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A 15" TFT-LCD

TFT-LCD (Thin Film Transistor-Liquid Crystal Display) is a variant of Liquid Crystal Display (LCD) which uses Thin-Film Transistor (TFT) technology to improve image quality. TFT LCD is one type of active matrix LCD, though it is usually synonymous with LCD. It is used in both flat panel displays and projectors. In computing, TFT monitors are rapidly displacing competing CRT technology, and are commonly available in sizes from 12 to 30 inches. As of 2006, they have also made inroads on the television market.

A diagram of the pixel layout

Normal Liquid Crystal Displays like those found in calculators have direct driven image elements – a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of picture elements (pixels), since it would require millions of connections - top and bottom connections for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor also means that the voltage applied to the pixel does not leak away between refreshes to the display image. Each pixel is a small capacitor with a transparent ITO layer at the front, a transparent layer at the back, and a layer of insulating liquid crystal between.

The circuit layout of a TFT-LCD is very similar to the one used in a DRAM memory. However, rather than building the transistors out of silicon which has been formed into a crystalline wafer, they are fabricated from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel, and the silicon film is etched away in the remaining areas, allowing light to pass through.

The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon is also used in some displays where higher performance is needed from the TFTs, typically in very high resolution displays or ones where performing some data processing on the display itself is desirable. Both amorphous and polycrystalline silicon TFTs have very poor performance compared with transistors fabricated from single-crystal silicon.

The 'TN (Twisted Nematic) + Film' display is the most common consumer display type, due to its low production cost and wide development. The pixel response time on modern TN panels is sufficiently fast to most users to avoid the shadow-trail and ghosting artifacts that were a cause for complaint in the past. This fast response time has been a heavily marketed aspect of TN displays, although in most cases this number does not reflect performance across the entire range of possible color transitions. Traditional response times were quoted as an ISO standard black > white transition and did not reflect the speed of transitions across grey tones (a much more common transition for liquid crystals to make in practice). Modern use of RTC (Response Time Compensation - Overdrive) technologies has allowed manufacturers to significantly reduce grey to grey (G2G) transitions, while the ISO response time remains pretty much unchanged. Response times are now quoted in G2G figures, with 4ms and 2ms now being commonplace for TN Film based models. This marketing strategy, combined with the relatively lower cost of production for TN panels, has led to the dominance of TN in the consumer market.

The TN display suffers from limited viewing angles, especially in the vertical direction, and most are unable to display the full 16.7 million colors (24-bit truecolor) available from modern graphics cards. These particular panels, with 6 bits per color channel as opposed to 8, can approach 24-bit color using a dithering method which combines adjacent pixels to simulate the desired shade. They can also use FRC (Frame Rate Control), the less conspicuous of the two. FRC quickly cycles pixels over time to simulate a given shade. These color simulation methods are noticeable to most people and discomforting for some. FRC tends to be most noticeable in darker tones. Dithering has the tendency to appear as if the individual pixels of the LCD were actually visible. Overall, color reproduction and linearity on TN panels is poor. Shortcomings in display color gamut (often referred to as a percentage of the NTSC 1953 color gamut) can also be attributed to backlighting technology. It is not uncommon for displays with CCFL (Cold Cathode Fluorescent Lamps) based lighting to range from 40% to 76% of the NTSC color gamut, whereas displays utilizing white LED backlights may extend past 100% of the NTSC color gamut - a difference quite perceivable by the human eye.

The number of bits (6 or 8) is often emphasized, but with LCD displays, the transmittance of a pixel is typically not linear with the applied voltage^[1], and even if it were, the sRGB standard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of the RGB value. From the curve in Ref. ^[1] it can be derived that especially for accurate display of low (dark) RGB values even an 8-bit D-A converter is not sufficient to display 8 bits per color channel with sufficient resolution without dithering.

IPS (In-Plane Switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Most also support true 8-bit color. These improvements came at a loss of response time, which was initially on the order of 50ms. IPS panels were also extremely expensive.

IPS has since been superseded by S-IPS (Super-IPS, Hitachi in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing. Though color reproduction approaches that of CRTs, the contrast ratio remains relatively weak. S-IPS technology is widely used in panel sizes of 20" and above. LG and Philips remain one of the main manufacturers of S-IPS based panels.

AS-IPS - Advanced Super IPS, also developed by Hitachi in 2002, improves substantially on the contrast ratio of traditional S-IPS panels to the point where they are second only to some S-PVAs. AS-IPS is also a term used for NEC displays (e.g. NEC LCD20WGX2) based on S-IPS technology, in this case, developed by LG.Philips.

A-TW-IPS - Advanced True White IPS, developed by LG.Philips LCD for NEC, is a custom S-IPS panel with a TW (True White) color filter to make white look more natural and to increase color gamut. This is used in professional/photography LCDs.

Image of a (switched on) transreflective color TFT LCD taken under a microscope with reflected light illumination lamp off (top, self-illumination only) and on (bottom).

MVA (Multi-domain Vertical Alignment) was originally developed in 1998 by Fujitsu as a compromise between TN and IPS. It achieved fast pixel response (at the time), wide viewing angles, and high contrast at the cost of brightness and color reproduction. Modern MVA panels can offer wide viewing angles (second only to S-IPS technology), good black depth, good colour reproduction and depth, and fast response times thanks to the use of RTC technologies. There are several "next generation" technologies based on MVA, including AU Optronics' P-MVA and A-MVA, as well as Chi Mei Optoelectronics' S-MVA.

Analysts predicted that MVA would corner the mainstream market, but instead, TN has risen to dominance. A contributing factor was the higher cost of MVA, along with its slower pixel response (which rises dramatically with small changes in brightness). Cheaper MVA panels can also use dithering/FRC.

PVA (Patterned Vertical Alignment) and S-PVA (Super Patterned Vertical Alignment) are alternative versions of MVA technology offered by Samsung. Developed independently, it suffers from the same problems as MVA, but boasts very high contrast ratios such as 3000:1. Value-oriented PVA panels also use dithering/FRC. S-PVA panels all use true 8-bit color electronics and do not use any color simulation methods. PVA and S-PVA can offer good black depth, wide viewing angles and S-PVA can offer additionally fast response times thanks to modern RTC technologies.

Due to the immense cost of building TFT factories, the number of major OEM panel vendors is small. Several of the most well-known are Sharp Corporation, Samsung Electronics, Samsung SDI, LG.Philips LCD and AU Optronics. As of April 2006, the three biggest manufacturers are LG, Philips, and Samsung with 22% of the market, followed by AU Optronics with a 19% share.

Raw LCD TFT panels are usually factory-sorted into three categories, with regard to the number of dead pixels, backlight evenness and general product quality. Additionally, there may be up to +/- 2ms maximum response time differences between individual panels that came off the same assembly line on the same day. The poorest-performing screens are then sold to no-name vendors or used in "value" TFT monitors (often marked with letter V behind the type number), the medium performers are incorporated in gamer-oriented or home office bound TFT displays (sometimes marked with the capital letter S), and the best screens are usually reserved for use in "professional" grade TFT monitors (usually marked with letter P or S after their type number).

Value TFT screens and most 15 inch sized LCDs usually fail to include a digital signal compatible DVI interface, thus their future proofing may be limited. Most of 17 inch or 19 inch gamer and office TFT screens have dual analog-VGA and DVI sockets; almost all professional screens have DVI and pivot mode for letter-mode display.

• TFT specifications explained (TFTCentral.co.uk)
• LCD Panels with Response Time Compensation, xbitlabs.com, 12/20/2005
• Contemporary LCD Monitor Parameters and Characteristics, xbitlabs.com, 10/26/2004
• Gaming issues with TFT LCD Displays, digital silence, August 10, 2004
• TFT panel database / search tool
• What is TFT LCD - detailed description of the technology inside a TFT LCD at plasma.com.

• Liquid crystal display television
• Transreflective liquid crystal display, for adaptation to environment brightness