the total length of the grating pieces 13)
It is.

  Note that 60 °, 55 °, and 65 ° are given as some detailed numerical examples of the incident angle of about 60 ° of light incident on the top surface 11U including the diffraction grating DG (essentially, the diffraction grating DG). . In addition, when light incident at these incident angles is reflected as −1st-order diffracted reflected light, the reflection angles are −60 ° reflection angle when the incident angle is 60 °, and 55 ° incident angle. Is a reflection angle of −65.56 °, and a reflection angle of −55.41 ° when the incident angle is 65 °.

  Moreover, when the above phenomena are summarized, the diffraction efficiency increases when the −1st-order diffracted reflected light is reflected in a direction opposite to the incident direction (incident angle) to the diffraction grating DG (reflected angle). The directivity is also increased. Therefore, in the relational expression (M0 ′), θ1 = −θ2 = θ (θ: see later) and m = −1 can be obtained, and the following relational expression (M1) is also derived.

d = λ / (2 · nd · sinθ) ... Relational expression (M1)
However,
nd: Refractive index with respect to d-line of the material forming the diffraction grating DG d: Diffraction of light by each of the grating piece groups 13gr.B, 13gr.G, 13gr.R
Arrangement period of the grating piece 13 (nm)
λ: wavelength of light (nm)
θ: Angle (°) when the incident angle of light incident on the diffraction grating DG matches the diffraction reflection angle of the incident light
It is.

  Further, in each of the grating piece groups 13gr.B, 13gr.G, and 13gr.R, the arrangement period (nm) of the grating pieces 13 that diffract light is about half the wavelength range of visible light. The total length (H) of the grating piece 13 is determined by correlation with the diffraction efficiency obtained by the RCWA method (strict coupling wave theory) (note that the total length of the grating piece 13 is often 50 nm or more and 1000 nm or less. ).

  By the way, the lattice piece groups 13gr.B, 13gr.G, and 13gr.R are alternately arranged in the J direction (without the same kind of lattice piece groups 13gr adjacent to each other), and are also arranged in the same direction along the K direction. . Then, inside the light guide plate 11 covered with the same type of lattice piece group 13gr, only light in the wavelength region corresponding to the lattice piece groups 13gr.B, 13gr.G, and 13gr.R is transmitted to the lattice piece group 13gr.B. , 13gr.G, 13gr.R so as to return to the side proceeding toward this side (hereinafter, this reflection is referred to as back reflection).

  That is, as shown in FIG. 1A, the blue light included in the white light is reflected backward as −1st-order diffracted reflected light when incident on the grating piece group 13 gr. The included green light and red light are totally reflected by the lattice group 13gr.B). Further, as shown in FIG. 1B, the green light included in the white light is reflected backward as −1st order diffracted reflected light when incident on the grating piece group 13gr.G at about 60 ° (note that the white light is converted into white light). The blue light and red light contained are totally reflected by the lattice piece group 13gr.G). Further, as shown in FIG. 1C, the red light contained in the white light is reflected backward as the −1st order diffracted reflected light when incident on the grating piece group 13gr.R at about 60 ° (in addition to the white light) Blue light and green light contained are totally reflected by the lattice piece group 13gr.R).

  That is, light that reaches the diffraction grating DG in the process from the light receiving surface 11Sa to the opposite surface 11Sb while being subjected to multiple reflections at the light guide plate 11, and light that is -1st order diffracted and reflected by the diffraction grating DG is reversed (opposite It faces from the surface 11Sb to the light receiving surface 11Sa; rearward) and does not go to the top surface 11U. Therefore, a prism 15 (refractive optical element) is formed on the bottom surface 11B of the light guide plate 11 in order to guide such first-order diffracted reflected light (light diffracted and reflected back by the diffraction grating DG) to the top surface 11U. The

  This prism 15 is a triangular prism, and as shown in FIGS. 1A to 1C and 3B, it protrudes from the bottom surface 11B of the light guide plate 11 and has two prism side surfaces 15 (front prism side surface 15Sf and rear prism side surface 15Sr). Is inclined with respect to the bottom surface 11B.

  Of these two prism side surfaces 15S, the front prism side surface 15Sf closer to the opposite surface 11Sb of the light guide plate 11 (away from the LED 22) is formed at a position where it can receive the −1st order diffracted reflected light from the diffraction grating DG. Is done. Further, the front prism side surface 15Sf is formed with an inclination capable of reflecting the received first-order diffracted reflected light toward the rear prism side surface 15Sr closer to the light receiving surface 11Sa of the light guide plate 11 (closer to the LED 22). .

  The rear prism side surface 15Sr is formed at a position where the first-order diffracted reflected light from the front prism side surface 15Sf can be received. Further, the rear prism side surface 15Sr is formed to have an inclination capable of reflecting the received first-order diffracted reflected light toward the top surface 11U.

  In particular, the rear prism side surface 15Sr is preferably formed to have an inclination that can reflect the −1st-order diffracted reflected light so as to be perpendicular to the top surface 11U. For this purpose, the prism 15 is preferably formed so as to satisfy the following relational expressions (C1) and (C2).

γ = θ ± Δ Relational expression (C1)
γ + 2 · δA + 2 · δB = 180 ° ... Relational expression (C2)
However,
θ (°): incident angle of light incident on the diffraction grating DG and diffraction by the incident light
Angle Δ (°) when the reflection angle coincides: Has a diffraction efficiency of 0.5 times or more than the diffraction efficiency of the diffracted reflected light at θ.
Is an angle that generates diffracted reflected light, and is an angle γ (°) in the range of 0 ° <Δ <10 °: the sum of θ and Δ, and 0 for the diffraction efficiency of diffracted reflected light at θ. .5
Reflection angle of diffracted and reflected light having diffraction efficiency more than double δA (°): The prism 15 is a triangular prism protruding from the bottom surface 11B.
Of the three corners of the prism, two corners touch the bottom surface 11B.
Angle δB (°) that the angle farther from the LED 22 has with respect to the bottom surface 11B: The prism 15 is a triangular prism protruding from the bottom surface 11B.
Of the three corners of the prism, two corners touch the bottom surface 11B.
The angle closer to the LED 22 is the angle with respect to the bottom surface 11B.

  The relational expressions (C1) and (C2) will be described with reference to the enlarged sectional view of FIG. In addition, the broken-line arrow in drawing shows -1st order diffraction reflected light similarly to FIG.

  First, the first-order diffracted reflected light traveling toward the prism 15 has a reflection angle “γ” with respect to the top surface 11U. Then, the −1st order diffracted reflected light up to the prism 15 is on one side, the normal line N to the bottom surface 11B (and the top surface 11U) is one side, and the bottom surface 11B and the first extended surface E1 of the bottom surface 11B extending to the prism 15 are one side. , The first virtual triangle includes a “γ” angle and a 90 ° angle. Therefore, the remaining angle is “90 ° −γ”. Further, this remaining angle faces the angle formed by the first extended surface E1 and the −1st order diffracted reflected light. Therefore, the angle formed by the first extended surface E1 and the −1st order diffracted reflected light is also “90 ° −γ”.

  Then, in the second virtual triangle with one side of the front prism side surface 15Sf, one side of the −1st order diffracted reflected light toward the front prism side surface 15Sf, and one side of the first extended surface E1, the front prism side surface 15Sf and the next time The angle formed by the folded reflected light is a value obtained by subtracting “δA” from the angle formed by the first extended surface E1 that is “90 ° −γ” and the −1st order diffracted reflected light (that is, “90 °”). −γ−δA ″).

  In addition, when the first-order diffracted reflected light incident on the front prism side surface 15Sf is totally reflected, the first-order diffracted reflected light that is totally reflected is one side, the front prism side surface 15Sf is one side, and the rear prism side surface 15Sb is one side. In the third virtual triangle, the angle formed between the first-order diffracted reflected light that is totally reflected and the front prism side surface 15Sf is also “90 ° −γ−δA”.

  In the third virtual triangle, the angle formed by the front prism side surface 15Sf and the rear prism side surface 15Sb is “180 ° − (δA + δB)” due to the shape of the triangular prism. Then, the angle of the remaining angle in the third virtual triangle, that is, the angle formed between the -1st order diffracted reflected light that is totally reflected and the rear prism side surface 15Sb is “γ + 2 · δA + δB−90 °”.

  Then, when the −1st order diffracted reflected light traveling from the front prism side surface 15Sf is totally reflected by the back prism side surface 15Sb, the −1st order diffracted reflected light that has undergone the second total reflection and the back prism side surface 15Sb The formed angle is also “γ + 2 · δA + δB−90 °”. Further, of the angles formed by the second extended surface E2 obtained by extending the rear prism side surface 15Sb and the bottom surface 11B, the angle facing the “δB” angle in the rear prism 15 is “δB”.

  Then, the total value (“γ + 2 · δA + 2” of the angle formed by the second extended surface E2 and the bottom surface 11B and the angle formed by the -1st-order diffracted reflected light that has undergone the second total reflection and the rear prism side surface 15Sb. .Delta.B-90 DEG ") is the exit angle of the -1st order diffracted reflected light that has undergone the second total reflection with respect to the bottom surface 11B (and hence the top surface 11U). Therefore, if the angle “γ + 2 · δA + 2 · δB−90 °”, which is the total value, becomes 90 °, the minus first-order diffraction reflected light from the diffraction grating DG is emitted perpendicularly to the top surface 11U. Become.

  That is, when the prism 15 is designed so as to satisfy “Relation (C2); γ + 2 · δA + 2 · δB = 180 °” derived from “γ + 2 · δA + 2 · δB−90 ° = 90 °”, the diffraction grating DG is designed. -1st order diffracted reflected light from the light exits perpendicularly to the top surface 11U.

  If this is the case, the blue light, the green light, and the red light having the relatively high directivity from the diffraction grating DG and the −1st order diffracted reflected light of the red light reach the prism 15. The prism 15 is guided so as to be perpendicular to the top surface 11U, and further exits from the top surface 11U. As a result, blue light, green light, and red light are emitted from the top surface 11U toward a desired position while being separated. Then, the liquid crystal display panel 59 that receives such separated light does not have a color filter and separates white light, and can display a color image.

  When the color filter is omitted, the liquid crystal display device 69 can reduce the cost for the color filter. Furthermore, in the liquid crystal display device 69, it is not necessary to consider the thickness of the color filter. That is, the liquid crystal display device 69 tends to be thin.

  In addition, the color-separated blue light, green light, and red light have a relatively high directivity due to the −1st order diffraction reflection, so that a desired direction is not required, for example, without requiring a separate member such as a condenser lens array. Proceed to Therefore, the backlight unit 49 can be reduced in thickness by the thickness of the condenser lens array, and the cost can be reduced by the condenser lens array.

  However, in the liquid crystal display device 69, the liquid crystal display panel 59 may include a color filter 55 as shown in FIGS. 8A to 8C (however, the manner of illustration in FIGS. 8A to 8C is illustrated in FIGS. 1A to 8C). Same as FIG. 1C). More specifically, in the color filter 55, three types of color patches 55B, 55G, and 55R that transmit light in different wavelength ranges (a color patch 55B that transmits blue light, a color patch 55G that transmits green light, and a red light) Color patch 55R), and color patches 55B, 55G, and 55R overlap lattice group 13gr.B, 13gr.G, and 13gr.R corresponding to the same wavelength region as the wavelength region of the transmitted light. May be.

  In this case, the blue light emitted from the grating piece group 13gr.B reaches the color patch 55B, and the green light emitted from the grating piece group 13gr.G reaches the color patch 55G, and the grating piece group 13gr. The red light emitted from .R reaches the color patch 55R. Therefore, the color patches 55B, 55G, and 55R do not absorb the energy of light when transmitting the light that reaches the color patches 55B, 55G, and 55R.

  Therefore, in such a liquid crystal display device 69, the utilization efficiency of the light from the backlight unit 49 is improved (in short, the liquid crystal display device 69 does not give a loss to the light from the backlight unit 49). . In addition, as the light use efficiency is improved, the backlight unit 49 does not have to emit excessively high energy light, and suppresses power consumption (and thus reduces power consumption of the liquid crystal display device 69). Connected).

One example of the numerical value of the prism 15 is as follows.
δA = 4 °
δB = 58.5 °
F = 10 μm
However,
F: width of the prism 15 in contact with the bottom surface 11B of the light guide plate 11 (K direction of the prism 15)
Length; see FIGS. 1A-1C)
It is.

  The reason why the above numerical examples are desirable is as follows. That is, when δA is an angle of 5 ° or more, a part of the −1st-order diffracted reflected light that travels back from the diffraction grating DG toward the prism 15, particularly the reflection angle (θ 2) is relatively large. After the small light is reflected by the front prism side surface 15Sf, it becomes difficult to go to the rear prism reflection surface 15Sb. More specifically, even if light reaching the front prism surface 15Sf is reflected while having a relatively small reflection angle (θ2), it proceeds toward the bottom surface 11B without going toward the rear prism surface 15Sb.

If the amount of such light increases, the amount of light reaching the rear prism side surface 15Sb decreases, and thus the amount of light emitted so as to rise from the top surface 11U decreases. Therefore, it is desirable that the following condition (C3) is satisfied.
δA <5 ° Condition (C3)

  Even if −1st-order diffracted reflected light that has passed through the prism 15 exists, the light is returned to the bottom surface 11 </ b> B of the light guide plate 11 by the reflective sheet 42.

[Embodiment 2]
A second embodiment will be described. In addition, about the member which has the same function as the member used in Embodiment 1, the same code | symbol is attached and the description is abbreviate | omitted.

  The diffraction grating DG of the backlight unit 49 of the first embodiment includes three grating piece groups 13gr.B, 13gr.G, and 13gr.R that are arranged at different periods, and these three grating piece groups 13gr. .B, 13gr.G, and 13gr.R are alternately arranged along the J direction and correspond to light in different wavelength ranges.

  Further, each of the lattice piece groups 13gr.B, 13gr.G, and 13gr.R returns only the incident light with the incident angle in the specific range to the side where the light travels in the corresponding specific wavelength region. (In other words, the diffraction grating DG causes white light to travel by being color-divided by diffraction reflection). Then, the prism 15 formed on the bottom surface of the light guide plate 11 reflects the light diffracted and reflected so as to return to the top surface 11U.

  The diffraction grating DG that causes such light behavior is formed on the top surface 11U of the light guide plate 11 having a single refractive index by being formed of a single material. However, it is not limited to this. For example, as shown in FIGS. 9A to 9C, a two-layered light guide plate 11 having different refractive indexes may be used (how to illustrate FIGS. 9A to 9C and FIGS. 1A to 1C). The same).

  In the two-layer light guide plate 11, the first light guide plate (first light guide layer) 11L1 is formed of, for example, polycarbonate (refractive index nd: 1.59), and the second layer light guide plate (second guide light plate). The light layer 11L2 is formed of, for example, an acrylic resin (refractive index nd: 1.49) (note that the light from the LED 22 is first incident on the first light guide layer 11L1, and the first light layer). The refractive index of the light guide layer 11L1 is larger than the refractive index of the second light guide layer 11L2.

  As in the first embodiment, the second light guide layer 11L2 includes lattice piece groups 13gr.B, 13gr.G, which are formed of lattice pieces 13 that are densely packed with three types of periods d (dB, dG, dR). Contains 13 gr.R. In addition, the lattice piece groups 13gr.B, 13gr.G, and 13gr.R are alternately arranged in the J direction as well as in the first embodiment, and are also arranged in the same direction along the K direction.

  In the case of such a two-layer type light guide plate 11, the light traveling through the first light guide layer 11L1 also travels into the second light guide layer 11L2 and further enters the diffraction grating DG at about 60 °. The light is reflected back as -1st order diffracted reflected light (note that the behavior of this back reflected light is substantially the same as in the numerical example in the case of the above-described polycarbonate diffraction grating DG).

  However, in the case of the light guide plate 11, a boundary surface S11 is generated by two types of optical members (that is, the first light guide layer 11L1 and the second light guide layer 11L2). Therefore, using the boundary surface S11, the light inside the light guide plate 11 can travel as follows. That is, the light that reaches the diffraction grating DG is totally reflected at least once at the boundary surface S11 in the process of reaching the diffraction grating DG.

  For this total reflection, the light traveling through the first light guide layer 11L1 may have an emission angle ε1 of about 20 ° with respect to the light receiving surface 11Sa (note that the emission angle ε1 is, for example, 45 If the LED 22 having an emission angle ε2 of about 0 ° is disposed facing the light receiving surface 11Sa, this is realized from Snell's law). In this case, when the light traveling through the first light guide layer 11L1 reaches the second light guide layer 11L2 (in short, when reaching the boundary surface S11), the light exceeds the critical angle. This is because it easily enters the boundary surface S11 and is totally reflected.

  Furthermore, the prism 15 formed on the bottom surface 11B of the light guide plate 11 (specifically, the bottom surface 11B of the first light guide layer 11L1) is designed to guide the totally reflected light to the diffraction grating DG. More specifically, the prism 15 is formed at a position where it can receive the totally reflected light traveling from the boundary surface S11. The front prism side surface 15Sf of the prism 15 reflects the received light and makes the light critical to the boundary surface S11. It is formed to have an inclination that is incident at an angle within an angle.

  In addition, the light incident on the boundary surface S11 must be at an angle of about 60 ° with respect to the diffraction grating DG in order to reflect back. Therefore, the incident angle of the light traveling from the front prism side surface 15Sf to the boundary surface S11 requires about 55 ° (for example, 54.2 °). This is because, at this incident angle, light enters the diffraction grating DG at about 60 ° from Snell's law. Therefore, the front prism side surface 15Sf has an inclination capable of guiding light reflected by itself to the boundary surface S11 at an incident angle of about 55 °.

  Then, the light reflected by the front prism side surface 15Sf designed as described above is refracted by transmitting through the boundary surface S11, and enters the top surface 11U including the diffraction grating DG at an incident angle of about 60 °. Then, the grating piece groups 13gr.B, 13gr.G, and 13gr.R included in the diffraction grating DG, as shown in FIGS. 9A to 9C, are light (dashed lines) in the corresponding wavelength region of the light (white light). (Refer to the arrow) is reflected backward, and the light in the other wavelength region (see the dashed line arrow) is totally reflected.

  The light reflected backward travels while having a reflection angle of about 60 ° with respect to the top surface 11U including the lattice pieces 13gr.B, 13gr.G, and 13gr.R, and about 60 ° with respect to the boundary surface S11. The incident angle is. Further, the light passes through the boundary surface S11 while having an emission angle γ of about 55 ° with respect to the boundary surface S11 (note that this angle of about 55 ° corresponds to γ in FIG. 7). ).

  The light passing through the boundary surface S11 and traveling to the second light guide layer 11L2 reaches the front prism side surface 15Sf of the prism 15. However, when the incident angle of this light to the front prism side surface 15Sf and the reflection angle of the light traveling from the front prism side surface 15Sf to the diffraction grating DG with respect to the front prism side surface 15S are compared, the angles of both are often slightly different. Due to the difference in angle, the light that passes through the boundary surface S11 and enters the front prism side surface 15Sf is reflected to enter the rear prism side surface 15Sr.

  That is, the front prism side surface 15Sf is formed to have an inclination that can receive the light totally reflected by the boundary surface S11 and that is reflected and incident on the boundary surface S11 at an angle within a critical angle. Further, the front prism side surface 15Sf is formed at a position where it can receive light (-1st order diffracted reflected light) transmitted through the boundary surface S11, and has an inclination capable of reflecting the light toward the rear prism side surface 15Sr. It is formed.

  The rear prism side surface 15Sr is formed at a position where the −1st order diffracted reflected light from the front prism side surface 15Sf can be received, and is further inclined to reflect the received −1st order diffracted reflected light toward the top surface 11U. Is done.

  As in the first embodiment, the rear prism side surface 15Sr is preferably formed to have an inclination capable of reflecting the minus first-order diffracted light so as to be perpendicular to the top surface 11U. For this purpose, the prism 15 is preferably formed so as to satisfy the above relational expressions (C1) and (C2), and further (C3).

  In the relational expression (C1), assuming that θ is 60 °, the assumed range of γ is 50 ° <γ <70 °. Then, the minus first-order diffracted reflected light traveling while having an emission angle γ of about 55 ° with respect to the boundary surface S11 is caused by the prism 15 satisfying the relational expressions (C1) and (C2) as in the first embodiment. The light is emitted vertically from the top surface 11U.

  Based on the above, even in the backlight unit 49 in the second embodiment, the -1st order diffracted reflected light of the blue light, the green light, and the red light from the diffraction grating DG is the prism as in the first embodiment. 15 is guided by the prism 15 so as to be perpendicular to the top surface 11U, and is further emitted from the top surface 11U. As a result, blue light, green light, and red light are emitted from the top surface 11U while being separated. Then, the liquid crystal display panel 59 that receives such separated light can use the color-separated light without having a color filter, and can display a color image.

  Therefore, the operational effects of the backlight unit 49 according to the first embodiment, that is, the reduction in thickness and cost due to the omission of the color filter and the condenser lens array, and the light in the case where the backlight unit 49 is covered with the color filter 55. The improvement of the use efficiency and the suppression of power consumption are also achieved by the backlight unit 49 of the second embodiment.

  Further, in the backlight unit 49 of the second embodiment, the light of the LED 22 is easily totally reflected at the boundary surface S11 before reaching the diffraction grating DG from the light receiving surface 11Sa of the light guide plate 11. Then, the light totally reflected in this way becomes the −1st order diffracted reflected light through the prism 15 and the diffraction grating DG, and further, when returning to the prism 15 and exiting from the top surface 11U, The distance P2 from the light emission point to the light receiving surface 11Sa is longer than the distance P1 from the light emission point to the light receiving surface 11Sa on the top surface 11U of the light guide plate 11 in the first embodiment (FIG. 9A to FIG. 9). 9C and FIGS. 1A-1C).

  Usually, in the light guide plate 11, when comparing the amount of emitted light between the vicinity of the top surface 11U near the light receiving surface 11Sa and the vicinity of the top surface 11U near the opposite surface 11Sb, the vicinity of the top surface 11U on the light receiving surface 11Sa side near the LED 22 is More than the vicinity of the top surface 11U on the opposite surface 11Sb side. The difference in the amount of emitted light causes unevenness in the amount of light from the backlight unit 49 (backlight light), which contributes to image quality deterioration of the liquid crystal display device 69.

  However, in the backlight unit 49 according to the second embodiment, as shown in FIGS. 9A to 9C, the light guide plate 11 emits color-separated light from the vicinity of the top surface 11U that is relatively far from the light receiving surface 11Sa. In addition, an excessively large amount of color-separated light is not emitted from the vicinity of the top surface 11U close to the light receiving surface 11Sa. For this reason, light amount unevenness hardly occurs in the light from the backlight unit 49.

  In order to totally reflect the light incident on the boundary surface S11 from the first light guide layer 11L1, the light traveling through the first light guide layer 11L1 has an emission angle ε1 of about 20 ° with respect to the light receiving surface 11Sa. However, the emission angle ε1 preferably satisfies the following relational expression (F1). In short, the LED 22 may make light traveling from the light receiving surface 11Sa to the boundary surface S11 incident at an angle exceeding the critical angle CA. As an example, for example, when an LED 22 having an emission angle ε2 of about 45 ° is arranged facing the light receiving surface 11Sa, from Snell's law, the LED 22 emits light that satisfies ε1 in the relational expression (F1). Can supply.

ε1 <90 ° -CA ... Relational expression (F1)
However,
In the light guide plate 11, the crossing angle between the light receiving surface 11Sa and the top surface 11U is 90 °,
ε1: Light traveling from the light receiving surface 11Sa of the light guide plate 11 is coupled to the light receiving surface 11Sa.
The emission angle CA is a critical angle at the boundary surface between the first light guide layer 11L1 and the second light guide layer 11L2.

[Other embodiments]
By the way, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

  For example, in Embodiment 1, as an example of the material of the light guide plate 11, polycarbonate that satisfies the above-described conditions (A1) to (A5) is cited. Further, the second light guide layer 11L2 made of acrylic resin including the diffraction grating DG in the second embodiment satisfies the conditions (A2) to (A5) and the condition (A1 ′) of nd = 1.49, By satisfying these conditions, back reflection has occurred {if the conditions (A1 ′) · (A2) to (A5) are satisfied, the relational expression (M1) is also satisfied}.

However, there are other conditions that cause back reflection. For example, the light guide plate 11 may be formed of silicon resin. In particular, even in the light guide plate 11 that satisfies the following conditions (B1) to (B5), the light behaves as shown in FIGS. 4A to 6C {note that the conditions (B1) to (B5) If it is satisfied, the relational expression (M1) is also satisfied}.
nd = 1.3 Condition (B1)
dB = 210 nm Condition (B2)
dG = 245 nm Condition (B3)
dR = 270 nm Condition (B3)
H = 300 nm Condition (B5)

  That is, even in such a light guide plate 11 made of silicon resin, each of the three lattice piece groups 13gr.B, 13gr.G, and 13gr.R is specified by light of a specific wavelength range corresponding to itself. Light that reaches itself with a range of incident angles (about 60 °) is diffracted and reflected at a reflection angle of about 60 °, which is a specific direction (to return to the light traveling side).

  Even in such a light guide plate 11 made of silicon resin, a detailed numerical example of an incident angle of light incident on the diffraction grating DG of about 60 ° is the same as that of the light guide plate 11 made of polycarbonate. That is, when the incident angle of light incident on the diffraction grating DG is 60 °, the reflection angle of the −1st order diffracted reflected light is −60 °, and when the incident angle is 55 °, the reflection angle is −65.56 °. When the incident angle is 65 °, the reflection angle becomes −55.41 °.

  Further, even in such a silicon light guide plate 11, when the relational expressions (C1) and (C2) are satisfied, the −1st order diffracted reflected light from the diffraction grating DG is emitted perpendicularly to the top surface 11U. become. Therefore, the -1st order diffracted reflected light of the blue light from the grating piece group 13gr.B, the green light from the grating piece group 13gr.G, and the red light from the grating piece group 13gr.R reaches the prism 15, Further, the prism 15 guides and emits light so as to be perpendicular to the top surface 11U.

  In short, when the color-diffracted diffracted reflected light is reflected by the prism 15 so as to be perpendicular to the top surface 11U, the light emitted from the light guide plate 11 has a directivity of being perpendicular to the light guide plate 11. Will have. As a result, even in the backlight unit 49 on which such a silicon light guide plate 11 is mounted, the color filter can be omitted (in essence, the same effects as those of the first embodiment can be achieved).

  In summary, in the light guide plate 11, the top surface 11U is formed with a diffraction grating DG that diffracts and reflects a part of the light reaching the surface of the light guide plate 11 back to the light traveling side, and the bottom surface 11B. If the prism 15 that reflects the light that is diffracted and reflected back toward the top surface 11U is formed, there are no restrictions on various conditions (however, in order to avoid color mixing of the light that is reflected backwards). The lattice pieces 13gr.B, 13gr.G, and 13gr.R must be arranged alternately along the J direction).

  Accordingly, the refractive index of the material forming the light guide plate 11, the diffraction grating DG, and the prism 15 is not particularly limited, and the shape of the grating piece 13 may be a columnar shape other than a rectangular parallelepiped, a cone shape, or the like. . Further, the arrangement period of the grating pieces 13 is not limited to about half of the wavelength range of visible light, and other arrangement periods may be possible. Of course, the total length of the grating piece 13 is 300 nm as an example, but is not limited thereto.

The numerical example of the silicon resin prism 15 is as follows.
δA = 3 °
δB = 59.5 °
F = 10 μm

Further, it is desirable that the following condition (C4) is satisfied instead of the above condition (C3). When this condition (C4) is satisfied, the same effects as when the condition (C3) is satisfied are exhibited.
δA <4 ° Condition (C4)

Further, the following conditional expression (C5) can be derived from the numerical examples of the prisms 15 made of polycarbonate and silicon resin. That is, when this conditional expression (C5) is satisfied, the prism 15 can emit −1st-order diffracted reflected light traveling from the diffraction grating DG perpendicular to the top surface 11U.
δA + δB = 62.5 ° Conditional expression (C5)

  In addition, although LED22 was mentioned as a light source above, it is not limited to this. For example, a linear light source such as a fluorescent tube, or a light source formed of a self-luminous material such as organic EL (Electro-Luminescence) or inorganic EL may be used.

  In the above description, the number of grating piece groups 13gr included in the diffraction grating DG is three. However, a larger number of grating piece groups 13gr may be included. If the specific wavelength region required for generating white light by color mixture is four or more, four or more grating piece groups 13gr may be included in the diffraction grating DG accordingly.

  In the above description, the prism 15 is given as an example of the optical element that guides the −1st-order diffracted reflected light to the top surface 11U. However, the optical element is not limited to this. For example, it may be a mirror.

(A) is a cross-sectional view taken along line AA ′ of the backlight unit shown in FIG. 2, and (B) is a cross-sectional view taken along line BB ′ of the backlight unit shown in FIG. FIG. 4C is a cross-sectional view taken along the line CC ′ of the backlight unit shown in FIG. FIG. 3 is an exploded perspective view of a liquid crystal display device. Then, (A) is a top view which shows the top | upper surface of a light-guide plate, (B) is a top view which shows the bottom face of a light-guide plate. Then, (A) is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 470 nm is incident on a group of lattice pieces in which lattice pieces with a total length of 300 nm are densely arranged at an arrangement period of 170 nm, and (B) is a full length diagram. It is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 470 nm is incident on a group of lattice pieces in which 300 nm lattice pieces are densely arranged at an arrangement period of 200 nm. FIG. It is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 470 nm is incident on a group of lattice pieces that are densely arranged in an arrangement period. Then, (A) is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 550 nm is incident on a group of lattice pieces in which lattice pieces with a total length of 300 nm are densely arranged at an arrangement period of 170 nm, and (B) is a full length diagram. It is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 550 nm is incident on a group of lattice pieces in which 300 nm lattice pieces are densely arranged at an arrangement period of 200 nm. FIG. It is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 550 nm is incident on a group of lattice pieces densely arranged in an arrangement period. (A) is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 620 nm is incident on a group of lattice pieces in which lattice pieces with a total length of 300 nm are densely arranged at an arrangement period of 170 nm, and (B) is a full length diagram. It is a polar coordinate diagram showing the behavior of reflected light when light having a wavelength of 620 nm is incident on a group of lattice pieces in which 300 nm lattice pieces are densely arranged at an arrangement period of 200 nm. FIG. It is a polar coordinate diagram which shows the behavior of the reflected light when the light of wavelength 620nm injects into the lattice piece group densely arranged by the arrangement period. These are the expanded sectional views of the light-guide plate shown by FIG. 1A-FIG. 1C. (A) is a liquid crystal display device that covers the backlight unit shown in FIG. 1A with a liquid crystal display panel, and (B) is a liquid crystal display device that covers the backlight unit shown in FIG. 1B with a liquid crystal display panel, (C) is a liquid crystal display device that covers the backlight unit shown in FIG. 1C with a liquid crystal display panel. (A) is a backlight unit on which a two-layer light guide plate different from the light guide plate shown in FIG. 1A is mounted, and a back on which a two-layer light guide plate different from the light guide plate shown in FIG. 1B is mounted. It is a light unit and is a backlight unit on which a two-layer light guide plate different from the light guide plate shown in FIG. 1C is mounted. These are sectional drawings of the conventional liquid crystal display device.

Explanation of symbols

11 Light guide plate 11L1 1st light guide layer (light guide plate)
11L2 Second light guide layer (light guide plate)
S11 Boundary surface 11U Top surface of light guide plate (opposing surface, one surface of opposing surfaces)
11B Bottom surface of light guide plate (opposite surface, the other of the opposing surfaces)
11S Side surface of the light guide plate 11Sa Light receiving surface of the light guide plate 11Sb Opposite surface which is the side surface of the light guide plate disposed opposite to the light receiving surface 13 Grid piece 13gr Grid piece group 13gr.B Grid piece group corresponding to blue light group)
13 gr.G Lattice piece group corresponding to green light (green light corresponding lattice piece group)
13gr.R Lattice piece group corresponding to red light (red light corresponding lattice piece group)
PH diffraction grating patch DG diffraction grating 15 prism (refractive optical element)
15S prism side surface 15Sf front prism side surface (prism side surface farther from the light source)
15Sr Rear prism side (side of the prism closer to the light source)
21 Mounting board 22 LED (light source)
42 Reflective sheet 49 Backlight unit 55 Color filter 55B Blue color patch 55G Green color patch 55R Red color patch 59 Liquid crystal display panel 69 Liquid crystal display device J LED parallel direction (cross direction)
K Direction from the light receiving surface to the opposite surface of the light guide plate L Direction in which the reflection sheet and the light guide plate 11 are stacked

Claims (11)

A light source;
A light receiving plate having a light receiving surface for receiving light from the light source, an opposite surface opposite to the light receiving surface, and two opposing surfaces sandwiching the light receiving surface and the opposite surface;
In the backlight unit including
If the direction intersecting the direction from the light receiving surface to the opposite surface is the crossing direction,
A diffraction grating including at least three groups of grating pieces arranged at different periods is formed on the top surface, which is one of the opposing surfaces,
The three lattice piece groups are arranged alternately along the crossing direction and correspond to light in different wavelength ranges,
Each grating piece group is diffracted and reflected so as to return only the incident light with an incident angle in a specific range to the side where the light travels, in the corresponding specific wavelength range.
On the bottom surface, which is the other surface of the opposing surfaces, the light that is diffracted and reflected so as to return,
A backlight unit in which a refractive optical element that reflects toward the top surface is formed.   In the three lattice piece groups, one is a blue light corresponding lattice piece group corresponding to the blue light wavelength region, one is a green light corresponding lattice piece group corresponding to the green light wavelength region, and one is red. The backlight unit according to claim 1, wherein the backlight unit is a group of lattice pieces corresponding to red light corresponding to a wavelength range of light. The backlight unit according to claim 2, wherein the blue light corresponding lattice piece group, the green light corresponding lattice piece group, and the red light corresponding lattice piece group satisfy the following relational expression (M1).
d = λ / (2 · nd · sinθ) ... Relational expression (M1)
However,
nd: refractive index with respect to d line of material forming diffraction grating d: arrangement period of grating pieces for diffracting light in each grating piece group λ: wavelength of light θ: incident angle of light incident on diffraction grating The angle when the diffraction reflection angle by the incident light coincides.   The backlight unit according to claim 2 or 3, wherein an overall length of the lattice piece is 50 nm or more and 1000 nm or less. The backlight unit according to claim 3 or 4 which satisfies the following relational expressions (C1) and (C2).
γ = θ ± Δ Relational expression (C1)
γ + 2 · δA + 2 · δB = 180 ° ... Relational expression (C2)
However,
Δ (°): The diffraction efficiency of 0.5 times or more of the diffraction efficiency of the diffracted reflected light at the above θ
The angle γ (°) within the range of 0 ° <Δ <10 °, which is the sum of the θ and the Δ, and the diffraction efficiency of the diffracted reflected light at the θ versus
The reflection angle of diffracted and reflected light having a diffraction efficiency of 0.5 times or more δA (°): a triangular prism in which the refractive optical element is raised with respect to the bottom surface.
Out of the three corners of the prism, the two corners that touch the bottom
Angle δB (°) of the distant corner is a triangular prism in which the refractive optical element is raised with respect to the bottom surface, and its triangle
Out of the three corners of the prism, the two corners that touch the bottom
This is the angle that the nearest corner has. The backlight unit according to claim 5, which satisfies the following condition (C3).
δA <5 ° Condition (C3)   The backlight unit according to claim 1, wherein the light guide plate is formed of a single material having a single refractive index. The light guide plate is formed of two light guide layers having different refractive indexes,
The backlight unit according to claim 1, wherein the first light guide layer on the bottom surface side has a higher refractive index than the second light guide layer on the top surface side. In the light guide plate, the crossing angle between the light receiving surface and the top surface is 90 °,
The backlight unit according to claim 8, comprising a light source that supplies light that satisfies ε1 of the following relational expression (F1).
ε1 <90 ° -CA ... Relational expression (F1)
However,
ε1: The outgoing angle of light traveling from the light receiving surface of the light guide plate with respect to the light receiving surface CA: the critical angle at the boundary surface between the first light guide layer and the second light guide layer. The backlight unit according to any one of claims 1 to 9,
A liquid crystal display panel having a color filter that transmits light from the backlight unit;
Including a liquid crystal display device. In the color filter of the liquid crystal display panel overlapping the top surface of the light guide plate,
Three types of color patches that transmit light in different wavelength ranges are included,
The liquid crystal display device according to claim 10, wherein the color patch overlaps the lattice piece group corresponding to the same wavelength range as the wavelength range of light to be transmitted.

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