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 a 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. LCD screens are very useful, though they can sometimes show image persistence if the same picture stays on for too long.

General characteristics

Each pixel of an LCD consists of special molecules between two transparent electrodes and polarizing filters. Without a liquid crystal between the filters, light would be blocked. By applying an electric field, the molecules change position, allowing light to pass through in varying amounts, creating different shades of gray.

Most color LCDs use red, green, and blue filters to create images. These filters are made using a special process on glass sheets, which are then cut and combined to form the display. Different types of LCDs, such as TN and IPS, are used in various devices like smartphones and televisions.

Main articles: passive matrix and active matrix

Manufacturing

Main article: LCD manufacturing

LCD screens are made in special factories where experts carefully put together tiny parts. They use clean rooms to keep everything free from dust, which could ruin the display. This careful process helps create the clear and bright screens we see in many devices today.

History

Main articles: Liquid crystal and Thin-film transistor

Liquid-crystal displays, or LCDs, have an interesting history that began in the late 1800s when scientists first discovered the special properties of liquid crystals. Over the years, many inventors and companies worked together to improve these displays.

In the 1960s, important steps were taken to create the first practical LCDs. By the 1970s, LCDs were being used in watches and calculators. The 1980s saw the development of color LCDs and the first small televisions. The 1990s brought 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 continue to evolve 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 over six million tiny lights, called subpixels, each controlled by wires embedded in the screen. These wires form a grid, with vertical wires on one side and horizontal wires on the other, allowing each pixel to be powered individually.

LCD panels are connected 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 guide the electricity, and special films or connectors are used to link the panel to other parts of the device. The same ideas apply to screens in smartphones, though they are much smaller.

Passive-matrix

Monochrome and later color passive-matrix LCDs were commonly 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 consumption and cost are important.

Passive-matrix LCDs use special technologies like super-twisted nematic (STN) to create images. These displays do not need continuous 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 found 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), where 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), where 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. IPS displays can sometimes make text look fuzzy if not made correctly.

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, and only need power when the image changes. Companies like ZBD Displays and Kent Displays have made such screens, including ones that can cover a whole phone and change colors without power. Researchers at the University of Oxford also worked on new types of these displays that don’t need constant power.

Specifications

LCD screens have different qualities that make them look better or worse. One key quality is resolution, which tells us how many tiny dots, called pixels, are on the screen. For example, a screen might have a resolution of 1024×768, meaning it has 1024 pixels across and 768 pixels down. Each pixel is made of three smaller dots—red, green, and blue—that 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, which can be either 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 CRT displays. They use less power, stay cool, and can be made in many sizes and shapes. They also don't glow with unwanted colors like older screens did.

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 can't 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 display images. These molecules are shaped like rods and have unique properties that allow them to change how light passes through them. To work in different temperatures, a mix of several chemicals is often used.

Making LCD screens also uses a gas called nitrogen trifluoride, which can affect the environment. However, most of this gas is broken down during production, so its impact is smaller than previously thought.