Character Display

Direct vs. multiplex driving


Segment drive method:

Used for simple displays, such as those in calculators


Dot-matrix drive method:

Used for high-resolution displays, such as those in portable computers and TFT monitors.



Two types of drive method


Simple driving method:

In the static, or direct, drive method, each pixel is individually wired to a driver.


Multiplex drive method:

As the number of pixels is increased, the wiring becomes very complex, we used this alternative method, in which the pixels are arranged and wired in a matrix format.



Passive and Active Matrix LCDs


Passive Matrix (AMLCDs) :

There are no switching devices, and each pixel is addressed for more than one frame time. The effective voltage applied to the LC must average the signal voltage pulses over several frame times, which results in a slow response time of greater than 150 msec and a reduction of the maximum contrast ratio.


Active-matrix LCDs (AMLCDs) :

A switching device and a storage capacitor are integrated at the each cross point of the electrodes. There are many kinds of AMLCD. For their integrated switching devices most use transistors made of deposited thin films, which are therefore called thin-film transistors (TFTs).
The most common semiconducting layer is made of amorphous silicon (a-Si).




Passive Matrix

• Row & Column approach
• Apply small bias to perpendicular lines of electrodes
• Bias strong enough to darken bit at line intersection
• Multiplexed addressing scheme
• Advantage: Simple to implement
• Disadvantage: Can cause distortion (‘ghosting’ or ‘crosstalk’)
  

Active Matrix

• Each cell has its own thin-film transistor (TFT)
• Addressed independently from behind LCD
• Direct addressing scheme
• Advantages: Sharp display, better viewing angle, 40:1 contrast
• Disadvantages: Need better backlight, complex hardware

LCD Structure

Backlight:

When the backlight shines through the liquid crystal display, the liquid crystals alternately block the light or let it through, which creates the image on the screen. For that reason, you'll need to beware of ultra-cheap LCD computer monitors because the backlights might not be evenly distributed, resulting in a brighter picture in only one spot on the monitor.

Polarizing Layers:

The light from the backlight passes through a polarizing layer, and it will pass through a second polarizing layer after passing through the rest of the display. The polarizing layers are arranged at a 90-degree angle, which means the light won't show through the screen unless the liquid crystals react to change the direction of the light so that it passes through the second polarizing layer.

Transistors:

A glass plate with transistors helps carry the electrical current to the liquid crystals, which prompts them to react and either block or show light.
Liquid Crystals: "LCD" stands for "liquid crystal display," and these crystals are the material that reacts to the electrical current, helping to create the image that you see. The crystals twist and untwist, and this action is what lets the light through.

Color Filter:

The crystals may block and unblock the backlight, but the screen is also composed of tiny filters-one red, one green and one blue-that create the colors you see. Each set of these three filters makes up a pixel. The transistors are associated with the pixels, and if a transistor ever burns out, the liquid crystal won't be able to react, which results in a "dead pixel."

A single-pixel LCD structure;

The sandwiched liquid crystal layer (typically on the order of 5 microns in thickness, or 1/20 of the width of human hair) is straddled by a pair of optical filters with orthogonal polarizations. When no voltage is applied across the crystal layer, incoming unpolarized light gets polarized as it passes through the entrance polarizer, then rotates by 900 as it follows the molecular spiral, and finally emerges from the exit polarizer, giving the exited surface a bright appearance. A useful feature of nematic liquid crystals is that their spiral untwists under the influence of electric field (induced by a voltage difference across the layer). The degree of untwisting depends on the strength of the electric field. With no spiral to rotate the wave polarization as the light travels through the crystal, the light polarization will be orthogonal to that of the exit polarizer, allowing no light passes through it. Hence, the pixel will exhibit a dark appearance.

LCD vs CRT

CRTs


Advantages


Color rendering:

The contrast ratios and depths of colors displayed were much greater with CRT monitors than LCDs. While this still holds true in most cases, many strides have been made in LCDs such that this difference is not as great as it once was. Many graphic designers still use the very expensive large CRT monitors in their work because of the color advantages. Of course, this color ability does degrade over time as the phosphors in the tube break down.


Easily scale to various resolutions:

This is referred to as multisync by the industry. By adjusting the electron beam in the tube, the screen can easily be adjusted downward to lower resolutions while keeping the picture clarity intact.


Disadvantages


Size and weight of the tubes:

An equivalent sized LCD monitor is upwards of 80% smaller in size and weight compared to a CRT tube. The larger the screen, the bigger the size difference.


Power consumption:

The energy needed for the electron beam means that the monitors consumer and generate a lot more heat than the LCD monitors.

Pros:

• Multisync Capable
• High Refresh Rates
• Color Clarity and Depth

Cons:

• Very Heavy and Large
• Use Large Amounts of Energy
• Generate Excess Heat

LCDs

Advantages

Size and weight:

As was mentioned earlier, the size and weight of an LCD monitor can be upwards of 80% lighter than an equivalent dimension CRT screen. This makes it possible to users to have larger screens for their computers than was possible before.

Produce less eye fatigue:

The constant light barrage and scan lines of a CRT tube tend to cause strain on heavy computer users. The lower intensity of the LCD monitors coupled with their constant screen display of pixels being on or off produces less fatigue for the user.

Disadvantages

Fixed or native resolution:

An LCD screen can only display the number of pixels in its matrix and no more or less. It can display a lower resolution in one of two ways. Using only a fraction of the total pixels on the display or through extrapolation. Extrapolation is a method whereby the monitor blends multiple pixels together to simulate a single smaller pixel. This can often lead to a blurry or fuzzy image particularly with text when running the screen below is native resolution.

Lower response times:

This has been overcome by many improvements, but there are some that still have low response times. Purchasers should be aware of this when purchasing a monitor. However, the improvements are often work arounds that can actually lead to another problem of reduced color clarity. Unfortunately, the industry is very poor about properly listing the specifications for monitors to help buyers understand and compare monitors.

Pros:

• Smaller and Lighter
• Energy Efficient
• Causes Less Eye Fatigue
  

Cons:

• Blurry Images Outside Native Resolution
• Motion Blur on Fast Moving Images
• Some Models Have Reduced Color Clarity

Basic Principle

1. Offline (no voltage is applied)

Molecules movement

• Along the upper plate : Point in direction 'a'



• Along the lower plate : Point in direction 'b‘
  


• Forcing the liquid crystals into a twisted structural arrangement. (Resultant force)

Light movement

• Light travels through the spacing of the molecular arrangement.
  
• The light also "twists" as it passes through the twisted liquid crystals.


• Light bends 90 degrees as it follows the twist of the molecules.


• Polarized light pass through the analyzer (lower polarizer).
  
  

2. Online (voltage is applied)

Molecules movement

• Liquid crystal molecules straighten out of their helix pattern


• Molecules rearrange themselves vertically (Along with the electric field)


• No twisting thoughout the movement


• Forcing the liquid crystals into a straight structural arrangement. (Electric force)

Light movement

• Twisted light passes straight through.


• Light passes straight through along the arrangement of molecules.
• Polarized light cannot pass through the lower analyzer (lower polarizer).


• Screen darkens.

Sequences of offline and online mode

Offline

1. Surrounding light is polarized on the upper plate.


2. Light moves along with liquid crystals and twisted at right angle.


3. Molecules and lights are parallel to the lower analyzer.


4. Light passes through the plate.


5. Screen appear transparent.

Online

1. Surrounding light is polarized on the upper plate.


2. Light moves along with liquid crystals which moves straight along the electric field.


3. Molecules and lights are perpendicular to the lower analyzer.


4. Light cannot pass through the plate.
5. Screen appear dark.

Physical principle

Liquid crystals are neither a pure solid nor a pure liquid, but rather a hybrid of both. One particular variety of interest is the twisted nematic liquid crystal whose molecules have a natural tendency to assume a twisted spiral structure when the material is sandwiched between finely grooved glass substrates with orthogonal orientation. Note that the molecules in contact with the grooved surfaces align themselves in parallel along the grooves. The molecular spiral causes the crystal to behave like a wave polarizer; unpolarized light incident upon the entrance substrate follows the orientation of the spiral, emerging through the exit substrate with its polarization (direction of electric field) parallel to the groove’s direction.

THE STORY OF LCD

1888- Friedrich Reinitzer first discovered liquid crystalin cholesterol extracted from carrots

1962-Richard Williams generated experiment on a thin layer of liquid crystal material by applied the voltage. This effect is based on an electro-hydrodynamic instability called “Williams domains” inside the liquid crystal.

1964-1968- A team from David Sarnoff Research Center in Princeton, New Jersey led by George Heilmier with Louis Zanoni and Lucian Barton, devised a method for electronic control of light reflected from liquid crystals and demonstrated the first liquid crystal display.

1969- James Fergason invented the twisted nematic field effect of liquid crystals

1972-International Liquid Crystal Company (ILIXCO) owned by James Fergason produced the first modern LCD watch based on James Fergason's patent.