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Saturday, 10 July 2021

Working Principle of LCD Display


Colour and monochrome liquid crystal displays (LCDs) of various sizes have been created for TV receivers. LCDs in video display devices are currently available in a variety of sizes and colours, ranging from a few inches to several inches. LCD TVs are television sets that create pictures using LCD technology.

When voltage is applied to an LCD panel, it shines a backlight through a layer of liquid crystals, twisting the changing amounts of light passing through colour filters to form a picture on the screen. By carefully filtering white light, LCD televisions generate a black and colourful image. A series of cold cathode fluorescent lamps (CCFLs) are generally used to give illumination at the back of the panel. A grid of millions of individual LCD shutters is used. It will open and close to enable a specific amount of white light to pass through.

Each shutter is coupled with a coloured filter that removes everything from the original white source except the red, green, and blue (RGB) components. A single sub-pixel is formed by each shutter filter combination. Because the sub-pixels are so small, the three sub-pixels with red, green, and blue colour filters are combined to create a single colour spot known as Pixel.

The relative strength of the light flowing through the sub-pixels is used to adjust the colour shade. The front and rear of a typical shutter assembly are formed by a sandwich of many layers placed on two thin glass sheets. The polarising film, glass sheet, active matrix components, and addressing electrodes appear first on the back sheet, followed by the director.

Fig: General concept of the LCD unit.

The active matrix components are replaced by patterned colour filters on the front sheet, which is comparable to the back sheet. A patterned plastic sheet is sandwiched between the two sheets, dividing the liquid crystal into separate shutters. The shutter assembly is coupled with control electronics and a backlight to form a full television.

A single bulb or a group of lights can be used to create a backlight. To disperse the light, a diffuser or frosted mirror is utilised. A column of liquid crystal molecules is suspended between two transparent electrodes and two polarising filters in each pixel. The polarizer's axes are perpendicular to one another. Light flowing through one would be blocked by the other if the liquid crystals were not there.

The polarization of light entering one filter is twisted by the liquid crystal, allowing it to pass through the other. The liquid crystal molecule is relaxed before an electrical charge is applied. The helical shape or twisting of these molecules is caused by charges on the molecules.

The light that passes through one polarised filter is rotated as it travels through the liquid crystal, allowing it to pass through the second polarised filter and making the assembly transparent. The molecules of the liquid crystal align themselves parallel to the electric field when an electric charge is given to the electrodes, thus restricting the rotation of incoming light. The light travelling through the liquid crystals will be polarised perpendicular to the second filter if they are entirely untwisted, and therefore completely blocked.

The pixel will appear to be dark. Light may be permitted to flow through in various amounts by regulating the twist of the liquid crystal in each pixel, lighting the pixel accordingly. The LCD panel provides excellent colour reproduction and contrast. CRT displays are limited in size, therefore they may be manufactured in bigger proportions. Despite their size, they are light in weight and maybe readily placed, especially if mounted on a wall.


Advantages of LCD are:

1. LCD's have great Compactness.

2. LCD's are thinner and lightweight devices.

3. It is inexpensive.

4. It uses a few microwatts for display, compared to a few milliwatts for LEDs.

5. When compared to CRT and LED, it uses less electricity.

Disadvantages of LCD:

1. Contrast and brightness are lacking.

2. Additional light sources are required.

3. The operating temperature range is restricted.

4. Lack of trustworthiness.

5. The speed is really slow.

6. AC drives are required for LCDs.

Applications of Liquid Crystal Display (LCD)

1. Thermometer with liquid crystals.

2. Optical imaging is a type of imaging that uses light to create images.

3. The liquid crystal display technology may also be used to visualise radiofrequency waves in waveguides.

4. It's used in medical settings.

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