We watch tv and glance at smartphone for its finest screen, but HOW LCD works?

Like laptop computers, most new televisions have flat screens with LCDs (liquid-crystal displays)—the same technology we’ve been using for years in things like calculators, cellphones, and digital watches. What are they and how do they work? Let’s take a closer look!

For many people, the most attractive thing about LCD TVs is not the way they make a picture but their flat, compact screen. If you sit up close to a flatscreen TV, you’ll notice that the picture is made from millions of tiny blocks called pixels (picture elements). Each one of these is effectively a separate red, blue, or green light that can be switched on or off very rapidly to make the moving color picture. The pixels are controlled in completely different ways in plasma and LCD screens. In an LCD television, the pixels are switched on or off electronically using liquid crystals to rotate polarized light. That’s not as complex as it sounds!

What are liquid crystals?

Liquid crystals dried and viewed through polarized light. David Weitz and NASA Marshall Space Flight Center (NASA-MSFC).

Iin 1888, an Austrian chemist named Friedrich Reinitzer (1857–1927) discovered liquid crystals, which are another state entirely, somewhere in between liquids and solids. Liquid crystals might have lingered in obscurity but for the fact that they turned out to have some very useful properties.

How LCD televisions use liquid crystals and polarized light

Looking through polarizing sunglasses, or a polarizing filter, an LCD display looks bright or dark depending on the viewing angle.

Left: Liquid crystals blocking light from passing through them and appearing opaque. Right: Liquid crystals allowing light to pass through them and appearing transparent. Both photos courtesy of David Weitz and NASA Marshall Space Flight Center (NASA-MSFC).

An LCD TV screen uses the sunglasses trick to switch its colored pixels on or off. At the back of the screen, there’s a large bright light that shines out toward the viewer. In front of this, there are the millions of pixels, each one made up of smaller areas called sub-pixels that are colored red, blue, or green. Each pixel has a polarizing glass filter behind it and another one in front of it at 90 degrees. That means the pixel normally looks dark. In between the two polarizing filters there’s a tiny twisted, nematic liquid crystal that can be switched on or off (twisted or untwisted) electronically. When it’s switched on, it rotates the light passing through it through 90 degrees, effectively allowing light to flow through the two polarizing filters and making the pixel look bright. Each pixel is controlled by a separate transistor (a tiny electronic component) that can switch it on or off many times each second.

Artwork showing how an LCD TV's pixels are switched off

How pixels are switched off

  1. Light travels from the back of the TV toward the front from a large bright light.
  2. A horizontal polarizing filter in front of the light blocks out all light waves except those vibrating horizontally.
  3. Only light waves vibrating horizontally can get through.
  4. A transistor switches off this pixel by switching on the electricity flowing through its liquid crystal. That makes the crystal straighten out (so it’s completely untwisted), and the light travels straight through it unchanged.
  5. Light waves emerge from the liquid crystal still vibrating horizontally.
  6. A vertical polarizing filter in front of the liquid crystal blocks out all light waves except those vibrating vertically. The horizontally vibrating light that travelled through the liquid crystal cannot get through the vertical filter.
  7. No light reaches the screen at this point. In other words, this pixel is dark.

How pixels are switched on

Artwork showing how an LCD TV's pixels are switched on

  1. The bright light at the back of the screen shines as before.
  2. The horizontal polarizing filter in front of the light blocks out all light waves except those vibrating horizontally.
  3. Only light waves vibrating horizontally can get through.
  4. A transistor switches on this pixel by switching off the electricity flowing through its liquid crystal. That makes the crystal twist. The twisted crystal rotates light waves by 90° as they travel through it.
  5. Light waves that entered the liquid crystal vibrating horizontally emerge from it vibrating vertically.
  6. The vertical polarizing filter in front of the liquid crystal blocks out all light waves except those vibrating vertically. The vertically vibrating light that emerged from the liquid crystal can now get through the vertical filter.
  7. The pixel is lit up. A red, blue, or green filter gives the pixel its color.

source:explainthatstuff.com

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