Android LCD (I): Basic LCD principles

Keywords: Android LCD tft LCD partial film color filter Backlight

Platform information:
Kernel: linux2.6/linux3.0
System: Android/android4.0
Platform: Samsung exynos 4210, exynos 4412, and exynos 5250

Author: xubin341719 (You are welcome to reprint it. Please note the author)

Correct the mistakes and learn and make progress together !!

Download link: LCD specifications (404 copies), LCD specifications used for previous work 00
LCD specifications 01,LCD Testing images, color bars, gray scale, etc.

Android LCD (I): Basic LCD principles

Android LCD (II): principle of common LCD Interfaces

Android LCD (III): Samsung LCD interface

Android LCD (iv): LCD Driver debugging

The first job, the first company, was doing TV solutions. During that time, I had to deal with LCD screens all day, from 7 "to 42. At that time, I didn't graduate. My school was very different from my work. However, my colleagues in that company are very good. I would like to thank them for their careful guidance, job opportunities, generous treatment (many for students), and good job opportunities. After that, I changed my business to a tablet and left my first company. If I had the opportunity or had enough capabilities, I would definitely return to those who have helped me. "The grace of dripping water is reported by Yongquan ". Take some time, work hard, and record every mile carefully.

Next let's talk about the construction and display principle of the TFT-LCD, like the previous blog, I will write about the composition of the device, and simple working principle, these relationships with programs and Android are not too great, but we need to debug a module and have a system understanding of its construction, the understanding of modules and work ideas are still of great help (only personal opinions ).

There are many LCD categories, which can be divided into passive matrix type and Active Matrix type by driving method.

Passive matrix:Passive matrix LCD can be divided into TN-LCD (twistednematic-LCD, twisted column LCD), STN-LCD (supertn-LCD, ultra-distorted column LCD) and DSTN-LCD (doublelayer STN-LCD, double-Layer ultra-distorted column LCD ). This part is not explained in detail. We will focus on TFT-LCD.

Active Matrix

The most widely used active matrix LCD, also known as TFT-LCD. TFT-LCD is thin-filmtransistor liquid-crystal display abbreviation (Thin Film Crystal Liquid Crystal Display) TFT-LCD how to light up? TFT-LCD is now widely used, we start from the TFT-LCD.

TFT-thin film transistorThin film transistor

LCD-Liquid Crystal DisplayLCD

TFT-LCD Transistor Liquid-crystaldisplayAbbreviation (Thin Film Crystal LCD)

Because the TFT-LCD has the advantages of small size, light weight, low radiation, low power consumption, full color, so it has been widely used in all kinds of display equipment.

I. Structure of TFT-LCD

1. The structure of the TFT-LCD is shown in

Reflector: Reflector CCF lamps: Cooling light tube Ligh pipe: Guide Film Extraction pattern: Diffuser: light panel, which plays a role in astigmatism and ensures even distribution of light.	This part is mainly the light source part, CCFL or LED Backlight light source, other reflector, light source, the role of these light distribution more even
Rear polarizer: Posterior eccentric film Front polarizer: front-end partial Film	The role of the partial film changes the natural light into a partial Aurora
Selected subpixels: sub-pixel TFT: Liquid Crystal: Liquid Crystal Color Filters: Color Filter	This part is the core part of the LCD. Choose the light source for conduction, blocking, and color control.
Back Glass: Back Glass Substrate Front Glass: front glass substrate	Protection

 

Functions of various TFT-LCD Structures

(1) backlight module: Provides the light source;

(2) Upper and Lower reflectors, TFT glass substrate, and liquid crystal: Form polarized light and control the passage of light;

(3) color filter: provides the source of tft LCD red, green, blue (three primary colors of light;

(4) ITO transparent conductive layer: Provides Transparent Conductive channels;

(5) Photo spacer: provides a fixed height for color filters and TFT glass substrate. It is used as the space for filling liquid crystal and as the support for the upper and lower layers of glass.

2. Side view of TFT-LCD Structure

I. TFT-LCD display principle

1. LCD Backlight

Backlight is the light source of the display. Two LCD backlights are commonly used: CCFL backlights and LED backlights.

(1) CCFL

Cold Cathode Fluorescent Lamp (CCFL) is a Cold Cathode Light tube. It has the advantages of high power, high brightness, low energy consumption, and is widely used in display and lighting fields.

(2) LED Backlight

Compared with CCFL, LEDs have the advantages of low power consumption, uniform light source, long life, and small size, which may be expensive in terms of price. However, the TFT-LCD on the flat panel now seems to be LED backlights, I broke it last time and opened it by the way.

(3) led and LED backlights

The so-called LED display on the market is actually an "LED-backlit LCD display". The popular LCD display is a "CCFL-backlit LCD display ". Therefore, the two are still LCD, but the backlight is different. If you don't see the LED display, you will mistakenly think it is the next-generation technology display. In fact, the latest technology is called OLED. So don't be fooled when buying TV.

2. LCD Overview

(1) Shape of Liquid Crystal

The liquid crystal used by the TFT-LCD is TN (twist nematic) type liquid crystal, and the liquid crystal molecules are elliptical.

(2) LCD features

TN Liquid Crystal is generally connected along the direction of the long axis, and the long axis is arranged in parallel. When it comes to the groove surface, the liquid crystal molecules will be arranged in the direction of the groove.

(3) vertical distribution of Liquid Crystal

When the liquid crystal is contained in the middle of two groove surfaces and the direction of the groove is perpendicular to each other, the liquid crystal molecules are arranged as follows:

Upper surface molecules: along the direction

Lower surface molecules: along the B direction

Molecules between the upper and lower surfaces: produce the rotation effect. Therefore, Liquid Crystal Molecules rotate 90 degrees between two groove surfaces.

(4) deflection between light and Liquid Crystal Molecules

(5) balanced distribution of liquid crystal at voltage

When electric pressure is added between the upper and lower surfaces, liquid crystal molecules are arranged along the electric field to form an upright arrangement. At this time, the incident light is not affected by the liquid crystal molecules, and is directly routed to the lower surface.

3. Features of the reflector

Purpose: Filter non-partial Aurora (General light) into partial Aurora. When the non-partial Aurora goes through the-direction partial light, the light is filtered into a linear partial Aurora parallel to the-direction.

: Linear Partial Aurora continues. When the second partial film is passed, the light passes through.

: When the second part is passed, the light is completely blocked.

The optical effect produced by the combination of a light source, a groove surface, and a liquid crystal, as shown in

(1) When the upper and lower skewness are perpendicular to each other, if no voltage is applied, the light can pass through

(2) When voltage is applied, the light is completely blocked

When the current changes through an electronic crystal, causing the deflection of liquid crystal molecules, the polarization of light is changed, and the pixel State is determined by the partial film. In this way, we can control the brightness and darkness of the light. If we want to display the color, we will talk about the color filter later.

4. Color Filter Principle color fliters

(1) structure of C/F

A display with low pixels can see these squares carefully. In a simple way, place a water drop on the monitor and you will see red, green, blue, and three colors.

(2) common arrangement methods of C/F pixel array

As shown in, they are mosaic, straight bar, triangular form, and four elements.

(3) display of different colors

Let's take a look at the corresponding pixel electrode when we want to display the corresponding color. As shown in:

The C/f color unit corresponds to the TFT control unit, which can control the color of pixels. Shows the TFT array equivalent circuit:

Iii. TFT-LCD imaging principle

We have explained the principle of liquid crystal light transmittance, the principle of partial light transmittance, the working principle of color filter, these understand him as a pixel control unit, and then we will sort out the overall display principle of the TFT-LCD.

(1) scan IC transmission signals;

Complete image signal input;

(2) The driver IC transmits the control signals of imaging;

Complete TFT unit control;

(3) When a sub-pixel is turned on, the sub-pixel is black due to its inability to transmit light;

Whether the completed pixels are bright or dark.

(4) If the sub-pixel is not on, the color is displayed because the light passes through cf.. After the synthesis of light, the display can produce the color effect. As shown in:

Now, let's look back at whether the structure is clear: in the light source part, the natural light is first converted into a partial Aurora through the partial film --> the TFT subpixels unit controls whether the LCD unit is light-guiding and color --> the color image is displayed through the front partial film. In fact, images are light of different colors. The light we see is actually Aurora.

This figure is like a clear description:

Iv. Common LCD knowledge points

1. Shadow

The shadow is a phenomenon in which the previous image does not disappear immediately but gradually disappears after the screen is switched. The shadow and the response time are not the same. It may take two or three seconds for the shadow to disappear completely, the LCD response time is dozens to dozens of milliseconds. Even if the response time of a well-designed LCD is 15 + 35 ms, it is impossible for users to see the shadow.

The residual film mechanism is somewhat complicated. Normally, when the screen shows for too long, the charged ions in the liquid crystal are adsorbed on both ends of the upper and lower glass to form a built-in electric field. After switching the screen, these ions are not immediately released, this causes the liquid crystal molecules to not immediately turn to the desired angle. Another possible cause is the poor design of the phoneme electrode, which leads to disordered arrangement of Liquid Crystal Molecules During state switching. In this case, it is also possible to see the shadow.

2. Bad Points (DOT defect)

The so-called bad point refers to the constant bright or constant dark points that cannot be controlled on the LCD. The bad point is caused by various factors in the production of the LCD panel, which may be caused by particles falling into the panel, the damage may be caused by electrostatic damage to the Panel, or poor process control. There are two types of bad points: Highlights and hidden points. Generally, the highlights are more unacceptable than the hidden ones, so many manufacturers will ensure that there are no bright spots, but it seems that there are few hidden spots. Some panel manufacturers will make the highlights a dark spot before shipment.

Panel manufacturers sell panels with bad points at a lower price. Generally, a grade is calculated for non-bad points, B grade for three points, and C grade for six points or less.

In the market, many companies now use ipad2, ipad3, and minipad screen, and then talk about Apple during product promotion. Compared with Apple, these companies in China are really funny. In fact, the screen they use is the screen that Apple cannot pass the test during the production process. For example, apple only uses a + screen, and a-including a-or lower-level screens all enter the market, tablets in China are rubbish. Although I have been doing these things, the environment is not good.

3. Mura

Mura was originally a Japanese word with uneven meanings and spots. As Japanese LCD displays spread across the world. Mura refers to the phenomenon of various traces caused by uneven brightness of the monitor. The simplest method is to switch to the black screen and other low-gray-scale images in the darkroom, and then look at them from different angles, problematic display is easy to see.

4. color saturation (color gamut)

Color saturation refers to the degree of brightness of the display. The display is composed of red, green, and blue colors to form any color light. If the RGB color is brighter, the display can have a wider color range.

5. brightness

Brightness refers to the brightness of the display under the white screen, in the unit of CD/m ^ 2, or nit. Brightness is an important factor that directly affects image quality. In the lab, we often say, "A bright screen makes three ugly faces ". Even if the color saturation is poor or the color is yellow or other adverse factors, a bright display may still look pretty.

The brightness is related to the light, and the lamp has a long life, especially the early CCFL backlight. After a long time, the light will yellow. If there is an old display in the house, you can obviously feel it.

6. Perspective

Because of the inherent physical characteristics, LCD displays enable users to view the image quality from different angles.
It has changed. Compared with the current video, the quality of the image changes to fail.
The critical angle is called the display angle.

7. color temperature (color temperature ):

The color temperature is used to describe the white color of the display. It is not limited to LCD. All displays are universal, when the color of the monitor is the same as the temperature of the black body to an absolute temperature, the color temperature of the monitor is equal to the temperature. For example, when the white color of the monitor is designed to be close, the light color (the sun in the morning when the temperature of the black body is 6500 K) is called the color temperature of the monitor as K.
The lower the color temperature, the more yellow the color. The higher the color temperature, the more blue the color. When a display with a high color temperature shows a picture, the entire screen looks blue.

8. gamma curve:

Gamma curve refers to the relationship between gray scale and brightness. Take the gray scale from zero to two to five as the X axis and the brightness as the Y axis. The drawn curve is gamma curve. gamma curve, which is usually not a straight line,
Because the human eye has different recognition effects on different brightness, for example, the recognition capability of low brightness is higher (a slight brightness change will make it feel ).

5. Image downloads frequently used during LCD debugging

Download link: Commonly Used LCD test images, including colored pictures and colored bars. Debugging results.