Construction and Working Principle of LCD; Dynamic Scattering type LCD; LED

  

     Television is the most wonderful invention in mankind's history. Over time, television had evolved drastically. The major attraction of the television is its display. The display is responsible for showing images to us in the form of light. There are many types of displays today, among which some displays are most attractive to people even today. Those displays are,

 LCD (Liquid Crystal Display

 LED (Light Emitting Diode)

These displays are one of the types of display technology evolved. Let’s look at the construction and working principles of these displays.

 LCD (Liquid Crystal Display):

          The LCD is the basic model of every modern display on the market. As the name suggests, this type of display contains liquid crystals that are semisolid. This crystal is mainly responsible for displaying images on the display. Based on their molecular arrangements, there are three major types of crystals used in displays. Those are Nematic crystals, Smectic crystals, and Cholesteric (Twisted Nematic) crystals. All these crystals are in the type of rod shape molecules.

Different types of Liquid Crystals

     In Nematic crystals the orientation order is present but the positional order is missed.  In Smectic crystals, both orientation order and positional order are present. In Cholestric crystal, the positional order is present but the orientation order is present in a helix shape. Based on the construction and operation, LCD is classified into two types. They are,

1.       Field-Effect type LCD.

2.       Dynamic Scattering type LCD.

Field Effect type LCD:

      Most LCD display uses this type of method. This display contains backlight and light guides, glass substrates, two polarizer films (vertical and horizontal), a color filter, a Thin Film Transistor (TFT), and two polarizers covering the liquid crystal between the electrodes. The backlight placed next to the rear glass plate is mainly fluorescent light, which is responsible for an image being formed. Next to the light source, light guides are used. After that polarizers are used. A polarizer is nothing but a sheet that is grooved in either a vertical or horizontal pattern. So when an unpolarized light (containing both vertical, horizontal, and diagonal lines) from the backlight, hits the vertical polarizer, only vertical light is passed out and the horizontal light is blocked; as this polarizer is vertically grooved. In the case of a horizontal polarizer, only horizontal light is passed out and the vertical light is blocked.

Difference between Vertical and Horizontal Polarizer

     After passing through the vertical polarizer, the vertically polarized light passes across the Thin Film Transistor (TFT) and reaches the liquid crystal. The crystal used here is a Twisted-nematic crystal (Cholestric crystal). The nature of this crystal is that, when no supply is given across the liquid crystal, the crystals are arranged in such a way that it forms a helix shape. So the light from the polarizer is twisted around 90 degrees in the liquid crystal. So the vertical light is converted into horizontal light and passes into the horizontal polarizer. So in the end, the light is visible at the output.

 

Liquid Crystal arrangement with and without voltage

Pixels in display

    But when an electric voltage is given across the liquid crystal, the crystals arrange themselves in a straight path. So the vertical light from the first polarizer does not get twisted by the liquid crystal and gets blocked by the horizontal polarizer. So no light is visible at the output. Therefore by controlling the voltage given to the liquid crystal, black, grey, and white light are produced at the output and an image can be formed. We know that an image is formed by many pixels. So, in order to provide a colorful image, various colored pixels are used. Each pixel in the display is divided into three sub-pixels as Red, Green, and Blue. These three colors can be combined in different ways and can be made to produce several colors, by simply varying the voltage. By altering the voltage, these pixels will glow at different brightness according to the voltage given, and a colorful image is made to form. As seen above, in LCD an excess film called TFT is used. This film contains, thousands of transistors, thereby each sub-pixel is controlled by a single transistor for better resolution. TFT is also used in LED displays.

Construction and Working of LCD

Dynamic Scattering type LCD:

          This type of LCD is commonly used in watches, advertisement boards, etc. It mainly consists of a nematic crystal type, sandwiched between the front and back glass plates. Other materials are also added to the crystal, to increase its impurity, and to make it more conductive. Both glass plates are also coated with a conductive material at a specific part. The front glass plate is photo-etched as per the requirement by the user, like forming a 7-segment digit and many more. When no voltage is applied across the conductive coatings the crystals remain in normal order. So the display remains transparent. When the voltage is given to the crystals, the crystals get disturbed and the incoming light is scattered in different directions by the crystals. So the light strikes at the output efficiently.

Construction and Working of Dynamic Scattering type LCD

LED (Light Emitting Diode):

       The LED display works in the same method as the LCD display, but unlike the background fluorescent light used in the LCD, the LED display use LED lights in the background to provide the required light. LEDs used in the backplate may be edge placed or directly placed. As LED display uses LEDs in the background, it is thinner than LCD. Also, the power consumption of LED is much lesser than LCD. The only disadvantage of LED is, that it is more expensive than LCD.

Construction and Working of LED