How Do LCD Screens Work?

There's more to building an LCD than simply creating a sheet of liquid crystals. The combination of four facts makes LCDs possible:

Light can be polarized. (See How Sunglasses Work for some fascinating information on polarization!)

Liquid crystals can transmit and change polarized light.

The structure of liquid crystals can be changed by electric current.

There are transparent substances that can conduct electricity.

An LCD is a device that uses these four facts in a surprising way.

To create an LCD, you take two pieces of polarized glass. A special polymer that creates microscopic grooves in the surface is rubbed on the side of the glass that does not have the polarizing film on it. The grooves must be in the same direction as the polarizing film. You then add a coating of nematic liquid crystals to one of the filters. The grooves will cause the first layer of molecules to align with the filter's orientation. Then add the second piece of glass with the polarizing film at a right angle to the first piece. Each successive layer of TN molecules will gradually twist until the uppermost layer is at a 90-degree angle to the bottom, matching the polarized glass filters.

As light strikes the first filter, it is polarized. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.

If we apply an electric charge to liquid crystal molecules, they untwist. When they straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. Consequently, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas.

Building a simple LCD is easier than you think. Your start with the sandwich of glass and liquid crystals described above and add two transparent electrodes to it. For example, imagine that you want to create the simplest possible LCD with just a single rectangular electrode on it. The layers would look like this:

The LCD needed to do this job is very basic. It has a mirror (A) in back, which makes it reflective. Then, we add a piece of glass (B) with a polarizing film on the bottom side, and a common electrode plane (C) made of indium-tin oxide on top. A common electrode plane covers the entire area of the LCD. Above that is the layer of liquid crystal substance (D). Next comes another piece of glass (E) with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film (F), at a right angle to the first one.

The electrode is hooked up to a power source like a battery. When there is no current, light entering through the front of the LCD will simply hit the mirror and bounce right back out. But when the battery supplies current to the electrodes, the liquid crystals between the common-plane electrode and the electrode shaped like a rectangle untwist and block the light in that region from passing through. That makes the LCD show the rectangle as a black area.

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What Is Liquid Crystal Display (LCD)?

Abbreviated LCD, liquid crystal display is a flat, thin display device that has replaced the older CRT display. LCD provides better picture quality and support for large resolutions.

Generally, LCD refers to a type of monitor utilizing the LCD technology, but also flat-screen displays like those in laptops, calculators, digital cameras, digital watches, and other similar devices.

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There's also an FTP command that uses the letters 'LCD.' If that's what you're after, you can read more about it on Microsoft's website, but it doesn't have anything to do with computers or TV displays.

How Do LCD Screens Work?

As liquid crystal display would indicate, LCD screens use liquid crystals to switch pixels on and off to reveal a specific color. Liquid crystals are like a mixture between a solid and a liquid, where an electric current can be applied to change their state in order for a specific reaction to occur.

These liquid crystals can be thought of like a window shutter. When the shutter is open, light can easily pass through into the room. With LCD screens, when the crystals are aligned in a special way, they no longer allow that light through.

It's the back of an LCD screen that's responsible for shining light through the screen. In front of the light is a screen made up of pixels that are colored red, blue, or green. The liquid crystals are responsible for electronically turning a filter on or off in order to reveal a certain color to or keep that pixel black.

This means that LCD screens work by blocking light emanating from the back of the screen instead of creating the light themselves like how CRT screens work. This allows LCD monitors and TVs to use much less power than CRT ones.

LCD vs LED: What's the Difference?

LED stands for light-emitting diode. Although it has a different name than liquid crystal display, it's not something entirely different, but really just a different type of LCD screen.

The major difference between LCD and LED screens is how they provide backlighting. Backlighting refers to how the screen turns light on or off, something that's crucial for providing a great picture, especially between black and colored portions of the screen.

A regular LCD screen uses a cold cathode fluorescent lamp (CCFL) for backlighting purposes, while LED screens use more efficient and smaller light-emitting diodes (LED's). The difference is that CCFL-backlit LCDs can't always block out all black colors, in which case something like a black on white scene in a movie may not appear so black after all, while LED-backlit LCDs can localize the blackness for a much deeper contrast.

If you're having a hard time understanding this, just consider a dark movie scene as an example. In the scene is a really dark, black room with a closed door that's allowing some light through the bottom crack. An LCD screen with LED backlighting can pull it off better than CCFL backlighting screens because the former can turn on color for just the portion around the door, allowing all the rest of the screen to remain truly black.

Not every LED display is capable of dimming the screen locally, like you just read. It's usually full-array TV's (versus edge-lit ones) that support local dimming.

Additional Information on LCD

It's important to take special care when cleaning LCD screens, whether they be TVs, smartphones, computer monitors, etc.

Unlike CRT monitors and TVs, LCD screens don't have a refresh rate. You might need to change the monitor's refresh rate setting on your CRT screen if eye strain is a problem, but it's not needed on the newer LCD screens.

Most LCD computer monitors have a connection for HDMI and DVI cables. Some still support VGA cables, but that's much less common. If your computer's video card only supports the older VGA connection, double-check that the LCD monitor has a connection for it. You might need to purchase a VGA to HDMI or VGA to DVI adapter so that both ends can be used on each device.

If there isn't anything showing up on your computer monitor, you can run through the steps in our How to Test a Computer Monitor That Isn't Working troubleshooting guide to find out why.

FAQ

What is LCD burn-in?

CRT hardware, LCD's predecessor, was famously susceptible to screen burn-in, a faint image imprinted on the electronic display that could not be removed.

What is LCD conditioning?

LCD conditioning solves minor problems that occur on LCD monitors, including persistent images or ghost images. The process involves flooding the screen or monitor with various colors (or with all white). Dell includes an image conditioning feature in its LCD monitors.

What is the likely problem if you see small white, black, or colored spots on your LCD screen?

If you see a black spot that never changes, it's likely a dead pixel and may require a professional repair or screen replacement. Stuck pixels are usually red, green, blue, or yellow (although they can be black in rare cases). A dead-pixel test distinguishes between stuck and dead pixels.