How Does the LCD Computer Monitor Work?

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The LCD appeared mainly to make computers truly portable. Early models were monochrome , and offered a very limited resolution .

Currently two technologies the market reaprten : passive matrix and active matrix. The latter provides exceptional image quality .

The active matrix displays are more expensive to manufacture, since a very high proportion of screens produced is defective and should be rejected (over 75 %). This proportion increases rapidly with the size of the screen . For this reason its resolution is limited , so on a laptop screen resolutions will be much smaller than a desktop computer . Most modern monitors are manufactured with Dula Scan ( DSTN ) or TFT (Thin Film Transistor ) technology. The latter is the most advanced and enabling better decisions and quality of vision.

Since early 1998 he is taking display technology for laptops to desktops , thus achieving a greater saving space and energy. The big drawback is , besides its high cost , the problem being the size of the screen, since older LCD monitors and TFT technology currently manufactured are 15 ” , with standard 12.2 ” while monitors desktop can reach up to 24 . “

However Gauchos believe that the short term, the LCD ( Liquid Crystal Display – Liquid Crystal Display ) replace traditional monitors , which is supported by the continued fall in prices of these devices.

How it works?

Basically , the liquid crystals are transparent substances with characteristics of liquids and solids qualities .

As solid , light passing through a liquid crystal is the alignment of the molecules , but as the liquid , applying an electric charge to these crystals , a change in the alignment of the molecules , and thus the way in which light passes through them .

A LCD screen is formed by two polarizing filters with the liquid crystal rows aligned perpendicular to each other , so that by applying or stop applying a electric current filters is achieved light to pass or not pass through them , as the second filter does not block or pass light that has passed through the first.

The COLOR is achieved by adding an additional 3 color filters ( one red, one green, one blue). However, for the reproduction of a variety of shades , it should apply different levels of intermediate brightness between light and no – light, which is achieved by variations in the voltage applied to the filters.

LCD monitors generally have the following characteristics :

They take up less space and are lighter .

Excellent picture, very good sharpness.

Better protection for our eyes because they have special materials.

Start to become accessible to the common user.

They save energy

No use scanning system and does not reflect the picture as it has lighting for each crystal cell .

It has a maximum resolution of 1024 x 768 dpi (dots per inch)

The planes have a greater vision for their ancestors , so a liquid crystal display is equivalent to a larger viewing surface than a CRT monitor with more inches .

Better ease of movement. There are versions that can rotate up to ninety degrees.

But they also have a downside: They occupy a place with a special plaque in the cabinet.

Comparisons between CRT and LCD

The Advantages of LCD versus CRT are size , power consumption , flicker and geometry.

By not requiring the use of a single picture tube , LCD monitors have a much smaller SIZE , especially a line, making them ideal for laptops or in environments where space is scarce .

CONSUMPTION of these monitors is also much lower , hence its suitability to the world of laptops, where battery durability is crucial .

The FLASH on LCD displays is highly reduced by the fact that each cell where the liquid crystals are housed is on or off , so that the image does not need a facelift (drink ) but on or off .

The PERFECT GEOMETRY is given that each cell containing a liquid crystal is switched on or off individually , and therefore no convergence problems .

The DISADVANTAGES are given by the cost , the viewing angle , the lower range of colors and color purity .

The COST manufacturing of LCD monitors is superior to CRTs, not only the technology , but also for its poor implementation makes manufacturing volumes are small.

Since light of LCDs is produced by fluorescent tubes located behind the filter , instead of illuminating the front and CRT monitors with a diagonal view the light passes through the pixels (crystals) contiguous for so the image is distorted from a viewing angle of 100 º on the monitors passive matrix ( DSTN ) and from 140 ° on monitors active matrix (TFT ) .

Voltage variations of current LCD screens, which is what generates the color tones , only allows 64 levels per color (6 BIT ) compared to 256 levels (8- BIT) CRT monitors , so three up to 262,144 colors different colors ( 18 BIT) are achieved compared to 16,777,216 colors (24- BIT) CRT monitors . Although 262,144 colors are sufficient for most applications , this PALETTE is clearly insufficient for photographic reproduction work for video work .

Due to the lighting system with fluorescent LCDs inevitably show a lower PURITY OF COLOR , as shown brighter than others , which results in a very light, too dark image areas affecting adjacent areas of the screen , creating a rather unpleasant effect .

An additional problem affecting the quality of the image on the LCD screens are given by the current operation of graphics cards and LCD screens: the graphics card receives a digital signal processor and analog transforms to send off signal , for its part receives the LCD screen and the analog signal must be converted to digital signal , with the logic loss that occurs between the two transformations .

Current LCDs connect to VGA analog ports , but it is expected that in the future all graphics cards also incorporate a digital output.

The diagonal LCD screen size equivalent to a CRT monitor oversize . Here we have the equivalences , together with resolutions usually supported:

13.5” 15” 800×600
14.5”/15” 17” 1024×768
17” 21” 1280×1024/1600×1280


And SHARP TOSHIBA with its HDP system ( Passive Hybrid Display) try to introduce bridging between DSTN and TFT technologies using lower viscosity liquid crystals , so that the response time is less ( 150ms ) and higher contrast (50:1) with a small increase in cost on DSTN screens.

HITACHI , with its technology HPA (High Performance Addressing) gets closer to the DSTN TFT technology for quality video playback and viewing angle.

CANON has tested the use of ferroelectric crystals , reducing response time and allowing the crystals need not receive electricity constantly , only to change its voltage , reducing consumption ( especially important on laptops ), but its manufacturing cost is too close to the TFT screens so its future is uncertain.

Some Japanese manufacturers talk about reflective LCD screens , instead of backlit panels. Its unique advantages are the low consumption , thinness and lightness, but their cost makes them more appropriate for PALMTOP and sub-notebooks .


Plasma panels , which are also known as gas discharge displays are constructed by filling the area between two glass plates with a gas mixture that usually includes neon . Here the definition of the image is stored in a buffer and delayed charging voltages applied to refresh the pixel positions . The thin film electroluminescent displays are similar to a plasma panel . The difference is that between the glass plates is filled with a phosphor such as zinc sulfide, manganese compound , instead of a gas .

The Plasma vision system consists of a set of cells called pixels each composed of three sub -pixels that correspond to the red, green and blue. The gas present in the device reacts with neon particles within each sub – pixel and thus produces a colored light (red , green or blue). Each sub – pixel is controlled by a sophisticated electronic system and produce over 16 million different colors . Thus obtained images of impeccable sharpness using a device with a thickness less than 15 inches.

It is based on the principle that by passing a high voltage by a low pressure gas light is generated . That screen uses phosphorus as CRT monitors but are emissive as LCD , achieving improved color and great viewing angle.


The field emission displays ( FED ) combine the match with the cell structure of the LCD screens. Mini – tubes are used (instead of the bulky CRT tube ) for each pixel and lets achieve a similar thickness of LCDs .

The light is generated in front of the pixel, as in CRT, so that an excellent viewing angle is achieved .

These monitors have a response time better than TFT screens and playback similar to CRT color, but the cost and difficulty of manufacture (480,000 vacuum tubes small screen) and the need for a screen shield does its dubious viability.

If you get lower costs and improve reliability , this technology may threaten the LCD technology in the future .

Thin CRT

Thin cathode tubes are based on the use FED technology 3’5mm tube thickness instead of bulky CRT tube .

In 1999 came the first screens with this technology similar to the cost TFT screens .


It is based on the application of a voltage to a plastic surface. The technique to manufacture color displays LEP technology uses inkjet printing to form a fine dot matrix red , green and blue polymer in a grid of electrodes. In principle , this was based on large monitors and TV screens that were as flexible sheets . The LEP also offer the advantage of having an autonomous lighting, they do not need a separate back light , and could be seen from any angle. They would be more than desirable to substitute desktop monitors .

The advantages over LCD screens is that only one layer of plastic is required , compared with two glass for LCD , need feedback, it is the surface that emits light , have low power consumption and good viewing angle . In addition , this technology allows curves and even flexible displays .


It is a proprietary technology TEXAS INSTRUMENTS and is widely used in projectors.

Design is a static memory in which the bits are stored in cells in the form of silicon and the electric charge image is achieved by means of very complex optics .

The problems arise from this technology by the heat produced and the need for cooling , which generates enough noise. In addition , color technology is a major complication , using triple swivel lens , and slowly makes her unsuitable for video playback .


What should be kept in mind before buying a monitor, especially an LCD ?

· Resolution: The resolution acceptable depends on what you want to give to the monitor , usually for a common user standard resolutions are 1600×1200 , 1280×1024 , 1024×768 .

· Size: The size of a screen is a very important fact , but , unlike CRT monitors , it should be noted that the diagonal measurement of an LCD equivalent viewing area .

Ie the diagonal LCD screen size equivalent to a CRT monitor oversized . With the popularization of the 13.5 -inch LCD screens in notebook computers , the user gets ease and effect display desktop monitor .

· DSTN ( passive matrix ) or TFT ( Active Matrix) : significant differences are outlined in the following table:

Viewing Angle Contrast Slew Rate
DSNT 49º a 100º 40:1 300 milliseconds
TFT Más de 140º 140:1 25 milliseconds

Most desktop LCD monitors today use TFT technology, why have such price, ( but do not forget to check this fact ) but increased sales of such monitors is becoming more lowering its price.

The world of laptops , however , is showing a drop of more timid prices, but this is due to the imperative and increasingly sought miniaturization of the screens (in terms of thickness ) which makes the difference price between the two types of display is still important.


Clearly the monitor is a very important part in a computer , since it is he who usually provides the output data .

As we said above the speed of evolution of these devices is much lower than the other , so it is worth investing our money in a good monitor that can change computers without changing the monitor. In addition , with the passage of time our eyes will thank us .

Undoubtedly, we must also look at the use of it will, because it would be useless to buy the best monitor it on a server , which is hardly used . Therefore we choose the monitor whose characteristics are suited to our needs.


· CRT: Cathode ray tube .

· DB – IS : Connector marked by IBM, standard monitors.

· RGB: Monitor that accepts different signals of green , red and blue

· LCD : Liquid Crystal Display .

· Hz refresh rate per second

· Pixel : Dot

· AGP : Graphics card

· Buffer: System audio output

· Bitmap : bitmap

· Pixmap : pixmap

· DVST : Tube direct view storage

CPU: Central Processing Unit

· CAD : Autocad , drawing program

· Scanner : O Device that allows transferring printed text to Word

· LED: Light Emitting Diode

· DSTN : Dual Scan passive matrix ( LCD type )

· TFT : Active Matrix ( LCD type )

· IPS : Technology that increases the viewing angle on the LCD

· HDP : passive hybrid screen

· FED : Field Emission Display

· LEP : polymeric matrix display dot ( red, green and blue )

· VLF – ELF : Electromagnetic field that creates the power system

· CGA : They are the first graphics monitors with a resolution of 200×400 to 400×600 . They are 4 colors maximum

· EGA : Color Monitors , 16 Max colors or gray tones , with resolutions of 400×600 , 600×800 .

· VGA : Monitor colors to 32 bit true color or grayscale , graze 600×800 , 800×1200 .

· SVGA : Known as super VGA resolution and increases the number of colors a64 32 bit true color , 600×400 to 1600×1800 .

· UVGA : No super VGA varies widely , only increases the resolution 1800×2000 .

· XGA : These are high-resolution monitors , special for design, graphics capability is very good, plus the number of colors is greater .


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