LCD backlight high color gamut display principle and fluoride phosphor solution

Based on the characteristics of LED's high luminous efficiency, low carbon, environmental protection, and energy saving, most LCD TVs and mobile phones on the market currently use LEDs as backlight sources. At the same time, in order to meet the demand for high color gamut coverage of LED backlight sources, the industry is gradually introducing some new fluorescent materials with a narrow half-wave width, such as fluoride phosphors and quantum dot phosphors. In order to better understand the characteristics of these phosphor materials, we will focus on briefly introducing and explaining fluoride phosphors and their applications below.

First of all, before understanding and understanding fluoride phosphors, we first introduce some basic knowledge of LED light sources in backlight applications.

1. Introduction to LCD related knowledge

A. LCD meaning

LCD: The abbreviation of Liquid Crystal Display, the full name of liquid crystal display, which includes TFT, UFB, TFD, STN and other types of liquid crystal display.

LCD TVs and mobile phones: TVs or mobile phones that use LCD screens as displays;

B. The structure of LCD

Take a TFT type liquid crystal display as an example. Its structure mainly includes backlight, light guide plate, upper and lower polarizers, liquid crystal, color filters, thin film transistors, etc. (see Figure 1 for the structure diagram). The main structure functions are explained as follows:

1. Backlight (Back

Light): LCD's imaging principle is to control the brightness and darkness by blocking the light component of the liquid crystal. A light source is necessary to see the image on the screen, so the backlight source is responsible for providing the most basic light source for LCD display.

2. Light Guide Plate: Make the light evenly distributed on the entire screen;

3. Lower polarizer (Up/Down

Polarizer): The directionality of the light sent from the backlight is inconsistent and radial. If such light passes through the twisting of the liquid crystal molecules, we still cannot see the image we want to see on the screen. At this time, the polarizer below is responsible for it. In order to standardize the direction of light to be consistent, then send it to the work of the liquid crystal layer.

4. Thin Film Transistor (TFT): Control the twist angle of liquid crystal molecules

5. Liquid crystal (Liquid

Crystal): The liquid crystal molecules in this layer are twisted under the control of TFT, so that the light in the same direction can be turned on and controlled, so that the brightness of the light to the rear pixel unit is changed.

6. Color filter

Filter): After the white passes through the filter, we can see that the light of the color corresponding to the filter is transmitted, so in the LCD screen, the function of the color filter is to color.

Figure 1 LCD structure diagram

C. LCD's imaging principle

The imaging principle of LCD is to place the liquid crystal between two pieces of conductive glass. By controlling the driving of the upper and lower polarizers and the electric field between the upper and lower electrodes, the liquid crystal molecules are twisted and the nematic electric field effect is caused to control the transmission or transmission of the backlight. Masking, combined with other control and auxiliary function layers to realize the function of restoring the picture.

D, LCD backlight introduction

Since the liquid crystal must use an additional light source to emit light,

Commonly used LCD backlight sources include CCFL (cold cathode fluorescent lamp), LED (light emitting diode), HCFL (hot cathode fluorescent lamp), surface light source VFD (flat fluorescent lamp), EL (electroluminescent film), OLED (organic electric Luminescent film) and so on. Among them, CCFL is currently the most commonly used LCD backlight, usually also called traditional backlight;

Comparison of CCFL and LED:

CCFL-made of hard glass and three primary color phosphors sealed, there is a proper amount of mercury and inert gas in the tube, the inner wall of the tube is coated with phosphor, and there is an electrode at each end. The disadvantage is that the color displayed is limited .

LED—— is a kind of semiconductor solid light emitting device, which uses solid semiconductor chip as the luminescent material. In the semiconductor, the excess energy is released by the recombination of carriers to cause photon emission, which directly emits red, yellow, blue, green, cyan, and orange , Purple and white light. Because LED light-emitting diodes have good color performance, they have completely replaced the light source of traditional cold cathode fluorescent tubes.

2. Introduction to knowledge about color gamut coverage

A. Meaning of color gamut coverage

Color gamut coverage: The horseshoe-shaped chromaticity triangle marked with colors on the CIE-xy chromaticity diagram is the color area that the human eye can see. If a system can reproduce all the colors in this horseshoe-shaped area, it can be said to be its color The domain coverage is 100%. When the three primary colors of R, G, and B are used to reproduce colors, the triangular area formed by the coordinates of the three primary colors of R, G, and B is the color reproduction area determined by the three primary colors. The ratio of this area to the horseshoe-shaped area is the color gamut coverage. Therefore, the color gamut coverage is the ratio of the area of a triangular area composed of three primary color coordinates of R, G, and B to the area of a triangular area composed of three primary color coordinates of standard R, G, and B.

B. Judgment criteria for color gamut coverage

In different fields, the requirements for the three primary color coordinates of the standard R, G, and B are different, which involves different color gamut evaluation standards. Color gamut standards generally include NTSC, ITU-R

BT.709, sRGB, Adobe RGB, ITU-R BT.1361, xvYCC, etc., the following three common standards (sRGB, NTSC and Adobe

RGB) for a brief introduction:

1) NTSC standard: In 1953, the National Television Standards Committee (National Television Standards

Committee, NTSC for short) is based on the NTSC standard established by the CIE1931 chromaticity diagram, which is also a commonly used standard in China.

2) sRGB standard: In 1996, the International Electrotechnical Commission IEC established a color gamut standard for digital images. This standard is mainly used in digital image acquisition equipment, but has not been fully popularized on displays.

3) Adobe RGB standard: proposed by Adobe in 1998, it has a wider color space than sRGB, and is generally used in fields such as printing and publishing and image processing.