Randy Hoffner /
03.10.2004 12:00 AM
Advanced Displays, Pt. II
Last month, we looked at television and video displays, old and new, examining the venerable cathode ray tube in both its direct view and its projector roles, and the newest projection technology commonly found, micromirror semiconductors. Two other display technologies are finding increasingly frequent use today: liquid crystal and plasma. Both of these displays use somewhat exotic, but not at all new, technologies. Let's see how they work.

In basic science class, we learned that there are three states of matter: solid, liquid, and gaseous. Later, in chemistry and physics, we learned that things were not always that simple. Solids are classically characterized as having a crystalline structure. Glass, however, to cite an example, certainly appears to be solid at room temperature, yet it has no crystalline structure, making it resemble, on a micro scale, a liquid more than a solid. It has been called a "fourth state" of matter.

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Intel, which introduced its LCOS microdisplay technology at CES, claims the a technology will offer new large-screen HDTVs with clearer pictures than current systems for less than $2,000.

Another of these anomalous substances is liquid crystal. Solids, with their crystalline structures, are highly ordered, with the atoms that compose them possessing both orientational and positional order, while the atoms of liquids and gases have no order at all.

In the late 19th century, scientists were pondering biological systems that, while they are not crystalline solids, are highly ordered. In 1888, cholesteryl acetate was discovered-a liquid exhibiting optical properties that had previously been seen only in crystals. The term "liquid crystal" was first suggested in 1889, to characterize the substances that, like cholesteryl acetate, combine many of the mechanical properties of fluids, e.g., they flow and adopt the shape of their containers, with the optical and electromagnetic properties of crystals.

TWISTED NEMATICS

In the early 1970s, a breakthrough in the field of liquid crystals occurred with the discovery of the twisted nematic effect. Twisted nematics are the type of liquid crystals most frequently used in liquid crystal displays (LCDs) today. One of the several mesophases of liquid crystals is the nematic phase, in which the molecules, which are shaped somewhat like sausages, prefer to line up in a particular directional orientation, but in no particular positional order. The direction of preferred orientation is called the nematic director. This order can, however, be easily influenced by the application of weak electrical, magnetic or optical fields.

When a nematic liquid is introduced between two surfaces with their alignment preparations perpendicular to each other, the nematic director rotates in a helical fashion from one surface to the other, creating a condition known as the 90-degree twisted nematic phase. When linearly polarized light is propagated through the liquid crystals parallel to the helical axis, its plane of polarization follows the twist, so that when it emerges from the opposite side, its polarization has been rotated 90 degrees.

Such a cell is sandwiched between two light polarizers, which may be either parallel or perpendicular to each other in polarization. If they are perpendicular, as is the case in watch and calculator LCDs, and no voltage is applied to the liquid crystal material, light enters through one polarizer, follows the director's helical twist and is rotated 90 degrees, enabling it to pass through the output polarizer unimpeded.

When a sufficient AC voltage is applied to the material, the molecules unwind from their helical orientation and align themselves parallel to the direction of the electrical field. This causes the polarization of the light entering the crystals not to be twisted 90 degrees, preventing it from passing through the output polarizer and causing the output of the cell to appear dark. The degree of helix and polarization untwisting may be linearly controlled by varying the applied RMS voltage between the threshold of reorientation and the saturation field, thereby facilitating the generation of an analog gray scale at the output of the cell.

In simple LCD displays such as those on watches and calculators, the large segments may be directly energized, but directly energizing the large numbers of pixels in more complex displays would require too much etched wiring, so the pixels must be arranged into a matrix and addressed using time-multiplexed signals.

Monochrome LCDs frequently use passive LCD matrices, in which the pixels are directly driven by the multiplexed electrical signals. Color LCDs, which require red, blue, green and sometimes (for greater contrast) gray pixels, typically use the more expensive active matrix, in which each pixel is driven by its own thin-film transistor (TFT) etched onto the cell's glass surface along with the electrical conductors. Each pixel is covered with a red, green, blue or gray filter as appropriate.

The advantages of LCD displays include low weight, thin depth and, with sufficient backlighting, high brightness. Their disadvantages include high manufacturing costs, particularly as size increases, limited viewing angles and slow response time, which manifests itself as blurring or smearing artifacts on fast-moving images.

LCD displays typically range in size from microdisplays to direct view, flat-panel displays in the 40-inch range. Microdisplays, as their name implies, are quite small and may be used in near-eye applications such as camcorder monitor screens, or they may be used to form images for projection, in which case, the light source behind them must be made sufficiently strong, and focusing lenses must be used to direct the light beams at the screen.

PLASMAS

The second advanced display technology in common use is the plasma display panel (PDP). "Plasma" is defined by Webster's Third New International Dictionary as, "... a collection of charged particles... containing about equal numbers of positive ions and negative charges and exhibiting some characteristics of a gas but differing from a gas in being a good conductor of electricity and in being affected by a magnetic field."

A plasma is formed when a gas is heated to the point at which some of its electrons break free from their orbits around the atoms and move independently. This causes the gas atoms to become positively charged, i.e., ionized. The "cloud" of positively charged ions and free electrons is a plasma.

Like glass and liquid crystal, plasma has been called a "fourth state" of matter. Although PDPs are relatively new on the scene, plasmas are not: They are found forming the atmosphere of the sun and also, here on earth, in operating fluorescent lights. A plasma display is an emissive display, i.e., it generates its own light. A plasma display pixel is really a tiny fluorescent light containing a mixture of gases, largely xenon with small amounts of other gases added to optimize the plasma's emission in the ultraviolet range.

When the gas is energized to the plasma state, it radiates ultraviolet light, which strikes a phosphor that fluoresces with a red, green or blue color. Plasma displays are used as direct-view panels only, in which case they are relatively bright, but they are not capable of generating sufficient light to be used as projection devices.

The plasma excitation can be turned on and off, but its intensity cannot be modulated by varying the driving voltage or current, so in plasma displays, a grayscale is generated in a way similar to that used in DLP devices: By adjusting the on/off cycle of the pixels at a high frequency.

As plasma pixels become smaller, the manufacturing process becomes more difficult and more expensive. Typical PDPs have pixels in the 0.8- to 1.1-millimeter range, making them suitable for SD and larger HD displays. An HDTV-resolution display in the mid-30-inch category requires pixels in the range of 0.5 millimeter. Thus, HD PDPs are most commonly found in the 50- to 60-inch range, while SDTV PDPs range down to 30 inches. In addition to high brightness, thin physical depth and fast response time, PDPs have the advantage of not being influenced by external magnetic or electrical fields. They have the disadvantage of generating considerable electromagnetic radiation themselves, however.

LCOS

One of the newest projection display technologies to appear on the scene is Liquid Crystal on Silicon (LCOS). LCOS is a sort of hybrid of micromirror and liquid crystal technologies. The LCOS chip consists of micromirror pixels, upon which polarized light is shined. There is a liquid crystal layer covering each micromirror that, when driven by a modulated signal, controls the amount of light the mirror reflects by dynamically changing the polarization axis, so this is a micromirror projector that has an analog grayscale. LCOS projectors typically use three chips, one each for R, G and B.

OLED

A display technology still in the early stages of development is organic light emitting diode, or OLED. These devices take advantage of some of the many organic materials that have very high fluorescence quantum efficiencies in the visible light range. In spite of their high quantum efficiencies, their power conversion efficiencies can be quite low, requiring relatively high driving voltages in order to generate sufficient light output. Other obstacles include the fact that their luminance level is controlled by current, rather than voltage, which limits their practical size. Further research and development is required on these interesting devices.

These are some of the direct-view and projection technologies that are vying to replace the venerable cathode ray tube. Next time, we will look at the spatial resolutions that may be achieved with some of these displays.


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