“Brightness uniformity is a problem often faced by LCD TV products with LED backlight, and it is also one of the problems that users are more concerned about. This article combines practical work to introduce an LCD TV backlight design scheme to improve brightness uniformity and the role of each part. , the main feature of which is the design of the special-shaped light bar.

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0 Preface

With the popularization of LED applications, the current backlight sources of LCD TVs are almost all LEDs, and the direct-lit backlight solution DLED is the mainstream backlight solution, which is widely used because of its convenient design and low price.

This article introduces a backlight design scheme tailored for special export customers, the main feature is the design of special-shaped light bars.

1 LCD TV backlight scheme structure

Figure 1 shows the backlight scheme of a 32-inch LCD TV and the schematic diagram of the front, middle and rear shells. The backlight part from the back to the front is a reflector, a light bar, a diffuser, a brightening film, and a diffuser.

The following describes their functions:

(1) The function of the light bar is to provide a light source. The 2×6 scheme is adopted, that is, two light bars, each with 6 LED lamp beads and lenses, and the function of the lens is to disperse the light emitted by the LED lamp beads. The LED lamp beads are selected in 2 W specifications, with a lifespan of more than 50,000 h, a current of 600 mA, and a total power of 24 W.

(2) The working principle of the reflective sheet is shown in Figure 2 and Figure 3: the light propagating to the bottom of the module is reflected back to the front, and the light emitted by the LED is fully utilized. The substrate is generally PET, polycarbonate, etc., with a thickness of about 0.15 mm. The refractive index of TiO2 is very high (n=2.62), and TiO2 is mixed with a resin with high optical transparency such as PET or polycarbonate, and the reflective sheet is formed by micro-foaming. The diameter of the bubble is about several micrometers. The finer the dimension and the higher the density of the bubble, the higher the reflectivity. The bubble is a material with a refractive index of about 1.00, and forms a good refractive index interface with the transparent resin. After the light refracted from the diffuser enters the reflector, most of the light is returned to the diffuser through complex refraction action. Corresponding to different light mixing distances and DLED direct-type all-in-one machines with different powers, it is necessary to summarize the corners of the four corners of the reflector.

(3) The working principle of the diffuser plate is shown in Figure 4 and Figure 5. The function is to disperse the light emitted by the LED lamp to improve the uniformity; to cover up the shadow of the lamp and the print of the bracket to avoid causing too bright or too dark in some places ; At the same time, it also supports other optical films. The working principle is to use the physical phenomena of refraction, reflection and scattering when light encounters two media with different refractive indices on the way. Add inorganic or organic light diffusing agents, or artificially adjust the light through the array arrangement of the micro-features on the surface of the substrate, so that the light is refracted, reflected and scattered in different directions, thereby changing the light’s traveling route and realizing the sufficient scattering of the incident light. This produces the effect of light diffusion. In addition, the microstructure diffuser plate has a microstructure on the upper surface of the diffuser plate, which can improve a certain light intensity. And the ratio of the discharge distance of the light source to the distance OD between the light source and the diffuser plate affects the homogenization effect of the light source from the point light source to the surface light source.

(4) The function of the brightening sheet is to concentrate the light of the larger propagation direction into the smaller angle of the front, so as to improve the intensity of the front light. The working principle is shown in Figure 6. The basic brightness enhancement sheet is essentially a prism sheet (BEF), which refracts, reflects, and collects light to achieve the purpose of brightening.

The BEF structure is shown in Figure 7. A precise acrylic resin prism structure is formed on a PET substrate (thickness of about 0.25mm) with a size of about 30μm, with an anti-adhesion coating underneath. The characteristics of this film: the rounded corners of the prism have strong scratch resistance, weak static electricity, UV coating on the back, no protective film on the upper and lower surfaces, and the haze can be adjusted by the back coating. The actual material diagram is shown in Figure 8.

There is also a more efficient brightening sheet shown in Figure 9: DBEF.

DBEF is a reflective polarizer that selectively reflects the light of the backlight system so that it is not absorbed by the lower polarizer of the LCD, so that part of the light can be reused, so the brightness is increased by about 50%.

(5) The working principle of the diffuser is to use the light that will continuously pass through two media with different refractive indices when passing through the diffuser, and the phenomena of refraction, reflection and scattering will occur at the same time, so that the light will be scattered to achieve a blurred light source. The effect of making the light even. As shown in Figure 10 and Figure 11, the base material is generally PET, with a diffusion layer on the top and an anti-adhesion coating on the bottom.

This backlight solution achieves a brightness of 250 nits when the transmittance of the LCD panel is 6.2%. Generally, the design of the light bar is shown in Figure 12. The light bar and light beads are basically equally spaced, so the design is simple, Easy to produce, especially for the high utilization rate of light strip PCB, thus reducing the cost. In addition, the lamp beads of the light bar are relatively concentrated in the central area to ensure that the brightness of the central part can meet the requirements, and most people pay more attention to the brightness of the middle part, so that the design not only meets the standards and viewing requirements, but also takes care of more low cost.

2 Brightness uniformity test

The brightness uniformity of ordinary 9 points is about 70%. The test method is shown in Figure 13: the distribution of surrounding test points follows the principle of 1/9 height and width. The circle indicates the range of the brightness instrument probe test, and the brightness of P0~P8 is tested respectively. L0~L8.

Then the brightness uniformity is, i is any number from 1 to 8. Each Pi must be greater than or equal to 70% to qualify.

However, for some special customers, high brightness uniformity is required. Not only the above-mentioned 9-point uniformity is tested, but also the corner uniformity. The test method is shown in Figure 14: The corner brightness uniformity is Pi=Li/ L0×100%, i is any number from 1 to 4. Although this indicator only requires 50% to be satisfied, the normal equidistant linear light bar design cannot meet the requirements. Under normal circumstances, the center of the picture is the brightest, and it is also the place where the maximum brightness is tested. The closer to the corners, the darker, and the four corners are the darkest.

3 Design scheme of special-shaped light bar

In view of the above-mentioned problem that the four corners are dark, we have adopted the solution of special-shaped light strips, as shown in Figure 15: the two ends of the light strip are inclined to the corners, and the 4 lamp beads at the edge are closer to the four corners, thus improving the four corners. brightness. According to the test, the brightness of the four corners can be increased by about 30%, while the center brightness remains unchanged. The advantage of this design is that the standardization is guaranteed to the greatest extent, and the conversion between the two methods can be realized by only changing the shape of the light strip PCB without changing the structure size and other materials. The only disadvantage is that the utilization rate of the PCB board is slightly reduced, resulting in a slightly higher cost, but it can meet the special requirements of customers.

references:

[1]SJT 11343-2015 General specification for digital television liquid crystal Displays.
[2]SJT 11348-2016 Measurement methods for digital TV flat panel displays.
[3]Xu Lin, Zhu Yingying, Liu Ming. Design of a direct-lit LED-backlit LCD TV[J]. Communication and Broadcasting and Television, 2013(1):33-43.
[4]Zhang Yimin, Wu Qingfu. Development and research on key components of optical film for TFT-LCD backlight module[J]. Modern Display, 2010(4):49-54.

About the Author:

Bai Xiang, Bachelor, Senior Engineer, Director of Outsourcing Design Institute of Multimedia R&D Center, Senior Product Designer.
Advanced Worker of Multimedia R&D Center of Konka Group in 2002
Outstanding Scientific and Technological Worker of Konka Group in 2003
Outstanding Team of Konka Group in 2004: LCD4400 Project Team (Project Manager)
Outstanding Color TV Person of the Color TV Division of Konka Group in 2008
Konka BT4390W rear projection color TV passed the appraisal of the Ministry of Information Industry in 2004 and reached the international advanced level of similar products (the chief designer of the signal processing part)