With a 100-year head start over competing screen technologies, the CRT is still a formidable technology. It’s based on universally understood principles and employs commonly available materials. The result is cheap-to-make monitors capable of excellent performance, producing stable images in true colour at high display resolutions. However, the CRT’s most obvious shortcomings are well known: it sucks up too much electricity; its single electron beam design is prone to misfocus; misconvergence and colour variations across the screen; its clunky high-voltage electric circuits and strong magnetic fields create harmful electromagnetic radiation; it’s simply too big. Despite vested interest in CRTs it is inevitable that one of the several flat panel display technologies will win out in the long run and some estimate that even by the year 2000.

Liquid crystal displays : Liquid crystals were discovered in the late 19th century by the Austrian botanist, Friedrich Reinitzer, and the term ‘liquid crystal’ itself was coined shortly afterwards by German physicist, Otto Lehmann. Liquid crystals are almost transparent substances, exhibiting the properties of both solid and liquid matter. Light passing through liquid crystals follows the alignment of the molecules that make them up - a property of solid matter. In the 1960s it was discovered that charging liquid crystals with electricity changed their molecular alignment, and consequently the way light passed through them; a property of liquids. Since its advent in 1971 as a display medium, liquid crystal displays have moved into a variety of fields, including miniature televisions, digital still and video cameras and monitors and today many believe that the LCD is the most likely technology to replace the CRT monitor. The technology involved has been developed considerably since its inception, to the point where today's products no longer resemble the clumsy, monochrome devices of old. It has a head start over other flat screen technologies and an apparently unassailable position in notebook and handheld PCs where it is available in two forms: low-cost, dual-scan twisted nematic (DSTN) ,high image quality thin film transistor (TFT).

Principles Most liquid crystals are organic compounds consisting of long rod-like molecules which, in their natural state, arrange themselves with their long axes roughly parallel. It is possible to precisely control the alignment of these molecules by flowing the liquid crystal along a finely grooved surface. The alignment of the molecules follows the grooves, so if the grooves are exactly parallel, then the alignment of the molecules also becomes exactly parallel. The first principle of an LCD consists of sandwiching liquid crystals between two finely grooved surfaces, where the grooves on one surface are perpendicular (at 90 degrees) to the grooves on the other. If the molecules at one surface are aligned north to south, and the molecules on the other are aligned east to west, then those in-between are forced into a twisted state of 90 degrees. Light follows the alignment of the molecules, and therefore is also twisted through 90 degrees as it passes through the liquid crystals. However, following RCA America’s discovery, when a voltage is applied to the liquid crystal, the molecules rearrange themselves vertically, allowing light to pass through untwisted. The second principle of an LCD relies on the properties of polarising filters and light itself. Natural light waves are orientated at random angles. A polarising filter is simply a set of incredibly fine parallel lines. These lines act like a net, blocking all light waves apart from those (coincidentally) orientated parallel to the lines. A second polarising filter with lines arranged perpendicular (at 90 degrees) to the first would therefore totally block this already polarised light. Light would only pass through the second polariser if its lines were exactly parallel with the first, or if the light itself had been twisted to match the second polariser. An LCD consists of two polarising filters with their lines arranged perpendicular (at 90 degrees) to each other, which, as described above, would block all light trying to pass through. But in-between these polarisers are the twisted liquid crystals. Therefore light is polarised by the first filter, twisted through 90 degrees by the liquid crystals, finally allowing it to completely pass through the second polarising filter. However, when an electrical voltage is applied across the liquid crystal, the molecules realign vertically, allowing the light to pass through untwisted but to be blocked by the second polariser. Consequently, no voltage equals light passing through, while applied voltage equals no light emerging at the other end. The crystals in an LCD could be alternatively arranged so that light passed when there was a voltage, and not passed when there was no voltage. However, since computer screens with graphical interfaces are almost always lit up, power is saved by arranging the crystals in the no-voltage-equals-light-passing configuration. LCDs follow a different set of rules than CRT displays offering advantages in terms of bulk, power consumption and flicker, as well as ‘perfect’ geometry. They have the disadvantage of a much higher price, a poorer viewing angle and less accurate colour performance. While CRTs are capable are displaying a range of resolutions and scaling them to fit the screen, an LCD panel has a fixed number of liquid crystal cells and can display only one resolution at full-screen size using one cell per pixel. Lower resolutions can be displayed by using only a proportion of the screen. For example, a 1024 x 768 panel can display at resolution of 640 x 480 by using only 66% of the screen. Most LCDs are capable of scaling lower-resolution images to fill the screen through a process known as rathiomatic expansion. However, this works better for continuous-tone images like photographs than it does for text and images with fine detail. Also, the diagonal measurements of LCDs equal the viewable area, so there’s no loss of the traditional inch or so behind the monitor's faceplate or bezel. The combination makes any LCD a match for a CRT 2 to 3 inches larger: CRT has three electron guns whose streams must converge faultlessly in order to create a sharp image. There are no convergence problems with an LCD panel, because each cell is switched on and off individually. This is one reason why text looks so crisp on an LCD monitor. There’s no need to worry about refresh rates and flicker with an LCD panel - the LCD cells are either on or off, so an image displayed at a refresh rate as low as between 40-60Hz should not produce any more flicker than one at a 75Hz refresh rate. Conversely, it's possible for one or more cells on the LCD panel to be flawed. On a 1024 x 768 monitor, there are three cells for each pixel - one each for red, green, and blue - which amounts to nearly 2.4 million cells (1024 x 768 x 3 = 2,359,296). There's only a slim chance that all of these will be perfect; more likely, some will be stuck on (creating a ‘bright’ defect) or off (resulting in a ‘dark’ defect). Some buyers may think that the premium cost of an LCD display entitles them to perfect screens - unfortunately, this is not the case. LCD monitors have other elements that you don't find in CRT displays. The panels are lit by fluorescent tubes that snake through the back of the unit; sometimes, a display will exhibit brighter lines in some parts of the screen than in others. It may also be possible to see ghosting or streaking, where a particularly light or dark image can affect adjacent portions of the screen. And fine patterns such as dithered images may create moiré or interference patterns that jitter. Viewing angle problems on LCDs occur because the technology is a transmissive system which works by modulating the light that passes through the display, while CRTs are emissive. With emissive displays, there’s a material that emits light at the front of the display, which is easily viewed from greater angles. In an LCD, as well as passing through the intended pixel, obliquely emitted light passes through adjacent pixels, causing colour distortion. Currently, most LCD monitors plug into a computer's familiar 15-pin analogue VGA port and use an analogue-to-digital converter to convert the signal into a form the panel can use. However, the VESA is working on a specification for a digital video port

DSTN displays A normal passive matrix LCD comprises a number of layers. The first is a sheet of glass coated with a transparent metal oxide. This operates as a grid of row and column electrodes which passes the current needed to activate the screen elements. On top of this, a polymer is applied that has a series of parallel grooves running across it to align the liquid crystal molecules in the appropriate direction, and to provide a base on which the molecules are attached. This is known as the alignment layer and is repeated on another glass plate that also carries a number of spacer beads, which maintain a uniform distance between the two sheets of glass when they're placed together. The edges are then sealed with an epoxy, but with a gap left in one corner. This allows liquid-crystal material to be injected between the sheets (in a vacuum) before the plates are sealed completely. In early models, this process was prone to faults, resulting in stuck or lost pixels where the liquid crystal material had failed to reach all parts of the screen. Next, polarising layers are applied to the outer-most surfaces of each glass sheet to match the orientation of the alignment layers. With DSTN, or dual scan screens, the orientation of alignment layers varies between 90 degrees and 270 degrees, depending on the total rotation of the liquid crystals between them. A backlight is added, typically in the form of cold-cathode fluorescent tubes mounted along the top and bottom edges of the panel, the light from these being distributed across the panel using a plastic light guide or prism. The image which appears on the screen is created by this light as it passes through the layers of the panel. With no power applied across the LCD panel, light from the backlight is vertically polarised by the rear filter and refracted by the molecular chains in the liquid crystal so that it emerges from the horizontally polarised filter at the front. Applying a voltage realigns the crystals so that light can't pass, producing a dark pixel. Colour LCD displays simply use additional red, green and blue coloured filters over three separate LCD elements to create a single multi-coloured pixel. However, the LCD response itself is very slow with the passive matrix driving scheme. With rapidly changing screen content such as video or fast mouse movements, smearing often occurs because the display can’t keep up with the changes of content. In addition, passive matrix driving causes ghosting, an effect whereby an area of ‘on’ pixels causes a shadow on ‘off’ pixels in the same rows and columns. The problem of ghosting can be reduced considerably by splitting the screen in two and refreshing the halves independently and other improvements are likely to result from several other independent developments coming together to improve passive-matrix screens. New signal-processing algorithms being used in LCD panels analyse incoming video signals and correct for the distortion that causes streaking - the ghost lines that continue across the screen after a real line stops. Sharp, which claims 30 to 40 percent of the desktop and notebook LCD market, calls its version of this feature Sharp Addressing, and says that most new dual-scan panels incorporate some variation on this technique. Other evolutionary developments are simultaneously increasing dual-scan displays' speed and contrast. Most conventional DSTN displays use materials that have a response time of around 300ms, almost a third of a second. This sluggish response and the accompanying decay rate is largely responsible for the ghosting or image trails that have made dual-scan notebooks unacceptable for full-motion video applications. Other LCD materials offer response times as quick as 150ms, but simply using a faster material without making other changes causes flickering. In order to create the shades required for a full-colour display, there have to be some intermediate levels of brightness between all-light and no-light passing through. The varying levels of brightness required to create a full-colour display is achieved by changing the strength of the voltage applied to the crystals. The liquid crystals in fact untwist at a speed directly proportional to the strength of the voltage, thereby allowing the amount of light passing through to be controlled. In practice, though, the voltage variation of today’s LCDs can only offer 64 different shades per element (6-bit) as opposed to full-colour CRT displays which can create 256 shades (8-bit). Using three elements per pixel, this results in colour LCDs delivering a maximum of 262,144 colours (18-bit), compared to true-colour CRT monitors supplying 16,777,216 colours (24-bit). As multimedia applications become more widespread, the lack of true 24-bit colour on LCD panels is becoming an issue. Whilst 18-bit is fine for most applications, it is insufficient for photographic or video work. Some LCD designs manage to extend the colour depth to 24-bit by displaying alternate shades on successive frame refreshes, a technique known as Frame Rate Control (FRC). However, if the difference is too great, flicker is perceived. Hitachi has developed a technique, whereby the voltage applied to adjacent cells to create patterns changes very slightly across a sequence of three or four frames. With it, Hitachi can simulate not quite 256 greyscales, but still a highly respectable 253 greyscales, which translates into over 16 million colours - virtually indistinguishable from 24-bit true colour.