• LCD is the slim, lightweight successor to CRT.lcd tv image by Eray Haciosmanoglu from Fotolia.com
  
  Liquid crystal display (LCD) and cathode ray tube (CRT) are display technologies used in the screens of television sets and computer monitors. In fact, the presence of a CRT---a vacuum tube containing a source of negatively charged particles called electrons---gives traditional TV sets their characteristic shape. By contrast, the shape, footprint and weight of LCD screens do not suffer from this limitation. LCD technology also holds several other advantages over CRT.
  
  

Physical

• Unlike CRT, LCD technology uses the predictable behavior of liquid crystal molecules under an applied voltage to control the appearance of each picture element, or "pixel," in the display. Liquid-crystal molecules do not emit light themselves but change their orientation to permit or deny the passage of light and, in so doing, create an image on the screen. In practical terms, the absence of a CRT makes LCD TV and monitor screens slim and lightweight when compared with their CRT counterparts; a typical 17-inch LCD TV, for example, weighs in at around 13 lbs., whereas a typical CRT TV of the same size could easily weigh two or three times that figure. This relatively light weight, in turn, means that manufacturers can build LCD screens in much larger sizes---entry-level models start at 32 inches, measured diagonally---but they remain portable enough for consumers to mount them on a stand or a wall bracket.
  
  

Picture Quality

• Of course, size isn't everything. Images displayed on LCD screens do not suffer from the flickering associated with CRT screens because they do not rely on a scanning electron beam. Similarly, the electron beam cannot spread and cause geometric distortion at the edges of an image, as often happens with CRT screens. Furthermore, LCD screens offer uniform brightness across their entire surface and are less susceptible to glare than CRT screens, which often require special filters and treated glass. A frequent criticism of early LCD technology held that it offered an inferior contrast ratio---the difference between the darkest (black) and lightest (white) shade that can appear on the screen simultaneously---to CRT. This criticism remains true to a certain extent, but modern LCD screen have extremely high contrast ratios---up to 1,000,000 or more---and any difference is usually only noticeable in unusually dark scenes.
  
  

Other Advantages

• LCD screens use considerably less energy than their CRT predecessors. A 15-inch LCD screen, which draws around 25 watts (W) when operational, can offer reduction of up to 60 percent in energy consumption compared with a 17-inch CRT screen, which draws around 80 W; the casing and black border around a typical CRT screen intrude upon the viewing area, so, in terms of effective viewing area, a 15-inch LCD screen compares with a 17-inch CRT screen. A LCD screen generates very little heat, while the back of a CRT screen becomes noticeably warm when left on for any length of time.