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TFT LCD

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A flat panel computer display

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. [1]

Construction

A diagram of the pixel layout

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

TN display at 300x

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 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 many people and bothersome to some.[2] 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,[3] 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 that of CRTs?, the dynamic range is lower but this was improved over the years. Fringe Field Switching is a technique used to improve viewing angle and transmittance on IPS displays.[4] IPS technology is widely used in panel sizes of monitor 20"~30" and LCD TV 17"~52".

IPS technology uses an electrode configuration in which the electric field used to polarize the optical director is applied parallel to the glass plates of the LCD. The hemispherical viewing angle is not a consequence of the particular switching technology, but arises from the fact that the optical axis of the material used is in the plane of the cell, or pixel, rather than perpindicular. IPS differs from other technologies that utilize a material with an in-plane optical axis in that the field is applied in plane as well, rather than perpindicular to the glass plates.

Hitachi IPS evolving technology [5]
Name Nickname Year Advantage Transmittance /
Contrast ratio 		Remarks
Super TFT IPS 1996 Wide viewing angle 100 / 100
Base level		 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.
Super-IPS S-IPS 1998 Color shift free 100 / 137 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.
Advanced Super-IPS AS-IPS 2002 High transmittance 130 / 250 AS-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.
IPS-Provectus IPS-Pro 2004- High contrast ratio 137 / 313 The latest panel from IPS Alpha Technology with a wider color gamut and contrast ratio matching PVA and ASV displays without off-angle glowing.
LG IPS evolving technology
Name Nickname Year Remarks
Super-IPS S-IPS 2001 LG.Philips remain as one of the main manufacturers of panels based on Hitachi Super-IPS.
Advanced Super-IPS AS-IPS 2005 Increased contrast ratio with better color gamut.
Horizontal IPS H-IPS 2007 Improves contrast ratio by twisting electrode plane layout. H-IPS is used in the NEC LCD2490WUXi and LCD2690WUXi, Mitsubishi RDT261W, HP LP2475w, Planar PX2611W,[6] and Apple's newest Aluminum 24" iMac. H-IPS up close.
  • Introduces an optional Advanced True White (A-TW) polarizing film from NEC, which results in a TW (True White) color to make white look more natural. This is used in professional/photography LCDs. One such monitor using this technology is the NEC LCD2690WUXi.
Enhanced IPS E-IPS 2009 A low-cost AS-IPS display with lowered aperture ratio to increased transmittance.
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

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 (Response Time Compensation) 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 (Frame Rate Control).

PVA

PVA (patterned vertical alignment) and S-PVA (super patterned vertical alignment) are alternative versions of MVA technology offered by Samsung's and Sony's joint venture S-LCD. 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. S-PVA also largely eliminated off angle glowing of solid blacks and reduced the off angle gamma shift. Some newer S-PVA panels offered by Eizo offer 16-bit color internally, which enables gamma and other corrections with reduced color banding. Some high end Sony Bravia LCD-TV offer 10bit and xvYCC color support. PVA and S-PVA offer the best black depth of any LCD type along with wide viewing angles. S-PVA also offers fast response times using modern CRT technologies.

ASV

ASV (Advanced Super View), also called Axially Symmetric Vertical Alignment was developed by Sharp, it is a VA mode where LC molecules orient perpendicular to the substrates in the off state. The bottom sub-pixel has continuously covered electrodes, while the upper one has a smaller area electrode in the center of the subpixel.

When the field is on, the LC molecules start to tilt towards the center of the sub-pixels because of the electric field; As a result, a continuous pinwheel alignment (CPA) is formed; the azimuthal angle rotates 360 degrees continuously resulting in an excellent viewing angle. The ASV mode is also called CPA mode. [7]

Display industry

Due to the very high cost of building TFT factories, there are few major OEM panel vendors for large display panels. The glass panel suppliers are as follows:

LCD glass panel suppliers
Panel type Company Remarks
IPS-Pro Panasonic Solely for LCD TV markets and known as IPS Alpha Technology Ltd. [8]
H-IPS LG Display They also produce other type of TFT panels such as TN for OEM markets such as mobile, monitor, automotive, portable AV and industrial panels.
S-IPS Hannstar
Chuangwa Picture Tubes
A-MVA AU Optronics
S-MVA Chi Mei Optoelectronics
S-PVA Samsung/Sony
ASV Sharp Corporation Solely for LCD TV markets

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 a 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.

Electrical interface

External consumer display devices like a TFT LCD mostly use an analog VGA connection[citation needed], while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort. Inside external display devices there is a controller board that will convert CVBS, VGA, DVI, HDMI etc. into digital RGB at the native resolution of the display panel. In a laptop the graphics chip will directly produce a signal suitable for connection to the built-in TFT display. A control mechanism for the backlight is usually included on the same controller board.

The low level interface of STN, DSTN, or TFT display panels use either single ended TTL 5V signal for older displays or TTL 3.3V for slightly newer displays 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 transmits the same contents as the parallel interface (Hsync, Vsync, RGB) 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. Usually with 3 data signals and one clock line. Transmitting 3x7 bits for one clock cycle giving 18-bpp. An optional 4th signal enables 24-bpp.

Backlight intensity is usually controlled by varying a few volts DC, or generating a PWM signal, adjusting a potentiometer or simple fixed. This in turn controls a high-voltage (1.3 kV) DC-AC inverter or a matrix of LEDs.

The bare display panel will only accept a digital video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore colour LSB bits to present a consistent interface (8bit->6bit/colour).

The reason 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 from a quadrature amplitude modulation (QAM) to Luminance (Y), Blue-Y (U), Red-Y (V) representation which in turn is transformed into Red, Green Blue is needed.[citation needed]

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 and seek doctor's advice immediately. [9]

The leading [10] 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 [11]

See also

Wikimedia Commons has media related to LCD computer monitors.

References

  1. ^ LCD Panel Technology Explained
  2. ^ Oleg Artamonov (2004-10-26). "X-bit's Guide: Contemporary LCD Monitor Parameters and Characteristics (page 11)". xbitlabs.com. Retrieved 2009-08-05.
  3. ^ Marek Matuszczyk, Liquid crystals in displays. Chalmers University Sweden, ca. 2000.
  4. ^ "Fringe field switching mode LCD - Patent 6678027". 070927 freepatentsonline.com
  5. ^ IPS-Pro (Evolving IPS technology)
  6. ^ "Planar PX2611W review".
  7. ^ The World of Liquid Crystal Displays
  8. ^ IPS Alpha Technology Ltd
  9. ^ "Liquid crystal display module model: MTF-T057AMSLN-V1" (PDF). Microtips. 2007-09-25.
  10. ^ "Merck Celebrates 100 Years of Liquid Crystal Technology Innovation". 2004-05-28. Retrieved 2009-08-08. Today Merck KGaA has a global market share of 69% for all types of liquid crystals, and is the leading supplier for the LCD industry globally.
  11. ^ Becker, Simon-Hettich, Hoenicke (2002-09). "Toxicological and Ecotoxicological Investigations of Liquid Crystals; Disposal of LCDs" (PDF). Merck KGaA. Archived from the original (PDF; 640.4KiB) on 2007-12-21. Retrieved 2009-08-08. {{cite web}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
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