LCD

A liquid crystal display (LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It is often utilized in battery-powered electronic devices because it uses very small amounts of electric power.

Overview

There are two main types of LCD displays, passive and active.

Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing super-twisted nematic (STN) or double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with STN), and (CSTN) color-STN (a technology where color is added by using an internal color filter). Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called passive-matrix addressed because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix addressed LCDs. A new Reflective LCD has been developed for eBook use. It features low power and high resolution in a monochrome display.

High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix addressed displays look "brighter" and "sharper" than passive-matrix addressed displays of the same size, and generally have quicker response times, producing much better images.

LCD displays are often used with Touchscreens in mobile devices.

Light source

An LCD display needs a light source to cause the screen to be visible to the user. The light source is what controls the brightness of the screen. While it is possible to use a mirror to provide a light source using reflective light this is not very satisfactory on a color display since the color and amount of the source light cannot be controlled. Thus most LCD display's have a built-in "backlight" behind the display surface.

The range of color available in the display is dependent on the whiteness available in the light source. A pure white source will provide the maximum color range. The light source is also the main source of power consumption for the display and generally dominates the power usage compared to all of the other power used on the device. Early displays used a florescent light source (CCFLs) but lately LEDs are being used due to their lower power consumption. Pure white LEDs are hard to come by, but it is possible to use sets of 3 LEDs, all on, to produce a white backlight. Using 3 LEDs can also be tuned to produce different color temperatures in the display. Sometimes the color is adjusted to make the screen easier on the eyes. Blue-light exposure is specifically addressed in some devices that can change the background color depending on the time of day.

It is possible to place an array of LEDs behind the display but this tends to make the screen thicker. On mobile devices the LEDs themselves are at the edge of a light guide (to reduce the number of LED's needed which improves battery life) but the light guide is still behind the screen and is usually defused a bit to spread the light evenly. There is also a reflective surface on the bottom of the light guide to send as much light as possible through the LCD display. It behaves the same as if the LEDs themselves were behind the screen.

How it works

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes and two polarizing filters. The filters are generally arrange perpendicular to each other so that no light will pass through.

The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces.

In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. Light passing through one polarizing filter is rotated by the liquid crystal helix as it passes through the liquid crystal layer, allowing it to pass through the second polarized filter. Half of the incident light is absorbed by the first polarizing filter, but otherwise the entire assembly is transparent.

When a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field, distorting the helical structure. This reduces the rotation of the polarization of the incident light, and the device appears gray. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.

When a large number of pixels is required in a display, it is not feasible to drive each directly since then each pixel would require independent electrodes. Instead, the display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink. The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.

A color display uses different color filters for the 3 main colors, similar to a CRT display. These filters allow only that one color to pass through and a 3 dot triad is used to produce the various colors. Since this system depends on a backlight source the actual range of colors produced depends on the backlight being a pure white, a mix of all colors. If the color is not present in the backlight then the filter cannot add it and the user will not see it.

TFT LCD

A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin film transistor (TFT) technology to improve image quality. TFT LCD is one type of active matrix LCD, though it is usually synonymous with LCD. It is used in televisions, flat panel displays and projectors.

The circuit layout of a TFT-LCD is very similar to the one used in a DRAM memory. However, rather than building the transistors out of silicon which has been formed into a crystalline wafer, they are fabricated from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel, and the silicon film is etched away in the remaining areas, allowing light to pass through. A transistor is connected to each pixel and a matrix selection of the X/Y array is used to select an individual crystal. These devices store the electrical state of each pixel on the display while all the other pixels are being updated. This method provides a much brighter, sharper display than a passive matrix of the same size.

Improvements in TFT LCD

There have been many improvements in TFT LCD displays. For example OCB (Optically Compensated Bend) is a technology that realizes performance capabilities comparable to those of cathode ray tube (CRT) displays while SOG (System On Glass) technology offers improved response speed and picture quality and reduced power consumption.

Much of the power consumed by an LCD display is because of the backlight. Recently LED backlights have been used to reduce power consumption. They use much less power than the normal florescent tubing. However, the range of color can be impacted by the spectrum available in the backlight.

Newer improvements in TFT displays include IGZO (Indium gallium zinc oxide) technology can provide improved resolution and faster response than earlier techniques. LTPS (Low temperature PolySilicon) is another improved display technology which allows TFT to be use with plastic backed displays.

IPS LCD

IPS stands for In-Plane Switching Display. It’s one of the newer innovations in LCD displays. The new Apple iPad, the nook color, and the ViewSonic VP2365wb are LCD devices that uses IPS.

IPS offers far superior image quality and color matching to a TFT display. IPS works because it aligns each tiny liquid crystal horizontally, which covers more of the screen and offers better coverage, as well as much better viewing angles. This is done by applying an electrical field to both ends of each crystal, causing them to stretch out.

For an LCD monitor like the VP2365wb, it works great, enabling the $399 unit to compete with much more expensive LED-backlit displays. However, there are power consumption drawbacks to IPS:

• A TFT displays requires one transistor, which twists the crystal to create an image. With IPS, there are two transistors for every single pixel, one for each end. This doubles the power consumption of screen.
• Because more of the surface area of the screen is “covered” by images, a much more powerful backlight is required to shine through. And that means either more florescent tubes or much brighter ones.

Future of LCD

For eBook reading the future seems to be pointed at e-paper but there is certainly room for competing systems. One such promising system is the display used on the XO from olpc. It has both a passive mode and an active TFT mode. Here are the specs:

• Liquid-crystal display: 7.5” dual-mode TFT display;
• Viewing area: 152.4 mm × 114.3 mm;
• Two modes:
  • (1) grayscale (B&W) reflective mode (for outdoor use—sunlight-readable); and
    • reflective mode: high-resolution (200 DPI), 1200(H) × 900(V) grayscale pixels, power consumption 0.1–0.2Watts;
  • (2) color backlight Mode (for indoor use);
    • backlight mode: built in sub-pixel sampling of the high-resolution display results in approximately 800(H) × 600(V) color pixels, power consumption 0.2–1.0Watts;
• The display-controller chip (DCON) with memory that enables the display to remain live with the processor suspended. The DCON also formats data for the display.

This Liquid-crystal display is the basis of the olpc's extremely low power architecture. The XO is usable while the CPU and much of the motherboard is regularly turned off (and on) so quickly that it's imperceptible to the user.

The display comes from http://pixelqi.com/ - see PixelQi

For more information

• See also Reflective LCD for an LCD display using reflective mode.
• LCD E-Book Reader Matrix for specifications on eBook Readers using LCD displays.
• IPS explanation
• PixelQi - a low power design with multiple modes of operation.
• Boogie board - an LCD eWriter
• AMOLED is a competitive system.