Liquid Crystal Display
The Liquid Crystal Display (LCD) is an electronic display device that operates by applying a varying electric voltage to a layer of liquid crystal, thereby inducing changes in its optical properties. LCDs are commonly used in portable electronic games, in digital cameras and camcorders, video injection systems, electronic billboards, and monitors for computers. The main advantage of an LCD is its low power consumption when compared to that of an LED. The power consumption in LCDs is in the order of microwatts for the displays as compared to milliwatts in LEDs. To study the operation of an LCD, we must know about liquid crystals and their properties.
Liquid crystals
There are three common states of matter that we know: Solid, liquid, and gas. Liquid crystal is the fourth state that certain kinds of matter can enter into under the right conditions. Solids always maintain their orientation and stay in the same position. The molecules in a liquid change their orientation and moves anywhere in the liquid. The molecules in solids exhibit both positional and orientation order.
In other words, the molecules are constrained to point only a certain direction and to be only in a certain position concerning each other. In liquids, the molecules do not have position or orientation order, and the direction the molecules point and positions are random.
The liquid crystal phase exists between the solid and liquid phases. The molecules tend to maintain their orientation, like the molecules in the solid but also move different positions like the molecules in a liquid. The molecules do not point in the same direction at all the time. They tend to point more in one direction over time than other directions. The direction is referred to as the director of the liquid crystal.
Liquid crystals are temperature sensitive. It takes a fair amount of heat to change into a liquid. They turn into solid if it is too cold. This phenomenon can, for instance, be observed on the laptop screen when it is very hot and very cold.
The liquid crystal may be nematic, smectic, or cholestatic, depending on the arrangement of molecules. They are birefringent, meaning that they possess two different indices of refraction. One index of refraction corresponds to light polarized along with the director of the crystal and the other for light polarized perpendicular to the direction. Nematic LC is the simplest liquid crystal and is close to the liquid phase. The molecules float around as in a liquid phase but are still ordered in their orientation.
Smectic liquid crystal has a soapy texture and is found at a lower temperature than nematic. They are close to the solid phase. Inside these layers, the liquid crystals normally float around freely, but they cannot move between the layers. The chiral nematic (or cholesteric) liquid crystal exhibits a twisted structure. The director rotates about an axis as you move through the material. They often reflect visible light in different bright colors depending on the temperature. They can therefore be used in temperature sensors.
There are two fundamental ways in which liquid crystals are used to control the properties of light and thereby alter its appearance.
They are
(i) dynamic scattering method
(ii) Absorption method.
Dynamic scattering method
In the dynamic scattering method when an electrical potential is applied, the molecule in the liquid crystal acquires a random orientation. As a result, light passing through the material is reflected in many different directions, and a bright, frosty appearance emerges.
Absorption method
In this method, the molecules are oriented in such a way that they alter the polarization of light passing through the material. Polarizing filters are used to absorb or pass the light. Depending on the polarization it has been given, light is visible only in those regions where it can energize from the filter.
The above two methods can be operated in two modes.
Transmissive mode
In transmissive mode, the LCDs are designed such that light passes completely through them. The light is altered in the desired pattern as it passes through the liquid crystal.
Reflective mode
In this mode, a mirrored surface is used that reflects the light to the viewer. The light is allowed to pass through the material when it is altered and is reflected by a mirror to emerge from the same side it entered.
Construction
A liquid crystal display is composed of multiple layers. First, a sheet of glass is coated with a transparent metal oxide film which acts as an electrode. These electrodes are used to set the voltage across the cell necessary for the orientation transition. The electrodes are etched in patterns or individually accessible segments that can be selectively energized to create the desired display.
Dynamic scattering LCD- transmissive mode
Fig shows a dynamic scattering LCD operated in the transmissive mode. In the region activated by an external electric field, the molecules have random orientation. The molecules in the inactivated region have definite alignment. In the activated region, due to the random orientation of molecules, the light will be scattered and it escapes with a bright appearance.
Dynamic scattering LCD- reflective mode
The construction of a dynamic scattering LCD operated in reflective mode is the same as that of transmissive type except that a mirrored surface is replaced or added behind one of the glass sheets. However, unwanted reflections limit the readability of display of this type.
Twisted nematic crystal
1. The liquid crystal display depends on light absorption rather than light scattering. A twisted nematic cell is made up of two glass slides (such as indium tin oxide) that act as a detector.
2. Spacers to control the cell gap.
3. Two crossed polarizers.
4. The nematic liquid crystal material.
The underlying principle in a TN display is the manipulation of polarized light. When light enters the TN cell it undergoes a 90° change in polarization. For example, consider a light polarized parallel to the director at the top of the cell. As it travels through the cell, its polarization rotates with the molecules. When the light emerges, its polarization is rotated 90° from where it entered. However, if a voltage is applied to the electrodes, the molecules are reoriented in such a way that the change in polarization does not occur. The absorption type LCD operates on the principle that the horizontally polarized light is not visible through a vertically polarized filter.
In Fig light enters the TN cell through a vertical polarizer. If the applied potential is zero due to molecular twist, the vertically polarized light becomes horizontally polarized light and is absorbed by the vertical polarizer at the other end. Thus the inactivated region appears dark. In an activated region there is no change in polarization. Hence the vertically polarized light which enters the cell leaves the cell without any change and is not absorbed by the vertical polarizer. Here the activated region appears bright.
For applications such as digital -watches and calculators, a mirror is used under the bottom polarizer. A horizontal polarizer is placed between one of the glass sheets and the mirror. At the top, the vertically polarized light enters the inactivated region and becomes horizontally polarized. This light can pass through horizontally polarized light and deflect back due to the mirror. The unsaturated regions shift their horizontal polarized light to vertically polarized light. Hence the reactivated region appears bright. In the case of the activated region, the vertically polarized light does not undergo any changes and is absorbed by the horizontally polarized. Hence above region appears dark.
Applications of LCD
LCDs are normally used in
1. Watches
2. Calculators
3. Laptop computers
4. Higher-end CROS
5. Portable instrumentation
Advantages of LED
1. Low power consumption.
2. They are economical.
3. Greater range of color choice.
Disadvantages of LED
1. Response time is below 100 to 300 msec 3. Lifetime is a major concern. Lifetime is less when used with dc.
2. Occupies large area.
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