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Backlight unit

What is a backlight unit?

In the liquid crystal display, the liquid crystal works only as a filter (like a shutter to turn on / off the light). It cannot emit light itself. Therefore, a light source is needed. Backlight units mainly refer to the light source in the liquid crystal display.
It is roughly divided into the two types of edge-light method and direct backlight method. For the backlight of previous LCD TVs, a fluorescent lamp called CCFL (Cold Cathode Fluorescent Tube) was used. However, LEDs, which are thin and consume less power, have been recently used.

①Edge-light method
The edge-light method is also called a light guide plate method. The light emitted from the LED at the end of the display is converted to planar light through the light guide plate, diffusion film, prism sheet, and reflective film, and then sent to the liquid crystal.
This edge-light method backlight is usually used in smartphones, tablets, laptops, and desktop monitors.

②Direct-backlight method
The direct backlight method creates a planar light source by placing a diffusion plate on top of the LEDs arranged below the liquid crystal, and then sends the light directly to the liquid crystal. The direct backlight method backlight unit is used in LCD TVs. Because of a large number of LEDs, the brightness can be controlled in a very delicate way. 
However, the utilization of a large number of LEDs tends to make the body thicker and more expensive. When used in TVs, the contrast ratio (contrast between darkness and brightness) can be improved by using the local dimming function (a function to turn on LED lamps for the bright part while turning off lamps for the dark part), so the direct backlight method of backlighting is adopted.
In recent years, with the introduction of mini-LEDs and micro-LEDs that miniaturize LEDs, consideration has been given to using direct backlight method backlight systems in monitors, laptop computers, tablets, and smartphones for contrast ratio improvement purposes.

Contributions of diffusion film

In the edge-light method, the light guide plate creates a planar light source. In that case, the area near the LED will be brightened while the area far away from the LED will be dark. Even in the direct backlight method, uneven luminance for the parts with LED and without LED will be obvious. 
Unevenness of luminance, or uneven brightness, makes the screen very difficult to watch when turned into products.
However, the need for liquid crystal displays shows no signs of stopping or slowing down, and the applications are diverse. The demand for brighter, more energy-saving, and thinner LCDs has also become inevitable. The result is a high demand for the brightness of the LED, which is the light source. However, it is difficult to achieve brightness uniformity with the light guide plate and the diffusion plate.
In this case, a diffusion film is added onto the light guide plate and the diffusion plate. The film is coated with a light-diffusing resin so that the light passing through it can be diffused to form a more even planar light.

Contribution of reflective film

However, when the brightness of the backlight unit is enhanced, it is necessary to improve the optical diffusion performance by using a plurality of light diffusion films or one sheet with higher diffusive property.
However, light transmittance tends to decrease with enhanced light diffusivity. 
To solve this problem, a reflective film is used to brighten the liquid crystal display without increasing the brightness of the backlight. By reflecting light emitted from the opposite direction to the liquid crystal panel back to the panel, the reflective film can reduce light loss and improve the brightness of the LCD.

About local dimming

A technology called local dimming can be used to control the backlight.   
It can greatly affect the image quality of the display and control the brightness of each part of the backlight separately.
In the direct backlight method backlight unit, as described above, LEDs are placed on the entire back of the LCD panel. By controlling the brightness of these LEDs separately within a certain range, the brightness of the black part of the image will be decreased, while the bright part will be highlighted. Therefore, the brightness of the LED can be finely tuned for the detailed parts of the image. 
It has the following two advantages:

①Expression of high-purity black 
The display of the black color is considered a weakness for liquid crystals. However, black can be clearly expressed by using this local dimming function. Since the black color is perfectly expressed, other colors can also be firmly supported to enable an overall tight image.

②Enhanced contrast ratio
High contrast ratio, which means a huge difference between brightness and darkness, results in lively and beautiful images.

LCD TVs manufactured using this method is structurally expensive, but it shall also be an essential function if priority is given to image quality.

It is desirable not to let the backlight unit "work for a long time".

Liquid crystal displays are used not only in PCs, TVs, and smartphones, but also on various occasions, such as device management, building surveillance, and displays at plants and stores. In particular, LCDs for business use are used throughout the day and even continuously on holidays. In other words, backlight units used as light sources are operated without any breaks.
Several problems may arise from continuous operation.

①Decreased brightness and changed colors
After being used for a long time, it is inevitable that the backlight unit itself will lose its brightness, and the performance of other optical materials will deteriorate. The brightness of the display may decrease, or the color may change to yellow. Brightness unevenness and color variation are also likely to occur.

②Black spots appear
After long-term displaying of the screen, non-uniformity may occur in the liquid crystal components. As a result, black spots may be generated on the screen due to poor liquid crystal drive.

Burn-in is a phenomenon in which the trace of the image remains faintly on the screen after the same image has been displayed for a long time. Unlike CRTs in LCD monitors, it is said that burn-in is unlikely to occur in this case. 
However, if the display has been continuously used to show the same image for a long time or in a severe environment, such as high or low temperatures, burn-in is likely to occur.

Several methods to avoid these symptoms as much as possible are provided below.

①Turn off the power frequently
By turning off the power, ongoing degradation can be temporarily stopped or reduced.

②Use the power-saving mode
If you use the power saving setting of the ОS system on a PC, etc., you can enter the power saving mode to automatically turn off the monitor when not in use. In this way, like turning the power off frequently, ongoing deterioration can be temporarily stopped or reduced.

③Use presence sensor
The sensor can detect the absence of humans and automatically enter the power saving mode

Backlight unit with mini LED and micro LED

Compared to ordinary LEDs, which have a chip size of 300 to 350 μm, mini LEDs have a smaller chip area of 100 to 200 μm. Furthermore, next-generation technology for producing micro LED with a chip size of 100 μm or smaller is attracting considerable attention.
Attempts to use this mini LED as a backlight for LCDs are increasing, as well as for vehicles and lighting equipment.
Recently, the display industry has increasingly applied organic EL instead of liquid crystal. Compared to liquid crystals, organic EL has high contrast and color reproducibility. Furthermore, since it can emit light itself, a backlight unit is not needed, and the module can be made thinner. This material is also bendable.  
However, when the mini LED is used as the backlight of the LCD, since the backlight for the black part of the screen can be turned off, the contrast will be improved.
Mini-LED backlights have the potential to become a rival technology against OLED.

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