Liquid-crystal display

A liquid-crystal display (LCD) is a type of flat-panel display that uses special materials called liquid crystals to show pictures and information. These liquid crystals don’t produce light on their own, but they can control how light passes through. By using a backlight or a reflector, LCDs can create colorful or monochrome images that we see on screens.

LCDs can show many different things, from moving pictures on an LCD television to simple numbers on a digital clock. They work by arranging tiny dots called pixels in a grid, which can turn on or off to form images. Whether it’s a big screen or a small one, LCD technology is used in lots of devices we use every day, like computer monitors, watches, calculators, and smartphones.

Before LCDs, people used heavy and power-hungry cathode-ray tube displays, or CRTs. But since the late 2000s, LCDs have become the standard because they are thinner, lighter, and use less energy. They can show bright, clear pictures and are found in everything from aircraft cockpit displays to LCD projectors and digital cameras.

General characteristics

Each pixel of an LCD has special molecules between two clear layers. Without a liquid crystal, light cannot pass through these layers. When an electric field is applied, the molecules move, letting light pass through more or less. This creates different shades of gray.

Most color LCDs use red, green, and blue filters to make pictures. These filters are made on glass sheets and then put together to form the display. Different types of LCDs, like TN and IPS, are used in devices such as smartphones and televisions.

Main articles: passive matrix and active matrix

Manufacturing

Main article: LCD manufacturing

LCD screens are made in special factories. Experts put together tiny parts with great care. They work in clean rooms to keep dust away. This careful work helps make the clear and bright screens we use in many devices.

History

Main articles: Liquid crystal and Thin-film transistor

Liquid-crystal displays, or LCDs, have a fascinating history that started in the late 1800s when scientists found special properties in liquid crystals. Over time, many inventors and companies worked together to improve these displays.

In the 1960s, important steps were taken to make the first practical LCDs. By the 1970s, LCDs were used in watches and calculators. The 1980s brought color LCDs and the first small televisions. The 1990s saw big improvements in picture quality, making LCDs great for computer monitors and TVs. Today, LCDs are used in many devices, from phones to large screens, and they keep getting better with new technologies.

Illumination

Main article: Backlight

LCDs do not make their own light, so they need an external light source to show images. In most LCDs, this light comes from a backlight placed behind the screen. Some LCDs use a reflective surface instead, which uses the light around them.

There are different ways to create this backlight. One common way uses white LEDs placed behind the screen. Another uses special lamps called CCFLs placed at the edges of the screen. More modern LCDs often use LEDs at the edges with a special material to spread the light evenly. These different methods help make the screen brighter, thinner, and more efficient.

Connection to other circuits

A standard 1080p LCD television screen has many tiny lights, called subpixels. Wires embedded in the screen control each subpixel. These wires form a grid, with vertical wires on one side and horizontal wires on the other. This lets each pixel light up separately.

LCD panels connect to the rest of the device using special drivers attached to the edge of the screen. Common methods include Chip-On-Glass and Tape-automated bonding (Tape-automated bonding). These panels use thin metal paths on glass to move electricity. Special films or connectors link the panel to other parts of the device. The same ideas work for screens in smartphones, though they are much smaller.

Passive-matrix

Monochrome and later color passive-matrix LCDs were used in early laptops and the original Nintendo Game Boy until the mid-1990s. After that, color active-matrix displays became standard for laptops. Passive-matrix LCDs are still used today in devices like inexpensive calculators where lower power use and cost matter.

Passive-matrix LCDs use special technologies like super-twisted nematic (STN) to create images. These displays do not need power to keep the image visible, which is useful for devices like e-readers. However, as the number of pixels increases, passive-matrix displays can show slower and less sharp images compared to modern active-matrix displays in computer monitors and televisions. Some special types of passive-matrix LCDs can show color images by quickly changing the backlight color.

Active-matrix technologies

Main articles: Thin-film-transistor liquid-crystal display and Active-matrix liquid-crystal display

See also: List of LCD matrices

Active-matrix technologies use special layers to control liquid crystals in displays. One common type is twisted nematic (TN). In TN displays, liquid crystals twist and untwist to let light pass. This makes TN displays cheap and fast, but they don’t show colors as well and have narrow viewing angles.

Another type is in-plane switching (IPS). In IPS displays, liquid crystals move in the same plane as the display. IPS displays are used in many TVs, computer monitors, and smartphones. They show colors more accurately and work well over time, but they cost more to make.

Quality control

Some LCD screens can have small mistakes, like pixels that always stay lit or dark. These are called stuck or dead pixels. Even with a few of these, the screen can still work. Different companies have different rules about how many bad pixels are okay. Some will fix the screen even if there is just one bad pixel.

LCD screens can also sometimes show uneven dark patches, making parts of the picture look strange. Good-quality screens try to have no mistakes at all. The place where the bad pixels are matters too—if they are close together, it can be more annoying.

"Zero-power" (bistable) displays

See also: Ferroelectric liquid crystal display

Some special liquid-crystal displays can show an image without needing power all the time. These displays can keep an image in one of two states, like black or white. They only need power when the image changes. Companies like ZBD Displays and Kent Displays have made such screens. These screens can cover a whole phone and change colors without power. Researchers at the University of Oxford also worked on new types of these displays.

Specifications

LCD screens have different qualities that make them look better or worse. One key quality is resolution. Resolution tells us how many tiny dots, called pixels, are on the screen. For example, a screen might have a resolution of 1024×768. This means it has 1024 pixels across and 768 pixels down.

Each pixel is made of three smaller dots—red, green, and blue. These mix to make different colors.

Other important qualities include how colors look, how bright the screen is, and how well the screen shows moving images. The brightness comes from a light source behind the screen. This light can be a special kind of light called fluorescent or small lights called LEDs. A brighter light can make the screen look clearer and show more detail, but it also uses more power.

Advantages and disadvantages

LCD screens are very thin and light, especially compared to older screens. They use less power, stay cool, and can be made in many sizes and shapes. They also do not glow with unwanted colors like older screens.

However, some LCD screens can show darker colors differently depending on where you sit. They sometimes have uneven lighting, especially around the edges. They can also struggle with fast-moving images and may cause eye strain for some people. Some cheaper LCDs cannot show as many colors as more expensive ones. They also can sometimes show "burn-in" from long-time display of the same image.

Chemicals used

LCD screens use special molecules called liquid crystals to show images. These molecules look like tiny rods and can change how light passes through them. Different chemicals are mixed to make the screens work in various temperatures.

Making LCD screens also uses a gas called nitrogen trifluoride. Most of this gas breaks down during production, so it does not harm the environment as much as we used to think.