The Role of Backlight in LCD Technology

By Cici Wang

The backlight is the optical component that provides light source in LCD display. Since the LCD panel itself does not emit light, it needs an external surface light source system to help it display, that is, the backlight. The backlight lies on the back of the LCD display. It not noly directly determines the brightness of the LCD display  but is an important component of the LCD display.

1.How the backlight works

The working principle of the backlight is to turn the point light source (LED) into a surface (backlight panel, backlight source). So it can provide LCD display with the external light source required for display.

2.The backlight classification

According to the position of the light source, the backlight can be classified into edge-light type and bottom-light type (bottom backlight type). According to the type of light source, the backlight can be divided into three types: EL, CCFL, and LED.

LED Backlight

LED backlight: LED, which also called Light Emitting Diode, are the most popular type of backlight for small and medium-sized LCD. The advantages of LED type are low cost, long life, no EMI interference, low operating voltage, and easy to control brightness. From blue to red, LED lights have many colors, which can be roughly divided into two types: high brightness and low brightness. There are two basic LED backlight designs: bottom (array) and edge-light. Bottom (array) type backlight have several LEDs evenly mounted behind the display, providing even and brighter lighting, but also  with more power consuming and more thickness. The edge-light type has low power consumption and thin thickness . It consists of LED installed on edge or in the corner. The thinner it is, the utilization rate of the light source is smaller about 50%. The core of this technology is the design and production of light guide plates.

Led light type

EL backlight: Electroluminescence, which generates light by phosphor powder excited by an alternating electric field, rather than by heat. Its advantages are that the backlight is very thin, light in weight and the light provided is very even. The disadvantages are that the brightness is low, the life is short (generally 3000~5000 hours). Driven by an inverter, the circuit and results in flicker and noise can interfere it easily.

lcd backlight

CCFL backlight has low power consumption and very bright white light. CCFL are light sources with diffusers that distribute light evenly across the displaying area. It needs an inverter to provide the voltage used by CCFL. Graphic LCD uses it a lot and has a longer lifetime (typically 10,000 to 20,000 hours) than EL backlight. Their biggest disadvantage is that the operating temperature range is narrow, only 0-60 degrees, while other backlight such as LED can reach -20-70 degrees.

CCFL backlight

3.Composition of edge-light backlight:

The edge-light backlight mainly consists of Light bar (LED+FPC), light guide plate/LGP, reflector/R, diffuser/D, brightness enhancement film /BEF, etc.;

LED: Light Emitting Diode, the light source of the backlight, which with FPC form what we often call a light bar.

Light guide plate: A linear lighting source array composed of LED is irradiated from one side of the light guide plate. The light is transmitted inside the light guide plate and will be irradiated on the dots of the light guide plate, resulting in scattering, so that the light is output from the side of the light guide plate without dots , forming a surface light source.

Reflector, which reflects the light leaked from the bottom of the light guide plate back to the light guide plate for reuse, so as to increase the efficiency of light use.

Diffuser, the light passing through the diffuser is refracted and scattered through the acrylic ball, rough surface or diffusion particles inside the diffuser, and finally the light is uniformized, which can effectively homogenize the light and cover the bad.

Brightness enhancement film: BEF has light-gathering effect that enhances brightness.

4. The development trend of backlight Specification

Specification

With the rapid development of the international IT industry, the relevant LCD industry continues to introduce new ones. The size of LCD products is developing towards diversification and portability. Backlight, as one of the core components of LCD products, must match this development trend. Committing to product diversification and portability is important.

Light guide plate

The core technology in the backlight module is the optical technology of the light guide plate. There are mainly two types of light guide plates: printing and injection molding. Others, such as injection molding plus printing, laser dotting, corrosion, etc., account for a small proportion. Not suitable for mass production principles. Printing has become the mainstream technology for a long time. Because of its low cost, but its main disadvantage is low quality products. LCD products require a more precise light guide plate structure, and injection-molded light guide plates will inevitably become the mainstream of backlight development. However, only a few major Japanese manufacturers can overcome the corresponding mold technical problems.

Brightness

LCD has always had high requirements on the brightness of the backlight. The high brightness also makes the power consumption of th LCD remain high. The backlight is the most power-consuming accessory in the LCD module. The backlight is no longer suitable for the requirements of portable information products. Therefore, increasing the brightness of the backlight without increasing power consumption and thus increasing the L is the most power-consuming accessory which is no longer suitable for the requirements of portable products. Improving the brightness of the backlight without increasing power consumption is also one of the main development trends. The current goal is to achieve a brightness of 500-700cd/square meter and a flat fluorescent lamp with a life span of 100,000 hours.

Questions:

  1. The main technology difficulties the backlight are facing.
  2. Which country obtain the most advanced backlight technology.

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