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TFT LCD
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A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin film transistor (TFT) technology to improve image quality (e.g. addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones and computers, handheld video game systems , personal digital assistants, navigation systems, projectors, etc.
Construction
Small liquid crystal displays as used in calculators and other devices have direct driven image elements—a voltage can be applied across one segment without interfering with other segments of the display.
Discussion
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Encyclopedia
A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin film transistor (TFT) technology to improve image quality (e.g. addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones and computers, handheld video game systems , personal digital assistants, navigation systems, projectors, etc.
Construction
Small liquid crystal displays as used in calculators and other devices 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 prevents the voltage applied to the pixel from leaking away between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.
The circuit layout of a TFT-LCD is very similar to that of a DRAM memory. However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow 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 (frequently LTPS, low-temperature poly-Si) is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors are the best performers.
Types
TN
The inexpensive TN (twisted nematic) display is the most common consumer display type. The pixel response time on modern TN panels is sufficiently fast to avoid the shadow-trail and ghosting artifacts of earlier production. The fast response time has been emphasised in advertising TN displays, although in most cases this number does not reflect performance across the entire range of possible color transitions. Response times were quoted for an ISO standard black-to-white transition and did not reflect the speed of much more common transitions from one shade of grey to another. More recent use of RTC (Response Time Compensation—Overdrive) technologies has allowed manufacturers to significantly reduce grey-to-grey (G2G) transitions, without significantly improving the ISO response time. Response times are now quoted in G2G figures, with 4ms and 2ms now being commonplace for TN based models. The good response time and low cost has led to the dominance of TN in the consumer market.
TN displays suffer from limited viewing angles, especially in the vertical direction. Also, TN panels represent colors using only 6 bits per color, instead of 8, and thus are not able to display the 16.7 million color shades (24-bit truecolor) that are available from modern graphics cards. Instead, these panels display interpolated 24-bit color using a dithering method that combines adjacent pixels to simulate the desired shade. They can also use Frame Rate Control (FRC), which cycles pixels on and off to simulate a given shade. These color simulation methods are noticeable to most people and bothersome to some . FRC tends to be most noticeable in darker tones, while dithering appears to make the individual pixels of the LCD 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) are also due 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 transmittance of a pixel of an LCD panel typically does not change linearly with the applied voltage, and the sRGB standard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of the RGB value.
IPS
IPS (in-plane switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Most panels also support true 8-bit per channel color. These improvements came at the cost of a slower response time, initially about 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 dynamic range is lower. S-IPS technology is widely used in panel sizes of 20" and above. LG.Philips remain as 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. One such monitor to use this technology is the NEC LCD2690WUXi.
- H-IPS: Released in late 2006, it improves upon its predecessor, the S-IPS panel. The H-IPS panel is used in the NEC LCD2490WUXi and LCD2690WUXi, Mitsubishi RDT261W, HP LP2475w, Planar PX2611W, and Apple's newest Aluminum 24" iMac.
Fringe Field Switching is a technique used to improve viewing angle and transmittance on IPS displays.
MVA
MVA (multi-domain vertical alignment) was originally developed in 1998 by Fujitsu as a compromise between TN and IPS. It achieved pixel response which was fast for its 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 color reproduction and depth, and fast response times due 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 dominate the mainstream market, but the less expensive and slightly faster TN overtook it. The pixel response times of MVAs rise dramatically with small changes in brightness. Less expensive MVA panels can use dithering and FRC.
PVA
PVA (patterned vertical alignment) and S-PVA (super patterned vertical alignment) are alternative versions of MVA technology offered by Samsung. Developed independently, they offer similar features to MVA, but with higher contrast ratios of up to 3000:1. Less expensive PVA panels often use dithering and FRC, while S-PVA panels all use at least 8 bits per color component and do not use color simulation methods. Some newer S-PVA panels offered by Eizo offer 16-bit color internally, which enables gamma and other corrections with reduced color banding. PVA and S-PVA offer good black depth and wide viewing angles and S-PVA also offers fast response times using modern CRT technologies.
CPA
CPA (Continuous Pinwheel Alignment) was developed by Sharp.
Electrical interface
External consumer display devices like a TFT LCD mostly use an analog VGA connection, while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort.
Inside an external display device there is a controller board that will convert VGA, DVI, HDMI, CVBS etc. to digital RGB at native resolution that the display panel can make use of. In a laptop the graphics chip will directly produce a signal suitable for connection to the builtin TFT. A control mechanism for the backlight is usually included on the same controller board.
The lowlevel interface of STN, DSTN, or TFT display panels use either single ended TTL 5V or TTL 3.3V that transmits Pixel clock, Horizontal sync, Vertical sync, Digital red, Digital green, Digital blue in parallel. Some models also feature input/display enable, horizontal scan direction and vertical scan direction signals.
New and large (>15") TFT displays often use LVDS or TMDS signaling that is the same as the parallel interface but will put control and RGB bits into a number of serial transmission lines synchronized to a clock at 1/3 of the data bitrate.
Backlight intensity is usually controlled by varying a few volts DC to the backlight highvoltage (1.3kV) DC-AC inverter. It can also be controlled by a potentiometer or be fixed. Some models use PWM signal for intensity control.
The bare display panel will only accept a video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore colour LSB bits to ease interfacing (8bit->6bit/colour).
The factors why laptop displays can't be reused directly with an ordinary computer graphics card or as a television, is mainly because it lacks a hardware rescaler (often using some discrete cosine transform) that can resize the image to fit the native resolution of the display panel. With analogue signals like VGA the display controller also needs to perform a highspeed analog to digital conversion. With digital input signals like DVI or HDMI some simple bitstuffing is needed before feeding it to the rescaler if input resolution doesn't match the display panel resolution. For CVBS or "TV" usage a tuner and colour decode and transform is needed as well.
Safety
The liquid crystals inside the display are poisonous and must not be ingested or brought into contact with skin. Spills from a cracked display should be washed off immediately with soap and water.
The leading manufacturer of liquid crystal materials for display applications states as follows:Merck KGaA has committed itself to not introduce into the market liquid crystal materials which are either acutely toxic or mutagenic.
The complete report "Toxicological and Ecotoxicological Investigations of Liquid Crystals; Disposal of LCDs" is available from Merck KGaA
Display industry
Due to the very high cost of building TFT factories, there are few major OEM panel vendors for large display panels. The top seven glass panel suppliers are as follows:
- Samsung
- LG Display
- AU Optronics
- Chi Mei Optoelectronics
- Sharp Corporation
- Chuangwa Picture Tubes
- Hannstar
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 (often marked with letter P or S after their type number).
Value TFT screens and most 15 inch (381 mm) sized LCDs usually lack a digital input like DVI connector, so their future proofing may be limited. Most displays larger than 17 inch (432 mm) have both a VGA analog input and a DVI digital input sockets. Almost all professional screens include a DVI socket and some also include a pivot mode for portrait-mode display.
See also
External links
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- , X-bit labs, December 20 2005
- , X-bit labs, October 26 2004
- , Digital Silence, August 10 2004
- , Plasma.com – detailed description of the technology inside a TFT LCD
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