LCD Panel Construction

The two most popular methods of manufacturing LCD panels are twisted nematic (TN) and in-plane switching (IPS). Each method has its strengths and weaknesses, but of the two, IPS is regarded as having the best color representation in all angles, while TN is faster and less expensive.

TN places two electrodes on opposite sides of a liquid crystal layer. The electrodes are attached to the inside of two polarizing surfaces, each rotated 90 degrees with respect to the other. When light enters one side and travels straight through, the other side blocks the light. The light must twist 90 degrees to be allowed through. Think of the crystals as thin rods. When an electrical field is generated by the electrodes, the crystals align lengthwise with the field, perpendicular to the electrodes, and they allow the light to pass straight through. The result is that the light is blocked from making it to the eye of the observer.

When the electrodes are off, the crystals rotate parallel to the them and are influenced by the coating on the electrodes to align to each one so they are rotated 90 degrees with respect to the crystals at the opposing electrode. The crystals are naturally influenced to remain parallel to adjacent crystals, but the influence of the electrodes to remain parallel to the crystals is stronger and causes the chain of crystals in between to twist gradually as they progress from one electrode to the other, following a helical path. The helix rotates the passing light 90 degrees so that it now aligns with the opposite polarizer and appears to pass through to the observer’s eye.

TN exhibits the unfortunate characteristic of shifting the colors of the image as the observer views the screen from wide horizontal and vertical angles. At extreme angles, the light and dark shades appear to swap places, almost as a negative of the actual image. TN panels also tend to react to pressure when touched, blanching the area under a finger pressed against the screen, for instance. However, the faster response rates that lead to more fluid image changes make TN a favorite technology of gamers and action video enthusiasts.

IPS panels have electrodes that are positioned parallel to one another on the same side of the liquid crystal panel, creating an electrical “switch in the same plane.” The two opposing outside polarizing layers are aligned in the same direction, so to create the helical structure with the crystals in the off state, the opposing glass panels have a 90-degree rotated internal coating. The coating influences adjacent crystals to align with the direction of the coating, as with TN. Unlike with TN, however, the parallel polarizer on the opposite side blocks the light when the electrodes are off.

Turning the electrodes on results in a parallel alignment of the crystals lengthwise from one electrode to the other, as they follow the electrical field being generated. This narrows the stack of crystals so that the light passes by them without twisting, thus passing through the similarly aligned polarizer on the opposite side.

IPS technology reproduces colors more accurately and does not suffer from the shifts in color when the screen is viewed from wide angles. These characteristics makes IPS ideal for those who require a true-to-life representation of the original colors of the image displayed. For that reason, as well as for their lack of reaction to being touched, IPS panels are more suitable for touchscreens, including those on handheld portable devices, such as smartphones. The drawbacks of slow response and lusterless display of black hues have been mitigated through the generations of IPS technological advancements. Nevertheless, IPS remains a more expensive solution that requires more power to operate than TN.

Table 4.1 summarizes the differences between TN and IPS, including their cost, image stability, and the power required to operate them.

Pixel Addressing

Two major types of LCD displays have been implemented over the years: active-matrix screens and passive-matrix screens. Another type, dual scan, is a passive-matrix variant. The main differences lie in the quality of the image. However, when used with computers, each type uses lighting behind the LCD panel (backlighting) to make the screen easier to view. Legacy LCD panels had one or more fluorescent bulbs as backlights. Modern LCD panels use LEDs to light the display more precisely by individually dedicating a separate light source to each pixel. The following discussions highlight the main differences among the pixel-addressing variants.

Active matrix An active-matrix screen is made up of several independent LCD pixels. A transistor at each pixel location, when switched among various levels, activates two opposing electrodes that align the pixel’s crystals and alter the passage of light at that location to produce hundreds or thousands of shades. The front electrode, at least, must be clear. This type of display is very crisp and easy to look at through nearly all oblique angles, and it does not require constant refreshing to maintain an image because transistors conduct current in only one direction and the pixel acts like a capacitor by holding its charge until it is refreshed with new information.

The major disadvantage of an active-matrix screen is that it requires larger amounts of power to operate all of the transistors—one for each red, green, and blue subpixel. Even with the backlight turned off, the screen can still consume battery power at an alarming rate, even more so when conventional fluorescent backlights are employed.

Passive matrix A passive-matrix display does not have a dedicated transistor for each pixel or subpixel but instead a matrix of conductive traces. In simplified terms for a single pixel, when the display is instructed to change the crystalline alignment of a particular pixel, it sends a signal across the x- and y-coordinate traces that intersect at that pixel, thus turning it on. Figure 4.1 illustrates this concept.

The circuits controlling the columns are synchronized to fire when that row’s transistor is active and only for the pixels that should be affected on that row. Angles of visibility and response times (the time to change a pixel) suffer greatly with passive-matrix LCDs. Because neighboring pixels can be affected through a sort of “crosstalk,” passive-matrix displays can look a bit “muddy.”

Dual scan Dual scan is a variation of the passive-matrix display. The classic passive-matrix screen is split in half to implement a dual-scan display. Each half of the display is refreshed separately, leading to increased quality. Although dual scan improves on the quality of conventional passive-matrix displays, it cannot rival the quality produced by active matrix.

The main differences between active matrix and typical passive matrix are image quality and viewing angle. Because the computer takes hundreds of milliseconds to change a pixel in passive-matrix displays (compared with tens of milliseconds or less in active-matrix displays), the response of the screen to rapid changes is poor, causing, for example, an effect known as submarining; that is, on a computer with a passive-matrix display, if you move the mouse pointer rapidly from one location to another, it will disappear from the first location and reappear in the new location without appearing anywhere in between. The poor response rate of passive-matrix displays also makes them suboptimal for displaying video.

If you watch the screen and gradually reposition yourself laterally, farther away from the center of a passive-matrix LCD, you eventually notice the display turning dark. In contrast, active-matrix LCDs have a viewing angle wider than 179 degrees. In fact, if you didn’t know better, you’d think a passive-matrix display was a standard display with a privacy filter on it. A privacy filter is a panel that fits over the front of a display and, through a polarization affect, intentionally limits the viewing angle of the monitor. These same filters, as well as specialty versions, can act as antiglare filters, brightening and clarifying the image appearing on the monitor’s screen.

We’ll discuss additional concepts that apply to LCDs and other flat-panel displays later in this chapter.

Backlight Sources

A source of confusion for users and industry professionals alike, LED displays are merely LCD panels with light emitting diodes (LEDs) as light sources instead of the fluorescent bulbs used by legacy LCD monitors. No doubt, the new technology would not be nearly as marketable if they were referred to merely as LCDs. The general consumer would not rush to purchase a new display that goes by the same name as their current display. Nevertheless, calling these monitors LED displays is analogous to calling the conventional LCD monitors fluorescent displays; it’s simply the backlight source, not the display technology.

Because there are many individually controlled LEDs in an LED display, most often as many as there are transistors in the LCD panel, the image can be intelligently backlit to enhance the quality of the picture. Additionally, there is no need for laptops with LED displays to convert the DC power coming into the laptop to the AC needed to power traditional fluorescent backlights because LEDs operate on DC power just like the rest of the laptop. As a result, these systems have no inverter board (discussed later in Chapter 9, “Understanding Laptops”), which are the DC-to-AC conversion devices present in traditionally backlit laptop displays. LED displays rival plasma displays in clarity and variations in luminance. This variation is referred to as contrast ratio, and it is discussed later in this chapter.

Rear Projection

Another popular implementation of projection systems has been the rear-projection television, in which a projector is built into a cabinet behind a screen onto which a reverse image is projected so that an observer in front of the TV can view the image correctly. Early rear-projection TVs as well as ceiling-mounted home-theater units used CRT technology to drive three filtered light sources that worked together to create an RGB image.

Later rear-projection systems, including most modern portable projectors, implement LCD gates. These units shine a bright light through three LCD panels that adjust pixels in the same manner as an LCD monitor, except that the projected image is formed, as with the CRT projector, by synchronizing the combination and projection of the red, green, and blue images onto the same surface.

Digital light processing (DLP) is another popular technology that keeps rear-projection TVs on the market and benefits portable projectors as well, allowing some projectors to be extremely small. Special DLP chips, referred to as optical semiconductors, have roughly as many rotatable mirrors on their surface as pixels in the display resolution. A light source and colored filter wheel or colored light sources are used to switch rapidly among primary, and sometimes secondary, colors in synchronization with the chip’s mirror positions, thousands of times per second.

Brightness

Projection systems are required to produce a lighted image and display it many feet away from the system. The inherent challenge to this paradigm is that ambient light tends to interfere with the image’s projection. One solution to this problem is to increase the brightness of the image being projected. This brightness is measured in lumens. A lumen (lm) is a unit of measure for the total amount of visible light that the projector gives off, based solely on what the human eye can perceive and not on invisible wavelengths. When the rated brightness of the projector in lumens is focused on a larger area, the lux—a derivative of lumens measuring how much the projector lights up the surface on which it is focused—decreases; as you train the projector on a larger surface (farther away), the same lumens produce fewer lux.

The foregoing discussion notwithstanding, projection systems are rated and chosen for purchase based on lumens of brightness, usually once a maximum supported resolution has been chosen. Sometimes the brightness is even more of a selling point than the maximum resolution that the system supports because of the chosen environment in which it operates. Therefore, this is the rating that must be used to compare the capabilities of projection systems.

Some loose guidelines can help you choose the right projector for your application. Keep in mind that video versus static image projection requires more lumens, and 3D output requires roughly double the lumens of 2D projection. Additionally, use of a full-screen (4:3 aspect ratio) projector system in a business environment versus a widescreen (16:9) home theater projector requires approximately double the lumens of output at the low end and only 1.3 times at the high end.

For example, if you are able to completely control the lighting in the room where the projection system is used, producing little to no ambient light, a projector producing as little as 1,300 lumens is adequate in a home theater environment, while you would need one producing around 2,500 lumens in the office. However, if you can only get rid of most of the ambient light, such as by closing blinds and dimming overhead lights, the system should be able to produce 1,500 to 3,500 lumens in the home theater and 3,000 to 4,500 lumens in the office. If you have no control over a very well-lit area, you’ll need 4,000 to 4,500 lumens in the home theater and 5,000 to 6,000 lumens in the business setting. These measurements assume a screen size of around 120″, regardless of aspect ratio.

By way of comparison, a 60W standard light bulb produces about 800 lumens. Output is not linear, however, because a 100W light bulb produces over double, at 1,700lm. Nevertheless, you couldn’t get away with using a standard 100W incandescent bulb in a projector. The color production is not pure enough and constantly changes throughout its operation due to deposits of soot from the burning of its tungsten filament during the production of light. High-intensity discharge (HID) lamps like the ones found in projection systems do more with less by using a smaller electrical discharge to produce far more visible light. A strong quartz chamber holds the filament in a projector lamp and can be seen inside the outer bulb. It contains a metal halide (where the word halogen comes from) gas that glows bright white when the tungsten filament lights up. Depositing the soot on the inside of the projector bulb is avoided by using a chemical process that attracts the soot created back to the filament where it once again becomes part of the filament, extending its life and reducing changes in light output.

Expect to pay considerably more for projector bulbs than for standard bulbs of a comparable wattage. The metal halide gases used in projector bulbs are more expensive than the noble gases used in standard bulbs. Add to that the fact that the bulb itself might have to be handmade and you can understand the need for higher cost.

Cooling Down

Although it doesn’t take long for the fan to stop running on its own, this is a phase that should never be skipped to save time. With projector bulbs being one of the priciest consumables in the world of technology, doing so may cost you more than a change in your travel arrangements. See the sidebar titled “Factor In Some Time” for some perspective.

Super VGA

Until the late 1980s, IBM set most personal-computer video standards. IBM made the adapters, everyone bought them, and they became a standard. Some manufacturers didn’t like this monopoly and set up the Video Electronics Standards Association (VESA) to try to enhance IBM’s video technology and make the enhanced technology an open standard. The initial result of this work was Super VGA (SVGA). This new standard was indeed an enhancement because it could support 16 colors at a resolution of 800×600 (the VESA standard), but it soon expanded to support 1024×768 pixels with 256 colors.

Since that time, SVGA has been a term used loosely for any resolution and color palette to exceed that of standard VGA. This even includes the resolution presented next, XGA. New names still continue to be introduced, mainly as a marketing tool to tout the new resolution du jour. While display devices must be manufactured to support a certain display resolution, one of the benefits of analog video technology was that later VGA monitors could advance along with the graphics adapter, in terms of the color palette. The analog signal is what dictates the color palette, and the standard for the signal has not changed since its VGA origin. This makes VGA monitors’ color limitations a nonissue. Such a topic makes sense only in reference to graphics adapters.

XGA

IBM introduced a new technology in 1990 known as the Extended Graphics Array (XGA). This technology was available only as a Micro Channel Architecture (MCA) expansion board (versus ISA or EISA, for instance). XGA could support 256 colors at 1024×768 pixels or 65,536 colors at 800×600 pixels. It was a different design, optimized for GUIs of the day, such as Windows and OS/2. It was also an interlaced technology when operating at the 1024×768 resolution, meaning that rather than scan every line one at a time on each pass to create the image, it scanned every other line on each pass, using the phenomenon known as “persistence of vision” to produce what appears to our eyes as a continuous image.

The advertised refresh rate specifies the frequency with which all odd or all even rows are scanned. The drawback to interlacing is that the refresh rate used on a CRT has to be twice the minimum comfort level for refreshing an entire screen. Otherwise, the human eye will interpret the uncomfortably noticeable decay of the pixels as flicker. Therefore, a refresh rate of 120Hz would result in a comfortable effective refresh rate of 60Hz. Unfortunately, 84Hz was a popular refresh rate for interlaced display signals, resulting in an entire screen being redrawn only 42 times per second, a rate below the minimum comfort level.

More Recent Video Standards

Any standard other than the ones already mentioned are probably extensions of SVGA or XGA. It has become quite easy to predict the approximate or exact resolution of a video specification based on its name. Whenever a known technology is preceded by the letter W, you can assume roughly the same vertical resolution but a wider horizontal resolution to accommodate 16:10 widescreen monitor formats (16:9 for LCD and plasma televisions). Preceding the technology with the letter Q indicates that the horizontal and vertical resolutions were each doubled, making a final number of pixels 4 times (quadruple) the original. To imply 4 times each, for a final resolution enhancement of 16 times, the letter H, for hexadecatuple is used.

Therefore, if XGA has a resolution of 1024×768, then Quad XGA (QXGA) will have a resolution of 2048×1536. If Ultra XGA (UXGA) has a resolution of 1600×1200 and an aspect ratio of 4:3, then Wide Ultra XGA (WUXGA) has a resolution of 1920×1200 and a 16:10 aspect ratio. Clearly, there have been a large number of seemingly minute increases in resolution column and row sizes. However, consider that at 1024×768, for instance, the screen will display a total of 786,432 pixels. At 1280×1024, comparatively, the number of pixels increases to 1,310,720—nearly double the number of pixels for what doesn’t sound like much of a difference in resolution. As mentioned, you need better technology and more video memory to display even slightly higher resolutions.

Table 4.2 lists the various video technologies, their resolutions, and the maximum color palette they support, if specified as part of the standard. All resolutions, VGA and higher, have a 4:3 aspect ratio unless otherwise noted.

Starting with SXGA, the more advanced resolutions can be paired with 32-bit graphics, which specifies the 24-bit Truecolor palette of 16,777,216 colors and uses the other 8 bits for enhanced noncolor features, if at all. In some cases, using 32 bits to store 24 bits of color information per pixel increases performance because the bit boundaries are divisible by a power of 2; 32 is a power of 2, but 24 is not. That being said, however, unlike with the older standards, the color palette is not officially part of the newer specifications.

Native Resolution

One of the peculiarities of LCD, plasma, OLED, and other flat-panel displays is that they have a single fixed resolution, known as the native resolution. Unlike CRT monitors, which can display a crisp image at many resolutions within a supported range, flat-panel monitors have trouble displaying most resolutions other than their native resolution.

The native resolution comes from the placement of the transistors in the hardware display matrix of the monitor. For a native resolution of 1680×1050, for example, there are 1,764,000 transistors (LCDs) or cells (PDPs and OLED displays) arranged in a grid of 1680 columns and 1050 rows. Trying to display a resolution other than 1680×1050 through the operating system tends to result in the monitor interpolating the resolution to fit the differing number of software pixels to the 1,764,000 transistors, often resulting in a distortion of the image on the screen.

The distortion can take various forms, such as blurred text, elliptical circles, and so forth. SXGA (1280×1024) was once one of the most popular native resolutions for larger LCD computer monitors before use of widescreen monitors became pervasive. For widescreen aspects, especially for widescreen LCD displays of 15.4″ and larger, WSXGA+ (1680×1050) was one of the original popular native resolutions. The ATSC 1080p resolution (1920×1080) is highly common today across all display technologies, largely replacing the popular computer-graphics version, WUXGA (1920×1200).

Contrast Ratio

The contrast ratio is the measure of the ratio of the luminance of the brightest color to that of the darkest color that the screen is capable of producing. Do not confuse contrast ratio with contrast. Contrast ratios are generally fixed measurements that become selling points for the monitors. Contrast, on the other hand, is an adjustable setting on all monitors (usually found alongside brightness) that changes the relative brightness of adjacent pixels. The more contrast, the sharper and edgier the image. Reducing the contrast too much can make the image appear washed out. This discussion is not about contrast but instead it’s about contrast ratio.

One of the original problems with LCD displays, and a continuing problem with cheaper versions, is that they have low contrast ratios. Only LED-backlit LCD panels rival the high contrast ratios that plasma displays have always demonstrated. A display with a low contrast ratio won’t show a “true black” very well, and the other colors will look washed out when you have a light source nearby. Try to use the device in full sunshine and you’re not going to see much of anything, although the overall brightness level is the true key in such surroundings. Also, lower contrast ratios mean that you’ll have a harder time viewing images from the side as compared to being directly in front of the display.

Ratios for smaller LCD monitors and televisions typically start out around 500:1. Common ratios for larger units range from 20,000:1 to 100,000:1. In the early days of monitors that used LEDs as backlights, 1,000,000:1 was exceedingly common. Today, vendors advertise 10,000,000:1 and “infinite” as contrast ratios. Anything higher than 32,000:1 is likely a dynamic contrast ratio. Plasma displays have always been expected to have contrast ratios of around 5000:1 or better.

Once considered a caveat, a dynamic ratio is realized by reducing power to the backlight for darker images. The downside was that the original backlight being a single fluorescent bulb meant that the signal to the brighter LCD pixels had to be amplified to compensate for the uniform dimming of the backlight. This occasionally resulted in overcompensation manifested as areas of extreme brightness, often artificial in appearance. This practice tends to wash out the lighter colors and make white seem like it’s glowing, which is hardly useful to the user. Today’s LED backlights, however, are controlled either in zones made up of a small number of pixels or individually per pixel, resulting in trustworthy high dynamic contrast ratios.

The environment where the monitor will be used must be taken into account when considering whether to place a lot of emphasis on contrast ratio. In darker areas, a high contrast ratio will be more noticeable. In brighter surroundings, widely varying contrast ratios do not make as much of a difference. For these environments, a monitor capable of higher levels of brightness is more imperative.

One caveat to contrast ratios that remains is that there is no vendor-neutral regulatory measurement. The contrast ratio claimed by one manufacturer can take into account variables that another manufacturer does not. A manufacturer can boost the ratio simply by increasing how bright the monitor can go, the portion of the monitor tested, or the conditions in the room where the test was performed. This doesn’t do anything to help the display of darker colors, though. So, although the contrast ratio is certainly a selling point, don’t just take it at face value. Look for independent comparison reviews that use multiple methods of measuring contrast ratio or compare displays in person to see which one works better for the situation in which you intend to use it.