Technology Behind A LCD/TFT Monitor

A majority of the computer monitors in use are based on the liquid crystal technology today. Liquid crystals inside the screen are the secret behind the new, sleek flat monitors of today. Thus the name Liquid Crystal Display (LCD) monitors. These crystals play around with the properties of light to produce the picture on the screen. Traditionally developed for the notebook computers, owing to their small size and picture quality, these monitors have caught on with the desktop users too.

Working: The LCD comprises liquid crystals sandwiched between two finely grooved plastic surfaces. The grooves on one plastic surface are perpendicular to the grooves on the other. Enclosed in these plastic sheets is a special liquid made of rod shaped nematic molecules. These nematic molecules get aligned in the grooves of the plastic sheet and polarize the light passing through them.

When the light is polarized, the vibration of light is restricted only to one plane: i.e. either the vertical plane or the horizontal plane. The plastic sheets act as polarizing filters and allow light of only one wavelength to pass through them.

A second polarizing filter with lines arranged perpendicular to the first would therefore totally block this already polarized light. Light will pass through the second polarizer only if its lines were exactly parallel to the first one. However as it is not so, the orientation of the light has to be twisted by 90 degrees. And this is where the liquid crystals play their role. They twist the light by 90 degrees, thus allowing it to pass through the second filter. When an electric current is applied across these liquid crystals, the molecules get vertically realigned allowing the light to pass though, without twisting them. This will now block the light in the second filter. Thus if a voltage is present, the light will be blocked and vice-versa.

The level to which the crystals will polarize the light depends upon the voltage which is applied to them. In order to get an LCD display it is mandatory to supply current to selected areas on the screen. There are two types of LCD displays: DSTN displays and TFT displays.

DSTN displays: A normal passive matrix LCD or DSTN consists of a number of layers. The first layer is a sheet of glass coated with a transparent metal oxide. This operates as a grid of row and column electrodes through which the voltage required to activate the screen element passes. A polymer with a number of parallel grooves running across it is applied above this grid of electrodes. The grooves align the liquid crystal molecules in the appropriate direction. They also provide a base on which the crystal molecules are attached. Called the alignment layer, the latter is repeated on another glass plate. Spacer beads are placed between the two glass plates to maintain an uniform distance between the sheets.

Polarizing multicolored layers are applied to the outermost surfaces of each glass sheet to match the orientation of the alignment layers. A backlight is added in the form of cold-cathode fluorescent tubes mounted along the top and the bottom edges of the panel. Light originating from these is distributed across the panel using a plastic light guide or prism. The image that we see on the screen results from the interplay of light and voltage as the light traverses these different layers and is polarized. When there is no power across the LCD panel, the rear filter vertically polarizes backlight, which is refracted by the liquid crystal. Finally the light emerges from the horizontally polarized filter at the front. Upon applying voltage, the crystal is realigned and the light is blocked, thereby producing a dark pixel. Color LCD displays simply use additional red, green and blue colored filters over three separate LCD elements to create a single multicolored pixel.

Since the response time is very slow, these displays cannot keep up with the radically changing screen content such as video, which often appears smeared. Moreover, passive matrix driving causes ghosting, an effect where areas of ‘on’ pixels cause a shadow on the ‘off’ pixels in the same rows and columns.

TFT displays: In Thin Film Transistor (TFT) technology, every pixel holds a transistor. A small current is sent to the transistor through the horizontal and vertical grid, which is amplified on the screen. The pixel is switched on and the LCD panel is activated. There is one transistor for each color of each pixel.

When current traverses the grid, the pixel can be turned on and off faster, thereby increasing refresh rates in the display. While DSTN pixels are updated about 24 times a second, TFT pixels refresh about ten times faster. Thus increased speed translates into faster response. Which in turn means that your mouse pointer is not going to disappear if it is moved suddenly across the screen.

Because of the transistors, the TFT screens can be made much thinner than the DSTN screens. Also the refresh rates of TFT’s are much higher and approach those of CRT’s as the voltage runs about ten times faster on a TFT than on a DSTN screen.

However, TFT design requires fabrication of one transistor for each pixel. Putting those transistors in place requires a combination of the LCD and semiconductor manufacturing processes. To get around this problem, double scan passive displays were developed, where the screen is scanned twice in the time that it was scanned once. This is done by dividing the screen horizontally into two halves, with both the halves being scanned at the same time. This allows faster refresh rates and does away with extra brightness.

That's all about the latest technology used in LCD/TFT monitors.