From the working principle of LCD, we know that the display of LCD device is realized by controlling the transmission intensity of the light emitted by the backlight source through the polarizer and liquid crystal box, that is, the light switch. Therefore, the brightness of LCD depends on the transmittance through the polarizer, liquid crystal box (LCD transmittance) and color film (CF transmittance) and the brightness of the backlight source itself. Therefore, to improve the brightness of LCD, we should start from three aspects:

• Improve the transmittance of the LCD itself, especially the aperture ratio of TFT pixels and the switching ratio of liquid crystal:

• Increase the transmittance of all materials;

• Increase the brightness of the backlight source.

The solution is

• Increase the number of lamps and the power of the light source. However, it will lead to high power consumption and larger volume:

• Thinking from the perspective of the light guide plate, mainly puts forward higher requirements on the material and design of the light guide plate:

• Considering the reflective film behind the lamp tube and the reflective sheet under the light guide plate, improve the reflection efficiency and increase the reflection brightness

• Add prism film and brightness enhancement film.

Improve the transmittance of LCD itself

1. Improve the on/off ratio

The on-off ratio refers to the fact that the LCD device has good transmittance when it is on, and good light shielding when it is off. To achieve this goal, efforts should be made in the selection of materials, design, process, and manufacturing of the device.

Selecting high-transmittance glass and polarizers, designing the right match between liquid crystal materials and box thickness, high-quality sub-orientation, and accurate and consistent box manufacturing processes are all necessary conditions to ensure a high on-off ratio.

2. Increase the opening ratio

In each pixel of a TFT liquid crystal display, a TFT active device needs to be made. This device will occupy a certain area. Since this set of active devices has a strong photoelectric effect, it must be covered with a shielding layer. This part will no longer be able to transmit light. The ratio of the area of the entire pixel to this part of the TFT that cannot transmit light is the aperture ratio.

There are many ways to increase the aperture ratio. For example: increase the electron mobility, change amorphous silicon to polysilicon or single crystal silicon. Due to the increase in electron mobility, the device can be made smaller, the conductivity of the substrate can be increased, the lead can be made thinner, and the use of nano-level micro-machining can increase the aperture ratio, etc.

3. Improve transmittance

Selecting materials with high transmittance is the basic method to improve transmittance.

Increase the brightness of the backlight

Improve the brightness of the light source, change the structure and installation method of the light source.

The backlight source of the LCD is the main factor in improving the brightness. There are three main types of light sources that can be used as backlight sources. They are EL, LED, and CCFL.

(1) The electroluminescent (EL) backlight is thin and light. The surface light source provided by EL is consistent. Its power consumption is very low, and the required working voltage is AC80~100V. The inverter that provides the working voltage can convert the input voltage of 5/12/24V DC into high-voltage AC output.

Since the service life of EL backlight is limited (the average half-brightness life is 3000~5000 hours), the ideal EL inverter can increase the output voltage and frequency as the EL ages, thereby extending the effective service life of the EL.

EL backlights are suitable for small and medium-sized LCDs such as watches, digital clocks, and monochrome PDAs where low-brightness backlighting is sufficient. However, their low efficiency, low brightness, and short life make them unsuitable for backlighting large LCDs such as notebook computers and desktop LCD monitors.

(2) LED backlight has a promising future. LED backlight has a longer service life than EL (over 50,000 hours) and uses DC voltage. In the past, it was usually used in small monochrome LCD displays, such as mobile phones, remote controls, microwave ovens, stereo audio and video equipment, etc. However, its low brightness and easy generation of light spots make it difficult to use in large color LCD displays.

Nowadays, LED has made great progress, and white light and ultra-high brightness LED have appeared one after another, opening up a new way for developing new LED backlight sources. The utility model of LED backlight can not only be made very thin, up to 0.7mm, but also use direct backlight to achieve a brightness of 3000cd/m2, achieving more than 90% of the N-system color gamut.

(3) Cold cathode fluorescent lamp (CCFL) is still the most ideal backlight source. CCFL can provide the brightness and color required for large LCDs, but its biggest disadvantage is heat dissipation and electromagnetic interference.

At present, single and double tubes are more commonly used. With the increase of product area and the improvement of brightness requirements, 4-tube and 6-tube designs have begun to appear. In the 4-tube design, one tube can be added to the left and right of the screen on the basis of the original double tube, or one parallel tube can be added to the top and bottom on the basis of the original double tube to form four parallel tubes. Most of the installation schemes for 6 tubes adopt the bottom light direct irradiation method. That is, all three tubes are bent into a "U" shape and evenly placed side by side under the LCD screen. Or it can be a multi-tube bottom light CCFL backlight source.

The development from dual lamps to 4 lamps, and then to 6 lamps, aims to improve brightness and uniformity, as well as the expressiveness of colors. In fact, the brightness difference between 4 lamps and 6 lamps is not easy to be felt by the naked eye. However, the heat generated by 6 lamps is much higher than that of 4 lamps. With the increase of LCD TV screens and the requirement of higher brightness, products with 8 lamps, 12 lamps, and 16 lamps have also appeared, and the power consumption can reach tens of watts, and the heat dissipation problem will be more prominent. Therefore, designers must pay attention to balancing and solving problems such as brightness, heat dissipation, life, and power consumption.

The driving of CCFL tubes requires high voltages of tens of kilohertz and thousands of volts. This is usually solved by an inverter driver. This inverter will interfere with other electrical appliances and even display screens through connecting wires and electromagnetic waves in space. The design and manufacture of inverters must comply with relevant technologies for electromagnetic compatibility.

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