Contents
1. LCD meaning and definition: What is TFT-LCD
2.Principle of TFT-LCD image display: how LCD works?
3.TFT-LCD electrical principle: how LCD works?
4.TFT-LCD driving principle: how LCD works?
5.TN type liquid crystal
6.Polarizer Function
7.Principle of backlight
8. Summary of TFT-LCD imaging principle: how LCD works?

1. LCD meaning and definition: What is TFT-LCD

TFT-LCD → Thin Film Transistor – Liquid Crystal Display
  • Thin Film Transistor：To control the pixel signal on/off
  • Liquid Crystal Display：To control the light polarization
TFT-LCD is a non-luminous (penetrating) Display panel. It needs a backlight and a switch valve with a picture element as the light source to determine its display color and brightness. It is necessary to cooperate with the backlight and use the pixel as the switch valve of the light source to control the color and brightness.
TFT-LCD Structure

2.Principle of TFT-LCD image display: how LCD works?

The Unit of image display is commonly referred to as a Pixel, and each pixel is composed of three colors small dots of Light (sub-pixels), red (R), green (G) and blue (B)

LCD Pixel&Sub pixel

Red (R); Green (G); and blue (b), also known as the primary colors of color, can be blended by mixing the light to dark ratios of the three colors.

  • 256 Color: 8*8*4=256 colors
  • High Color: 32*64*32= 65536 colors
  • Full Color: 64*64*64=262,144 colors
  • True Color: 256*256*256=16,777,216 colors
The more colors, the more realistic and natural the Image

Even such a colorful painting can be made up of dozens of pixels of various colors.

3.TFT-LCD electrical principle: how LCD works?

Operation Concept of TFT component: Electrical Potential(Water Level)
  • Transistor TFT(Water tap)
  • Interconnections(Water pipe)
  • Capacitors (Flume)
TFT-LCD electrical principle

The level in the tank represents the brightness of the red, green and blue colors.
TFT-LCD electrical principle (2)

4.TFT-LCD driving principle: how LCD works?

Distribution of equivalent circuit in the LCD panel
  • The lower the voltage, the more light penetrates
    (The lower the water level, the lighter the color)
  • The higher the voltage, the less light penetrates
(The higher the water level, the darker the color)
Relationship between penetration rate and voltage of LCD panel
The gate stays open long enough for the water level to reach equilibrium.

Open the first row of the gate and close the rest, then the first row of the storage tank is filled to the pre-set water level.
Then close the first row of the gate, the water level has been fixed, so the display color has been fixed.
Open the second row of the gate and the rest remain closed. Fill the second row of the storage tank to the present water level.
And soon, the whole picture of the display can be completed.
LCD driving principle [4]

5.TN type liquid crystal

The TN LCD uses TN (Twist Nematic) type liquid crystal, and the Liquid Crystal Molecules are elliptical under a microscope.
TN type liquid crystals are usually connected in series in the direction of the long axes, which are arranged in parallel with each other.
When they contact with the grooved surface, the Liquid Crystal Molecules are arranged in the direction of the groove.
When the liquid crystal is contained in the middle of the two grooved surface and groove are perpendicular to each other, the liquid crystal molecules are arranged as follows:
Upper surface molecules: along A direction
Lower surface molecules: along B Direction
Molecules intermediate between the upper and lower surfaces produce a rotational effect, then the liquid crystal molecules rotate 90 degrees between the two grooves.
The effect of light on liquid crystal molecules (1)
When linearly polarized light strikes the upper layer of the grooved surface, the light also rotates as the liquid crystal molecules rotate.
When linearly polarized light penetrates from the lower layer of the grooved surface, it has already produced a 90-degree rotation.
The effect of light on liquid crystal molecules (2)
When a voltage is applied between the upper and lower surfaces, the liquid crystal molecules are aligned in the direction of the electric field, forming a vertical alignment. At this point, the incident light is not affected by the Liquid Crystal Molecules, straight out of the lower surface.

6.Polarizer Function

PURPOSE: To filter non-polarized light (normal light) into polarized light
When the non-polarized light passes through the polarizer in A direction, the light is filtered into a linear polarized light parallel to A direction.
Upper Image: The linearly polarized light continues to pass through the second polarizer.
Below image: The light is completely blocked as it passes through the second polarizer.
Depending on the polarizer angle setting, there are two display modes when no voltage is applied:
1.Normal White (NW Mode)
2.Normal Black (NB Mode)
When the top and bottom polarizers are perpendicular to each other, light can pass through if no voltage is applied.
If a voltage is applied, the light is completely blocked.
When the top and bottom polarizers are parallel to each other, light cannot pass through without applying voltage.

7.Principle of backlight

The liquid crystal cannot emit light by itself, and the backlight should be added behind the LCD panel. A few years ago, CCFL cold cathode backlighting was the dominant market application, but with the development of technology, the LED backlight has gradually replaced the traditional CCFL backlight.
LCM backlight structure introduction
There are two types of backlights, depending on the location
(1) Side entrance type-the lamp tube is placed on the side of the light guide plate. According to the light guide plate type and the optical demand, the straight type, the l type, and the small type of lamp tube can be selected. The advantage of this design is that the thickness is thinner and the temperature is less; at present, more use of this shape.
(2) The straight-down type-the lamp tube is placed under the diffusion plate, and the lamp tube usually uses the w type lamp tube. The disadvantage of this type is thicker and the temperature will be higher (used above 21 inches).
The led backlight has the characteristics of low heat, concentrated light source, slow decay speed, and low power consumption. It also has the advantages of high color saturation, fast start, no mercury, and long life.

8. Summary of TFT-LCD imaging principle: how LCD works?

When a Sub-Pixel is turned on, the Sub-Pixel attenuates to black because it cannot penetrate light.
If the Sub-Pixel is not on, the color is displayed because the light passes through CF.
After the synthesis effect of light, the LCD display can produce a colorful effect.
For more information on the LCD panel or to purchase LCM products, please feel free to visit https://www.vislcd.com/.