DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0127] Embodiments of the present invention are now described with reference to the drawings.

[0128] FIG. 1 is a schematic view showing a lighting unit 10 in accordance with a first embodiment. FIG. 2 is a partially enlarged view of the lighting unit 10 for explaining the operation of the lighting unit 10 shown in FIG. 1.

[0129] FIGS. 5 and 6 are schematic views showing an example of a display device including a lighting unit. In FIGS. 5 and 6, a display device 100 includes a lighting unit 102 and a display panel 104. In FIG. 5, the lighting unit 102 is a front light, and the display panel 104 is a reflective liquid crystal display panel. A polarizer 106 is arranged between the front light 102 and display panel 102. In FIG. 6, the lighting unit 102 is a backlight and the display panel 104 is a transmissive liquid crystal display panel. Polarizers 106 are arranged on both sides of the display panel 104. The lighting unit 10 to be described below is usable as the lighting unit 102 shown in FIG. 5 or FIG. 6 or any other lighting unit.

[0130] Referring to FIG. 1, the lighting unit 10 comprises a light guide plate 12, a light source 14, and a truncated pyramid 16 located between the light guide plate 12 and the light source 14. The truncated pyramid 16 has a base 16a, a top (support surface) 16b smaller than the base 16a, and a slope (slopes) 16c linking the base 16a and the top 16b. The light source 14 is placed in close contact with the top 16b of the truncated pyramid 16. The light guide plate 12 is placed in close contact with the base 16a of the truncated pyramid 16. Light is propagated from the light emitting part of the light source 14 to the light guide plate 12 without passing through any air layer.

[0131] Referring to FIGS. 1 and 2, the truncated pyramid 16 is made of the same material as the light guide plate 12 and integrated with the light guide plate 12. The light source 14 is fixed to the top 16b of the truncated pyramid 16 by an adhesive. For example, the light guide plate 12 the truncated pyramid 16 are made of acrylic resin (refractive index is 1.48), Arton, or Zeonor (refractive index is 1.51). Polycarbonate resin (refractive index is 1.58) may be adopted. In this embodiment, the adhesive optically couples the light source 14 and the truncated pyramid 16 to each other in such a manner that air will not substantially caught in the space between the light source 14 and the truncated pyramid 16. Herein, a permanent adhesive, a removable adhesive and other bonding members are generically called the adhesive.

[0132] The light source 14 comprises an LED. FIG. 4 is a sectional view showing an example of the LED. The LED 14 is manufactured as an LED package having a semiconductor chip 14a, in which a pn junction is formed, sealed with a resin 14b such as an acrylic or an epoxy resin. A mirror 14d is formed behind a light emitting part 14c included in the LED 14. Light is irradiated forwards from the LED 14 in the radial directions as indicated by arrows. The directivity of light of the LED 14 is small, and light travels in forward and oblique directions at various angles.

[0133] In FIG. 2, light 18 emanating from the light source 14 at a relatively large angle with respect to the axis of the light guide plate 12 is shown. The light 18 enters the truncated pyramid 16 and travels towards the slope 16c. The light 18 is totally reflected by the slope 16c of the truncated pyramid 16 and travels further within the light guide plate 12. The light 18 is propagated through the light guide plate 12, repeating total reflection within the light guide plate 12, and leaves the light guide plate 12 toward the display panel 104 by means of a light discharging mechanism such as prisms or a diffuse reflection layer. Light 19 emanating from the light source 14 at a relatively small angle with respect to the axis of the light guide plate 12 is propagated through the light guide plate 12 without hitting the slope 16c.

[0134] Assuming that the refractive index of the truncated pyramid 16 is n and an angle at which the slope 16c meets a line parallel to the axis of the truncated pyramid 16 is α, if the angle α of the slope 16c of the truncated pyramid 16 is equal to or larger than arcsin(1/n), light made incident to the truncated pyramid 16 is entirely totally reflected by the slope 16c. However, if the angle α is large, the amount of light reflected by the slope 16s may decrease. Moreover, there is a possibility that light reflected by the slope 16c may pass through the surface of the light guide plate 12. Consequently, if the angle α of the slope 16c of the truncated pyramid 16 ranges from 30° to 45°, the amount of light allowed to enter the light guide plate 12 increases.

[0135] The light emitting part of the light source 14 is smaller in size than the top 16b of the truncated pyramid 16 or equal thereto. Moreover, the light emitting surface of the light source 14 is smaller in size than the top 16b of the truncated pyramid 16 or is equal thereto. Consequently, a larger amount of light emitted by the light source 14 can be introduced into the truncated pyramid 16.

[0136] FIG. 3 is an explanatory view for explaining the operation of a lighting unit of a comparative example. In FIG. 3, the truncated pyramid 16 is not included, and the light source 14 is not in optical contact with the light guide plate 12. Namely, an air layer is interposed between the light source 14 and the light guide plate 12. In this case, part of light emitted by the light emitting part of the light source 14 is reflected by the interface between the light source 14 and air layer and is not directed to the light guide plate 12. Part of the light is reflected by the end surface of the light guide plate 12 and does not enter the light guide plate 12.

[0137] According to the present invention, this kind of light that cannot be utilized can be considerably largely introduced into the light guide plate 12, and the light utilization efficiency can be improved. An experiment has revealed that the light introducing efficiency is improved to be 2.9 times higher.

[0138] FIGS. 7 to 23 show further examples of the lighting unit 10. In these examples, the lighting unit 10 comprises a light guide plate 12, a light source 14, and a truncated pyramid 16 linking the light guide plate 12 and the light source 14. The light source 14 is placed in close contact with the top 16b of the truncated pyramid 16, and the light guide plate 12 is placed in close contact with the base 16a of the truncated pyramid 16, so that light is propagated from the light emitting part of the light source 14 to the light guide plate 12 without any air layer between them.

[0139] In FIG. 7, the light source 14 and the truncated pyramid 16 are integrated with each other, and the truncated pyramid 16 is attached to the light guide plate 12.

[0140] In FIG. 8, the truncated pyramid 16 and the light guide plate 12 are integrated with each other, and the light source 14 is attached to the top 16b of the truncated pyramid 16 using a deformable body (adhesive) 20 that can be deformed due to a flow thereof, and is applied to a concave part of the top 16b of the truncated pyramid 16.

[0141] In FIG. 9, the light source 14 is attached to the top 16b of the truncated pyramid 16 using the adhesive 22. The design is such that not only the truncated pyramid 16 has the slope 16c but also the adhesive 22 has a slope 22c. Consequently, in this example, the adhesive 22 has the same function as that of the truncated pyramid 16. This leads to improvement in light utilization efficiency. In this example, the adhesive 22 is acrylic resin, similar to the light guide plate 12 and to the truncated pyramid 16 that are acrylic. This leads to improvement in adhesive strength. Moreover, an air layer is removed. Preferably, the shape of the adhesive 22 is determined so that the relationship between the refractive index n and the inclination α of the slope, which has been described with reference to FIG. 2, will be established.

[0142] In FIG. 10, the light source 14 is attached to the top 16b of the truncated pyramid 16 using the adhesive 22. The surface 22d of the adhesive 22 is a rounded inclined surface. In this example, similarly to the lighting unit 10 shown in FIG. 9, light utilization efficiency improves. In this example, first, the light source 14 is bonded to the top 16b of the truncated pyramid 16. Thereafter, an acrylic resin is added in order to form a partial sphere. When the light source 14 is embedded in the adhesive 22, light emitted by the sides of the light source 14 can also be used.

[0143] In FIGS. 11 and 12, the truncated pyramid 16 is made of an adhesive. Even in this example, preferably, the shape of the adhesive made into the truncated pyramid 16 is determined so that the relationship between the refractive index n and the inclination a of the slope, which has been described with reference to FIG. 2, will be established.

[0144] In FIGS. 13 and 14, a plurality of light sources 14 are attached to the top 16b of the truncated pyramid 16. In this case, in the vertical section of FIG. 14, the structure is similar to that of FIG. 9 or FIG. 10, but in the horizontal section, the structure is similar to that of FIG. 11 or FIG. 12. The LED light source array is an array of LED packages having LED chips, which are arranged on a printed circuit board as an array and adhered thereto. One LED package contains one LED chip that emits light in one of three primary colors, LED packages that emit light in red, light in green, and light in blue are orderly set in array. Alternatively, one LED package may contain three LED chips that emit light in three primary colors. This has the merit that when such LED packages are constructed as a plane light source, color deviation will be minimized.

[0145] In this case, regular reflection mirrors 13 are preferably attached to the side surfaces of the truncated pyramid 16 and the side surfaces of the light guide plate 12 that extend in the direction of travelling light (the sides above and below the sheet of FIG. 14). This can prevent light from getting out of the light guide plate through the side surfaces while not being converged internally.

[0146] In FIGS. 15 and 16, the linear light source 14 realized with a white organic electroluminescent (EL) lamp or the like is mounted on the top 16b of the truncated pyramid 16. The linear EL light source 14 is realized with an EL lamp formed on a glass substrate. From the viewpoint of a coefficient of thermal expansion, the linear EL light source 14 may be realized with an EL lamp formed on an organic film or an organic sheet.

[0147] In FIGS. 17 to 19, a light guide plate comprises a first light guide plate 12A and a second light guide plate 12B. The light source 14 is attached to both sides of the first light guide plate 12A in the same manner as the foregoing examples. The first light guide plate 12A is placed on the side of the second light guide plate 12B. The first light guide plate 12A may be called a light guide pipe. The first light guide plate 12A has a sawtooth mirror 24 formed on the side thereof opposite to the side where the second light guide plate 12B is arranged. The second light guide plate 12B has a light converging means 26 formed on the side where the first light guide plate 12A is arranged.

[0148] Light emitted by the light sources 14 enters the first light guide plate 12A, whereby the first light guide plate 12A serves as a linear light source. Light within the first light guide plate 12A is reflected by the sawtooth mirror 24 and enters the second light guide plate 12B. The second light guide plate 12B then serves as a plane light source. The optical coupling portions of each of the light sources 14 and the first light guide plate 12A respectively are optically closely coupled to each other using an acrylic resin, but the optical coupling portions of the first light guide plate 12A and the second light guide plate 12B respectively are not optically closely coupled to each other but are merely contacted each other so that light guided within the first light guide plate 12A may be intercepted.

[0149] In FIGS. 20 and 21, the truncated pyramid 16 is formed as an elongated wedge-shaped member. A plurality of truncated pyramids 16 are arranged on one side of the light guide plate 12. Light sources 14 formed with LEDs are mounted on the tops of the respective truncated pyramids 16. A light absorbing member 28 surrounds all the truncated pyramids 16 on or near the border between the truncated pyramids and the light guide plate 12 so that the light absorbing member 28 will cover the portions of the truncated pyramids 16 adjoining the light guide plate 12. This example is adapted to convert the divergent light rays from the light source 14 into parallel light rays.

[0150] In FIGS. 22 and 23, the truncated pyramid 16 is formed as an elongated wedge-shaped member. A plurality of truncated pyramids 16 are placed on one side of the light guide plate 12. Light sources 14 realized with LEDs are mounted on the tops of the respective truncated pyramids 16. An optical absorbing member 28 surrounds all the truncated pyramids 16 on or near the border between the truncated pyramids 16 and the light guide plate 12 so that the optical absorbing member 28 will cover the portions of the truncated pyramids 16 adjoining the light guide plate 12. Furthermore, a mirror or a scattering reflecting member 30 encloses all the truncated pyramids 16. This example is suitable for conversion of diverging light rays emitted by the light sources 14 into parallel light rays. The small light sources 14 such as LEDs are bonded to the tops of the respective truncated pyramids 16, but, the light sources 14 need not always be bonded to the tops of the respective truncated pyramids 16. As long as the light sources 14 are placed near the respective truncated pyramids (near the side surfaces thereof), no problem occurs.

[0151] FIGS. 24 to 27 show further examples of a lighting unit. In FIGS. 24 and 25, a lighting unit 10 comprises a light source 15, a light guide plate 12, a plurality of truncated pyramids 16 formed as elongated wedge-shaped members and attached to the light guide plate 12, an optical absorbing member 28 that surrounds all the truncated pyramids 16 on or near the border between the truncated pyramids 16 and the light guide plate 12 so that the optical absorbing member 28 will cover the portions of the truncated pyramids 16 adjoining the light guide plate 12, and a mirror or a scattering reflecting member 30 that encloses all the truncated pyramids 16. The light source 15 is realized with a cold cathode tube (fluorescent lamp) and placed by the truncated pyramids 16 to traverse all the truncated pyramids 16. Light emitted by the light source 15 enters the truncated pyramids 16 from the sides of the respective truncated pyramids 16, is repeatedly totally reflected within the truncated pyramids 16, and enters the light guide plate 12 at small angles with respect to the axis of the light guide plate 12. This example is suitable for introducing parallel light rays into the light guide plate 12.

[0152] In FIGS. 26 and 27, a lighting unit 10 comprises a light source 15, a light guide plate 12, a truncated pyramid 16 formed as an elongated wide wedge-shaped member and attached to the light guide plate 12, a light absorbing member 28 surrounding the truncated pyramid 16 on or near the border between the truncated pyramid 16 and light guide plate 12 so that the light absorbing member 28 will cover the portion of the truncated pyramid 16 adjoining the light guide plate 12, and a mirror or a scattering reflecting member 30 enclosing the truncated pyramid 16. The operation of this example is identical to that of the previous example.

[0153] In FIG. 24 to FIG. 27, the light source 15 may be not only a cold cathode tube but also any other rod-like light source, for example, an EL light source. Moreover, the light source 15 is not limited to the rod-like light source but may be a light source having any shape. In the examples, the light absorbing member 28 prevents stray light.

[0154] FIG. 28 is a schematic view showing a liquid crystal display device in accordance with a second embodiment of the present invention. FIG. 29 is a sectional view of the liquid crystal panel included in the liquid crystal display device shown in FIG. 28. The liquid crystal display device 40 comprises a liquid crystal panel 42, a lighting unit 44, a polarizer 46, and a low refractive index layer 48. Preferably, the liquid crystal panel 42 is of a reflective type and of a vertical alignment type (VA type).

[0155] In FIG. 29, the liquid crystal panel 42 has a liquid crystal 42c arranged between a pair of glass substrates 42a and 42b. One of the glass substrates 42a includes a common electrode 42d and a vertical alignment layer 42e, and the other glass substrate 42b includes pixel electrodes 42f and a vertical alignment layer 42g. Consequently, the liquid crystal molecules are aligned generally perpendicularly to the surfaces of the substrates when no voltage is applied, and the liquid crystal molecules are aligned generally parallel to the surfaces of the substrates when the voltage is applied. The pixel electrodes 42f are made of a material by which light is reflected. Preferably, one pixel is divided into a plurality of regions in which pretilt direction of liquid crystal molecules varies depending on the regions.

[0156] In FIG. 28, a (second) light guide plate 54 included in the lighting unit 44 is bonded to the polarizer 46 with the low refractive index layer 48. The polarizer 46 is bonded to the liquid crystal panel 42.

[0157] In this way, in the case where the light guide plate 54, the sheet polarizer 46, and the liquid crystal panel 42 are bonded to one another, a problem arises in that contrast is degraded because, of the light made incident to the light guide plate 54, light that is not totally reflected by the low refractive index layer 48 passes through the polarizer 46, as it is, and is then irradiated onto the liquid crystal panel 42. The inventors have recognized this problem as a serious problem when a high contrast vertical alignment type liquid crystal panel 42 is used in which the polarizer 46 is bonded to the light guide plate 54 and further the liquid crystal panel 42 is bonded to the polarizer 46.

[0158] As the low refractive index layer 48 whose refractive index is higher than the refractive index of air is formed on one side of the light guide plate 54, and the liquid crystal panel 42 is bonded to the light guide plate via the polarizer a large amount of light having entered the light guide plate 54 (light that is not totally reflected by the low refractive index layer 48) is made incident to the liquid crystal panel 42, resulting in a problem in that display quality deteriorates because contrast is degraded and becomes inhomogeneous and the brightness also becomes inhomogeneous. This problem first came to light when the light guide plate 54, the polarizer 46, and the liquid crystal panel 42 were bonded to one another.

[0159] Moreover, as the light guide plate 54 and the liquid crystal panel 42 that are plates are bonded to each other, the light guide plate 54 and liquid crystal panel 42 tend to peel off. In particular, if dust is caught in the adhesive, the light guide plate 54 and the liquid crystal panel 42 tend to locally peel off from each other in a special environment. This poses a problem in that the display quality deteriorates. The light guide plate 54 and the liquid crystal panel 42 are brought into contact with each other with the low refractive index layer 48 whose refractive index is lower than the refractive index of the light guide plate 54. Transmission of light through the interface is therefore not completely zero. The transmitted light causes a degradation of contrast. When the polarizer 46 is bonded to the light guide plate 54 and the liquid crystal panel 42 is bonded to the polarizer 46, the polarizer 46 absorbs a large amount of light. This degrades brightness. The embodiment to be described below attempts to solve the foregoing problems.

[0160] FIG. 30 is a plan view of the lighting unit 44 shown in FIG. 28, and FIG. 31 is a sectional view of the lighting unit 44 shown in FIG. 30. The lighting unit 44 includes light sources 50, a first light guide plate 52 that receives light emitted by the light sources 50, a second light guide plate 54 that receives light passing through the first light guide plate 52, and a light converging means 56 interposed between the first light guide plate 52 and the second light guide plate 54. The thickness of the second light guide plate 54 is greater than the thickness of the first light guide plate 52.

[0161] The lighting unit 44 is analogous to the lighting unit 10 shown in FIGS. 17 to 19. Specifically, the light sources 50 are realized with LEDS and are attached to the respective sides of the first light guide plate 52. The first light guide plate 52 is placed on one side of the second light guide plate 54. The first light guide plate 52 has a sawtooth mirror (see FIG. 17) formed on the side thereof opposite to the side where the second light guide plate 54 is arranged. The second light guide plate 54 has the light converging means 56 formed on the side where the first light guide plate 52 is arranged. In this example, the light converging means 56 is formed as an integral part of the second light guide plate 54 tapered towards the first light guide plate 52. The second light guide plate 54 has prisms (not shown) formed on the surface 54A thereof as a light discharging mechanism that directs light, which is propagated through the second light guide plate 54, toward the liquid crystal panel 42. The second light guide plate 54 includes the prisms even in an example to be described below.

[0162] The light sources 50 are formed with LEDs each having a thickness of 0.6 mm. The thickness of the first light guide plate 52 is 0.5 mm, and the thickness of the second light guide plate 54 is 1.0 mm. The light converging means 56 is the portion of the second light guide plate 54 having a slope 56A that extends between the first and second light guide plates 52 and 54. However, the light converging means 56 and the first light guide plate 52 are not optically coupled to each other but placed mutually closely to each other so that they may come into contact with each other.

[0163] The first light guide plate 52 and the second light guide plate 54 are made of Arton (whose refractive index is 1.51) manufactured by JSR Corp. The light converging means 56 is also made of Arton. The polarizer 46 shown in FIG. 28 is bonded to the second light guide plate 54 with an adhesive (whose refractive index ranges from 1.45 to 1.47), to which nano-level bubbles are added in order to attain a low refractive index, between them. The removable adhesive serves as the low refractive index layer 48 shown in FIG. 28. The polarizer 46 has a polarizing layer, layers of triacetylcellulose (TAC) coated on both sides of the polarizing layer and a plurality of layers formed below the TAC layer. The vertical alignment type liquid crystal panel 42 is bonded to the polarizer 46.

[0164] The light converging means 56 is included in order to improve the degree of parallelism of light entering the second light guide plate 54 from the first light guide plate 52.

[0165] The adhesive serving as the low refractive index layer 48 is made of an acrylic material which has a refractive index of 1.48 by nature. Invisible nano-level bubbles are contained in the adhesive in order to decrease the refractive index, irregularities (projections and cavities) whose heights range from several micrometers to ten micrometers are formed on the surface of the polarizer, the adhesive is applied to the polarizer, and the polarizer is bonded to a light guide plate. Thus, the polarizer is stuck to the light guide plate with air caught uniformly over the surface of the polarizer. In this stage, the bubbles are discernible, and therefore, the assembly of the polarizer and the light guide plate is autoclaved to be finished. It has been confirmed that the contained bubbles makes an amount of illumination light 1.2 to 1.8 times greater.

[0166] FIG. 32 is a view for explaining the operation of the light converging means 56. Reference numeral 58 designates light entering the second light guide plate 54 from the first light guide plate 52. An angle at which the slope 56A meets a line parallel to the axis of the second light guide plate 54 is α. An angle at which the light 58 meets a line parallel to the axis of the second light guide plate 54 is θ. An angle at which the light 58 meets the line parallel to the axis of the second light guide plate when the light travels from the lowermost point of the entrance of the light converging means 56 (adjoining the first light guide plate 52) to the uppermost point of the exit thereof (adjoining the second light guide plate 54) is θ0. A total reflection angle at which light is totally reflected within the second light guide plate 54 is θc.

[0167] The character W denotes the length of the light converging means 56, L denotes the incident position of the light 58, and H0 denotes the thickness of the first light guide plate 52, and H1 denotes the thickness of the second light guide plate 54. The character h equals (H1−H 0)/2. In this case, the relationships of H1=H0+2h, h=W tan α, and h+H0=W tan θ0 are established.

[0168] FIG. 33 is a view for explaining the operation of the light converging means 56 when the angle θ of the light 58 satisfies θ<α (when negative angles are taken into account, −α<θ<α). In this case, the angle of the light 58 is small and exhibits a high degree of parallelism. The light 58 does not hit the slope 56A and enters directly the second light guide plate 54. Light which enters the second light guide plate 54 is designated by the reference numeral 60.

[0169] FIG. 34 is a view for explaining the operation of the light converging means 56 when the angle θ of the light 58 satisfies α<θ<θo (and −θo<θ<α). In this case, depending on the incident position L of the light 58, there is a possibility in which the light 58 may not hit the slope 56A and directly enters the second light guide plate 54, and there is a possibility in which the light 58 may hit the slope 56A, be totally reflected, and then enter the second light guide plate 54. Light which enters the second light guide plate 54 in the latter case is designated by the reference numeral 60. When the light 58 and 60 is shifted downwards in parallel, light in the former case can be obtained.

[0170] FIG. 35 is a view for explaining the operation of the light converging means 56 when the angle θ of the light 58 satisfies θo<θ (or θ<−θo). In this case, all the light 58 hits the slope 56A, to be totally reflected by the slope 56A, and then enters the second light guide plate 54. Light which enters the second light guide plate 54 is designated by the reference numeral 60.

[0171] FIG. 36 shows an angular distribution of light traveling from the first light guide plate 52 to the light converging means 56. FIG. 37 shows an angular distribution of light adjusted by the light converging means 56. When the light 58 is totally reflected by the slope 56A, the angle θ of the light 58 is compensated for so that the light approaches a line parallel to the axis of the second light guide plate 54 by ±2α. A hatched part in FIG. 36 indicates a region permitting the light 58 to be reflected by the slope 56A. Consequently, the degree of parallelism of the light traveling from the first light guide plate 52 to the second light guide plate 54 improves as indicated in FIG. 37 (the quantity of light traveling at a small angle increases). Incidentally, P denotes a preferable range of angles.

[0172] Now, the length of the light converging means 56 and the angle of the slope will be discussed. Samples of the light converging means 56 are produced with the parameter of the length set to range from 0.5 mm to 7 mm. The angle α of the slope 56A of the light converging means 56 is determined, when the thickness of the first light guide plate 52, the thickness of the second light guide plate 54, and the length of the converging means 56 are determined.

[0173] FIG. 38 shows an angular distribution of light traveling from the first light guide plate 52 to the light converging means 56. The distribution is shown only on the positive angle side. The curve α is drawn by plotting the values of the angle α of the slope 56A of the light converging means 56. The curve θ0 is drawn by plotting the values of the angle θ0 indicated in FIG. 32. The region x existing below the curve α is a region in which light travels to the second light guide plate 54 without being reflected by the slope 56A, as shown in FIG. 33. The region Y between the curve α and the curve θ0 is a region in which light is partly reflected by the slope 56A, as shown in FIG. 34. The region above the curve θ0 is a region in which light is totally reflected by the slope 56A, as shown in FIG. 35.

[0174] FIG. 39 shows an angular distribution of the light 58 that has an angle permitting total reflection from the slope 56A and that has passed through the light converging means 56. In this case, the angular distribution of the light 60 exists in the region between the curve U and the curve θ′. The curve θ′ corresponds to a curve obtained by shifting the curve θ0 shown in FIG. 38 downwards. Comparing this region with the region Z shown in FIG. 38, the angular distribution of the light 60 is converted into the angular distribution of the light having smaller angles, and the degree of parallelism of the light is improved.

[0175] Region M is between the angle of 13° and the angle of −13° and shows the angular range in which, when light travels in the second light guide plate 54, light is totally reflected by the interface between the second light guide plate 54 and the low refractive index layer 48. When the light 60 is totally reflected by the interface between the second light guide plate 54 and the low refractive index layer 48, the light is sufficiently propagated all over the second light guide plate 54. This feature is preferable for the lighting unit. In the region between the angle of 13° and the curve θ′, and in the portion on the left side of the line a, light is reflected by the slope 56A and falls within the region M. In the portion on the right side of the line a, light is reflected by the slope 56A and falls within the region M, but even if the light is not reflected by the slope 56A, the light falls within the region M from the beginning.

[0176] FIG. 40 shows an angular distribution of light that has an angle permitting part of the light 58 to be reflected by the slope 56A (the region Y in FIG. 38) and that has passed through the light converging means 56. In this case, the angular distribution of the light 60 is between the curve V and the curve W. Comparing this region with the region Y in FIG. 38, the angular distribution of the light 60 is converted to the angular distribution of the light having smaller angles, and the degree of parallelism of the light is improved. Light which falls within the portion on the left side of the line b is reflected by the slope 56A and falls within the region M. Light which falls within the portion on the right side of the line b is reflected by the slope 56A and falls within the region M, but the light falls within the region M from the beginning, even if it is not reflected by the slope 56A.

[0177] In FIGS. 38 to 40, the line N passing through the length of the light converging means 56 of 0.8 mm is shown. In the range of the length of the light converging means 56 between 0.8 mm (the angle α of the slope is approximately 18°) to 3.5 mm (the angle α of the slope is approximately 4.1°), the quantity of light that is propagated by the total reflection increases, but if the length of the taper further increases, the effect of light convergence hardly changes (in particular, FIG. 40). Moreover, within the light that is propagated by total reflection, the quantity of light nearly parallel to the axis of the second light guide plate (ranging between +5° to −5°) increases. Thus, the light outputting prisms of the second light guide plate 54 are arranged at an angle approximate to 45°, so the amount of light to be irradiated perpendicularly to the liquid crystal panel 42 increases.

[0178] Consequently, assuming that a total reflection angle at which light is reflected by the second light guide plate 54 is θc and the angle of the slope 56A is α, α<1.5 θc should be satisfied. The relationship of α<1.5 θc is established when the total reflection angle θc is set to 13° and the angle α of the slope 57A is set to approximately 18°. Consequently, the