Illumination device and display device

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-144558, filed Aug. 6, 2019, the entire contents of which are incorporated herein by reference.

Embodiments described herein relate generally to an illumination device and a display device.

Display devices such as liquid crystal display devices, for example, comprise a display panel containing pixels and an illumination device such as a backlight for illuminating the display panel. The illumination device comprises a light source which emits light and a light guide to which light from the light source is irradiated. The light from the light source enters the light guide from a side surface thereof, propagates inside the light guide, and is emitted from an emission surface which corresponds to one of main surfaces of the light guide.

When non-uniformity of luminance occurs in the emission surface of the light guide, the quality of images displayed by the display panel can be degraded as well. For example, when the viewing angle of light emitted from the light source is narrow, a desired luminance may not be obtained in a region close to the light source on the emission surface of the light guide. In this case, in the region near the light source of the emission surface, light of a luminance sufficient for the display panel cannot be supplied.

FIG. 1 is a decomposed perspective view showing a configuration example of a display device of an embodiment.

FIG. 2 is a plan view of an illumination device shown in FIG. 1 .

FIG. 3 is a cross-sectional view showing a display device shown in FIG. 1 .

FIG. 4 is a diagram illustrating shapes of a first layer and a second layer, which is a perspective view of the illumination device shown in FIG. 3 .

FIG. 5 is a partial cross section of the light guide, the first layer and the light source shown in FIG. 3 .

FIG. 6 is a cross section of the illumination device near the boundary.

FIG. 7 is a cross section of the first layer and the light guide near the side surface.

FIG. 8 is a cross section of a modified example of the embodiment.

In general, according to one embodiment, there is provided an illumination device comprising: a first area; a second area adjacent to the first area in a first direction; a first light guide comprising a first main surface located in the first area and the second area, a second main surface located in the first area and the second area, on an opposite side to the first main surface, a first side surface located in the first area and a second side surface located in the second area on an opposite side to the first side surface in the first direction; a plurality of first light sources opposing the second side surface; a second light guide comprising a third main surface opposing the second main surface and located in the first area and the second area, a fourth main surface located in the first area and the second area, on an opposite side to the third main surface, a third side surface located in the first area, and a fourth side surface located in the second area and opposing the third side surface in the first direction; a plurality of second light sources opposing the third side surface; a first layer including a plurality of first prisms provided on the second main surface; and a second layer including a plurality of second prisms provided on the fourth main surface. The first layer extends from the first area over a boundary between the first area and the second area to between the second side surface and the boundary, and the second layer extends from the second area over the boundary to between the third side surface and the boundary.

According to another embodiment, there is provided a display device comprising: an illumination device; and a display panel which displays images. The illumination device comprises a first area; a second area adjacent to the first area in a first direction; a first light guide comprising a first main surface located in the first area and the second area, a second main surface located in the first area and the second area, on an opposite side to the first main surface, a first side surface located in the first area and a second side surface located in the second area on an opposite side to the first side surface in the first direction; a plurality of first light sources opposing the second side surface; a second light guide comprising a third main surface opposing the second main surface and located in the first area and the second area, a fourth main surface located in the first area and the second area, on an opposite side to the third main surface, a third side surface located in the first area, and a fourth side surface located in the second area and opposing the third side surface in the first direction; a plurality of second light sources opposing the third side surface; a first layer including a plurality of first prisms provided on the second main surface; and a second layer including a plurality of second prisms provided on the fourth main surface. The first layer extending from the first area over a boundary between the first area and the second area to between the second side surface and the boundary, and the second layer extending from the second area over the boundary to between the third side surface and the boundary. The display panel oppose the first main surface.

Embodiments will be described hereinafter with reference to the accompanying drawings. The disclosure is a mere example, and arbitrary change of gist which can be easily conceived by a person of ordinary skill in the art naturally falls within the inventive scope.

To more clarify the explanations, the drawings may pictorially show width, thickness, shape and the like, of each portion as compared with an actual aspect, but they are mere examples and do not restrict the interpretation of the invention. In the present specification and drawings, elements like or similar to those in the already described drawings may be denoted by similar reference numbers and their detailed descriptions may be arbitrarily omitted.

In the embodiments, a transmissive liquid crystal display device will be explained as an example of the display device DSP. Note that the main configuration disclosed in the embodiments is also applicable to liquid crystal display devices comprising a reflective function which reflects external light to utilize the reflecting light for displaying, in addition to the transmissive function, electronic-paper display devices comprising an electrophoretic element, display devices which apply micro-electromechanical systems (MEMS), display devices which apply electrochromism, or the like.

FIG. 1 is a decomposed perspective view showing a configuration example of a display device DSP according to an embodiment.

As shown in FIG. 1 , a direction X, a direction Y and a direction Z are orthogonal to each other, but they may intersect with an angle other than 90 degrees. The direction X and the direction Y correspond to directions parallel to the main surface of a substrate which constitutes the display device DSP, and the direction Z corresponds to a thickness direction of the display device DSP.

The display device DSP comprises a display panel PNL, an illumination device IL, an IC chip 1 and a wiring substrate 2 .

The display panel PNL comprises a first substrate SUB 1 and a second substrate SUB 2 . The display panel PNL includes a display area DA on which images are displayed. The display panel PNL includes, for example, a plurality of pixels PX arrayed in a matrix, in the display area DA.

The IC chip 1 and the wiring substrate 2 may read signals from the display panel PNL, but they mainly function as signal sources which supply signals to the display panel PNL. The signal sources are mounted on one piece of the display panel PNL, in which the first substrate SUB 1 is exposed from second substrate SUB 2 . Note that the IC chip 1 may be mounted on the wiring substrate 2 . The wiring substrate 2 is, for example, a flexible printed board which can be bent.

The illumination device IL illuminates the display panel PNL. The illumination device IL comprises a light guide LG 1 , a light guide LG 2 , a plurality of light sources LS 1 and a plurality of light sources LS 2 . The light guide LG 2 , the light guide LG 1 , the first substrate SUB 1 and the second substrate SUB 2 are arranged in this order along the direction Z.

The light guide LG 1 is formed into a flat plate parallel to an X-Y plane defined by the direction X and the direction Y. The light guide LG 1 comprises a main surface 1 A opposing the display panel PNL, a main surface 1 B on an opposite side to the main surface 1 A, a side surface SF 1 and a side surface SF 2 on an opposite side to the side surface SF 1 . The side surface SF 1 and the side surface SF 2 are X-Z planes defined by, for example, the direction X and the direction Z. The light guide LG 1 has a thickness T 1 . The thickness T 1 is a length taken from the main surface 1 A to the main surface 1 B in the direction Z.

The light sources LS 1 are arranged to be spaced apart from each other in the direction X. The light sources LS 1 each oppose the side surface SF 2 .

The light guide LG 2 is formed into a flat plate parallel to the X-Y plane. The light guide LG 2 comprises a main surface 2 A opposing the main surface 1 B, a main surface 2 B on an opposite side to the main surface 2 A, a side surface SF 3 and a side surface SF 4 on an opposite side to the side surface SF 3 . The side surface SF 3 and the side surface SF 4 are, for example, X-Z planes. The light guide LG 2 has a thickness T 2 . The thickness T 2 is a length taken from the main surface 2 A to the main surface 2 B in the direction Z. In the example illustrated, the thickness T 1 and the thickness T 2 are substantially the same as each other. Note that the thickness T 1 and the thickness T 2 may be same as or different from each other.

The light sources LS 2 are arranged to be spaced apart from each other in the direction X. The light sources LS 2 each oppose the side surface SF 3 .

The light sources LS 1 and the light sources LS 2 each are a laser light source such as a semiconductor laser which emits polarized laser beams, for example. Note that the light sources LS 1 and the light sources LS 2 are not limited to those emitting laser beams, but they may be, for example, light emitting diodes.

The light sources LS 1 and the light sources LS 2 each may comprise a plurality of light emitting elements emitting light of different colors, respectively. For example, if the light sources LS 1 and the light sources LS 2 each comprise three light emitting elements emitting red, green, blue light, respectively, light of mixture of these colors (for example, white) can be obtained.

FIG. 2 is a plan view of the illumination device IL shown in FIG. 1 .

As shown in FIG. 2 , the illumination device IL comprises a first area A 1 , a second area A 2 and a boundary BO between the first area A 1 and the second area A 2 .

The first area A 1 and the second area A 2 are adjacent to each other in the direction Y. The first area A 1 has a length 10 in the direction Y, and the second area A 2 has a length 20 in the direction Y. The length 10 and the length 20 are substantially equal to each other. Note that the length 10 and the length 20 may not necessarily be equal to each other. In the example illustrated, the light guide LG 1 and the light guide LG 2 are located respectively over the entire regions of the first area A 1 and second area A 2 . In other words, the main surface 1 A, the main surface 1 B, the main surface 2 A and the main surface 2 B shown in the FIG. 2 are located in the first area A 1 and the second area A 2 , respectively. The side surface SF 1 and the side surface SF 3 are located in the first area A 1 and the side surface SF 2 and the side surface SF 4 are located in the second area A 2 . The side surface SF 1 and the side surface SF 2 oppose each other in the direction Y and the side surface SF 3 and the side surface SF 4 oppose each other in the direction Y. The side surface SF 1 and the side surface SF 3 overlap each other and the side surface SF 2 and the side surface SF 4 overlap each other. The boundary BO is equivalent to each of a middle point between the side surface SF 1 and the side surface SF 2 and a middle point between the side surface SF 3 and the side surface SF 4 .

The light sources LS 1 each emit light in an emission direction DL 1 towards the side surface SF 2 . The intensity of light emitted from the light sources LS 1 is at maximum in an optical axis AX 1 , and the emission direction DL 1 is parallel to the optical axis AX 1 . The light sources LS 2 each emit light in an emission direction DL 2 towards the side surface SF 3 . The intensity of light emitted from the light sources LS 2 is at maximum in an optical axis AX 2 , and the emission direction DL 2 is parallel to the optical axis AX 2 .

FIG. 3 is a cross-section showing the display device DSP shown in FIG. 1 .

As shown in FIG. 3 , the display panel PNL further comprises a liquid crystal layer LC, a sealant SE, a polarizer PL 1 and a polarizer PL 2 .

The liquid crystal layer LC and the sealant SE are located between the first substrate SUB 1 and the second substrate SUB 2 . The sealant SE adheres the first substrate SUB 1 and the second substrate SUB 2 together and also seals the liquid crystal layer LC.

The polarizer PL 1 is adhered onto a lower surface of the first substrate SUB 1 . The second polarizer PL 2 is adhered on an upper surface of the second substrate SUB 2 . A polarizing axis of the polarizer PL 1 and a polarizing axis of the polarizer PL 2 are, for example, orthogonal to each other.

The illumination device IL further comprises a first layer P 1 , a second layer P 2 , a diffusion sheet DS, a prism sheet PS and a reflective sheet RS.

The diffusion sheet DS is located between the display panel PNL and the light guide LG 1 . The diffusion sheet DS diffuses light entering the diffusion sheet DS to equalize the luminance of the light. The prism sheet PS is located between the diffusion sheet DS and the light guide LG 1 . The prism sheet PS concentrates the light emitted from, for example, the main surface 1 A of the light guide LG 1 in the direction Z. The reflective sheet RS opposes the main surface 2 B of the light guide LG 2 . The reflective sheet RS reflects light leaking from within the light guide LG 2 , for example, to re-enter the light guide LG 2 .

The first layer P 1 and the second layer P 2 each are a layer including a plurality of prisms, which will be described later.

The first layer P 1 is located on the main surface 1 B. The first layer P 1 extends from the first area A 1 over the boundary BO to between the boundary BO and the side surface SF 2 . The first layer P 1 comprises an end portion E 10 and an end portion E 11 on an opposite side to the end portion E 10 . The end portion E 10 is located between the boundary BO and the side surface SF 2 , so as to be close to the boundary BO. The end portion E 11 is close to the side surface SF 1 .

The second layer P 2 is located on the main surface 2 B. The second layer P 2 extends from the second area A 2 over the boundary BO to between the boundary BO and the side surface SF 3 . The second layer P 2 has an end portion E 20 and an end portion E 21 on an opposite side to the end portion E 20 . The end portion E 20 is located between the boundary BO and the side surface SF 3 , so as to be close to the boundary BO. The end portion E 21 is close to the side surface SF 4 . The first layer P 1 and the second layer P 2 overlap each other the boundary BO and in the vicinity of the boundary BO in the direction Z.

The light source LS 1 is located remote from the side surface SF 2 . The emission direction DL 1 of the light source LS 1 is a direction crossing a normal direction of the side surface SF 2 . The light source LS 2 is located remote from the side surface SF 3 . The emission direction DL 2 of the light source LS 2 is a direction crossing a normal direction of the side surface SF 3 .

Next, light beams L 1 emitted by the light sources LS 1 and light beams L 2 emitted by the light sources LS 2 will be described.

The light beams L 1 emitted by the light sources LS 1 are refracted by the side surface SF 2 , so as to enter the light guide LG 1 . Of the light beams L 1 entering the light guide LG 1 , light beams traveling towards the main surface 1 A are reflected by an interface between the light guide LG 1 and an air layer. On the other hand, of the light beams L 1 entering the light guide LG 1 , light beams traveling towards the main surface 1 B are reflected by an interface between the light guide LG 1 and the air layer. Thus, in a region of the second area A 2 , where the first layer P 1 is not provided, the light beams L 1 travel in the light guide LG 1 while repeatedly reflected. Of the light beams L 1 traveling in the light guide LG 1 , a travel direction of light advancing towards the first layer P 1 from the light guide LG 1 is changed by a prism of the first layer P 1 and thus the condition for total reflection of the main surface 1 A is unapplied, thus enabling the emission of the light from the main surface 1 A. The light emitted from the main surface 1 A illuminates the display panel PNL via the prism sheet PS and the diffusion sheet DS. That is, in the region of the second area A 2 , where the first layer P 1 is not provided(, or in the vicinity of the side surface SF 2 ), the incident light beams L 1 are inhibited from entering the display panel PNL from the side surface SF 2 .

Similarly, light beams L 2 emitted by the light sources LS 2 are refracted by the side surface SF 3 and enter the light guide LG 2 . In a region of the first area A 1 , where the second layer P 2 is not provided, the incident light beams L 2 advance in the light guide LG 2 while repeatedly reflected by the main surface 2 A and the main surface 2 B. Of the light traveling in the light guide LG 2 , a travelling direction of the light advancing towards the second layer P 2 from the light guide LG 2 is changed by a prism of the second layer P 2 , and thus the condition for total reflection of the main surface 2 A is unapplied, thus enabling the emission of the light from the main surface 2 A. The light emitted from the main surface 2 A illuminates the display panel PNL via the light guide LG 1 , the prism sheet PS and the diffusion sheet DS. That is, in the region of the first area A 1 , where the second layer P 2 is not provided(, or in the vicinity of the side surface SF 3 ), the incident light beams L 2 are inhibited from entering from the side surface SF 3 to the display panel PNL.

The display panel PNL is illuminated by the emission light beams L 1 from the light sources LS 1 in the first area A 1 . The display panel PNL is illuminated by the emission light beams L 2 from the light source LS 2 in the second area A 2 .

Generally, the emission light beams from the light sources arranged at intervals each travel inside the light guide while scattering, but in the vicinities of the light sources, the emission light beams do not sufficiently mix each other. Therefore, in a display device which utilizes such light as illumination light, stripe-like non-uniformity in luminance and chromatic shift due to the difference in intensity may be visually recognized when planarly viewed. The difference in intensity between the illumination light beams is reduced further as the location is distant further from the light sources.

According to the embodiment, in the region of the second area A 2 , where the first layer P 1 is not provided, the incident light beams L 1 from the side surface SF 2 are contained in the light guide LG 1 and inhibited from entering the display panel PNL. In the second area A 2 , the light beams L 1 from the light sources LS 1 hardly enter the display panel PNL, but the light beams L 2 from the light sources LS 2 illuminate the display panel PNL. The first area A 1 is distant from the side surface SF 2 by a distance sufficient for the light beams L 1 to mix with each other. Thus, degradation of the illumination quality due to non-uniformity in luminance and the chromatic shift of the illumination light can be suppressed in the first area A 1 .

Similarly, in the region of the first area A 1 , where the second layer is not provided, the incident light beams L 2 from the side surface SF 3 are contained in the light guide LG 2 , and inhibited from entering the display panel PNL. In the first area A 1 , the light beams L 2 from the light sources LS 2 hardly enter the display panel PNL, but the light beams L 1 from the light sources LS 1 illuminate the display panel PNL. The second area A 2 is distant from the side surface SF 3 by a sufficient distance for the light beams L 2 to mix with each other. Thus, degradation of the illumination quality due to the non-uniformity of the illumination light can be suppressed in the second area A 2 .

Further, the first layer P 1 extends over the boundary BO to the second area A 2 , and the second layer P 2 extends over the boundary BO to the first area A 1 . Therefore, it is possible to avoid the reduction of the luminance level of the emission light of the illumination device IL in vicinity of the boundary BO. Note that when the end portion E 10 of the first layer P 1 and the end portion E 20 of the second layer P 2 are each located on the boundary BO, the luminance level of the emission light of the illumination device IL reduces in vicinity of the boundary BO.

FIG. 4 is a diagram illustrating the shape of the first layer P 1 and the second layer P 2 , and is a perspective view of the illumination device IL shown in FIG. 3 . FIG. 4 shows only a part of light guide LG 1 , a part of the light guide LG 2 , a part of the first layer

P 1 and a part of the second layer P 2 in the illumination device IL.

As shown in FIG. 4 , the first layer P 1 includes a plurality of prisms PA. The first layer P 1 is constructed by the prisms PA arranged intermittently in the direction Y. The second layer P 2 includes a plurality of prisms PB. The second layer P 2 is constructed by the prisms PB arranged intermittently in the direction Y. The prisms PA are provided in the main surface 1 B. The prisms PB are provided in the main surface 2 B.

The prisms PA project from the main surface 1 B towards the main surface 2 A. The prisms PA each have a cross section parallel to the Y-Z plane, which is triangular, and extend in the direction X. In the example illustrated, the cross sections of the prisms PA are similar to each other. The prisms PA each comprise a slope SL 1 , a slope SL 2 , a reference plane BL 1 , an intersection TL 1 and a height HA. The slope SL 1 is located on a side surface SF 1 side and the slope SL 2 is located on a side surface SF 2 side. The reference plane BL 1 is located on the plane same that of the main surface 1 B. The intersection TL 1 is a line where the slope SL 1 and the slope SL 2 intersect each other. The intersections TL 1 are arranged at equal intervals 30 in the direction Y. The intervals 30 are, for example, 0.1 mm. In the example illustrated, an angle θ 11 made between the slope SL 1 and the reference plane BL 1 and an angle θ 12 between the slope SL 2 and the reference plane BL 1 are equal to each other. Note that the angle θ 11 corresponds to one of the interior angles of the cross section of the prism PA, and the angle θ 12 corresponds to another one of the interior angles of the cross section of the prism PA. The cross section of the prism PA is an isosceles triangle. The height HA is the height of the prism PA in the normal direction of the main surface 1 B. The height HA is equivalent to the length taken from the reference plane BL 1 to the intersection TL 1 along the direction Z.

The height HA of the prism PA decreases as the location approaches from the side surface SF 1 to the side surface SF 2 (in a direction indicated by an arrow pointing the direction Y). For example, the height HA of the prism PA becomes higher as the location approaches from the end portion E 10 of the first layer P 1 to the end portion E 11 shown in FIG. 3 . As the location approaches from the end portion E 10 to the end portion E 11 , the ratio of the prism PA (the reference plane BL 1 ) per unit area in the X-Y plane increases and the ratio of the main surface 1 B per unit area in the X-Y plane decreases. Further, when the light travelling in the light guide LG 1 advances to the prism PA of the first layer P 1 and then is emitted from the light guide LG 1 , the quantity of light traveling in the light guide LG 1 decreases. Thus, the illumination device IL can irradiate the illumination light of a uniform brightness distribution to the display panel PNL in the first area A 1 .

The prisms PB each project from the main surface 2 B in the direction Z. The prisms PB each have a cross section parallel to the Y-Z plane, which is triangular, and extend in the direction X. In the example illustrated, the cross sections of the prisms PB are similar to each other. The prisms PB each comprise a slope SL 3 , a slope SL 4 , a reference plane BL 2 , an intersection TL 2 and a height HB. The slope SL 3 is located on a side surface SF 3 side, and the slope SL 4 is located on a side surface SF 4 side. The reference plane BL 2 is located on the plane same as that of the main surface 2 B. The intersection TL 2 is a line where the slope SL 3 and the slope SL 4 intersect each other. The intersections TL 2 are arranged at equal intervals 30 along the direction Y. In the example illustrated, an angle θ 13 made by the slope SL 3 and the reference plane BL 2 and an angle θ 14 made by the slope SL 4 and the reference plane BL 2 are equal to each other. Note that the angle θ 13 corresponds to one of the interior angles in the cross section of the prism PB, and the angle θ 14 corresponds to another one of the interior angles in the cross section of the prism PB. The cross section of the prism PB is an isosceles triangle. The height HB is the height of the prism PB in the normal direction of the main surface 2 B. The height HB is equivalent to the length taken from the reference plane BL 2 to the intersection TL 2 along the direction Z.

The height HB of the prism PB decreases as the location approaches from the side surface SF 4 to the side surface SF 3 (in an opposite direction indicated by the arrow pointing the direction Y). As the location approaches from the side surface SF 4 to the side surface SF 3 , the ratio of the prism PB (the reference plane BL 2 ) per unit area in the X-Y plane decreases and the ratio of the main surface 2 B per unit area in the X-Y plane increases. Further, when the light travelling in the light guide LG 2 advances to the prism PB of the second layer P 2 and then is emitted from the light guide LG 2 , the quantity of light traveling in the light guide LG 2 decreases. Thus, the illumination device IL can irradiate the illumination light of a uniform brightness distribution to the display panel PNL in the second area A 1 .

FIG. 5 is a partial cross-sectional view of the light guide LG 1 , the first layer P 1 and a light source LS 1 shown in FIG. 3 .

As shown in FIG. 5 , the light source LS 1 comprises a light emission point LP 1 and a light emission surface LF 1 . The light emission point LP 1 is a point from where a light beam L 1 having an optical axis AX 1 parallel to an emission direction DL 1 is emitted. The light beam L 1 emitted from the light emission point LP 1 travels while spreading. The emission surface LF 1 corresponds to, for example, an outer surface of the light source LS 1 .

The emission direction DL 1 is inclined to the direction Y and the direction Z. The emission direction DL 1 and the side surface SF 2 are not orthogonal to each other. That is, the emission direction DL 1 intersects a normal direction of the side surface SF 2 . With this configuration, the light beam L 1 is refracted when entering the light guide LG 1 . An incident angle θ 1 of the light beam L 1 to the light guide LG 1 is smaller than an angle made by the emission direction DL 1 and the direction Y. Further, the incident angle θ 1 is equal to the angle θ 11 .

In the example illustrated, the light beam L 1 traveling through the light guide LG 1 is reflected by the slope SL 1 of the prism PA. The light beam L 1 reflected by the slope SL 1 is inapplicable to the total reflection condition of the main surface 1 A and is refracted by an interface between the main surface 1 A and the air layer, to be emitted at an emission angle θ 2 from the main surface 1 A. The emission angle θ 2 is an angle made by the light emitted from the main surface 1 A and the normal line of the main surface 1 A. The refractive index of the light guide LG 1 is represented by n.

A relationship between the angle θ 11 of the prism PA and the refractive index n of the emission angle θ 2 , can be expressed by the following formula.

A structure similar to that of FIG. 5 can be applied to the side surface SF 3 and the light sources LS 2 as well. That is, the emission direction DL 2 of the light source LS 2 and the side surface SF 3 are not orthogonal, but intersects the emission direction DL 2 and the normal direction of the side surface SF 3 . Note that the structure of the side surface SF 3 and the light sources LS 2 may be different from the structure shown in FIG. 5 .

FIG. 6 is a cross section of the illumination device IL in the vicinity of the boundary BO.

As shown in FIG. 6 , the first layer P 1 has a length D 1 in the second area A 2 . The length D 1 is equivalent to a length taken from the boundary BO to the end portion E 10 in the direction Y. The second layer P 2 has a length D 2 in the first area A 1 . The length D 2 is equivalent to a length taken from the boundary BO to the end portion E 20 in the direction Y. In the example illustrated, the length D 2 is equal to twice the length D 1 . Note that it suffices if the length D 2 is greater than the length D 1 .

A relationship between the length D 1 , the thickness T 1 , the emission angle θ 2 and the refractive index n can be expressed by the following formula.

The length 40 is a length taken from the main surface 2 B to the main surface 1 A in the direction Z. The length 40 is substantially a total of the thickness T 1 and the thickness T 2 (or twice the thickness T 1 ). In the example illustrated, the angle made by the light beam L 1 reflected by the end portion E 10 of the first layer P 1 and the main surface 1 B is angle θ 3 . The angle made by the light beam L 2 reflected by the end portion E 20 of the second layer P 2 and the main surface 2 B is angle θ 4 . The angles θ 3 and θ 4 are acute angles, respectively, and are equal to each other.

According to this embodiment, the first layer P 1 has a length D 1 in the second area A 2 , and the second layer P 2 has a length D 2 in the first area A 1 . The light travelling through the light guide LG 1 is emitted from the main surface 1 A in the boundary BO by the prisms PA located in the end portion E 10 . The light travelling through the light guide LG 2 is emitted from the main surface 1 A in the boundary BO by the prisms PB located in the end portion E 20 . With this configuration, as compared to the case where the first layer P 1 does not extend to the second area A 2 and the second layer P 2 do not extend to the first area A 1 , the illumination device IL can also emit illumination light of a uniform luminance in the vicinity of the boundary BO as well.

FIG. 7 is a cross section of the first layer P 1 and the light guide LG 1 in the vicinity of the side surface SF 1 .

As shown in FIG. 7 , the end portion E 11 of the first layer P 1 is spaced apart from the side surface SF 1 by a distance of a length D 3 in the direction Y. A relationship between the length D 3 , the thickness T 1 , the emission angle θ 2 and the refractive index n, can be expressed by the following formula.

In the example illustrated, the light beam L 1 traveling to the end portion E 11 is reflected by the first layer P 1 , and then advances towards the main surface 1 A without advancing to the side surface SF 1 . Thus, the light beam Ll traveling in the light guide LG 1 and then reflected by the first layer P 1 can be inhibited from being reflected by the side surface SF 1 thereafter, which may cause increase in luminance of the illumination light of the illumination device IL partially in the vicinity of the side surface SF 1 .

A structure similar to that of FIG. 7 can be applied to the end portion E 21 of the second layer P 2 as well. That is, the end portion E 21 of the second layer is spaced apart from the side surface SF 4 by a distance of the length D 3 in the direction Y.

In the configuration example described above, the direction Y is equivalent to the first direction, the direction X is equivalent to the second direction, the light sources LS 1 are equivalent to the first light sources, the light sources LS 2 are equivalent to the second light sources, the light guide LG 1 is equivalent to the first light guide, the light guide LG 2 is equivalent to the second light guide, the main surface 1 A is equivalent to the first main surface, the main surface 1 B is equivalent to the second main surface, the main surface 2 A is equivalent to the third main surface, the main surface 2 B is equivalent to the fourth main surface, the side surfaces SF 1 to SF 4 are equivalent to the first to fourth sides, respectively, the prisms PA are equivalent to the first prisms, the prisms PB are equivalent to the second prisms, slopes SL 1 to SL 4 are equivalent to the first to fourth slopes, respectively, the intersection TL 1 is equivalent to the first intersection, the intersection TL 2 is equivalent to the second intersection, the reference plane BL 1 is equivalent to the first reference plane, the reference plane BL 2 is equivalent to the second reference plane, the end portion E 10 is equivalent to the first end portion, the end portion E 20 is equivalent to the second end portion, the length D 1 is equivalent to the first length, and the length D 2 is equivalent to the second length.

FIG. 8 is a cross section showing a modified example of the embodiment.

As shown in FIG. 8 , a prism PA of the modified example is different from a prism PA of the embodiment in that the angle θ 11 and the angle θ 12 are not equal to each other. The angle θ 12 is greater than the angle θ 11 . In the example illustrated, the angle θ 12 is 90 degrees, and the Y-Z cross section of the prism PA is a right-angled triangle. The area of the slope SL 1 is greater than the area of the slope SL 2 .

The light beam L 1 travelling in the light guide LG 1 advances in the light guide LG 1 while spreading. The diffused light EL shown in the FIG. 8 is equivalent to the light spreading from the light beam L 1 . In the modified example illustrated, the area of the slope SL 1 is greater as compared to the case of FIG. 5 , in which the cross section of the prism PA is a right-angled triangle. With this configuration, chances are higher that the diffused light EL are reflected by the slope SL 1 of the prism PA, thereby making it possible to increase the luminance of the illumination light of the illumination device IL.

A structure similar to that of FIG. 8 can be applied to the prism PB as well. That is, the angle θ 14 is greater than the angle θ 13 , and the area of the slope SL 4 is greater than the area of the slope SL 3 .

As explained above, according to the embodiments, an illumination device which can suppress the deterioration in illumination quality and such a display device can be provided.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.