KR20080063679A - Backlight unit using led - Google Patents

Abstract

A backlight unit using an LED is provided to restrain the generation of a dark area and to reduce the dimension of the backlight unit and a liquid crystal display by increasing a light transmittance near a dark area point. A backlight unit includes a plurality of LEDs(103) arranged on a substrate(101) and a diffusion plate(105). The diffusion plate is spaced away from the LED and has a lower surface facing the substrate. The diffusion plate has a density lower than that of a diffusion material at a straight upper section from the LEDs. The diffusion plate has a plurality of low density regions located at a position that is furthermost position from the adjacent straight upper section. A mean diffusion material density of the low density region is lower 7 to 25 % than that of the straight upper section. Alternatively, the mean diffusion material density of the low density region is lower 10 to 20 % than that of the straight upper section. The LEDs are arranged in the form of a matrix having columns and rows.

Description

Backlight unit using LED {BACKLIGHT UNIT USING LED}

FIG. 1A is a cross-sectional view of a conventional backlight unit using LEDs, and FIG. 1B is a plan view of the backlight unit of FIG.

2 is a plan view of a backlight unit according to an embodiment of the present invention.

3 is a cross-sectional view taken along the line SS ′ of the backlight unit of FIG. 2.

FIG. 4 is a partially enlarged view of a portion of FIG. 2 and schematically illustrates a distribution of diffuser particles in a diffuser plate. FIG.

5 is a graph showing the density distribution of the diffusing agent according to an embodiment of the present invention.

6 is a graph showing the density distribution of the diffusing agent according to another embodiment of the present invention.

7 is a plan view of a backlight unit according to another embodiment of the present invention.

8 is a plan view of a backlight unit according to another embodiment of the present invention.

<Description of the symbols for the main parts of the drawings>

100, 200, 300: backlight unit 101: substrate

102: reflector 103: LED

105: diffuser plate 106: optical sheet

115: diffusing agent particle A: dark

B, C, D: low density region

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight unit for a liquid crystal display device using a light emitting diode (LED), and more particularly, to a backlight unit that provides uniform light distribution on a lower surface of a liquid crystal panel by suppressing dark portion generation in the backlight unit.

Background Art In recent years, in accordance with the trend of thinning and high performance of image display devices, liquid crystal displays have been widely used in TVs and monitors. Since the liquid crystal panel does not emit light by itself, the liquid crystal display requires a separate light source unit, that is, a backlight unit (hereinafter, simply referred to as a BLU). Inexpensive and easy to assemble cold cathode fluorescent lamps (CCFLs) have been used as BLU light sources. However, BLU using CCFL has disadvantages such as environmental pollution due to mercury, slow response speed, and difficulty in implementing partial drive. To overcome this, LED was proposed as a BLU light source instead of CCFL. BLU using LED can realize high color reproducibility, environment-friendly, and partial drive such as local dimming.

Generally, BLU using LED is divided into direct type BLU (direct type) and edge type BLU (side type). In the edge type, a bar-shaped light source is positioned on the side of the liquid crystal panel to irradiate light toward the liquid crystal panel through the light guide plate, whereas in the direct type, a surface light source using LEDs is disposed under the liquid crystal panel. Direct light control from the liquid crystal panel. On a LED surface light source, optical sheets, such as a diffuser plate and a prism sheet, are arranged at a distance from the LED.

In direct BLUs using LEDs, it is an important design element to place a plurality of LEDs at an appropriate pitch (d: spacing between the LEDs in the LED array). If the pitch is increased to reduce the number of LEDs used, the diffuser portion directly above the LEDs (upper part of the LED) is relatively bright, while the area between the upper parts, in particular the horizontal distance from the upper part, In darker areas, darker dark areas occur more strongly.

1 is a cross-sectional view schematically illustrating a conventional BLU (FIG. 1A) and a plan view (FIG. 1B). Referring to FIG. 1A, a BLU includes a plurality of LEDs 13 arranged on a substrate 11 such as a PCB, and an optical sheet 16 such as a diffuser plate 15 and a prism spaced therefrom. It includes. The liquid crystal panel 17 is disposed on the BLU to receive white surface light emitted from the BLU. The substrate 11 is provided with a circuit unit (not shown) for driving and controlling the LED.

As shown in Fig. 1 (b), when viewed from the top of the BLU, bright light is emitted from the area (directly above) directly above the LED 13, while the horizontal distance from the nearest directly upper part is farthest. Dark spots A occur at a point (or an in-plane point that is the farthest horizontal distance from the nearest LED 13). This dark part is generated because the LED's direct angle limit light, etc., requires a certain distance (height) above the LED for sufficient light mixing. Due to this dark portion, the output light of the BLU provided to the liquid crystal panel is not sufficiently uniform, and the screen quality of the liquid crystal panel is degraded. The dark portion appears stronger as the height from the substrate 11 to the diffuser plate 15 is lower and the pitch between the LEDs is larger. Increasing the height of the diffusion plate 15 to suppress dark spots increases the thickness of the BLU and the liquid crystal display product. Also, densely placing many LEDs for dark suppression results in significantly higher manufacturing costs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object thereof is to provide a backlight unit capable of securing thickness and reducing costs while suppressing dark part generation.

In order to achieve the above technical problem, the backlight unit of the present invention is a direct type backlight unit disposed under the liquid crystal panel to irradiate light to the rear of the liquid crystal panel, a plurality of LEDs arranged on the substrate; A diffuser plate disposed on the LED spaced apart from the LED, the diffuser plate having a bottom surface facing the substrate, the diffuser plate having a lower density than the diffuser density at the tops of the LEDs; It has a plurality of low-density areas located at a point farthest from the horizontal distance from the top.

According to an embodiment of the present invention, the average diffuser density in the low density region is 7-25% lower, more preferably 10-20% lower than the average diffuser density in the upper portion.

The low density region may have a uniform diffuser density within that region. Alternatively, the low density region may have a distribution of diffusing agent density in which the diffusing agent density changes depending on the position in the region. The low density regions may have the same average diffuser density. The low density region may have various shapes such as a circle, a rectangle, and an oval.

The LEDs may be arranged in a matrix form having a plurality of rows and columns on the substrate. In this case, the low density region is located at the center of the quadrangle formed by the tops of the four adjacent LEDs. The distance between the rows and the columns may be equal to each other. In addition, the distance between rows and columns may be different.

According to an embodiment of the present invention, the diffuser plate may exhibit a periodic diffuser density distribution on a straight line connecting the low density regions. As an example, the diffusion plate may exhibit a density distribution of a diffusion agent in the form of a square wave. As another example, the diffuser plate may exhibit a sinusoidal diffuser density distribution. In addition, various types of diffusing agent density distribution, such as sawtooth type diffusing agent density distribution, are possible.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art. Accordingly, the shape and size of elements in the drawings may be exaggerated for clarity, and the elements denoted by the same reference numerals in the drawings are the same elements.

2 is a plan view of a backlight unit BLU according to an exemplary embodiment of the present invention, and FIG. 3 is a cross-sectional view taken along the line SS ′ of FIG. 2. In FIG. 2, the illustration of the optical sheet 106 is omitted for convenience of description.

2 and 3, the BLU 100 includes a plurality of LEDs 103 arranged on a circuit board 101 such as a PCB or an MCPCB, and a diffusion plate 105 disposed thereon. Various optical sheets 106 such as a prism sheet may be disposed on the diffuser plate 105. Each LED 103 acts as a point light source, but the light coming from their arrangement is mixed with each other in the space below diffuser 105 and diffuser 106 to provide two dimensional light distribution. The BLU 100 is a direct type BLU and is disposed below a liquid crystal panel (not shown) to irradiate light to the rear of the droplet panel.

The LED 101 is a unit light source for obtaining white light, and each of the LEDs 101 can output monochromatic light or white light. For example, white light can be obtained by combining a blue LED, a green LED, and a red LED in a suitable arrangement such as a triangular or square array. Alternatively, the individual LEDs 101 may output white light by combining a phosphor (or combination of phosphors) and a monochromatic LED chip (eg, a combination of blue LEDs and yellow phosphors, or a combination of blue LEDs and red and green phosphors). Combinations, etc.).

A reflector or reflecting sheet 101 may be disposed on the substrate region between the LEDs to return the light traveling from the LED 103 toward the substrate 101 upwards. As the board 101, a PCB board commonly used as a circuit board or an MCPCB board having good heat dissipation characteristics may be used, and other ceramic boards may be used.

A diffuser plate 105 (also referred to as a 'diffusion sheet') is spaced apart from and disposed on the LED 103 and has a bottom surface facing the substrate 101. The diffuser plate 105 is used to uniformly diffuse the light from the plurality of LEDs 103 over the front of the liquid crystal panel and to keep the luminance constant. The diffusion plate 105 is made of polyester, and diffuser particles such as spherical or elliptical are formed in the diffusion plate. The diffuser particles (see reference numeral 115 in FIG. 4) can be obtained from, for example, acrylic resin, and serve to diffuse and mix light passing through the diffuser plate 105.

In light of the fact that the diffusing agent not only diffuses the light, but also absorbs the light, the inventors have found that darkening can be improved by specially adjusting the distribution density of the diffusing agent in relation to the LED location under the diffuser plate. .

Referring to FIG. 2, the top portions a1, a2, a3, a4, a5, etc. of the plurality of LEDs 103 in the entire region of the diffusion plate 105: diffusion plates corresponding to the LEDs 103 on the plan view of FIG. 2. (Difference 105), the density of the diffusing agent is lower in the region B between these upper portions. In particular, in the low-density region B between the uppermost portions, the point where the horizontal distance from the nearest uppermost LED portion is farthest (in the plan view of FIG. 2, the four upper LED portions a1, a2, a4, a5 are shown). Corresponding to the center of the square).

Here, 'the point at which the horizontal distance from the nearest LED directly is the farthest' means the LED directly adjacent to each other in the unit cell region (the quadrangular region composed of a1, a4, a2, and a5 in Fig. 2) obtained subsequently. It refers to the point farthest from (for example, where each vertex of a square unit cell is directly above the LED, in the center of the square), the probability of dark areas is usually relatively high at this point. Hereinafter, for convenience of explanation, this point is referred to as 'dark point'.

As such, by adjusting the diffuser density in the diffuser plate to have a lower diffuser density in the region of the dark point between the upper portion than the upper portion of the LED 103 (i.e., the low density region B of FIG. 2) ( Without such control, it is possible to effectively suppress the occurrence of the cancer part (which occurs easily). This is because a relatively large amount of light can be transmitted at a portion where the density of the diffusing agent is relatively low due to the light absorption characteristic of the diffusing agent.

If the diffuser in the diffuser plate 105 is distributed substantially uniformly throughout the light transmissive region or is distributed without particular regularity as in the prior art, it is located directly above the LED 103 arranged below the diffuser plate 105. The upper portion of the LED (a1, a2, a3, a4, a5, etc.) has a relatively high brightness, and appears as a dark dark portion near the 'dark point' between the upper portions. However, as shown in Fig. 4, when the low density region B is provided at the 'dark point' between the upper portions of the LED, the low density region B is the upper portion of the LEDs a1, a2, a3, a4, a5. Compared to), the density of the diffusing agent 115 is lower to transmit more light. Therefore, the generation of the dark portion near the 'dark portion point' is effectively suppressed.

By suppressing the dark portion generation in this way, the output light of the BLU emitted through the diffusion plate 105 becomes more uniform in its luminance over the entire light emission area. This contributes to improving the color quality or screen state of the liquid crystal display.

In addition, it is possible to increase the light transmittance in the vicinity of the 'dark part point', so by adjusting the density of the diffuser in the low density region (B), the pitch of the LED can be further increased without the dark part (or the dark part is suppressed) or the You can lower the height further. This allows for reduced manufacturing costs and reduced BLU thickness by reducing the number of LED components, thus further thinning the liquid crystal display.

Preferably, the average diffuser density in the low density region B is 7-25% lower than the average diffuser density directly above the LED. If the average diffuser density in the low density region B is too low compared to the average diffuser density directly above the LED, the overall light diffusion characteristic of the diffuser plate may be degraded. In addition, it is preferable that the difference in the density of the diffusing agent between the low density region (B) and the upper portion of the LED is 7% or more for effective dark improvement. More preferably, the average diffuser density in the low density region B has a diffuser density 10-20% lower than the average diffuser density directly above the LED.

The distribution of the overall diffuser density, including the low density region B and the LED top portion, can be variously selected as long as the low density requirements at the 'dark point' described above are met.

5 is a graph showing the density distribution of the diffusing agent according to an embodiment of the present invention. Referring to FIG. 5, the diffusion agent density on the straight line X connecting the low density regions B of FIG. 2 shows a periodic distribution. In particular, it shows a diffuser density distribution in the form of a square wave, which shows that the diffuser density changes rapidly (or substantially discontinuously) between the low density region B and the region near the LED directly adjacent thereto. Another straight line Y following the low-density regions B may also exhibit a diffuser density distribution in the form of a square wave similar to that shown in FIG. 5.

In addition, as shown in FIG. 5, the low density region B may have a constant diffuser density within the region, and the diffuser density (average diffuser density) of the plurality of low density regions B may be the same. have. However, FIG. 2 only illustrates an example of a diffusing agent density distribution, and the diffusing agent may be distributed in other ways such as aperiodic distribution, continuous distribution, and spatial distribution of the diffusing agent density in the low density region.

6 is a graph showing a diffuser density distribution according to another embodiment. In FIG. 6, the low density region and the region near the LED directly adjacent to each other have a distribution in which the diffusing agent density continuously changes without the boundary of a distinct density distribution. In particular, FIG. 6 shows a diffuser density distribution in which the diffuser density changes to a sinusoidal shape along a straight line X. FIG. Therefore, also in each low-density area | region B, a diffuser density is not fixed but changes in a predetermined range along the straight line X. Along the other straight line (Y) can also show a similar sinusoidal diffuser density distribution. In the case where the diffuser density continuously changes without the boundary of a distinct density distribution as shown in FIG. 6, the portion having a density corresponding to the arithmetic mean of the lowest density (in the low density region) and the highest density (in the region near the LED). For convenience, it can be determined as the boundary of the low density region.

6 and 7 are only examples of the diffusing agent distribution density, but the present invention is not limited thereto. As another example, various types of diffuser density distributions are possible, such as sawtooth type diffuser density distributions. In either case, the low density region B has a lower diffuser density than the LED top.

The (planar) shape of each low density region may also be variously modified. In FIG. 2, although the circular low density region B is illustrated, like the backlight units 200 and 300 of FIGS. 7 and 8, the low density regions C and D may have various shapes such as a rectangle and an ellipse. The area occupied by each of the low density regions B, C, and D (on the horizontal plane) may also vary. However, if the low-density areas (B, C, D)) occupy an area that is too large, the light diffusion characteristics of the diffusion plate may be degraded. Do. In addition, if each of the low density areas B, C, and D occupies an area that is too narrow, when there is misalignment of the low density areas B, C, and D, it may be difficult to effectively suppress the occurrence of dark areas. It is preferable that the area of (B) is 1/10 or more of the said unit cell area.

In addition, the manner in which the LEDs 103 are arranged on the substrate may be variously selected. 2, 7 and 8, the LEDs 103 may be arranged in a matrix shape having a plurality of rows and columns. In particular, the distance between the LED rows and the LED columns may be the same as shown in FIGS. 2 and 7, and the distances may be different from each other as shown in FIG. 8. When the distance between the LED rows and the LED columns are different from each other, the low density region D may have an elliptical shape on a horizontal plane (see FIG. 8).

The present invention is not limited by the above-described embodiment and the accompanying drawings, but by the appended claims. Therefore, it will be apparent to those skilled in the art that various forms of substitution, modification, and alteration are possible without departing from the technical spirit of the present invention described in the claims, and the appended claims. Will belong to the technical spirit described in.

As described above, according to the present invention, the dark portion can be effectively suppressed without reducing the pitch of the LEDs or increasing the height of the diffuser plate. Rather, by increasing the light transmittance near the dark point, the pitch of the LEDs can be increased or the height of the diffuser plate can be reduced while suppressing dark occurrence. This enables dark suppression, cost reduction, and thinning of BLUs and liquid crystal displays.

Claims (15)

A direct type backlight unit disposed under the liquid crystal panel to irradiate light to the rear of the liquid crystal panel; A plurality of LEDs arranged on the substrate; And A diffusion plate disposed on the LED spaced apart from the LED and having a bottom surface facing the substrate, And the diffuser plate has a lower density than the diffuser density at the tops of the LEDs and has a plurality of low density regions located at the point where the horizontal distance from the adjacent tops is farthest. The method of claim 1, And the average diffuser density in the low density region is 7 to 25% lower than the average diffuser density in the upper portion. The method of claim 1, And the average diffuser density in the low density region is 10 to 20% lower than the average diffuser density in the upper portion. The method of claim 1, And the low density region has a uniform diffuser density within the region. The method of claim 1, And the low density region has a distribution of a diffusing agent density in which the diffusing agent density changes according to a position in the region. The method of claim 1, And the plurality of low density regions have the same average diffuser density. The method of claim 1, The low density region has a backlight unit, characterized in that any one selected from a circle, a square and an ellipse. The method of claim 1, And the LEDs are arranged in a matrix form having a plurality of rows and columns on the substrate. The method of claim 8, The low density region is a backlight unit, characterized in that located in the center of the quadrangle of the top of the adjacent four LED. The method of claim 8, And the distance between the lines and the distance between the columns are the same. The method of claim 8, And the distance between the lines and the distance between the columns. The method of claim 11, And the low density region has an oval shape. The method of claim 1, The diffuser plate has a backlight diffuser density distribution on a straight line connecting the low density regions. The method of claim 13, The thin diffuser has a diffuser density distribution in the form of a square wave. The method of claim 13, The diffuser plate has a diffuser density distribution in the form of a sine wave.

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