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How does an LCD screen work?

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Quick answer

An LCD screen uses liquid crystals that twist or untwist when voltage is applied. This changes whether polarized light can pass through. Each pixel has red, green, and blue subpixels with filters, and a backlight provides the light. By controlling the voltage per subpixel, the screen builds the image you see.

An LCD screen works by controlling the orientation of liquid crystal molecules with an electric field to twist or untwist polarized light. Each pixel combines these light valves with color filters and a backlight to produce the image you see.

Step by step

  1. A white backlight shines light evenly behind the entire screen.
  2. The light passes through a polarizing filter that allows only one orientation of light waves.
  3. Liquid crystal molecules between two alignment layers twist the light's polarization by 90 degrees.
  4. A second polarizing filter, rotated 90 degrees, lets the twisted light pass through.
  5. Applying voltage to electrodes untwists the liquid crystals, reducing the light that passes.
  6. Color filters split each pixel into red, green, and blue subpixels to create the final color image.

Key things to understand

  1. Liquid crystals are rod-shaped molecules that flow like a liquid but align like a crystal.
  2. In a twisted nematic cell, the molecules naturally twist 90 degrees between two grooved alignment layers.
  3. Two polarizing filters sit at right angles, so without the liquid crystal the cell would block light.
  4. Applying voltage across the cell untwists the molecules, changing how much light passes through.
  5. Color filters and a backlight turn each pixel into a controllable red, green, or blue light source.

Labelled diagram

  1. Backlight

    Provides white light from behind the screen, usually from LEDs.

  2. First polarizer

    Allows only light waves vibrating in one direction to pass.

  3. Liquid crystal layer

    Molecules twist 90 degrees and rotate the light's polarization.

  4. Second polarizer

    Aligned at 90 degrees to the first, letting twisted light through.

  5. Color filter

    Separates each pixel into red, green, and blue subpixels.

Layers of a twisted nematic LCD pixel

Real-world examples

  • A smartphone screen dims when you turn down brightness because the backlight intensity is reduced.
  • A calculator display uses a reflective LCD with no backlight, relying on ambient light to show numbers.
  • A laptop screen viewed from the side looks washed out because the liquid crystal layer has a limited viewing angle.
  • A digital watch uses a simple twisted nematic LCD to show time with low power consumption.

Common mistake

✗ Myth: LCD screens produce their own light like a light bulb.

✓ Fact: Liquid crystals do not emit light; they only control the passage of light from a separate backlight. The screen would be completely dark without that backlight.

People also ask

What is a liquid crystal?
A liquid crystal is a state of matter that flows like a liquid but has molecules that can align in ordered patterns like a crystal. In LCDs, rod-shaped molecules twist in response to an electric field, which is how they control light.
Why do lcd screens have dead pixels?
A dead pixel occurs when a transistor or electrode in that pixel fails, so the liquid crystal can no longer be controlled. It may stay permanently black or, if the backlight shines through, permanently bright. Manufacturing defects or physical damage cause this.
How does a touchscreen work with an lcd?
A touchscreen adds a transparent layer on top of the LCD that senses your finger's touch. Capacitive touchscreens detect changes in an electric field when your finger touches the glass. The LCD below shows the image, while the touch layer reports the position to the device.
Why do LCD screens need a backlight?
Liquid crystals do not emit light; they only block or pass it. A backlight, usually white LEDs, provides the light that the liquid crystal layer modulates. Without it, the screen would be dark.
What is the difference between LCD and LED screens?
An LED screen is actually an LCD screen that uses light-emitting diodes for its backlight instead of cold cathode fluorescent lamps. Both still rely on liquid crystals to control the image. The name LED refers only to the backlight type.
Why do LCD screens have a limited viewing angle?
The liquid crystal molecules and polarizers work best when viewed straight on. From an angle, light takes a longer path through the cell, so the contrast and colors shift. Modern screens use compensation films to widen the angle.
How does an LCD make different colors?
Each pixel is divided into three subpixels with red, green, and blue color filters. By varying the voltage on each subpixel, the liquid crystal controls how much of each color passes through. Mixing these three primaries produces the full range of colors.

Part of the guide: How the Internet Works →