The Basic Structure and Features of LED Backlight

1. The Basic Structure of LED Backlight

LED backlight can be divided into several important parts.

Light guide plate: It is divided into printed type and non-printed type by the production method of light guide points.

Non-printed type of light guide plate: An excellent mold has to be produced before the process begins. Light guide plate is injected in particular shape with the mold. Some granular materials which have different refractive index are put on the surface of the plate. It looks like a lattice point.

Printed type of light guide plate: light guide points are produced with UV screen printing technology at the bottom of PMMA sheet.

Ps: It is better to make the thickness of light guide plates between 2.5mm and 3.5mm. It takes at least 2 mm of thickness of light guide plate to install DIP LED, and the thicker the brighter.

Diffusion film: It distributes the light more uniform and avoids seeing the reflecting points emerging in the front of light guide plate so that it could focus the light and increase the brightness of LED backlight.

Reflection film: It is used to reflect the light from the bottom into light guide plate to improve the using effect of the light.

Light shade film: It is used to block the light from LED Lamps and make the light more uniform.

Light shade film on the edges: It is used to reflect the light from LED Lamps and keep the light in the light guide plate.

Blocking light area has to be made more than 7mm while using the DIP LED.

The blocking light area has to be made more than 4mm while using the SMD LED.

Connecting Methods of LED backlight

Connecting methods of LED backlight include wire harness, FPC, PCB with hard PIN, PCB without hard PIN, PIN and wire. You can see the picture above.

Light area equals the total length subtracts blocking light area and double-sided adhesive area. You can see the picture as follow:

PS: AA (Active Area)

2. The Light Sources of Backlights

It is very important to choose the suitable light source for the design of backlight. The light sources you are using not only determines opto-electronic parameters of consumption, luminance, color and other optical parameters, but also decides the features of using conditions and serving time.

Light source  shape Light source kind Light source wave length (temperature) Power Serving time Feature
Consumption (h)
(w)

Point source light lamp bulb 2800k 1.0 above 2000 Simple, big, serious heating
Any kind LED 430-700nm 0.038 100,000 Long serving time, low heat, low brightness
Linear source light CCFL Blue red green and the mix color 1.0-10.0 25 High brightness, long serving time, high inverter driving voltage
HCFL 1.0-220 5000-7000 Serious heating
VFD 200mW/cm2 below 5000 High brightness, good uniformity, double power source driving
EL 20mW/cm2 below 5,000 Thin, good uniformity, low brightness, short serving time
OELD 1,000 Thin, good uniformity, high brightness, short serving time
FED 10,000 High brightness, exploring

3. The Requirement of Backlight to LED Lamp

3.1 The requirement of edge LED backlight

Edge LED backlight is going to minimize the number of LED and enhance illumination, light efficiency and light power.

It is going to be super-light and thin and low cost so that the light guide plate is going to be thinner, and the illumination area of LED is going to be smaller.

Light guide plate is becoming long and narrow, and the number of LED lamps is decreasing, so that LED backlight is decreasing light-emitting angle to enhance the light efficiency in the narrow side, and it is also increasing the light-emitting angle to decrease shadow between LED lamps in the long side.

3.2 The requirement of bottom LED backlight

Bottom LED backlight is going to be thinner and lower cost so that the number of LED lamps is decreasing and light-emitting angle is increasing from 120 degrees to 160-170 degrees.

So far, the main solution is that using two times optical lens which is high cost and low light efficiency.

4. The Trends of Backlight

4.1 Size flexible and portable

Nowadays the IT industry is developing rapidly, and the LCD industry which is one of the related industries benefits from this trend. LCD products which have become an important part of our lives are developing into many kinds of products that are sizes flexible and very light, and also suitable for people to carry. Thus, the LED backlight which is one of the most important parts of LCD is for sure becoming size flexible and portable.

4.2 Non-Printing type of light guide plate becomes the major

The hardcore technology of the backlight is light guide plate which is divided into printing type of light guide plate and non-printing type of light guide plate. A mold is necessary for non-printing type of light guide plate, although it is more expensive but more accurate. Printing type of light guide plate is simple and cheap, so it is the major type for a long time. Non-accurate is always is its label. In the future, LCD is required to be more accurate, so it is pretty sure that non-printing type of light guide plate will become a major trend.

4.3 Much Brighter 

The backlight has to be very bright, so it could meet the demand of LCD. But it takes a lot of energy for the brightness, which is not suitable for portable digital products. In the future, the backlight will be much brighter while it is consuming the same amount of energy. Flat fluorescent lamp whose luminance is reaching 500-700cd/m2 and serving time is 100 thousand hours is its current goal.

5. 5 Advantages of LED Backlight

Thanks to Sony Corporation as the first company to launch the LED backlight liquid crystal television which has attracted wide attention. It has reached a new level in both vivid colors and super thin. We have concluded five advantages of LED backlight liquid crystal television.

5.1 It is more energy-saving.

 LED backlight products save 40% energy, compared with products with CCFL backlight in the civilian industry. According to our evaluation data, some high quality LED backlight products could save 60% energy, compared with mainstream products with CCFL backlight. Furthermore, along with the development of LED backlight technology, LED backlight module will be more energy saving in the future.

5.2 It is green and environment-friendly.

Mercury in CCFL backlight is considerable damage to the environment, but LED backlight does not contain it. As we know, the global display production in 2009 was 220 million units. If LED backlight display is popular in the future, it will be very helpful to protect the global environment.

5.3 It has a wider color gamut

LED backlight module has a more pure white light and a wider color gamut, compared with the conventional CCFL backlight module. LED backlight display can display more vivid colors and make a better color representation.

5.4 It has a longer serving time.

Theoretically, LED backlight has a long-serving time and a longer brightness decreasing cycle, compared with CCFL backlight. And the serving time will be extended with the development of technology.

5.5 It is lighter and thinner.

LED backlight products are much lighter and thinner than CCFL backlight products. The selling point of LED backlight display in the market is super thin, which meets the fashion needs of consumers.

6. Global Backlight Module Manufacturers Adopt RGB LED Backlight Module

The left image above shows the original and traditional technology which black border space exists among photoresist of R, G and B color, and it has only 65% of light open rate, and final result is that it only gets about 22% luminous efficiency because photoresist blocks the two other lights. Meanwhile, the right image above shows color sequential or color filter-less technology which uses no color filters, so that R, G, B transform sight quickly, opening rate is better and using efficiency of light is almost no loss. Besides, it saves about 15% of the cost of color filter. This technology has been adopted by smart phones for a few years. And large size LCD panels have just adopted this technology because of relatively slow speed of liquid crystal.

The feature of LED is monochromatic and it can reach 130% of NTSC if 6 color LED lamps are used, so we can see the excellent color performance that the golden color of coast dusk and the green color of inserts. LED has better luminous efficiency, compared with CCFL. We take small size display panels for example, since CCFL emits the light in all direction, the using light efficiency of CCFL gets worse, although semicircular cover reflects the non-useful light from outside. If the luminous efficiency of CCFL is 721 lm/w and the using efficiency is 75%, the final using luminous efficiency will be 541 lm/w. Meanwhile, if luminous efficiency of LED is 721 lm/w, and the using efficiency is 90% because of the good emitting direction of LED, the final using luminous efficiency will be 541 lm/w. The luminous efficiency of LED enhances with the development of technology and it will greatly increase its advantages in the future.

7. Super Thin LED Backlight Module is Going to Be Popular

LED backlight module is going to become super thin and light to meet the requirement of laptop computer

LED backlight is replacing CCFL backlight. As early as the year 2005, SONY company had launched a screen size of 11.1-inch VAIO laptop computer with white LED backlight, which drew great attention of LCD module manufacturers and LED backlight module manufacturers. Some people have noticed the trend of LED backlight for a few years. From that on, LCD and backlight industries began to develop LED backlight and relative fields positively. VAIO laptop computer was the largest size product which adopted LED backlight back then. People have adopted LED as the light source of backlight for a long time, but it can be made in the max size of 7 inches. Not only backlight module manufacturers are devoted into developing LED backlight, but also LED  factories are committed to improving luminous efficacy. The difference of structure between LED backlight and CCFL backlight is the light guide plate. When the backlight adopts CCFL light source, light guide plate is a wedge board which is at least 20.0 mm because the diameter of light tube is 18.0 mm. It is no way to make the laptop computer thinner and you can see the image above.

Meanwhile, when the backlight adopts LED light source, the limitation of thickness of the light guide plate is no longer existing, it can meet the requirement of laptop computer super-thin and light. So far, our factory can make LED backlight as thin as 1.0 mm.

8. The Temperature of LED Backlight Module Remains Stable While Time Goes on

The thermal problem is the big bottleneck for the development of LED backlight. The most concerning issue for large size LED is that the temperature increases with the time goes on, and it often accompanies by the variation of the wavelength and the reduction of the brightness. Therefore, the overall performance of stability of backlight module is very important for both middle-size laptop computer and large-size liquid crystal television.

The combo RGB 3 in 1 top view LED and bottom backlight module with PCB material not only presents the excellent color performance and stability, but also efficiently manages the thermal problem. From the image above, we can see the test result of temperature and time graph of 32 inches LCD TV. At the beginning of the first 20 minutes, the heat will increase rapidly with time, but then the heart increases very slow with time, which we name it the stabilization phase and the light performance is relatively stable. Therefore, we can conclude that LED backlight module can perform excellent stability of temperature as long as the LED package and the thermal management are good.

9. Backlight Manufacturers Overcome The Light and Dark Stripes

Overcoming the problem of light and dark stripes is an excellent technology performance

Technically, V-cut light guide structure is likely to cause the problem of light and dark stripes because of the optical properties. And the manufacturers of backlight module overcome the technical obstacle in urgent need. Taiwan’s LCD module manufacturers have a higher quality requirement to light guide plate than Korean and Japanese LCD module manufacturers. It is a key point for them to develop optical and precision processing technology that to make the best combo for precision machined prism surface of light guide plate and atomizing surface. However, there are some bottlenecks of technical development in significantly improving the brightness of v-cut technology. Some suppliers of brightness enhanced film of light guide plate question that the structure of v-cut light guide plate is easy to cause the problems of scratching, poor uniformity and weak mechanical strength and so on. If it adopts traditional light guide plate technology, the pass rate will be affected by the scratching. It requires the high quality manufacturers of backlight module who have advanced equipments and professional staffs with quality training.

Uniformity is about 75% for v-cut technology light guide plate, compared with 85% for traditional technology, which is an inevitable phenomenon when meeting customers’ requirements about the best performance of the middle part of the light guide plate and excellent luminance. There are a lot of ways to solve the problem of weak mechanical strength problem. For example: first laminating then fixing has passed most of the vibration and shocking tests.

greg

greg

Hi, I'm Greg, sales manager of displaybly.com. I've been working in the display & touch industry for more than 10 years, and the purpose of this article is to share the knowledge from a Chinese supplier's perspective.
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