What are Liquid Crystals and Why are They in my Television?

Picture a crystal. Unless you're a television manufacturer, you're probably imagining something like the image above (maybe we did introduce a little bias there). Generally, we think of crystals as clear, stony, possibly valuable, and—most of all—solid. So solid, in fact, that crystals can be some of the hardest substances on earth. While most people understand the meaning behind the LCD acronym—liquid crystal display—and the words therein, surprisingly few notice the apparent paradox: How can something be liquid and crystal at the same time?

Order from Chaos

A crystal's minimum qualification is order at the molecular level, when molecules line up with each other in a pattern. Typically, only solid molecules form these ordered patterns—such as a diamond's crystalline structure—but a solid state of phase isn't necessarily a prerequisite to crystal status.

To produce liquid crystals you need two components: the molecules forming the crystal, and the special carrier fluid in which the crystals can form.

Liquid crystals share characteristics with both crystals and liquids. The molecules are ordered similar to crystals, but still able to move around as they are in liquids.

When dissolved into the carrier fluid, the crystal-forming molecules will find each other and assemble into an ordered structure—the crystal—while still suspended in liquid form inside the carrier fluid.

Harnessing the Power of Liquid Crystals.

When the liquid crystal molecules are made up of a substance that responds to electricity, they can be controlled enough to create images. Imagine a bunch of compass needles floating on corks in a bowl of water. Eventually, the earth’s magnetic field would cause the needles to point north. But put a magnet on one side of the bowl and all of a sudden the needles line up with each other in a different direction, pointing towards the more powerful magnetic field.

magnet vs lc.png
The molecules in liquid crystals align with electric fields like compass needles align with magnetic fields.

That's pretty much how liquid crystals work in display technology: Simply replace compass needles on corks with elongated zwitter-ionic molecules (which have a positive and negative end analogous to a compass) and substitute electric fields for magnetic fields. Now you have a liquid full of oblong molecules that are lined up with each other and whose orientation can be controlled by applying varying strength of electric fields perpendicular to each other. While it doesn’t seem like much, this is the basis for the display technology that you're probably using right now to read this article. Mind blown?

Turning Liquid Crystals into Displays: Crystalline Shutters

Molecular compass needles on their own don't make televisions. You need some light. Unlike plasma, liquid crystals can't produce their own light directly or indirectly, so the red, blue, and green light is supplied by color filters and either fluorescent light (LCD displays) or light emitting diodes (LED displays). In an LCD television, the liquid crystals merely modulate how much light is transmitted from the backlight to the viewer.

Liquid crystals control how much light gets transmitted, just like window shutters.

Imagine the television's liquid crystals as window shutters. When they're aligned parallel with the path of the backlight to the viewer, the maximum amount of light can be transmitted through. As the electric field modulates, the crystals move slightly and block the light.

lcd function.png
The amount of light passing through the LC layer of a display depends on the orientation of the LC molecules.

With the liquid crystal shutters completely perpendicular to the light beams, the crystals block the maximum amount of light possible. Since each individual sub-pixel (red, green and blue) can be modulated independently, the overall amount of light and color of each pixel can be tightly controlled.

Boons and Drawbacks

Theoretically, LCD technology works great for displays that need to be very bright and portable, since the components are very light and the backlight luminance can be very high, due to advances in LED technology. Tablet and mobile phone displays commonly feature LCD panels for precisely those reasons, which make LCD displays very, very cheap to produce.

However, convenience comes with downsides. As with anything moving through liquids, liquid crystals take some time to arrive at their desired angles (measured in milliseconds), which means that LCD panels have lower response rates than their plasma counterparts. Additionally, no LCD shutter is perfect, so a little light will always bleed through. Without local dimming effects, this means LCD based panels will generally have grayer black levels than their plasma counterparts, which almost completely cease emitting light when displaying black pixels.

grey blacks.jpg
Even the best shutters let through some light. In LCDs light leakage leads to elevated black levels.

We're expiration dating this technology.

While LCD technology is completely ubiquitous today, our love affair with these displays is headed for a rough patch. There's another display technology in the picture. With the incredible performance and portability of OLED displays—get ready for clear and flexible displays—LCD technology doesn't have a chance when OLED display prices inevitably fall. But when they die, you'll at least know how they worked.

This is the future.

Contributing: Ethan Wolff-Mann

Photos by Didier Descouens CC-BY-SA-3.0-2.5-2.0-1.0, Hasnain Khattak CC-BY-SA-3.0-2.5-2.0-1.0, Meharris CC-BY-SA-3.0

TAGS: HOW IT WORKS television