An LCD screen is structured in layers to control light and display images. Each pixel in an LCD is made up of several components working in conjunction.
Key Components of an LCD Screen:
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Polarizing Filters: Two polarizing filters are crucial. These are oriented perpendicularly to each other. The first filter only allows light waves vibrating in a specific direction to pass through. The second filter, oriented at 90 degrees to the first, would normally block all the light.
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Glass Substrates with Electrodes: Two transparent glass substrates sandwich the liquid crystal material. These substrates have thin, transparent electrodes (usually made of Indium Tin Oxide or ITO) deposited on them. These electrodes are patterned to define the individual pixels or segments of the display.
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Liquid Crystal Material: This is the heart of the LCD. The liquid crystal molecules are situated between the glass substrates. These molecules align themselves in a predictable manner but can be twisted or untwisted when an electric field is applied.
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Thin Film Transistors (TFTs): In active-matrix LCDs (like those found in most TVs and computer monitors), each pixel is controlled by a TFT. This transistor acts as a switch, controlling the voltage applied to the liquid crystal cell.
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Color Filters (for color LCDs): For color displays, a layer of color filters (red, green, and blue) is used. Each pixel is subdivided into subpixels, each with its own color filter. By controlling the brightness of each subpixel, a wide range of colors can be produced.
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Backlight: Since LCDs don't produce their own light, a backlight is essential. Common backlight technologies include:
- CCFL (Cold Cathode Fluorescent Lamps): Older LCDs used CCFLs.
- LED (Light Emitting Diodes): Modern LCDs primarily use LEDs. These can be placed along the edges of the screen (edge-lit) or directly behind the LCD panel (full-array).
- Quantum Dots: (Advanced LED Backlights) Used to increase the range of colours.
How it Works:
- The backlight shines light through the first polarizing filter.
- The light then passes through the liquid crystal layer.
- When no voltage is applied, the liquid crystal molecules twist the light to align with the second polarizing filter, allowing the light to pass through.
- When voltage is applied to the electrodes, the liquid crystal molecules untwist. This changes the polarization of the light, so it becomes blocked by the second polarizing filter. This makes the pixel appear dark.
- By varying the voltage applied to each pixel (or subpixel in a color LCD), the amount of light that passes through can be precisely controlled, creating the image.
Simplified Analogy
Think of it like Venetian blinds. The polarizing filters are like two sets of slats, one horizontal and one vertical. Normally, no light gets through. The liquid crystal is like a mechanism that can rotate the light to match the angle of the second set of slats, allowing light to pass. Applying voltage changes the rotation, blocking the light.
