Without LCD (liquid-crystal display) technology, it’s unlikely that there would be such a thing as portable computing. Sure, you could probably strap on a back brace and endure “luggable” designs such as the Osborne 1 or Kaypro II from the early 1980s, which sported built-in monochrome CRT (cathode-ray tube) monitors that were smaller than a CD jewel case. These monsters weighed anywhere from 15 to 30 pounds, which made them much more of an oddity than anything really practical. Real portable computing didn’t arrive until LCD-based flat-panel screens that were larger than 10 inches became affordable in the early 1990s, and then the entire computing landscape changed. The notebook revolution had begun.Why are LCD screens such a big deal? Primarily, because they’re not “big.” At their simplest, LCD screens are two glass plates with some liquid-crystal material sandwiched between them. By sending a little electrical juice to key points in the sandwich, you can make patterns appear in the liquid crystal, which we then interpret as numbers on a calculator, text on a PDA (personal digital assistant), or a photograph on a notebook display.
Because there are no bulky vacuum tubes, which are the central mechanism in a CRT design, the whole apparatus needs to be only a few inches thick. One of the biggest advantages of this smaller size is that the display uses less energy. In fact, an LCD screen uses up to 90% less energy than a standard computer monitor. Sure, the viewing angle is much narrower than a television or conventional monitor, but text and graphics are often sharper than on CRTs and a good flat panel will offer the same if not better color quality than most monitors.
Fortunately, though, flat-panel prices are coming down, albeit slowly. East Asian mass production facilities are now ramping up with more streamlined processes, meaning there will be fewer shortages, higher brand competition, and thus lower prices at the retail level. Right now, the difference between a 15-inch flat panel and a 15-inch CRT is about $700. But if that number could be erased, or even halved, it would allow many families to send their kids to school with notebook computers instead of three-ring binders. Students in the back row wouldn’t have to squint at a blackboard if desks could be affordably equipped with notebook display jacks.
Professionals would be increasingly hard-pressed to justify the cost and space requirements of desktop PCs when more convenient notebooks are available. In the home, it may suddenly make sense to have a notebook in the living room and one in the bedroom, especially as wireless networking matures. In short, evolving flat-panel technology will continue to push notebooks ever deeper into our lifestyles. Here, we’ll take a look inside LCD displays and explain how this technology works and what it means to the computing consumer.
What Is Liquid Crystal?
Thanks to water, everybody is familiar with the three basic types of matter: solid, liquid, and gas. Turn on the tap or pour it from a fancy bottle, and the little H2O molecules bounce this way and that, free to move about each other in a liquid state while not being so excited (by heat) that they fly off in the form of gaseous water vapor. If you take the glass of water and leave it in the freezer, as the material cools to 0 degrees Celsius (C) or colder, the molecules slow to the point that they lock into place against one another, turning the material into the solid we recognize as ice.The birth of liquid-crystal research is generally attributed to an Austrian botanist named Friedrich Reinitzer. In 1888, Reinitzer, while investigating the role of cholesterol in plants, was experimenting with cholesteryl benzoate. He observed that while the substance was a solid at room temperature, it melted to a foggy liquid at 145.5 C and became clear at 178.5 C. Moreover, right at the transition temperature between clear and cloudy, the substance took on a blue color, and at the transition between cloudy and solid, there was a transitory blue-violet. Soon, this became known as the liquid-crystal phase.As you might guess, liquid crystal exists in a state that is sort of halfway between solid and liquid. Crystals are generally made up of rod-like molecules you might visualize as pieces of hard thin rice stacked tip to tip in long, straight, parallel rows. When heated to a liquid, the “rice” pieces fall out of ranks and swirl around any which way. When cooled to the solid state, they fall back into their orderly parallel ranks. Liquid-crystal molecules are like those orderly ranks of rice, except someone bumped the table slightly. They’re still in orderly parallel lines, only jostled a bit so the lines seem kinked or wavy. Liquid-crystal molecules can slide past each other as liquid molecules do, but they don’t lose their directional orientation.The direction in which liquid-crystal molecules line up is called the director while the order parameter is the amount that each molecule generally deviates from the director. The order parameter will determine some of the liquid crystal’s properties while the director will determine how the substance must be measured or observed. For example, imagine a row of cardboard tubes. Look at them straight on from the top and they look clear because you can see through them. However, if you move your viewing angle, suddenly they look like brown opaque cardboard. Thus, the orientation of the materials and the light coming through them in relation to the viewer is critical.
Get Polarized.
As you obviously know, applying heat to ice will change its properties and turn it into liquid. Similarly, in some liquid-crystal substances, applying an electrical charge, changing the environmental pressure, or altering the magnetic field surrounding it will cause the substance to change its properties, and thus also change its optical characteristics. Sending a small electrical current through a bit of this or that liquid crystal may turn it from opaque to transparent, or, more significantly for notebook display purposes, take on the ability to twist polarized light.Understanding polarized light requires understanding a couple of light’s basic principles. Light travels in waves with crests, troughs, and a clear direction. Light, of course, is all around us bouncing every which way, allowing us to see objects from any angle. However, it is possible to filter light so only waves with a certain directional orientation are allowed to pass through.Consider how curbs, dividers, and painted lines allow traffic to move in only one direction, polarizing it in a sense. Or, think about polarized sunglasses. Because the glare from flat surfaces such as water, snow, and tarmac tends to be horizontally oriented, polarized sunglasses use a filter to screen out horizontal light, allowing only vertical waves to continue to your eyes. If you take a second pair of polarized sunglasses and hold them at a 90-degree angle in front of the first pair, the two filters will block out all light.One simple class of LCD technology is called TN (twisted nematic). Nematic refers to the type of liquid crystal, and twisted refers to the changing director angle within the substance. With a TN cell, regular light first hits a polarizing filter and then moves into the nematic liquid crystal. At the other side of the cell is a second polarizing filter, which is at a 90-degree angle from the first. The liquid crystal’s director twists by 90 degrees as it moves from the first polarizing filter to the second. As polarized light moves through the substance, its own orientation twists with the liquid crystal.Without the nematic twist, light striking these two opposing filters would be blocked just like our two pairs of sunglasses that we described earlier. However, because light passing through the first filter twists such that its departing orientation matches that of the second filter, it passes through undisturbed.
In a calculator display, which uses ambient light (light that is visible because it reflects off material in a given environment), the first filter is located closest to the viewer, and behind the second polarizer is a mirror. When a charge is applied to a given TN cell, light passes through the first polarizer, twists through the liquid crystal, passes through the second polarizer, bounces off the mirror, and then repeats the process in reverse until the light strikes our eye and registers as “clear.” When there is no charge applied to the cell, the light is blocked at the second polarizer because it hasn’t been twisted into the proper orientation by the liquid crystal. Because no light is returned to our eye, the cell registers as black.In a notebook display, a backlight system is typically used to supply a light source, thus eliminating the need to use reflected ambient light. Backlights often are fluorescent bulbs that are mounted either behind or to the side of the screen. (This second method is also called edgelighting.) Some backlighting technologies use optical fibers to help evenly distribute light across the panel. Naturally, the brighter the light (and it must be fairly bright to push through the several obstacles in the LCD panel), the greater the battery drain. This is why most newer notebooks incorporate elaborate power-saving schemes that are complete with the ability to customize how dim the screen appears at different times.Of course, the world is not just about black and white. The trouble with a 90-degree twist is that it’s fairly inflexible and doesn’t allow for small gradations in the amount of light that passes through the apparatus. STN (supertwist nematic) displays rectify this by using a 210-degree twist. Minor voltage variations affect the precise amount of twist in the liquid crystal. If the twist is such that only half the original amount of light is allowed to pass through, the tone may show up as a medium gray. And that’s how grayscale displays are achieved.From this point, creating color is relatively easy. As white light passes through, for example, a blue color filter, other color wavelengths are screened out so only blue light is allowed to pass through. When used with an LCD cell, three actual color elements (or subpixels) exist within each cell, one each of red, green, and blue (RGB). Each of these subpixel filters is actually acting on white, black, or a shade of gray, turning it into a color. The amount of RGB light allowed to pass through each pixel will determine the cell’s overall color, and contemporary screens can now display millions of color tones.
From Dots To Displays.
Any digital display is made up of a grid of dots called pixels, and when the dots are small enough, our brains perceive patterns within this matrix, which form text or pictures. There are essentially two ways to activate a matrix cell. The first is called direct addressing, wherein each pixel has its own transistor to apply a charge. In contrast, multiplex addressing places a transistor at the head of each matrix row and column.Mutiplex addressing uses fewer circuits, so it is more cost-effective. Consider VGA (Video Graphics Array) resolution, which has 640 columns and 480 rows of pixels (640 x 480). With multiplex addressing, the display only needs 1,120 transistors (640 plus 480); with direct addressing, the screen requires 307,200 transistors (640 multiplied by 480). ultiplex addressing is found in passive-matrix displays, which also are called “dual-scan” displays. To confuse the nomenclature further, dual-scan displays often use a workaround called double-scanning to increase screen brightness. Because passive-matrix technology relies on electrical pulses to charge a cell rather than a solid current, passive-matrix screen images tend to be dimmer. To address this problem, double-scan technology splits the screen into top and bottom halves. This shorter distance allows more pulses to reach each pixel in a given time so the brightness is increased. Moreover, double-scan technology allows faster on-screen response times, which allows for smoother graphics motion.Active-matrix displays use direct addressing via a sheet of transparent transistors that rests immediately between the glass substrate closest to the backlight and the liquid-crystal chamber. This sheet is also what gives the technology its more common name, thin film transistor (TFT). Each pixel has three transistors charging it (one for each color subpixel), which is part of the reason why active-matrix screens are so much brighter than passive-matrix screens.The quality differences are as obvious in the numbers as they are in the viewing. Early passive-matrix displays had a contrast ratio of roughly 15:1 to 20:1, which means a fully illuminated (white) pixel was 15 to 20 times brighter than a dark pixel. The newest passive-matrix technologies, such as CSTN (color supertwist nematic), have managed to extend this up to 50:1. TFT displays, on the other hand, have contrast ratios of 200:1 to 400:1. This drastic difference in contrast and brightness is much of what makes TFT screens readable in outdoor conditions while passive-matrix displays are virtually illegible.
Nothing Is Perfect.
Still, both technologies have their drawbacks. With active matrix, the obvious drawback is price. As with semiconductors, the issue is being able to achieve high manufacturing yields. For example, in a typical 13-inch TFT display with an 800 x 600 resolution, there are 480,000 pixels. Because each pixel uses three transistors, that makes 1,440,000 transistors. If one transistor fails, its pixel will be defective and register a light color against an otherwise black background. Most vendors agree that up to four “bad” pixels still constitutes a salable display. But achieving such low failure rates in a manufacturing process where stray dust can destroy a product is extremely difficult, so low yields have kept prices high, although this is starting to change for the better.Passive-matrix technology also is subject to bad transistors, but when this happens, an entire screen-wide line goes bad. Repairing screens is essentially impossible, so when shopping for a notebook, check to see if the screen can be separated from the main unit and replaced.In both formats, viewing angle is an issue, but it is far more important in passive technologies. Because polarized light is being beamed out from a cell, the light generally travels straight out from the screen. This is why the image looks fine when you look at it straight on, but dim or even inverted if you move too far to the side. Ideally, a flat-panel screen should be comfortably viewable anywhere within 70 degrees of center, just in case multiple users are looking over your shoulder. In general, however, only the highest-quality units come close to achieving this.There are stopgap solutions to improve viewing angles. Some vendors apply a diffuser film over the screen, much like a “soft white” light bulb. This serves to scatter the polarized light in all directions, but because it is a cheap fix, its effectiveness is somewhat restricted.A better solution is IPS (in-plane switching). Here, two electrodes are mounted on the horizontal sides of each subpixel rather than the one typically used in standard TFT designs. Both polarizing filters are lined up along identical horizontal orientations. When no charge is applied, the liquid crystal’s regular horizontal director is present and light passes through horizontally so it spreads out sideways more easily upon exiting the screen. When a charge is applied, the liquid crystal twists, thus cutting off the light flow. With twice the number of transistors, IPS predictably costs more to implement and requires stronger backlighting to overcome the additional transistor obstructions, but the results are some of the best available today. The difference in quality is easy to see.Passive-matrix technology has rightfully been under fire for its mediocre performance since the early ’90s, but despite drops in TFT prices, vendors haven’t given up on passive-matrix screens, which still show up in budget notebooks. Several newer types of passive-matrix technology have evolved, each of which attempts to bridge the gap between active-matrix technology.The drawback to STN technology is that colors are inevitably either yellow- or blue-shifted. However, DSTN (double-layer supertwist nematic) technology uses two display layers, the second one of which corrects this color shifting. DSTN became the standard in passive-matrix notebook displays for years, but it still suffered from the added dimness incurred by the second layer, as well as a cell response time of 300ms (milliseconds; compared to a cell response time of 25ms for TFT screens). This slow response is why your mouse pointer seems to vanish on older passive-matrix displays when dragging it across the screen.More recently, passive technologies, such as HPA (high-performance addressing) and revisions to CSTN (color supertwist nematic), offer very usable results to those who don’t have heavy graphics needs. Both provide better contrast and brightness than DSTN, with CSTN now dipping down to 100ms cell response times and offering a 140-degree viewing angle.
Lastly, one often-overlooked hitch with notebook screens is the resolution issue. Because each cell corresponds to a pixel, just as the number of matrix cells on each monitor is fixed, so is the number of physical pixels, which is usually determined by the screen size. A 12-inch screen typically displays at SVGA (Super Video Graphics Array) resolution (800 x 600 pixels) while a 15-inch displays at XGA (Extended Graphics Array) resolution (1,024 x 768 pixels).Windows-based CRT users are accustomed to going into the Control Panel’s Display Settings and changing screen resolution on-the-fly. Although you can still do this on an LCD, the screen’s squarish pixels don’t handle antialiasing (the process of eliminating jags, which are the rough edges of a diagonal line or curve) as well, and the results can be jaggy. That is why it’s a good idea to match your display size to the type of work (and its usual resolution requirements) when shopping for a new notebook.Considering these few drawbacks, flat-panel LCDs still emerge as clear victors over conventional CRTs, except in cases where your budget is your top priority. Still, with advanced passive-matrix technology continuing to improve and TFT prices gradually falling, we may yet find that just as flat-panel desktop monitors will increasingly displace CRTs, so too will notebooks replace desktop systems because of their superior convenience and excellent displays.
Relatively:
How Polarizing Filters Work
How Filtering Light Works
0 comments:
Post a Comment
Thanks for your Valuable comment