LCD

From MobileRead

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.

[edit] 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.

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.

[edit] 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.

[edit] 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 and 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.

[edit] Future of LCD

For eBook reading the future seems to be pointed at e-Ink 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 also Reflective LCD for a new LCD display using reflective mode.

[edit] For more information

LCD E-Book Reader Matrix for specifications on eBook Readers using LCD displays.