WO2011033901A1

WO2011033901A1 - Planar illumination device and liquid crystal display device provided with the same

Info

Publication number: WO2011033901A1
Application number: PCT/JP2010/064002
Authority: WO
Inventors: 昌広横田; 健高橋; 修小野; 秀三松田
Original assignee: 株式会社東芝
Priority date: 2009-09-17
Filing date: 2010-08-19
Publication date: 2011-03-24
Also published as: JP2011065891A

Abstract

Disclosed is a planar illumination device provided with a plurality of light sources and a diffusion layer which receives light from the light sources. The diffusion layer is positioned without making contact with the light sources, and D ≤ 8 mm when the minimum separation from the light sources to the diffusion layer is taken as D.

Description

Planar illumination device and liquid crystal display device including the same

Embodiments described herein relate to a thin planar illumination device using a point light source such as an LED, and a liquid crystal display device including the same.

A planar illumination device is a device that emits light emitted from a light source from a planar radiation surface. Such a surface illuminator is used as an illuminator by itself, and is also used in a liquid crystal display device in combination with a liquid crystal display panel.

As a recent trend, from the viewpoint of mercury-free, the light source of the planar lighting device has been actively replaced with the LED from the mainstream cathode ray tube. In general, the planar illumination device includes an LED mounted on an LED substrate, and a diffusion layer disposed with a distance D from these light sources. The LEDs that are point light sources are arranged in a matrix over the entire surface of the LED substrate at a predetermined arrangement pitch. The light emitted from each light source is overlapped with the light from other light sources on the diffusion layer by expanding the space of the interval D, so that the illuminance is uniform over the entire surface. Thereby, the light radiated from the diffusion layer to the front surface is illuminated with uniform brightness over the entire surface.

In a large liquid crystal display device, a technique called local dimming tends to be introduced as a technique for driving a planar illumination device using a point light source (for example, Patent Document 1). Local dimming is a technique for individually adjusting the amount of light emitted from each point light source or a plurality of point light source groups, and is a technique for dramatically improving the contrast and power consumption of an image display device.

Explain the improvement effect by local dimming and conventional problems. Here, for example, assuming a screen displaying a crescent moon in the night sky, local dimming and conventional driving (non-local dimming) are compared. The planar illumination device is divided into a plurality of dimming areas composed of point light sources or point light source groups arranged in a matrix, and in local dimming driving, dimming is performed to the required luminance for each dimming area according to the display image.

In the conventional driving (non-local dimming), the planar illumination device is driven with the maximum luminance over the entire surface regardless of the display image, so that the power consumption increases. In a 50-inch class liquid crystal display device, the power consumption of the LED light source is 200 to 300 W in the case of a standard 500 cd / m 2 display, accounting for about 80% of the total power of the liquid crystal display device.

On the other hand, in local dimming, only the light emission amount required for the display image is driven, so that the power consumption of the LED light source is greatly reduced. For example, in the crescent moon image, the light emission area is very small, so the power consumption of the light source is almost zero. Further, even for a general broadcast image, the power consumption is reduced to about half compared to the conventional driving. In conventional driving, the entire planar illumination device emits light even when displaying a black image, so that the screen is whitened by the light leaking from the liquid crystal display panel and the contrast deteriorates, and the limit is about 1: 1000. Also in this case, in the local dimming, when the black image is displayed, the planar illumination device itself becomes dark, so the contrast ratio is substantially improved to 1: 100000 level, and the black image can be displayed even in a dark room.

However, in the local dimming drive using the conventional planar illumination device, the following two problems to be improved remain.
One of them is a decrease in luminance gradation during partial lighting. This is a phenomenon that occurs when there are few light control areas that emit light, and is a phenomenon in which the brightness of the corresponding light control area decreases because light leaks outward from the light control areas that emit light. In a conventional planar lighting device, for example, the luminance when only one area is lit by 512-division local dimming drive is about 10 to 20% of the luminance of one area when the planar lighting device is fully lit. It will fall. For this reason, in the image with much black, the brightness of the crescent moon is remarkably impaired.

The other one is contrast. In the above description, it has been described that the contrast is 1: 100000 due to local dimming, but this is a luminance difference between the illuminated region and a region far from the illuminated region. In actual display, the luminance difference (contrast) between adjacent regions is important. When light leaks from the light control area to the outside, the region that should be black naturally floats and the contrast deteriorates. For example, when only one area is turned on by 512-division local dimming driving, the luminance due to leakage light at a location 20 mm away from the edge of the area is about 50% of the center luminance of the area. For this reason, a sufficient contrast between adjacent regions cannot be ensured.

As another planar illumination device, a planar illumination device is proposed in which a point light source is enclosed by a reflective film, the brightness uniformity of each light source is ensured by the upper transmission reflective film, and a plurality of these point light sources are arranged side by side. (For example, Patent Document 2).

However, in such a planar lighting device, since each light source is surrounded by a reflecting wall and the independence of each light source is high, there are some problems. First, when the planar illumination device is used in a local dimming-driven liquid crystal display device, a change in luminance is clearly recognized at the boundary between light sources with different light control gradations. This is because the brightness changes abruptly at the reflection side wall portion, and in order to make this boundary unevenness inconspicuous, a profile that leaks into a gentle adjacent region and attenuates is essential. Second, since the light source has variations in individual chromaticity and brightness, in a surface illumination device that lights with uniform power over the entire surface, a sudden change in chromaticity or brightness is visually recognized at the boundary between the light sources. Will be. When the specification of the light source is limited so as to reduce the difference in chromaticity and luminance between the light sources, the cost increases. In order to avoid this, it is necessary to smooth the fluctuation at the boundary of chromaticity and luminance by natural light leakage to the adjacent area.

As described above, a plurality of point light sources such as LEDs are arranged as the light source of the surface illumination device, and these light sources are dimmed for each region so that illumination can be performed not only with uniform luminance but also with an arbitrary luminance distribution. There is a need for a planar lighting device. However, in the conventional planar illumination device, there is a large amount of light leaking from one dimming area to an adjacent area, and there is a problem that the luminance in the partially lit area decreases or the contrast between adjacent areas deteriorates. is there. Moreover, if it is going to solve these, there exists a subject by which the boundary between area | regions is visually recognized as a nonuniformity.

Japanese Patent No. 2582644 JP 2008-27886 A

FIG. 1 is a cross-sectional view illustrating a liquid crystal display device including a planar illumination device according to the first embodiment. FIG. 2 is a cross-sectional view showing a planar illumination device according to a comparative example. FIG. 3 is a diagram showing luminance attenuation of one light source luminance profile when the distance D between the light source and the diffusion layer is changed. FIG. 4 is a diagram showing the relationship between the actually measured interval D and the 1 dimming area lighting luminance rate. FIG. 5A is a diagram showing a display state of the planar illumination device and the liquid crystal display panel according to the present embodiment. FIG. 5B is a diagram showing a display state of a planar illumination device and a liquid crystal display panel by local mixing according to a comparative example. FIG. 6 is a cross-sectional view illustrating a liquid crystal display device including the planar illumination device according to the second embodiment. FIG. 7 is a diagram showing the relationship between the transmittance of the light guide layer and the attenuation of light. FIG. 8 is a diagram schematically showing an isoluminance shape of a light source of the planar illumination device according to the first and second embodiments. FIG. 9 is a diagram schematically showing an isoluminance shape of a light source of a surface illumination device according to a comparative example.

According to the embodiment, the planar illumination device includes a plurality of light sources and a diffusion layer that receives light from the light sources, and the diffusion layer is provided in a non-contact manner with respect to the light source, and the diffusion layer is formed from the light source. When the interval up to is D, D ≦ 8 mm.

The planar illumination device according to another embodiment is provided between a plurality of light sources, a diffusion layer that receives light from the light sources, and the light source and the diffusion layer, and at least partially reflects and partially transmits light. A light-transmitting film of one layer, and at least one light-guide layer provided between the light source and the reflective-transmitting film, and the transmittance T per mm of the light-guiding layer is T ≦ 90% It is said.

Hereinafter, various liquid crystal display devices including planar illumination devices according to embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a cross-sectional view illustrating a liquid crystal display device including the planar illumination device according to the first embodiment. As shown in FIG. 1, the liquid crystal display device includes a rectangular liquid crystal display panel 30 and a planar illumination device 20 disposed to face the back side of the liquid crystal display panel 30.

The liquid crystal display panel 30 includes a rectangular array substrate 32, a rectangular counter substrate 34 disposed to face the array substrate 32 with a gap therebetween, and a liquid crystal layer sealed between the array substrate 32 and the counter substrate 34. 36. The planar illumination device 20 is formed in a rectangular surface shape having a size corresponding to the liquid crystal display panel 30, and is provided adjacent to the array substrate 32 of the liquid crystal display panel 30.

The planar lighting device 20 includes a rectangular circuit board 10, a lower reflection film 12 that is formed on the upper surface of the circuit board 10 to diffuse or diffusely reflect light, and a lower reflection film 23 on the circuit board 10. A plurality of LEDs 16 disposed on the LED 16, and a rectangular diffusion layer 14 disposed above the LEDs 16 and facing the LEDs with a distance D therebetween. On the LED side of the diffusion layer 14, a transmission / reflection film 18 that partially transmits and partially reflects light is provided. A light guide layer 21 is sandwiched between the lower reflective film 12 and the transmissive reflective film 18. The light guide layer 21 is formed transparently or formed of air. The outer peripheral part of the diffusion layer 14 and the circuit board 10 is held by a frame (not shown).

The LEDs 16 that are point light sources are arranged in a matrix over the entire surface of the circuit board 10 at an arrangement pitch L, and are electrically connected to the circuit board 10. Each LED 16 irradiates light toward the transmission / reflection film 18. The planar lighting device 20 is disposed in a state where the diffusion layer 14 faces the back surface of the liquid crystal display panel 30. The light emitted from the planar illumination device 20 transmits the liquid crystal display panel 30 to display an image.

The planar illumination device 20 has a control unit 22 that controls the lighting of the LED 16. The control unit 22 is connected to the circuit board 10 and is connected to a main control unit (not shown) of the liquid crystal display device. Based on the video luminance signal sent from the main control unit of the liquid crystal display device, the control unit 22 adjusts the light emission amount for each LED 16 or for each of the adjacent LEDs 16 as one unit. A quantity adjusting unit 24 is provided. That is, the control unit 22 performs dimming of the planar lighting device 20 according to the video information by local mixing that individually drives the plurality of LEDs 16.

In a planar lighting device that is lit and driven by dividing the entire surface into a plurality of dimming areas, it has been stated that when only one dimming area is lit, the luminance deteriorates compared to the luminance when fully lit. This is because the point light source supplies light not only to the corresponding area but also to an adjacent area.

FIG. 2 shows a planar illumination device according to a comparative example, and shows the light reaching the point P on the diffusion layer from the LED. The planar illumination device includes a rectangular circuit board 10, a lower reflective film 12 that is formed on the upper surface of the circuit board 10 and diffuses or diffusely reflects light, and a lower reflective film 213 on the circuit board 10. A plurality of LEDs 16, and a rectangular diffusion layer 14 disposed above the LEDs 16 and facing the lower reflective film 12 with a distance D therebetween. The LEDs 16 are arranged in a matrix over the entire surface of the circuit board 10 at an arrangement pitch L.

The position of the point P on the diffusing layer 14 is separated from the LED by a surface direction distance r with respect to the optical axis of the LED 16 and a distance D between the LED and the diffusing layer 14, and the geometrical ray moving distance is √ (r ² + D ² ). Since the illuminance from the LED 16 at the point P is inversely proportional to the square of the light beam travel distance, it is proportional to 1 / (r ² + D ² ). This is a theoretical calculation that completely ignores the light distribution distribution of the point light source, but it includes the essence of considering leakage, and it has been confirmed experimentally that the luminance profile that attenuates from the point light source almost matches the actual measurement. It has been.

By interpreting this equation, it is necessary to make the distance D between the LED 16 and the diffusion layer 14 as small as possible in order to concentrate the light as much as possible in the corresponding dimming area and prevent the light from leaking out to the adjacent area. is there.

FIG. 3 shows the luminance attenuation of one light source luminance profile when the interval D is changed. The horizontal axis represents the surface direction distance r, and the vertical axis represents the relative luminance with the central luminance being 1.
In the conventional system, recently, a technique for reducing the distance D by attaching a lens to the light source unit and diffusing the light has started to appear, but even in this case, the distance D is about 15 mm. As can be seen from FIG. 3, the luminance is about half of the central luminance at a location 20 mm away from the lighting area, which causes a decrease in central luminance due to leakage and deterioration of contrast in the vicinity.

On the other hand, as the distance D is decreased, as shown in FIG. 3, it is understood that the attenuation increases and leakage to the adjacent area is suppressed. In order to ensure a contrast ratio of 1: 100,000 at a distance of 20 mm from the point light source in the plane direction, when the contrast ratio of the liquid crystal display panel is 1: 1000, the planar illumination device needs 1: 100. As shown in the figure, if the luminance is attenuated to 1/100 at r = 20 mm, it can be seen that the distance D should be 2 mm or less.

As shown in FIG. 1, according to the planar lighting device according to the present embodiment, the diffusion layer 14 is provided in a non-contact manner with respect to the LED 16, and the shortest distance D between the LED 16 and the diffusion layer 14 is set to 2 mm. ing. On the LED side of the diffusion layer 14, a transmission / reflection film 18 that partially transmits and partially reflects light is provided. The uneven pitch of the LED due to the narrowing of the interval D is compensated by the transmittance pattern of the transmissive reflection film 18 to form a uniform planar illumination device over the entire surface.

FIG. 4 shows data obtained by experimentally verifying the degree of improvement in luminance reduction of the one-area lighting luminance when the interval D is changed in the planar illumination device according to the present embodiment. The one-area lighting luminance is a relative luminance with 100% as the total area lighting luminance when only one area in local dimming is lit with full gradation.

In the conventional method, the interval D can only be reduced to about 15 mm, and the one-area lighting luminance has not reached 30%.
On the other hand, in the present embodiment, the interval D can be set small, and in order to make the one-area lighting brightness 35% or more, which was impossible with the conventional method, it is only necessary to set the interval D within 8 mm. It has been confirmed from. With D = 2 mm in the present embodiment, the one-area lighting luminance is improved to about 50%, although it does not reach the small piece experimental value shown in FIG. In addition, about the nonuniformity of the boundary between adjacent light control areas, the problem which arose with the conventional surface illuminating device has not occurred.

5A shows a display state of the planar illumination device and the liquid crystal display panel according to the present embodiment, and FIG. 5B shows a display state of the planar illumination device and the liquid crystal display panel according to the conventional local mixing according to the comparative example. Is shown in comparison. Here, a screen displaying a crescent moon in the night sky is shown. As shown in FIG. 5B, in the local mixing of the comparative example, light leaks over a relatively wide range around the crescent moon, and contrast is low around the crescent moon. According to the planar illumination device according to the embodiment, it can be seen that the amount of light leaking is small and the contrast around the crescent moon is improved.

Next, a liquid crystal display device including the planar illumination device according to the second embodiment will be described.
FIG. 6 is a cross-sectional view of the liquid crystal display device according to the second embodiment.

According to the second embodiment, as the light guide layer 21, an opaque light guide plate having a transmittance of 80% per 1 mm is used by dispersing light diffusion particles in the light guide plate. The shortest distance D (thickness of the light guide layer 21) D between the LED 16 and the diffusion layer 14 is, for example, 2 mm. The transmittance here is based on the measurement method shown in JIS standard K7361, and is the ratio of light that escapes to the front surface when light is vertically incident from the back surface of the light guide layer 21. The other configuration of the planar illumination device is the same as that of the first embodiment described above, and the same reference numerals are given to the same parts, and detailed description thereof is omitted.

In this surface illumination device, the luminance rate when only the LED 16 in one dimming area is lit is improved to over 60%. In the second embodiment, the contrast is improved by speeding up the luminance attenuation to the adjacent region. Details are described below.

By using the opaque light guide layer 21 with a transmittance T per 1 mm, the theoretical expression of luminance attenuation adds the transmission attenuation effect to the attenuation due to the distance described above.
1 / (r ² + D ² ) × exp (−T × | r |)
It becomes. Here, the exponent term indicates the light attenuation effect by the opaque light guide layer 21, and this attenuation indicates that the light attenuates to the transmittance T every time the light travels through the light guide layer by 1 mm.

FIG. 7 is a graph showing a luminance attenuation curve. The horizontal axis represents the distance r from the LED, and the vertical axis represents the relative luminance with the central luminance being 1. From this figure, it can be seen that the attenuation increases as the transmittance of the light guide layer 21 is lowered.

In a place where r = 20 mm, if it is intended to ensure a contrast ratio of 1: 100 as a planar illumination device, the distance D needs to be set to 2 mm in the first embodiment described above. According to the second embodiment, attenuation due to the light scattering transmittance of the light guide layer 21 is added. Therefore, when the interval D = 2 mm and the transmittance 80%, the luminance at a location r = 20 mm away from the LED is 1/10000. Attenuates to As can be seen from FIG. 6, when a light scattering light guide layer having a transmittance of 90% is used, the distance D may be 6 mm in order to attenuate to 1/100. Even when the distance D is set to 6 mm or more, an attenuation effect by the light scattering transmittance of the light guide layer can be obtained.

As described above, the local dimming drive is performed by narrowing the distance D between the point light source and the diffusion layer 14 to 8 mm or less, or by forming the light guide layer 21 as a light guide layer having a light scattering property of 90% or less. In the planar illumination device, the one-area lighting luminance can be improved to 35% or more of the total lighting luminance while ensuring a contrast ratio of 1: 100000 near r = 20 mm.

In the first embodiment, the light leakage is controlled by using the interval between the light guide layers, and in the second embodiment, the opacity of the entire light guide layer is used to improve the one-area lighting luminance. It is good also as a structure. That is, the interval D may be 4 mm, and 2 mm of this half may be an opaque light guide layer. In this case, since the opaque light guide layer portion leaks relatively less than the transparent light guide layer portion, it is possible to obtain substantially the same one-area lighting luminance as the transparent light guide layer having a spacing of 2 mm. In addition, the cost of the opaque light guide plate can be reduced by half compared with the case where all the gaps of 4 mm are filled.

FIG. 8 shows the isoluminance line shape formed by the LED 16 in the planar lighting device according to the first and second embodiments, and FIG. 9 shows the isoluminance formed by the LED 16 in the planar lighting device according to the comparative example. The line shape is shown.

In the first and second embodiments and the comparative example, the LEDs that are the point light sources are arranged in a grid pattern with the arrangement pitch L in the vertical and horizontal directions. In this case, since the diagonal LED spacing becomes wider than the vertical and horizontal spacing, as shown in FIG. 9, in the circular isoluminance line in which the luminance is isotropically attenuated, uneven luminance in the diagonal direction is obtained. It is easy to come out. Therefore, in the first and second embodiments, as shown in FIG. 8, by optimizing the transmittance pattern of the transmission / reflection film 18 for each direction, the non-circular shape in which the isoluminance lines look like a lattice shape. The shape makes it difficult to cause uneven brightness.

FIG. 8 shows a substantially rectangular isoluminance line, but this can be changed according to the LED arrangement. That is, if the LED array is a fine hexagonal array, the isoluminance lines may be substantially hexagonal.

With the configuration as described above, a planar illumination device that appropriately controls light leakage to an adjacent region, is less likely to reduce partial lighting brightness even when driven by local dimming, and can provide sufficient partial contrast is provided. can do.

In addition, the light-scattering light guide layer 21 shown in the second embodiment is configured to disperse transparent particles having a refractive index different from that of the base material, but other configurations can be used as long as the transmittance is controlled. But you can. For example, a configuration in which bubbles are contained in the transparent base material of the light guide layer, an unevenness on the surface of the base material, or a space in the base material may be scattered.

Further, the essence of the present invention is to control light leakage to the adjacent region by narrowing the distance D between the light source and the diffusion layer to 8 mm or less, or by reducing the transmittance of the light guide layer to 90% or less. As a specific realization means, a means other than the transmission / reflection film shown in the first and second embodiments may be used. For example, a lens for diffusing the light distribution attached on the light source, a partial reflection film attached to a position corresponding to the light source of the diffusion layer, or a scattering member may be used.

In the above-described embodiment, the planar illumination device used as the backlight unit of the liquid crystal display device has been described. However, the planar illumination device used as a single illumination device may be used. The LED as the point light source may be white or monochromatic, and is not limited in terms of the type of LED.

The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

Claims

A plurality of light sources, and a diffusion layer that receives light from the light source, the diffusion layer is provided in a non-contact manner with respect to the light source, and when the shortest distance from the light source to the diffusion layer is D,
A planar illumination device in which D ≦ 8 mm.
The planar illumination device according to claim 1, further comprising at least one reflection / transmission film provided between the light source and the diffusion layer and partially reflecting and partially transmitting light from the light source.
The planar illumination device according to claim 1, further comprising an optical lens system provided in the vicinity of the light source and diffusing light emitted from the light source in a lateral direction.
The planar illumination device according to claim 1, further comprising a reflector or a scatterer provided in the vicinity of the light source and diffusing light emitted from the light source in a lateral direction.
The planar illumination device according to claim 1, wherein the luminance profile formed by the one light source is such that the isoluminance lines are non-circular according to the regularity of the light source arrangement.
A plurality of light sources; a diffusion layer that receives light from the light sources; at least one reflective / transmissive film that is provided between the light source and the diffusion layer and that partially reflects and partially transmits light; and the light source And at least one light guide layer provided between the reflective transmission film,
A planar illumination device having a transmittance T per mm of the light guide layer of T ≦ 90%.
The planar illumination device according to claim 6, wherein D ≦ 8 mm, where D is the shortest distance from the light source to the diffusion layer.
The transmittance of the light guide layer depends on particles dispersed in the light guide layer and having a refractive index different from that of the base material of the light guide layer, or bubbles, irregularities on the surface of the light guide layer, or voids in the light guide layer. The planar illumination device according to claim 6, wherein the planar illumination device is defined.
9. The luminance profile formed by the one light source gradually decreases in brightness at a boundary between the adjacent light sources or a group of light sources subjected to collective dimming, and spreads in an adjacent region. Planar lighting device.
9. The luminance profile formed by the one light source spreads over the entire surface without being blocked at a boundary between the adjacent light sources or between the light source groups that are collectively dimmed. The surface illumination device described in 1.
The planar illumination device according to any one of claims 1 to 8, wherein the light source is a point light source.
The distance at which the luminance attenuates to 1/100 with respect to the central luminance when only the one light source is lit at full gradation is within 20 mm from the one light source. The surface illumination device described.
9. The light emission amount adjustment unit according to claim 1, further comprising: a light emission amount adjustment unit that partially adjusts the light emission amount of the light source for each light source or for each unit including a plurality of adjacent light sources as one light source unit. Planar lighting device.
The planar illumination device according to claim 13, wherein the center luminance of the luminance profile when the one light source unit is lit at full gradation is 35% or more of the luminance when all the light source units are lit at full gradation. .
A liquid crystal display panel;
The planar illumination device according to any one of claims 1 to 8, wherein the planar illumination device is disposed so as to face a back surface of the liquid crystal display panel and irradiates the liquid crystal display panel with light.
A liquid crystal display device.