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Liquid Crystal Displays (LCDs) are one of the most versatile display technologies currently in the market. Although Liquid Crystals many decades earlier, LCDs started to make an impact in the latter half of the 20th century. Initially passive LCDs were utilized in portable calculators, wrist watches, etc. with limited resolution. Advances in Thin Film Transistor (TFT) technology enabled large size defect free LCDs, one of the main components of modern laptops, cell phones, etc. LCDs are compact, light weight, consume low power compared to old CRTs, and can now be tailored for various sizes offering great brightness, contrast-ratios, and viewing angles. 

 Even though LCDs have come a long way, there are still several areas where they can be improved. All LCDs use backlights that are either Compact Fluoroscent Light (CFL) tubes or Light Emitting Diodes (LEDs) to generate white light. Each individual pixel in an LCD acts like a light switch and has many layers as shown in the image below. The pixels do not produce their own light but rather act as shutters to allow or block light depending on the voltage applied to them, controlled within each pixel by a transistor based on the image being displayed. 

Here the operating principle of a Twisted-Nematic (TN) LCD is shown. The liquid crystal layer is sandwiched between transparent electrodes (Indium Tin Oxide- ITO) that control the voltage across the pixel. Unpolarized light from the backlight is polarized by the input polarizer. When there is no voltage on the liquid crystal layer, the polarization state is rotated such that it passes through an output analyzer resulting in a bright state. When a voltage is applied as in part (b), the liquid crystals bend up and do not affect the input polarization which is blocked by the analyzer, resulting in a dark state.


Although the example here represents a particular LCD mode (one of the most popular), all LCDs utilize Liquid Crystals and polarizers of some sort for creating the image. One of the main limitations of this conventional approach is the fact that the input polarizer blocks more than 50% of the input light that is usually unpolarized from the backlight. Several approaches have been suggested and implemented to improve this efficiency but is still a concern in most flat panel LCDs.