What is the structure of an LCD display?

Its core working principle is to control the twist angle of liquid crystal molecules by applying voltage, thereby changing the polarization state of light. Polaroids then act like a "gate" to determine whether light can pass through, ultimately achieving brightness changes at each pixel. Combined with color filters, brilliant colors can be displayed.

Here's a breakdown of a typical LCD layer structure from back to front (from backlight to screen surface):

LCD Core Structure Breakdown

Part 1: Backlight Module

This is the "light source" of the LCD, as liquid crystals themselves do not emit light.
1. Backlight:
· Early: CCFLs (cold cathode fluorescent lamps), similar to small fluorescent tubes, arranged on either side or behind the screen.
· Modern: LEDs (light-emitting diodes). This is currently the absolute mainstream and is generally divided into two layouts:
Edge-lit: LED light strips are installed around the screen's bezel, evenly distributing light across the entire screen via a light guide. This approach offers the advantage of being very thin.
Direct-lit: LED lights are evenly distributed directly behind the panel, enabling zoned dimming, brightening desired areas and darkening desired areas, thereby improving contrast and image quality. (Commonly used in high-end TVs)

2. Light guide plate: (mainly used for edge-entry backlight) A transparent acrylic plate with a precise dot design on the surface, which converts the LED point light source from the edge into a uniform, surface-emitting surface light source.

3. Brightness enhancement film/optical film group: A stack of films located above the light guide plate, used to enhance and concentrate light.

Diffuser: Provides more uniform lighting, eliminating shadows from dots or LEDs. Prism/Brightness Enhancement Film: Focuses diffused light directly in front of the screen, significantly increasing frontal brightness.

The second part: LCD panel body
This is the core of the technology and usually consists of two parallel glass substrates.

1. Lower Glass Substrate - TFT Array Substrate · This is the most complex and sophisticated layer of the LCD. It's not flat, but instead features millions of tiny thin-film transistors fabricated using photolithography. Each transistor corresponds to a subpixel (red, green, and blue subpixels make up a pixel). · Each subpixel has a transparent pixel electrode (usually ITO, indium tin oxide).

2. Upper Glass Substrate - Color Filter Substrate · On the inner side, corresponding to each sub-pixel, are printed red, green, and blue color filters. These filters color the light passing through the liquid crystal.

Between the filters, there is a black matrix, which is a light-proof black grid that is used to isolate the light of adjacent sub-pixels, prevent cross-color, and improve contrast and clarity.

The inner side of the substrate is covered with a common electrode (also an ITO film), which together with the pixel electrode of the lower substrate forms an electric field to drive the liquid crystal.

3. Liquid crystal material · Precision poured between two substrates, the material is only a few microns thick. To control the initial alignment of the liquid crystal molecules, an alignment layer is applied to the inner sides of the upper and lower substrates. Microscopic grooves are formed through friction and other methods, allowing the liquid crystal molecules to align in a specific direction.

4. Sealant Frame and Spacers · Use sealant to bond the two glass substrates together while maintaining a precise cell gap. · Tiny plastic balls or photolithographically formed pillars are scattered within the cell as spacers to prevent the two glass sheets from sticking together due to external forces.

Part 3: Light "Gates" and Control
1. Polarizers
· LCDs have two polarizers, like two "gates."
· Lower polarizer: Attached between the backlight unit and the TFT substrate. It only allows light with a specific vibration direction to pass through.
.Upper polarizer: Attached to the color filter substrate. Its polarization direction is 90 degrees (or 0 degrees, depending on the liquid crystal mode) perpendicular to the lower polarizer.
Basic Principle: When no voltage is applied, liquid crystal molecules twist the polarization direction of light by 90°, allowing it to pass through the second polarizer, resulting in the pixel appearing "bright." When voltage is applied, the liquid crystal molecules no longer twist the light, blocking it from the second polarizer, resulting in the pixel appearing "dark." By controlling the voltage, the shades of gray can be precisely controlled.

Therefore, the structure of LCD is a sophisticated system that is linked together. White light is emitted from the backlight, filtered by the polarizer, regulated by the liquid crystal, and colored by the color filter, ultimately forming each colorful pixel we see on the screen.