LCD - Liquid Crystal Display

Definition of Liquid Crystal: defines Liquid Crystal as the following:
   Any of various liquids in which the atoms or molecules are regularly arrayed in either one dimension or two dimensions, the order giving rise to optical properties, such as anisotropic scattering, associated with the crystals.

Another definition:
   A thermodynamic stable phase characterized by anisotropy of properties without the existence of a three-dimensional crystal lattice, generally lying in the temperature range between the solid and isotropic liquid phase, hence the term mesophase.


The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. Because of his work, Friedrich Reinitzer is often credited for finding this phase of matter - the liquid crystal.

The most common application of liquid crystal technology is in liquid crystal displays (LCDs). LCD devices have become an important part of our lives, from the wrist watch, calculator to LCD monitors and now LCD TVs.

LCD Technology

LCD consists primarily of two glass plates with some liquid crystal material between them. Lack of bulky tubes makes LCD practical for many applications where size and weight is important like LCD TVs.

Passive Vs Active Matrix Displays

Passive Matrix:
A type of liquid crystal display which relies on persistence to maintain the state of each display element (pixel) between refresh scans. The resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.

The passive matrix display is addressed by a set of transparent electrodes, parallel wires run both vertically and horizontally and pixels are turned on when the wires intersecting at that pixel are both energized

When there is a sufficient voltage across it to cause the liquid crystal molecules (in a pixel) to align parallel to the electric field, the passive pixel is addressed. A display can have more than one pixel on at any one time because of the response time of the liquid crystal material. When addressed, a pixel has a short turn-on time during which the liquid crystal molecules align in such a way as to make the pixel opaque. When the voltage is removed the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their natural orientation.

Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed. As the voltage is removed the cell remains opaque briefly before it becomes clear again.

Active Matrix:
A type of liquid crystal display where each display element (each pixel) includes an active component such as a thin film transistor (TFT) to maintain its state between scans. Considered superior to passive matrix

Active matrix displays can be found in computer monitors, laptops and LCD TVs. In this type of display, the addressing takes place completely behind the liquid crystal film. TFT acts as a switch for each pixel. The TFT is addressed by a set of electrodes, gate lines and source lines, running along the gaps between pixels. A pixel turned on by applying current to a gate line which allows charge from the source line to flow on to the rear electrode. This sets up a voltage across the pixel and turns it on. An image is created similar to the passive display as the addressing circuitry scans across the matrix. An active matrix display does not suffer from many of the limitations of the passive display. It can be viewed at an angle of up to 45 degrees. It does, however, require a more intense back lighting system because the TFT's and the gate and source lines are not very transparent and therefore block a fraction of the light.

Active matrix

Color Displays

To get full colour, each individual pixel is divided into three sub pixels: red, green and blue (RGB). If you go close enough to an LCD/Plasma/LCD Monitor you will notice that each pixel is made up of 3 smaller rectangular sub pixels. These sub pixels are created by applying colour filters which only allow certain wavelengths to pass through them while absorbing the rest. With a combination of red, blue and green sub pixels of various intensities, a pixel can be made to appear any number of different colours. If each red, green and blue sub pixel can display 256 different intensities of their respective colour, then 256x256x256 = 16777216 different colours can be emitted from the one pixel. RGB sub pixels are analogous to a colour cathode ray tube (CRT) like a television or computer monitor in which different phosphors glow red, green or blue when excited by an electron beam. The number of colours that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display. The picture below shows an LCD pixels at close up. You will notice the 3 red, green and blue sub pixels that make a a pixel.

Production Challenges

These colour displays take an enormous number of transistors. For example, a typical laptop computer supports resolutions up to 1,024x768. If we multiply 1,024 columns by 768 rows by 3 sub pixels, we get 2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen. Manufacturers of Plasma screens usually allow for a maximum of 3 broken pixels, otherwise the monitor is replaced. A broken pixel maybe a pixel where all RGB sub pixels don't work (leaving a black dot), altelast one sub pixel does not work (be it Red Green or Blue sub pixel) causing the pixel to display an undesired colour or all sub pixels are "on" causing a white pixel.

Display size is limited by the quality-control problems faced by manufacturers. Simply put, to increase display size, manufacturers must add more pixels and transistors. As they increase the number of pixels and transistors, they also increase the chance of including a bad transistor in a display. Manufacturers of existing large LCDs reject a significant percent of the panels that come off the assembly line. The level of rejection directly affects LCD price since the sales of the good LCDs must cover the cost of manufacturing both the good and bad ones. Only advances in manufacturing can lead to affordable displays in bigger sizes.


LED TVs are really LCD TVs. That is, LED TVs have a backlight that is an LED array rather than fluorescent lamp that "standard" LCDs use. LED TVs still use the LCD array to reproduce colours. LED TVs in their current incarnation are not point source red, green and blue light emitters. The LED back light should last for hundreds of thousands of hours before failure. Because LED TVs use LED as the backlight, they use less power to run and can be manufactured in thinner sizes.

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