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Liquid Crystal Display (LCD)

 

Basic Working Principle of LCD Panel 

A LCD display consists of many pixels, this is what the resolution stands for, the number of pixels. Each of these pixels is an LCD panel, and it is seen as a multi-layer sandwich supported by a fluorescent backlight. At the 2 far ends of the LCD panel are non-alkaline, transparent glass substrates with smooth surface and free of surface scratches. The glass substrates are attached to polarizer film that transmits or absorbs a specific component of polarized light. In between the 2 glass substrates is layer of the nematic phase liquid crystals. There is also a colour filter containing the 3 primary colours (red, green and blue). Each of the polarized glass is arranged at right angles to each other, so when electric current was passed through the LCD panel, the liquid crystals are aligned with the first polarized glass encountered and will make a 90o twist when approaching the other polarized glass at the end. When this happens, the light from the fluorescent backlight is able to pass through and thus giving us a lighted pixel on the monitor. When there is no electric current, the liquid crystals will not twist and thus the light will not pass through and a black pixel will be shown. The reason we see the coloured images are due to the colour filter, light passes through the filtered cells creates the colors.

Figure 5: How LCD works

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What are Liquid Crystals? 

A liquid crystal is a liquid substance that has solid-like properties. There are many types of liquid crystals and each having different properties but typically are rod-shaped molecules which are free to move about, as in an ordinary liquid, but they are of a certain orientation as in a solid.  

Liquid crystal substances exist as solid crystals at low temperatures, and upon heating, they will slowly become a liquid while retaining some of their crystalline properties. At higher temperatures, they become completely liquid.  

Figure 6: Liquid Crystals

Therefore a substance is only called a liquid crystal during the liquid crystal state or mesophase. The liquid crystalline mesophase can divide into several distinct phases, depending on the temperature and nature of the liquid crystal substance. And in LCD, the most commonly used liquid crystals are the nematic phase liquid crystals. The work of LCDs is possible because of the special properties of these liquid crystals and light.

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Types of Liquid Crystal (LC) Operating Modes

Liquid crystals molecules possess anisotropic properties such as refractive index and dielectric constant. Therefore it is possible to use electric field to realign the liquid crystals molecules to control the way they polarize light. This realigning state is called the Freedericksz transition and it is widely used in LCDs.

 There are 2 common LC operating modes:

  •  TN (twisted nematic) mode
     

  •  STN (super-twisted nematic) mode
     

TN Mode

The TN mode is one of the oldest operating modes and it is still the dominant one. This basically involves the rotation of the polarization axis of the linear polarized light. This was also roughly explained in the section “Basic Working Principle of the LCD”.

First of all, the polariser films are fixed onto the exterior surfaces of the two glass substrates so that the polarization axes (transmission axes) of the 2 substrates are 90o to each other. Since the nematic liquid crystals will twist when an electric current is applied, the linear polarized light passing through the first molecular layer of liquid crystals rotates along the molecules and enters the second molecular layer. Therefore the linearly polarized light will twists in an angle equal to the angle by which the liquid crystals molecular layer is twisted. When this occurs repeatedly, the light reaching the bottom layer of liquid crystals, the polarization axis of the light has rotate 90o, equal to the amount twist of the molecules. 

And this phenomenon is called “optical rotation”.

Figure 7: TN mode

STN Mode

STN mode is a mode that shows a better quality image due to the faster change in the transmission (realignment of liquid crystal molecules). In the TN mode, the rate of change is gradual, thus this led to the development of a fundamental change in operating mode, TN to STN mode, which gives a clearer resulting image. 

The main difference between the STN and TN mode is the twisting angle of the liquid crystals. For TN mode, the liquid crystals only twist by 90o whereas in STN mode, the twist angle is 180o-270o. This is achieved by add some chiral nematic liquid crystals and chiral compounds to the nematic liquid crystals which cause it to assume a helical shape.

Figure 8: STN mode

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Liquid Crystal Drive Scheme 

There are 2 common drive methods (how voltage is applied), passive matrix and active matrix. 

Passive Matrix

The passive matrix drive method makes use of striped electrodes to apply the voltage across the LCD panel. Striped electrodes are attached on the inner side of the 2 glass substrates, and in a manner that they are staggered 90o to each other. These electrodes are made up of transparent and conductive material and are usually indium tin oxide. All these rows or columns of electrodes are connected to integrated circuits (ICs) that control the voltage charge sent down a particular row or column of electrodes. When a voltage is applied to each of he mutually crossing electrodes, a potential difference will fall across at the intersection of the rows or columns of electrodes, which is across the liquid crystals. When this happens, the liquid crystals will twist or untwist, turning the pixel on or off. This drive scheme can be used for any types of liquid crystals and thus is applied to TN and STN mode LCDs.

Figure 9: Passive Matrix

Active Matrix

Active matrix drive method depends on thin film transistors (TFT). They are basically transistors and capacitors and are arranged in a matrix on the glass substrate. This method involves transmitting a drive voltage to a liquid crystal element through a switch. And the liquid crystal elements and capacitors are connected in parallel in an active matrix pixel. The structure is shown in the following diagram.

Figure 10: Active Matrix

 

And also, capacitor in the TFT is able to hold or store the voltage charge. So by carefully controlling the amount of voltage supplied to a crystal, the liquid crystals can be untwist or twist only enough to allow some light to pass through or not pass through.

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Character Display

There are 2 ways that liquid crystal can produce an image display. There is the segment method, which name suggests the characters and pictures are displayed or arranged in segmental order due to patterned electrodes. Also there is the dot-matrix method, in which the characters are displayed in sets of dots.

Segment method is usually used for simple displays such as in calculators.

 

Figure 11: Segment method 

For the dot-matrix method, each dot is called a pixel. So the greater number of character dots or greater number of pixels, the clearer the resulting displays. This is also represented by the resolution, the multiplication of the 2 numbers will give the number of pixels. And the set of pixels that forms a character is referred to as a character font. This method is usually used for high-quality displays like LCD screens.   

Figure 12: Dot-matrix method

For this dot-matrix method there are 2 common drive methods for driving a voltage through the pixels or individual liquid crystal panels.

Static Drive Method 

This method involves each pixel to be wired individually, so that by applying voltage to the pixel by an electrode, it can be turned on and off. Although this method requires complex wiring, the pixels are immune to effects of adjacent electrodes because each one of them operates independently. This also means that if there is something faulty about a voltage supply, it will be only one pixel that will be affected. But this method is very limited in use, as this method cannot be applied in displays with lots of pixels. For example, if a display density were 1920 x 480 dots, the number of drive electrodes needed would be 921600. This is will complicate the whole display with wiring and is not very feasible. So this method is usually used in simple displays like calculator. 

Multiplex Drive Method 

This is a dynamic way of driving method as it involves driving several pixels at a time. It requires the number of drive electrodes equal to the number of rows and columns of pixels. In other words, only one wiring connection between the row of pixels and one drive electrode. So using the same example, for a display density of 1920 x 480 dots, 2400 electrodes is needed. So this method is suitable for large display density. But the disadvantage is, if a drive electrode spoils, the whole row of pixels will not be working.

A combined used of the dot-matrix and multiplex drive method for display is shown in the following figure.

 

Figure 13: Multiplex Drive Method

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Displaying of Colours 

In a Colour Thin Film Transistors (TFT) display, colour filters comprising the three primary colours: red, green, and blue (RGB) are provided on the common electrode substrate of the TFT liquid crystal display. Using a suitable balance between RGB, colours can be produced by additive mixing.

To accomplish this, first the intensity and the colour of light must be defined quantitatively, and the resulting value must be adjusted to match the characteristic of human visual perception. In this manner, the desired colour can be produced on a colour TFT display.

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Comparing LCD and Traditional CRT monitors

 

Characteristics

LCD Displays

CRT Monitors

Physical size

Thin, compact and lightweight. Takes up less space.

Bulky, heavier and takes up more space.

Brightness

LCDs are backlit and have different levels of brightness and capable of producing very bright images and shows extremely uniform brightness. Very suitable for environments that are brightly lit.

Fairly bright, but not as bright as LCDs. Brightness is not a necessary concern with CRTs. Not appropriate for brightly lit conditions.

Power Consumption and Heat

Energy efficient. Consume less electricity than a CRT and produce little heat.

Use more power and produce more heat than a LCD.

Radiation Emission

Emit considerably lesser radiation.

Emit harmful radiation.

Display Size

Advance LCD technology make colour LCD monitors comparable in screen size to CRT monitors

Wide ranges of screen sizes.

Colours

Capable of displaying hundreds or thousands of colours, but newer ones are capable of unlimited colours.

Capable of displaying unlimited colours.

Resolution

Works best at its own native resolution. The native resolution is generally the highest resolution that the LCD can display. Changing to other resolutions require adjusting procedures which can cause considerable deterioration of the image.

Capable of displaying multiple video resolutions, each with the same good quality. Highest pixel resolution was available for operation.

Viewing Angle

Restricted viewing angles. Needed to be viewed in front to have a better view. Not much of an issue for newer LCDs.

Viewable from almost every angle.

Price

More expensive but saves more electricity.

Less expensive but more electricity consuming.

Table 2: Comparison between LCD and CRT

       - LCD Advantage                 - CRT Advantage                - Equal

 

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