Liquid crystal displays come in a variety of aspect ratios to fit specific applications. Aspect ratio is defined as the ratio of the horizontal dimension to the vertical dimension of the active area of a display and is typically expressed as:
Aspect Ratio = Horizontal Dimension : Vertical Dimension
The dimensions are usually not the real dimensions of the active area but are expressed in terms of the "least common denominator". For example conventional video displays and most computer displays have an aspect ratio of 4:3. The actual dimensions could be 4 inches by 3 inches or it could be 40 inches by 30 inches. In both cases, the aspect ratio is 4:3. Another aspect ratio that is common (especially for HDTV) is 16:9. For specialized applications other aspect ratios may be used. Examples would be displays designed to fit in "industry standard" size such as a "single DIN" size for the automotive industry.
The most common method for backlighting a display is the cold cathode fluorescent tube (CCFT) not unlike the fluorescent tubes used for office building lighting. They are narrow diameter tubes that are extraordinarily bright. These types of backlights are relatively easy to manufacture. They can be configured as an array directly behind the display or as (single, double, or L-shaped) edge lights.
LEDs have following features-
Small form factor
Extremely LowOperating Temperature
Brightness or Luminance is a measure of the amount of light output from the panel. It is typically measured in "candelas per square meter", also called a "nit".
Luminance = (1/p) candelas per meter2
CCFT’s are glass tubes with electrodes in both ends. The gas is pressurized Neon/Argon mixed with Mercury. Electrically excited mercury produces ultra-violet light. The UV radiation reacts with the phosphor coating which glows visible light.
CCFTs use high pressure -1 to 2 atmospheres. This creates a more excited reaction and greater light efficiencies. CCFT’s require a high voltage to start the current flowing from cathode to anode. Typically 1000+ VAC This voltage causes the mercury to ionize, releasing electrons from the molecules. Once ionized, the voltage drops to a nominal level, approximately 400-450 VAC. The higher the pressure, the higher the voltage required to strike. The creation of light is due to ionized Mercury emitting ultraviolet light that strikes the phosphor to produce visible light.
Mercury Depletion - The ionization of mercury will eventually exhaust the mercury, reducing the light output. (Below 10°C accelerates this process). Mercury absorbed by the glass also reduces the transparency of the glass.
Phosphor Depletion - Mercury is absorbed by the phosphor reducing efficiency (Below 10°C accelerates this process). Poor ignition of the bulb will also reduce phosphor life.
Electrode Depletion - Sputtering of the CCFT (firing voltages below kick-on point) will erode the electrodes. (Operation below 10°C accelerates this process).
Low Temperature Effects - Operating CCFT’s between 10°C and 50°C has no effect on life. Starting and operating a CCFT below 10°C greatly shortens life. At 0°C, a CCFT will last approximately 1000 hours.
Most LCD displays use a Cold Cathode Fluorescent Tube (CCFT) for a backlight. This is essentially a miniature version of the tube used for household lighting. These tubes typically require a 600-1100 volt AC driving source. This voltage is generally supplied by an inverter from a DC source. It is very important to select an inverter with proper electrical characteristics. The two critical parameters are the initial strike voltage and the run-time current.
The strike voltage is the maximum level of the minimum voltage required to assure the tube will always turn on. Once a tube begins to conduct, the voltage required to maintain the light drops dramatically. As a display ages, the strike voltage will generally go up. LCD Panel specification will show a minimum strike voltage and it is important to insure that the inverter can meet or exceed this value. It doesn't matter if the value is exceeded because the tube will self-regulate and cut off the voltage as soon as it strikes.
Most inverters are designed as constant current devices that will adjust the voltage to supply the required current. The constant current level must match the display. If an inverter with higher current than the rated value is used, the tube will operate, but the tube life will drop dramatically.
Its important to operate an inverter within the total power range for which it is specified. Inverters are designed for short term excessive power output in order to satisfy the initial strike voltage requirement, but cannot deliver this power level on a continuing basis. For example, a 3 watt inverter will actually drive a 6 watt backlight, but the inverter will quickly burn out or shut down automatically via a self-contained thermal protection device. Depending on the amount of stress, these early failures may occur after several months of operation.
Current Limiting -By turning down the output current to the display, it will dim. The output voltage will increase with dimming, so it is limited by the range of inverter (lower the current, higher the voltage)
Pulse Width Modulation - Pulsing the input voltage to the inverter will lower the output. Frequency of pulse width modulation may cause flicker and/or failure to ignite bulb.
Voltage Limitation - Turning down input voltage will lower inverter output. Wide ranging input inverters will prevent the use of this technique.
Cleaning the Polarizer
Blow off dust with a blower for which static electricity preventive measures have been taken, like an ionized air gun.
Polarizer is vulnerable therefore its recommended to wipe it carefully with a lens cleaning cloth. If metal parts of the TFT-LCD module (shielding lid and rear case) are soiled, wipe them with a soft, dry cloth.
Wipe off liquid immediately since it can cause color changes and staining.
Oils contaminate electronics and will smear the LCD surface.
Use 90% Isopropyl Alcohol or Ligroin (Petroleum Ether) for cleaning.
Use a lint free, soft cloth.
Do not use ammonia-based window cleaner.
Do not use paper towels
Color consists of three characteristics:
Luminance (intensity), Hue, and Saturation.
Color luminance is the wavelength (spectral radiance) and it’s conversion to luminance values.
Hue is the dominant wavelength of color as subjectively perceived by the human eye
Saturation is the degree to which the hue of the color is undiluted by its complimentary color to form white
Uniformity (or the lack thereof), is the gradual change of brightness (luminance) and/or color (chrominance) from one area of a display to another
Color depth is a measure of the total number of colors an LCD can display. Color depth can be specified as the total number of colors (e.g. 256 K or 262,144 colors) or as the number of digital "bits" used to generate a color (e.g. 18 bits - 6 bits/color or 18 bits/pixel).
Plasma display are large area flat panel displays and due to the nature of the process used to create plasma panels, it possible to make large size arrays of light emitting elements. Earlier only monochrome and now full color Plasma Displays are steadily produced in size range of approximately 30" to 60” diagonally.
Plasma Display Panels (PDPs) work by creating a plasma unlike that in a fluorescent tube. The plasma generates ultra-violet light that strikes a phosphor and causes the phosphor to give off its characteristic color light (red, green, or blue). The luminant efficiency, while not extraordinarily high, is good enough to create a fine looking display.
Large diagonal sizes.
Emits light that supports wide viewing angle emitter.
Fast Response Time as CRTs.
Better Colour Gamut than even CRTs
As a result, PDPs have gained acceptance in applications ranging from home theater to dynamic advertising signage. We are now at the point where we are going to see the emergence of LCDs in sizes up to 42" (diagonal) competing with plasma panels. LCDs will clearly gain market share with their increasingly favorable cost structures, intrinsic reliability and elimination of burn-in effects.
Image burn-in - like every light emitter, the efficiency of emission for PDPs diminishes over time. So if it continually shows the same image to one area, it eventually becomes "burnt in". And, because the human eye can see fourteen orders of magnitude of brightness, it is possible to see the very subtle differences of the burn-in effect.This is an intrinsic property of all plasma panels. Showing continuous video, is not as much of an issue as with graphics. But even for home theater or HDTV where you show a logo such as the Channel Logo, it will not take very long to be burnt in - typically 20,000 hours.
Another problem with plasma displays is cooling because of less efficiency. Typical ventilation with a fax is recommended. This restricts the mounting options of a plasma display. It can't be easily embedded into a wall or an enclosure unless provisions are made for cooling.
The fast response time of plasma displays sometimes will cause visible flicker that is visible to viewers too.
Plasma displays do not have the dynamic contrast range to show highlights and features that are now possible with LCDs.
The CRT are tough to replace. It is practically impossible to fully replace CRT because of their intrinsically simple reliable design. Their limitations are-
• Bulky size and heavy weight
• They create light that is Lambertian
• High Power consumption
• Fast Response times
• Not a Digital devices.
• Relatively easy to manufacture
• Washes out in bright ambient light
• Images are easy to scale since they are analog.
• Operates predictably in any temperature range
• Viewing angle uniformity
• Crisp images
• Slower response times
• Intrinsically Digital
• Limited Operating Temperature range
• Thin package
• Low Power consumption
• Suitable for mobile applications
An LCD is intrinsically a Digital device and produces images that are sharp and crisp. There are no issues of linearity where it is required to correct for pincushion.
CRTs respond very fast and have response time in microseconds. The phosphors with a decay time long enough results in non-visible flickers.
LCDs are increasing the color saturation to approach (and even surpass) the high-performance CRTs.
The obvious advantages of LCDs are that they can be made very thin, use considerably less power, and can be made very lightweight. They are usable in applications that are impossible for CRTs because of their bulk.
Another advantage is that LCDs reflect far less light where as the shadow mask of a CRT is an intrinsic reflector. LCD reflect much less light in areas with high ambient light level, so it is better for applications such as ATM machines or kiosks where it is very difficult to protect a CRT from direct sunlight without turning up the brightness to extreme levels. LCDs can be made reflective or transflective to perform much better in these situations.
Environmentally, LCDs are the clear winner because of their significantly reduced power consumption. While there have been attempts to recycle CRTs, the overall benefits of LCDs in terms of energy use and the materials they are made from leads to much better environmental solutions in the long term.
While CMOS and TTL display interfaces are old interfaces they are still in use today, whereas Low Voltage Differential Signaling (LVDS) or LVDS variants are relatively newer interfaces; being used — especially for larger displays. This provides the advantage of being able to send signals over a relatively long distance through a cable. The disadvantage is that the cable is more difficult to fabricate and a transmitter-receiver chip is required on both ends.
Another set of interfaces exist for video applications where video interfaces such as NTSC, PAL and RGB are built into the display. These are quite different from conventional graphics interfaces.
The DVI interface was authored by the Digital Display Working Group (DDWG), formed in September of 1998 to provide an open IP, royalty free-environment for digital display standardization. The DDWG is lead by Intel, Silicon Image, Compaq, Fujitsu, HP, IBM and NEC.
The standard provides digital-only and digital/analog connector formats similar to the Plug & Display connectors.
In reflective mode, an external source of light is impingent on an LCD and then reflected from a surface behind the cells. The intensity of the reflected light is modulated by the cells. There is no light source within the LCD structure.
In transmissive mode, the source of light is contained within the LCD structure behind the cells. The intensity of the transmitted light is modulated by the cells.
In transflective mode, there are two sources of light. One is contained within the LCD structure behind the cells and the other is reflected light. The resulting intensity is a combination of the two sources of light modulated by the cells.
IPS Technology from Hitachi uses pairs of electrodes on the sides of each cell with a horizontal electric field through the material. The liquid crystals are parallel to the front of the panel for an increased viewing angle.
Third Party A/D Boards
A third-party A/D board which takes the video output from a computer (that is intended for a CRT) and translates the signals into LCD flat panel compatible signals. These interfaces are the simplest because they use existing solutions that are commonly available for computers. These solutions are often used to replace an existing CRT monitor with an LCD. These offerings often include cable solutions and power supplies so it is almost a plug-and-play approach.
The next level of interface is a single-board computer with an LCD controller built-in. These are often available in the form of kits with all of the bits and pieces included. The difference from the first approach is that the controller is embedded with a processor which must be included. The advantage of this approach is the tight integration all on a single board.
The previous levels of interface are basically self-contained solutions that do not require extensive design expertise. The next level of interface requires design. It is appropriate for the customer who is already developing their own custom CPU board and wants to integrate a flat panel controller such as that which is available from Chips & Technology or Epson. The advantage is that the components are much less expensive. This approach requires software expertise to write (or integrate) the required drivers for the operating system.
The lowest level of integration is the TFT-specific interface. This is often used for small displays where space and power are at a premium. In this case both the controller solution and the interface electronics, and power must be provided. This is the most difficult approach requiring a significant amount of engineering and development time and skill. This is also more difficult to manufacture because many of these interfaces have to be individually tuned.
Active matrix displays can be further divided into three categories:
Amorphous Silicon TFTs
Continuous Grain (CG) Silicon TFTs (see CG-silicon Technology)
Amorphous silicon semiconductor materials are deposited onto ordinary glass. In doing so, it is possible to build arrays of Field Effect Transistors (FETs) connected together with row and column buss bars. By applying a signal to the column, and applying a signal to a row, its possible to activate the intersecting transistor and apply a field to a liquid crystal material. By sequentially activating row by row as the signals to the columns are changed, it is possible to individually control each element in a rectangular display area. The drain pad of each transistor is connected to a LCD with a color filter that can switch red, green, and blue light. This comprises an amorphous silicon TFT.
But to drive those columns and rows, one must use a higher performance semiconductor material than you can put on the glass itself. If looked microscopically at the array on the glass, the deposited silicon material has very small crystalline domains that are randomly aligned in a lattice. (This is referred to as amorphous silicon). This material has relatively poor semiconductor performance, so it needs an LCD driver that is typically built on a separate piece of silicon and is attached to transparent electrodes through a Tape Automated Bonding (TAB) circuit. The TAB circuit is attached to the transparent electrode (indium-tin oxide) through an anisotropic conductive film (ACF). The TAB bonding process provides the path to send the controlling data signals to the array on the glass.
High Temperature Polysilicon
With polysilicon displays, the objective is to put the drive electronics – particularly the drivers and the shift registers – onto the same piece of glass. This can be done with high-temperature processes on a piece of fused silicon. With a single piece of quartz (fused silicon), a uniform lattice can be made with superior semiconductor characteristics. The methodology is to first deposit the silicon layer, and then heat it up to the point where it melts and slowly re-crystallizes, making semi-conductors with much higher performance than amorphous silicon TFTs.
This process can be used to make very high performance devices. It is used today in high performance video projectors.
It is not possible to make a large display using this high temperature process, because its only possible make a quartz substrate up to about eight inches diagonal.
By replacing the high temperature annealing process with a laser annealing process (which is low temperature) you get domains larger than amorphous silicon, but not as large as the ordinary high temperature annealing process. This gives semiconductor performance (electron mobility) that are 200 times faster than amorphous silicon TFTs. Now its possible to place the drivers and shift registers on the same substrate as the LCD cells. This approach allows very high resolution displays where the density of interconnects goes beyond that which is possible with TAB structures. For example, this process can produce VGA displays in a 4" format.
LTPS technology has evolved in two different forms. The first is known as n-channel version, but it has limitations due to electron mobility. It is the most commonly available LTPS technology that has been in production the longest. Its possible to produce quarter-VGA or smaller formats. These are used for camcorder and digital still camera displays.
The second form of LTPS uses both n-channel and p-channel CMOS requiring many more layers (10-12 vs. 6-7) which is far more complex. This allows XGA resolutions in small formats. It is difficult to get this process to the cost point required for highly mobile applications such as PDAs and cell phones.
CG-silicon is a next generation technology and is a variant of the LTPS process using laser annealing to get larger domains. In LTPS, the electron mobility is impaired when moving from domain to domain. The advancement of CG-silicon has seen the development of ways to make these domain boundaries less of an impediment through some additional processing steps which triples the carrier mobility (performance) of CG-silicon relative to LTPS and is 600 times the performance of amorphous silicon.
The result is that high resolution devices can be made in small formats with far fewer layers. CG-silicon has a cost structure much like the n-channel process. These advances allow more system devices on the panel. This process eliminates the weakest element in an ordinary display that is the Tape Automated Bonding (TAB) bond. Among the many benefits of CG-silicon are far higher transmissivity and contrast ratios.
OLEDs are that just like an LCD but they must be used in conjunction with an active matrix array except for the very simplest character type displays. To get sufficient current to drive an OLED array, a low temperature polysilicon process is used. OLEDs are not very efficient, so when multiplexed, they have to be driven with relatively high currents to get an adequate amount of light.
Since it is a light emitter, it creates light that is Lambertian so it can be seen uniformly at all angles and gives a very pleasing effect.
The biggest strength of OLEDs is that they do not require a backlight and can be made thinner than all other technology used today. A 2 mm thick OLED is a reality today where as the thinnest LCD is 3 mm.
Dynamic display efficiency. While writing only few lines of static text is greatest efficiency, playing videos requires more power than an LCD. OLEDs are more efficient for small graphics or text because they only consume power in the area where they are addressed.
Till date, these are not as reliable as LCDs.
It is particularly difficult to drive the blue colors where the luminance efficiency is very low. As a consequence, the lifetime is reduced, and burn-in is also an issue.
OLEDs has limitations in terms of market. They work best at low temperatures and are prone to failure at higher temperatures. Since its required to control burn-in effects, the driving levels are important. As the high light level is required in high ambient light, there is considerable risk of burn-in. In general, they are not used in highly mobile applications, and it is unlikely that we will see these devices in automobiles and PDAs and other applications that are better matched to transflective LCDs, because of the expectations of operability in all lighting ambients.
Pixel pitch is the physical distance between the pixels (picture elements) in a display device. If it’s not given in the specifications, it’s the display’s Active Area divided by the number of pixels. Pixel pitch can affect view-ability when viewing large displays up close because in some displays (plasma, more so than LCD) the pixels do not blend well and spaces between them can be discerned. It’s for this reason that giant LED displays look beautiful from a distance but get uncomfortable to view from close proximity.
Pixel pitch in larger plasma display consume more current due to their size and therefore need for larger power supply resulting more heating. In case of smaller plasma panels, pixels become size-limited due to the lower light output of the smaller elements.
Large plasma displays can be uncomfortable to watch up close due to the large amount of space between the pixels; and when the viewer is close enough to be able to distinguish these individual pixels, the inherent qualities of the display begin to be lost – because the bright emitters and the dark spaces in between them can be distinguished easily.
LCDs are made in all sizes with a much narrower pixel pitch, because the sub-pixels are much closer together, due to miniaturized circuit runs between the switching elements. CRTs can have extremely small pixel pitches because the phosphor dot arrangements are separated by only a few microns of space on the face of the CRT. Monochrome CRTs are best for high-resolution displays, because its literally possible to address each individual molecule of the screen phosphor.
The interface signals of a LCD panel can divided in following categories-
Control signals are largely common to all LCD panels; however, they may also differ by type and manufacturer.
ENAB: Enable signal
R/L: Right or Left scan direction select
U/D: Up or Down scan direction select
V/Q: VGA or QVGA select
The Enable signal enables the LCD; and so must be HIGH to enable the LCD display. Furthermore, some control signals are specific to certain LCDs. For Example for LCD panels R/L, U/D, V/Q signals must be tied to VCC or GND; they affect the image presentation (such as mirror imaging).
These signals are for shifting RGB data into the LCD panel.
Horizontal Synchronization signals (HSync): There are ‘N’ number of CK periods per Hsync period where ‘N’ is the number of RGB pixels in a line. This signal marks the point at which the current line ends, and the new line begins.
Vertical Synchronization signals (VSync): There are ‘M’ number of Hsync periods per Vsync period where ‘M’ is the number of vertical lines in the display. This signal marks the point at which the current frame ends, and the new frame begins.
CK is the LCD’s clock source, a continuous square-wave. This frequency controls the flickering in a display. The Horizontal Synchronization signal is a pulse that is activated when one line of data has been transmitted to the LCD. The Vertical Synchronization signal is a pulse that is activated when one page (or frame) of data has been transmitted to the LCD. Correct clock frequency and correct Synchronization signal polarities will result in an appropriate image on the LCD, otherwise the image will exhibit twisting or flickering.
There are 18 or 24 data signals in total. They correspond to the LCD controller These signals are the LCD image data; they are active between the Horizontal Synchronization (Hsync) and Vertical Synchronization (VSync) pulses.
A Standard LCD panel requires VCC (3.3 V or 5 V) and GND.
Standard example of a LCD Panel Interface:
The Resolution of a display means the number of distinct pixels that can be displayed in each dimension (width & height).
For Example- 1920x1080 means the width is 1920 pixels and height is 1080 pixels. This is commonly also spoken as Nineteen Twenty by Thousand Eighty.
In LCDs, the panel’s resolution is known as “native resolution” because the panel electronics cannot perform any interpolation to resize (or more accurately, redefine) an image for display. It’s up to the electronics (controller-board) driving this display to perform the resizing function. Therefore the processing quality of the display electronics (controller-board) can figure heavily in how any given signal looks on an LCD or plasma screen.
Some displays (typically DLP), can operate with great success at higher resolutions because through new firmware, they are able to not only re-scale any incoming signal, but also offer the enhancement of precisely dithering the reflected light from the DLP mirror array allowing them to resolve a higher definition image.