Display Materials and Components

How does LCD technology work?

25 November 2020

Liquid crystal displays (LCDs) are called passive devices, as they do not generate any light to show animations, videos, images or characters. The internal structure of a LCD defines how the orientation of light may be changed as it travels through the device. This article focuses on the basic operating principles of an LCD screen.

The most appealing thing for many people about LCD TVs is not how they render an image, but their lightweight flat screen. The traditional cathode ray tube TVs used to use three electron guns to produce a picture. Their function can be imagined as an extremely accurate paintbrush that hops forward and backward, completing the animated picture on the TV screen that can be watched. However, flat screen LCD functions in a new way.

Looking closely at a flat screen TV, it can be observed that the screen image is developed using thousands of small blocks known as pixels (elements of image). Each is essentially an individual red, blue or green light, which can be turned on or off easily to produce an animated color image. The pixels are electronically turned on or off using liquid crystals to move the polarized light in an LCD screen. To explain what's happening, we will first define what liquid crystals are then explore properties of light and how it propagates.

Liquid crystals

It is commonly known that a certain substance can exist in three states of matter i.e., liquid, solid or gas. However, in 1888, liquid crystals were discovered in a state of matter that lies in between solids and liquids. Liquid crystals may have been left in darkness back then, but now they have proven to be quite valuable. Solids are hardened blocks of matter, often bundled into a tidy, consistent structure known as a crystal. Liquids do not have the structure of solids, but while they remain in a bottle, they take their shape and would flow reasonably comfortably when taken out. Now, the liquid crystal is in an in-between state. It will exist in many likely "substates" anywhere in between the territory of liquid and solid.

The crucial substates of liquid crystals are the smectic state and nematic state. In a nematic state, they behave somewhat similar to a liquid. The molecules can move past each other and may travel about but within a confined area and all move in the same direction. They may become distorted, but can straighten out again if the electricity is applied. This is the secret to how LCD screens can switch off and on pixels. In a smectic state, the liquid crystals have been cooled down and molecule layers can move across each other easily. In a given layer, the molecules can travel around, but they do not travel to other layers.

Polarized light

Polarized light is filtered light. Consider sunlight shining upon the ground as its light waves vibrate and diffuse in almost every direction. This can light be filtered by using, for example, vertical grid lines. The light waves vibrating in all directions except the ones in vertical would be blocked this way. The light intensity would also be dimmed since most of the real sunlight waves have been blocked; sunglasses function according to this mechanism. Therefore, polarized light can move in only one direction and is a filtered light wave.

How LCD utilizes polarized light and liquid crystals

A flat LCD television monitor utilizes the sunglasses method to turn on or off its colored pixels. There's a big bright light on the back of the television that shines to the audience. And, millions of pixels are present at its Fig 1: An LCD subpixel. Source: Marvin Raaijmakers / CC BY-SA 2.5Fig 1: An LCD subpixel. Source: Marvin Raaijmakers / CC BY-SA 2.5front, and every one of them consists of smaller areas known as subpixels (shown in Figure 1), which are green, blue or red. Every pixel has two polarizing glass filters (polarizers) arranged in parallel to each other, one behind and the second at its front as shown in Figure 2. This implies that the pixels appear dark in a normal state. A nematic distorted small liquid crystal is present between these two glass filters, which can be straightened out and distorted by turning it on and off electronically.

When the nematic liquid crystal is turned off, it turns around nearby light by 90 degrees, efficiently permitting light to go via the two glass filters and allowing the pixels to become bright. On the other hand, if it is turned on, it would not bend the light that is obstructed by one of the Fig 2: LCD structure. Source: Spockoyny / CC BY-SA 4.0Fig 2: LCD structure. Source: Spockoyny / CC BY-SA 4.0filters, and the pixels appear dark. This switching of pixels is controlled by transistors, which can turn them on and off several times every second.

The current state of LCDs

The latest LCD monitors are also known as thin-film transistors, dual scan active matrix and flat panels. These are now incredibly common because of their sleek appearance, compactness, thinness and quality. LCD screens now offer vivid, bright and quality contrast pictures. Previously, LCD technology was sluggish, had a low contrast level and could not perform well. LCD applications are not confined to TVs or desktop computers and are also being widely used in other electronic devices such as tablets, laptops, video and photo cameras, GPS devices, smartphones, watches, MP3 players and e-readers.

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