Liquid-Crystal Display Technology

Learn About Flat-panel Displays

By far the most common of the flat-panel display technologies is the liquid crystal display, or LCD. Now used in everything from simple calculator, watch, and control panel displays to sophisticated full-color desktop monitors, the LCD is almost synonymous with “flat-panel display” in many market at present.

Unlike the other flat-panel types, the LCD is a non-emissive display. It acts only to modulate or switch an external light source, either as that light passes through the LCD (the transmissive mode of operation), or as the light is reflected from the LCD structure (operating in reflective mode). There are numerous specific means through which LCDs control light, but all operate in the same fundamental manner – the arrangement of molecules within a fluid is altered through the application of an electric field across the material. The effect on the light transmission or reflection may be through phase or polarization changes, the selective absorption of light, or by switching between scattering and nonscattering states.

Probably the most common operating mode, and certainly one of the most useful in explaining the basic of LC operation, is the twisted-nematic mode. Liquid crystals are so named because the molecules of the liquid tend to align themselves in ordered arrays, as in a solid crystalline substance. These materials are also generally organic compounds in which the molecules are relatively long and thin; for the purposes of analyzing their electro-optical behavior, they may be though of as extremely small rods in suspension in a fluid medium. In the nematic state, these molecules – the “rods” – align themselves in layers throughout the fluid, and such that those in adjacent layers tend to be oriented in the same direction. The molecules will also align themselves with fine physical structures in the substrate of the display. (In practice, these are created by physically rubbing a relatively soft layer of material deposited on top of the glass substrate, creating a very large number of very fine scratches, all aligned in the same direction.) If the liquid crystal material is placed between two such substrates, the tendency of the molecules to align themselves with those above and below, plus the tendency of the outermost layers to align with the “rubbing direction” of the substrate, a sort of helical arrangement of the molecules through the liquid crystal occurs. This helix has the effect of twisting the polarization of light passing through it by 90°. If crossed polarizing layers are then placed on either side of this structure, light can still pass through by virtue of the polarization rotation.

The LC molecules’ tendency to form the helical structure described above can be overcome by a field of sufficient rubbing directions orthogonal to one another, the tendency of the molecules of a given layer to align with those above strength, and the molecules will  then instead align themselves with the field. This destroys the helical structure, and with it the polarization rotation effect. Light that previously passed through the second polarizing layer is now blocked. Removal of the electric field permits the helical structure to re-form, and light once again will pass through. The transition between the two states is not especially abrupt, as may be seen in the graph of light transmission vs. applied voltage for a typical LC cell. This gives the TN LCD the inherent capability of producing a range of intensities, or a “gray scale”, although the shape of the response curve is less than ideal.

It should be noted at this point that the action described above depends solely on the magnitude of the electric field across the LC cell, not on its polarity; in other words, the liquid crystal display would operate as shown with the source connected in either direction. This turns out to be very important, as it was discovered early in the commercial history of LC displays that the display would be damaged if exposed to a long-term net DC voltage across the cells.

Many simple liquid-crystal displays are of the passive-matrix type. However, to provide sufficient contrast, the LC materials and cell design used for these result in relatively slow operation. This is necessary so that the individual pixels will remain in the desired state long enough between drive pulses, but makes this type ill-suited to applications requiring the display of rapid motion. Use of an active-matrix design enables faster response, and can result in an LCD suited to motion-imaging applications. Most LCD panels used in high-end applications, such as desktop monitors and notebook computers, are of the active-matrix type, also known as “TFT-LCD” (for “thin film transistor liquid crystal display”; the active components are constructed via thin films deposited directly onto the display substrate). However, in addition to the added complexity of the active-matrix pixels, this type generally requires more power than the passive-matrix LCDs, making the passive-matrix often the more attractive choice in power-critical portable applications.

The simple TN-LCD also suffers from a limited viewing angle, meaning that the appearance of the display is optimum only through a certain limit range of angles, centered around a line roughly perpendicular to display surface. (It should be noted that in almost all practical LC displays, the direction of maximum contrast will not be precisely normal to the plane of the display.) This results from the nature of the electro-optical effect behind the operation of the display, which clearly functions best along the axis of the helical arrangement of molecules. Light passing through the structure at an angle does not experience the distinct change in transmission states, and so the contrast of the display falls off rapidly off-axis. This can be compensated for, to some degree, through the addition of optically active film layers on top of the basic TN panel, or through the use of different LC modes. In the passive-matrix types, the most common approach is to employ the “super-twisted nematic”, or “STN” mode. Without going into unnecessary detail, this mode involves a 270° twist in the helical arrangement of the molecules, rather than the 90° of the standard TN, and provides both higher contrast and a wider viewing angle, along with a much sharper response curve.

Active-matrix LCDs may also use other LC modes rather than the simple TN (with or without compensating film) in order to obtain improved contrast and viewing angle. Two of the more common in current displays are the in-plane switching, or IPS type, and the vertical linear alignment (VLA) mode. These modes are not used in passive-matrix displays, due to their requirement for more complex pixel structures and/or higher power requirements, both of which are contrary to the low-cost/low-power aims of most passive-matrix designs. Both offer greatly improved viewing angle and response times over the conventional TN mode. However, the higher power requirement has limited their use to date to panels intended for desktop monitor or television applications (as opposed to notebook PC applications, which are of course more power-critical). More recently, both types have evolved into “multi-domain” variants; these address color and contrast uniformity issues in the original IPS and VLA types, which resulted from the fact that the LC molecules do not actually swing exactly 90° between states. The multi-domain solution is using the vertically aligned type as an example. In this approach, the display area is broken into many small areas, each with a different orientation of the LC molecules as shown. When viewed at a normal distance, the color errors introduced by each domain, as viewed from a given angle, cancel each other and the display appears uniform on average.

Both active- and passive-matrix designs may be used in either transmissive or reflective displays. Transmissive-mode displays most often incorporate an integral “backlight” structure. The light source itself may be one or more small fluorescent tubes (most often of the cold-cathode fluorescent, or CCFL, type), LEDs, or an electroluminescent panel. To provide acceptable brightness uniformity, some type of diffusing layer is generally also included. The backlight, along with the additional power supply generally required to drive it, again increases the complexity and cost of the complete display system, and so may limit the applicability of such displays to relatively high-end applications. In the reflective LCDs, ambient lighting is used to view the display; rather than a backlight, a reflective layer is placed “behind”the LC panel (as seen by the viewer). Due to the light losses involved in two passes through both polarizing layers and the LC material itself, reflective displays generally provide poor contrast compared to their backlit transmissive counterparts, but still are often the preferred choice where low power consumption is of paramount concern. A hybrid type, the transflective display, typically adds a limited-use backlight to a normally reflective display, to enable occasional use in low-ambient-light environments. Making the LCD into a full-color display is conceptually very simple.

With the exception of certain LC modes which involve wavelength-specific effects, this type of display has little or no inherent color, instead passing or reflecting an external light source essentially unchanged. In order to make a full-color display, then, all that is required is the addition of color filters over the LC cells, and the use of a white light source. Various pixel layouts have been used in the design of color panels, but one typical arrangement is simply to place three complete pixel structures – now becoming the three primary-color sub-pixels – into a single square area that is now the complete full-color pixel. Besides the additional complexity in the panel design (which now has at least three times as many “pixels” as in a monochrome panel of the same format), the fabrication and alignment of the color filter layer adds considerable cost to the display. An alternative method of producing a color LC display is to employ three stacked panels with filter layers corresponding to the subtractive-color primaries (cyan, magenta, and yellow). As a reflective display, this permits full-color operation by selectively absorbing these primary colors.

The term “LCD”covers a wider range of specific technologies than any other of the flat panel types. There are a very wide range of liquid-crystal types and operating modes which have not been covered in detail here, with varying advantages, disadvantages, and unique features. Some provide very high contrast; some provide bistability, and with it the ability to retain an image even after electrical power is disconnected from the display. However, LCDs have until very recently generally been limited to small-to-medium sized applications; from roughly 2.5 cm (1 inch) (or less) diagonal up to perhaps 63 cm (25 inches) at the upper end. The larger sizes are almost exclusively the domain of the active-matrix types, and the size is for the most part limited by the ability of manufacturers to process sufficiently large panels while maintaining acceptable uniformity and defect counts. There has, however, been some success demonstrated in tiling LCDs, using panels specifically designed to be placed adjacent to one another in order to form a much larger complete display system.

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