A Thin-Film Transistor Liquid Crystal Display (TFT LCD) operates by controlling light transmission through liquid crystals to form images. Here’s a detailed breakdown of how a TFT LCD works:
To improve efficiency, Pulse-Width Modulation (PWM) is used to drive LED backlight. it involves turning the LED ON/OFF at a specific rate that human eye perceives the LED being ON continuously but at a reduced brightness. The amount of the dimming is controlled by the duty cycle. Typically the LED is turned ON/OFF between 60 to 240 times a second (Hz). Any slower than 50 Hz and the eye perceives flicker.
Applied voltage: The liquid crystals align in a way that rotates the light to match the second polarizer, allowing light to pass through and making the pixel appear bright.
No voltage: The liquid crystals stop rotating the light, so it is blocked by the second polarizer, making the pixel appear dark.
Why does light change its direction? The birefringence of liquid crystals is the key. Birefringence is the property of certain materials (like liquid crystals) to refract light differently depending on the orientation of their molecules. When the liquid crystal molecules twist, they change the optical path of the light by altering its direction as it passes through. This interaction allows the liquid crystal layer to control the polarization and therefore the brightness of the light exiting the LCD. This forms the basic mechanism behind how images are created on an LCD.
When people refer to TFT LCDs as TN, IPS, or VA, they are talking about the different types of panel technologies used in the construction of the display. Each type has its own unique characteristics in terms of how the liquid crystals are arranged and controlled, affecting the display’s color accuracy, response time, contrast, and viewing angles. Let’s break down each technology:
Advantages:
Each of these panel technologies is built on TFT (Thin-Film Transistor) technology, which refers to how each pixel is individually controlled for better image quality and faster refresh rates. The difference lies in how the liquid crystals move or align, leading to different strengths and weaknesses for each type of panel.
For further information of TFT-LCD, please take a look at the linked document.
TFT LCD Basic Structure
A TFT LCD consists of several layers:- Backlight: Usually composed of white LEDs, it provides the necessary light to illuminate the display.
- Polarizers: Two polarizing filters are placed at the front and back of the liquid crystal layer. These filters control the direction of light.
- Liquid Crystal Layer: This layer contains liquid crystals, which change their orientation in response to electric fields, thus controlling light transmission angle.
- Thin-Film Transistor (TFT) Array: A matrix of tiny transistors, each connected to a pixel in the display. This array is crucial for precisely controlling each pixel.
TFT Display Working Principle
TFT LCD Backlight
The backlight provides uniform illumination across the entire screen. Since the liquid crystals do not produce light, the backlight is necessary for making the display visible. White LEDs are commonly used for this purpose, offering brightness and energy efficiency.To improve efficiency, Pulse-Width Modulation (PWM) is used to drive LED backlight. it involves turning the LED ON/OFF at a specific rate that human eye perceives the LED being ON continuously but at a reduced brightness. The amount of the dimming is controlled by the duty cycle. Typically the LED is turned ON/OFF between 60 to 240 times a second (Hz). Any slower than 50 Hz and the eye perceives flicker.
Polarization of Light
Light from the backlight passes through the first polarizer, which allows only light waves oscillated in one direction to pass through. The second polarizer, placed at 90 degrees to the first one, blocks light unless the liquid crystals between them change light’s polarization.Liquid Crystal Modulation
Liquid crystals used in displays (typically nematic liquid crystals) consist of long, rod-like molecules that can be aligned when an electric field is applied. These molecules twist or rotate when electric field is altered, which affects light’s angle and in turn how much light can pass through the second polarizer:Applied voltage: The liquid crystals align in a way that rotates the light to match the second polarizer, allowing light to pass through and making the pixel appear bright.
No voltage: The liquid crystals stop rotating the light, so it is blocked by the second polarizer, making the pixel appear dark.
Why does light change its direction? The birefringence of liquid crystals is the key. Birefringence is the property of certain materials (like liquid crystals) to refract light differently depending on the orientation of their molecules. When the liquid crystal molecules twist, they change the optical path of the light by altering its direction as it passes through. This interaction allows the liquid crystal layer to control the polarization and therefore the brightness of the light exiting the LCD. This forms the basic mechanism behind how images are created on an LCD.
Pixel Control via TFTs
Each pixel in a TFT LCD is controlled by a corresponding transistor in the TFT array. A matrix of transistors is connected to the display’s pixels:- Active Matrix: This arrangement enables individual control of each pixel ON/OFF. When voltage is applied to a specific row and column, only the pixel at that intersection is activated.
- Capacitor: A small capacitor maintains the pixel’s charge, ensuring that the pixel remains in its intended state until the next refresh cycle.
Color Creation
Color TFT LCDs add a layer of color filters on top of the liquid crystal layer. These filters are arranged in red, green, and blue (RGB) subpixels:- Each pixel consists of three subpixels (red, green and blue).
- By varying the intensity of the light passing through each subpixel, different colors are produced. When these subpixels are combined, they form the desired color for each pixel.
Refresh and Response Time
TFT LCDs refresh many times per second (typically 60Hz or higher) to update screen image. The response time refers to how quickly the liquid crystals can shift from one state to another, affecting how fast the display can change colors, which is crucial for motion clarity in fast-moving content (e.g., gaming or videos).Viewing Angles
Since the liquid crystals’ alignment affects how light is transmitted, viewing angles in TFT LCDs is limited by how much liquid crystal can twist. However, advanced techniques such as In-Plane Switching (IPS) and Vertical Alignment (VA) improve viewing angles and color consistency by modifying the way liquid crystals are aligned and controlled.TFT Display Categories
There are different terms in the market when referring to TFT LCD. Sometimes we hear TN, IPS or VA. The other time people say a TFT LCD is a-Si, LTPS or IGZO.When people refer to TFT LCDs as TN, IPS, or VA, they are talking about the different types of panel technologies used in the construction of the display. Each type has its own unique characteristics in terms of how the liquid crystals are arranged and controlled, affecting the display’s color accuracy, response time, contrast, and viewing angles. Let’s break down each technology:
TN (Twisted Nematic) Panels:
- How it works: The liquid crystals twist when an electric current is applied, controlling the passage of light. The “twisted nematic” refers to the 90-degree twist of the liquid crystal molecules in the off state.
- Advantages:
- Fast response times: TN panels are popular for gaming because of their quick pixel response, reducing motion blur.
- Low cost: TN panels are typically the most affordable type of TFT LCD.
- Disadvantages:
- Narrow viewing angles: Colors and contrast shift dramatically when viewed from the side or above.
- Poor color reproduction: Compared to other panel types, TN panels have inferior color accuracy and contrast
- Use cases: Gaming monitors, budget displays, general-purpose screens.
IPS (In-Plane Switching) Panels:
- How it works: In IPS panels, the liquid crystals are aligned parallel (in-plane) to the glass layers and move horizontally when voltage is applied, rather than twisting as in TN panels. This keeps the light passing through the display more consistent, especially at wide viewing angles.
- Advantages:
- Wide viewing angles: Colors and contrast remain stable even when viewed from the sides or at steep angles (up to 178°).
- Better color accuracy: IPS panels offer superior color reproduction and are preferred for tasks requiring precise color, like photo and video editing
- Disadvantages:
- Slower response times: Compared to TN, IPS panels usually have slower response times, though modern versions have improved significantly.
- Higher cost: IPS panels are generally more expensive to produce than TN panel
- Use cases: Professional monitors for graphic design, video editing, photography, high-end smartphones, tablets and medical screen.
VA (Vertical Alignment) Panels:
- How it works: In VA panels, the liquid crystals align vertically when no voltage is applied and tilt when voltage is introduced. This vertical alignment allows VA panels to block more light, resulting in deeper blacks and higher contrast.
Advantages:
- Superior contrast ratios: VA panels offer much better contrast than both TN and IPS, with deep blacks and bright whites, making them ideal for watching movies or working with high-contrast images.
- Good color reproduction: Though not as good as IPS, VA panels offer better colors than TN panels.
- Disadvantages:
- Moderate viewing angles: Better than TN but not as wide as IPS. There can be a slight shift in color and contrast when viewed from extreme angles,
- Slower response times: VA panels generally have slower response times than TN and IPS panels, which can cause motion blur, particularly in fast-moving scenes.
- Use cases: TVs, monitors for entertainment and general use, displays requiring high contrast.
| Panel Type | Response Time | Color Accuracy | Contrast | Viewing Angle | Cost |
|---|---|---|---|---|---|
| TN | Fast | Low | Low | Narrow | Low |
| IPS | Moderate | High | Moderate | Wide | High |
| VA | Slow | Moderate | High | Moderate | Medium |
Each of these panel technologies is built on TFT (Thin-Film Transistor) technology, which refers to how each pixel is individually controlled for better image quality and faster refresh rates. The difference lies in how the liquid crystals move or align, leading to different strengths and weaknesses for each type of panel.
For further information of TFT-LCD, please take a look at the linked document.