What are Liquid Crystals?
LCD panels can be categorized as flat-panel displays. What makes them distinct from other display technologies is the layer of liquid crystal material within. In this thin layer, liquid crystal molecules are aligned between two glass substrates. On the inner surfaces of each of those substrates lie electrodes that control charge carriers like electrons that then interact with the liquid crystals, creating an electric field that runs through them; this, in turn, can change the alignment of the crystals, also changing the overall behavior of the molecules. On the opposite sides of the substrate, polarizers are used to control the levels of light passage, affecting the overall image of the display.
How Do Liquid Crystal Displays Work?
Unlike CRT monitors, LCD monitors cannot illuminate themselves, and so they require a light source: the backlight. This backlight is most frequently made of the well-known LEDs which stand for light-emitting diodes. Sourced from the backlight, light is moved through the back polarizer and back substrate, into the liquid crystals. Now, the light waves can behave in a variety of ways. Backlight used in LCD displays can be LED (Light Emitting Diode) backlight or CCFL (Cold Cathode Fluorescent Lamp) backlight. LED backlights use less power which becomes more popular, while CCFL is lower cost for large size LCD displays such as large LCD TV. Recently, quantum dots technology is used to increase the LCD contrast.
Electrodes are the controlling factors of the liquid crystal behavior, and thus also the light behavior. By conducting or not conducting a current into the crystal layer, the light may or may not be able to pass through the liquid crystals in a manner that will allow passage through the polarizer. Because of this role, electrodes in LCDs are often made of indium tin oxide (ITO). ITO has good conducting properties and can also make for a transparent electrode which is essential to the appearance of displays today.
How the electrodes affect the liquid crystal alignment can vary depending on the method of alignment used (twisted nematic, multi-domain, in-plane switching). For example, twisted nematic liquid crystals are oriented in a twist when no electric field is present which then polarizes the light passing through the layer; when the electrodes apply the field in full, the twist will straighten out, no longer polarizing the light, and so no light passes. In each of these alignment types, the electrodes are placed differently within the structure, altering the properties of the display, such as width of viewing angle, power consumption, and response time. Despite these different alignment methods, the liquid crystal layer’s purpose remains the same: to polarize the light so that the polarized light passes through to the surface of the display. By polarizing the light transmitted from the backlight, the liquid crystal molecules play a role in how much of the light passes through the polarizing filters, whether it be all, none, or a partial amount.
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