CRT monitors use beams of electrons striking a phosphorescent material coated on glass to generate images. LCD monitors use electric charge to excite liquid crystal cells suspended between layers of glass. These cells either block or transmit the light to create a viewable image.

The LCD screen for a laptop or desktop is composed of several layers of different materials. A fluorescent light source, known as a backlight, is on one end. The light from this source passes through a layer of polarized glass. The polarized light then passes through thousands of liquid crystal cells arrayed in rows across the screen. This liquid crystal layer is followed by another layer of polarized glass, then by a protective covering. If the screen is to be color, a layer of red, green, and blue filters is placed between the crystal layer and the second layer of polarized glass.

There are two types of LCD screens used in computers: passive and active. Passive screens use a grid of horizontal and vertical wires to apply charge. The horizontal grid is attached to one of the glass layers and the vertical grid to the other glass layer with the liquid crystal cells sandwiched between them. Each intersection constitutes a single pixel that can either pass or block light, depending on the amount of charge. For color screens, a single color pixel is composed of three individual liquid crystal cells. Because the individual charges must be continually refreshed, passive displays suffer from slow response times and low brightness and contrast. There are some new passive technologies, CSTN (color super-twist nemantic) and DSTN (double-layer super-twist nematic), which have improved response times and generate brighter screens with more contrast.

Active screens, also known as TFT (twisted film transistor), use transistors and capacitors to control the charge on each liquid crystal cell. Tiny transistors and capacitors are etched unto the glass substrate at the intersection of each row and column. Color displays require an enormous number of transistors. For example, a color display with a resolution of 1,024 by 768 pixels requires 2,359,296 transistors (1024 columns x 768 rows x 3 cells) etched unto the glass. This technology provides high response times and much brighter and high contrast images. However, because of their complexity, they are also more expensive to produce. Practically all notebook displays and LCD monitors now on the market use TFT technology.

LCD monitors offer a number of advantages over CRT monitors, as well as some shortcomings. LCD monitors take up less space, weigh less, and consume less power then their CRT counterparts. Let's compare a 15-inch (diagonal) LCD monitor with a 17-inch CRT monitor. Remember that a 17-inch CRT only has a 15.6- to 16-inch viewable area, so its viewable image is only slightly larger then that of a 15-inch LCD. A LCD monitor is about one-third the size of the CRT monitor. Its depth is typically 7 inches versus 17 inches for a CRT. A typical LCD unit weighs 12 pounds versus 40 pounds for the CRT unit. It consumes only 30 watts of power compared to about 110 watts for an equivalent CRT. Another advantage of LCD monitors, unlike their CRT counterparts, is that they do not emit electromagnetic radiation.

An important issue with LCD monitors is resolution. CRT monitors are usually capable of displaying multiple resolutions. LCD monitors, however, usually work well in only one resolution, known as their native resolution. Other resolutions can usually be displayed, but either the image may not be full-screen or the image quality may be poor. The industry-standard 15-inch LCD monitor usually has a native resolution of 1024 x 768 pixels.

Another area in which LCD monitors still trail CRTs is the display of moving images. LCD displays have a slower response time than CRTs. This delay (or latency) may cause images from a video or animated game to blur or lose detail. LCD monitors also have a smaller viewing angle then CRT monitors. A CRT screen can be viewed from a wide angle, whereas the LCD screen must be viewed from the front.

Until recently, most desktop LCD monitors were built to handle only analog inputs. The reason for this is that most standard graphics boards in your computer convert information to be displayed to the analog form required by CRT monitors. Since the LCD monitor is a digital device, it contains circuitry to convert the analog data back into digital data. This double conversion tends to degrade the quality of the image that is displayed. Older LCD monitors sometimes had trouble synchronizing with the analog signals, resulting in pixel jitter. Tweaking the settings and/or working with the drivers was required to get a stable image. Many of the newer monitors are provided with a single button on the front panel that lets the user initiate an automatic synchronization process that eliminates pixel jitter.

Newer LCD monitors may be provided with a digital connector and cable that interfaces to a graphics board that provides a digital output. This digital output usually conforms to the Digital Visual Interface (DVI) standard promoted by the Digital Display Working Group (see www.ddwg.org). The DVI standard can accommodate both analog and digital interfaces with a single connector, although there are actually three different configurations. DVI-A is for analog-only connections; DVI-D is for digital connections; and DVI-I (for integrated) can handle either analog or digital signals.

Some other features to be considered when selecting a LCD monitor are brightness and contrast. Brightness of LCD screens, also known as luminance, is measured in nits (candelas per meter squared). Newer monitors have a maximum brightness that can range from 100 to 250 nits. The higher the nits, the brighter the display. The contrast ratio is the ratio between the average brightness for white and black areas on the screen. These ratios typically vary from 150 to more than 300. The higher the number, the better the image.

Following are some web sites of manufacturers that produce LCD monitors: Eizo Nanao Technologies, IBM, Samsung Electronics, Sony Electronics, and Viewsonic. The February 6, 2001 issue of PC Magazine has a good article in which 13 leading 15-inch LCD monitors were evaluated and compared.

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