Question

How do liquid crystal displays (LCDs) work?

Asked by: Brian Mitchell

Answer

In liquid crystal displays (LCDs), use is made of linear polarisers, familiar to most as the glass in polarising sunglasses. If you “cross” two polarisers (i.e. arrange them at 90 degrees to each other) then light does not pass through. This is the basis of the LCD, but between the cross polars the liquid crystals are arranged with a “twist”. This twist allows light to pass through. However, when an electric field is passed through the liquid crystals, the twist is removed, and so light cannot pass through – the area appears black.

For more info on liquid crystals, see my previous answer at:
http://www.physlink.com/Education/AskExperts/ae303.cfm

Answered by: Jan Skakle, Ph.D., Lecturer, University of Aberdeen, Scotland, UK



If you've ever played with polarized sunglasses (or polarized plates in physics class), and noticed that when you have two polarized lenses rotated at the right angle, you know the basic idea behind Liquid Crystal Displays (LCDs).

An LCD has two polarized layers on top of each other. Normally they are both polarized in the same way, so that light gets through both layers just fine. One (or both, I'm not sure) of the layers is made of liquid crystals which have the ability to change the direction of their polarization when a voltage is applied to them. When the voltage is applied, the crystals' polarization shifts so that it is at 90 degrees with respect to the second layer, and no light gets through the layers. This creates an area which looks dark. Different areas are controlled by voltages from whatever circuitry controls the device.

Answered by: Gregory Ogin, Physics Undergraduate Student, UST, St. Paul, MN



First of all, Liquid crystal displays do not emit light. They only control whether light gets through them or not.

The specifics are quite technical and rely on something called the "polarization" of light. I will not discuss polarization because it is better done here: http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l1e.html

Now, liquid crystals are actually small thin rod like molecules that like to move in unison when you apply a voltage across them. This is kind of like a school of fish. There are so many of them and not every one of them has exactly the same orientation with respect to another but they are all pointed in more or less the same direction. Yet, in the blink of an eye, they all turn and are moving together in another direction. That is the response of liquid crystal molecules to applied voltages.

Based on what direction the molecules are pointed compared to the polarization of incoming light and the thickness of the liquid crystal sample, the incoming light's polarization either gets rotated by 90 degrees passing through the sample or not at all.

You can take advantage of this polarization rotation with the use of polarizers (explained in the link above). Placing a polarizer on the output of the sample allows light to be let through ONLY when the polarization of the light matches the polarization orientation of the polarizer and you have the beginnings of a display!

For example, turning the voltage on, rotates the LC molecules one way and you get light through. Turning it off, and no light gets through.

Placing the LC molecules into pixel format and putting Red Green and Blue filters above them, you can get color! Now, some LCDs like your watch don't have what is called a "backlight" and therefore you only get black and a greyish background. They use the light around you to pass through the liquid crystals. LCD monitors used in gameboys and computers have backlights. They are necessary for vibrant color incorporation (RGB filters absorb a lot of light) and high brightness levels.

These LCDs are more sophisticated than the ones in your watch and require a more advanced controlling mechanism to operate it at the speeds and color levels desired when watching DVDs and playing games. They accomplish this through the incorporation of what is called an "active matrix". An LCD with an active matrix just has a matrix of transistors behind the screen controlling each pixel. These transistors are extremely fast and through the use of addressing and a controller computer, the pixels on your LCD can be efficiently managed such that it meets the requirements for movies, games and everyday applications such as "Word".

At least, in order for LCDs to be successful, they have to match CRTs in every way. At least the one I am writing with is a 17" LCD and it has more vibrant colors, better contrast and weighs considerably less (and takes up less room) than an equivalent 17" CRT (normal) monitor. I must also mention that when they say a CRT monitor is 17", its actually like 16" viewable area. With an LCD, if it says 17" then you can SEE 17". Now if they were to just bring down the price tag!

Answered by: Paul Speziale, B.S., Eng Physics Grad Student, McMaster U, Ontario



Light is polarized. That is, it has components which oscillate up and down and left and right. There are materials which can only allow certain polarizations through them. Polarized lenses on sunglasses help to reduce glare by not allowing the polarizations that come from reflections through but allowing other light through.

Because all light can be broken down into two perpendicular polarizations, two types of polarizing film can be used to block out all light. That is, if you take two pairs of polarized sunglasses and rotate them so that the lens of one is over top of the lens of the other and the glasses are at right angles to each other, no light should come through the combination of the two lenses. The first lens will block out light in one polarization and the second lens will block out the rest.

An LCD depends on this sort of blocking.

In an LCD, there are two polarizing films arranged in a very similar manner as what I just described, so that no light can pass through them. A special type of material -- a "liquid crystal" which has a certain structure but can tend to "unwind" in the presence of heat or electricity is placed in between them. This crystal's structure twists and, as it twists, can cause light of one polarization to twist with it.

As a consequence, if the two films are placed exactly the right distance away from each other with the liquid crystal between them, light will pass through the first film, get polarized, and will then twist down the liquid crystal until it is perpendicular to its original polarization and will pass through the second film. Thus, because of the liquid crystal, light WILL pass through this arrangement, however it will be polarized on the other end (this is one of the reasons for the way LCDs look -- the particular quality of the image, especially when looked upon at certain angles).

Now, what allows a computer or some other controller to actually make a display out of this is that those liquid crystals can actually be manipulated by electricity to "straighten out." By applying an electric current to the liquid crystal, it will stop twisting the light. As a consequence, light at that point will once again get blocked by the combination of the two polarized films.

A matrix of these LCD pixels can be built and each pixel can be turned on (causing a black lack of light) or turned off (causing light to pass through) in such a way that allows images to be displayed.

Other arrangements of film, crystal, and film can also be used to cause an inverse effect -- so that when electricity is not applied, no light can pass through. Similarly, the light that sits behind each pixel can be a different color. By putting a red pixel, a green pixel, and a blue pixel in close proximity, colors can be formed.

LCD technology is constantly evolving. These are just the basics of what makes it work. Different liquid crystals are being used to create different LCD materials, and different types of control are being used to create different types of LCD displays. It can be very complicated, but all of these new technologies depend on a liquid crystal which can bend and unbend light and polarization films which can block out light.

Answered by: Ted Pavlic, Electrical Engineering Undergrad Student, Ohio St.

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