Twisted Nematic Field Effect - Description


The twisted nematic effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. This is achieved with little power consumption and at low operating voltages.

The illustrations to the right show both the OFF and the ON-state of a single picture element (pixel) of a twisted nematic light modulator liquid crystal display operating in the "normally white" mode, i.e., a mode in which light is transmitted when no electrical field is applied to the liquid crystal.

In the OFF state, i.e., when no electrical field is applied, a twisted configuration (aka helical structure or helix) of nematic liquid crystal molecules is formed between two glass plates, G in the figure, which are separated by several spacers and coated with transparent electrodes, E1 and E2. The electrodes themselves are coated with alignment layers (not shown) that precisely twist the liquid crystal by 90° when no external field is present (left diagram). If a light source with the proper polarization (about half) shines on the front of the LCD, the light will pass through the first polarizer, P2 and into the liquid crystal, where it is rotated by the helical structure. The light is then properly polarized to pass through the second polarizer, P1, set at 90° to the first. The light then passes through the back of the cell and the image, I, appears transparent.

In the ON state, i.e., when a field is applied between the two electrodes, the crystal re-aligns itself with the external field (right diagram). This "breaks" the careful twist in the crystal and fails to re-orient the polarized light passing through the crystal. In this case the light is blocked by the rear polarizer, P1, and the image, I, appears opaque. The amount of opacity can be controlled by varying the voltage. At voltages near the threshold, only some of the crystals will re-align, and the display will be partially transparent. As the voltage is increased, more of the crystals will re-align until it becomes completely "switched". A voltage of about 1 V is required to make the crystal align itself with the field, and no current passes through the crystal itself. Thus the electrical power required for that action is very low.

To display information with a twisted nematic liquid crystal, the transparent electrodes are structured by photo-lithography to form a matrix or other pattern of electrodes. Only one of the electrodes has to be patterned in this way, the other can remain continuous (common electrode). For low information content numerical and alpha-numerical TN-LCDs, like digital watches or calculators, segmented electrodes are sufficient. If more complex data or graphics information have to be displayed, a matrix arrangement of electrodes is used. Obviously, the voltage controlled addressing of matrix displays, such as in LCD-screens for computer monitors or flat television screens, is more complex than with segmented electrodes. These matrix LCDs necessitate integration of additional non-linear electronic elements into each picture element of the display (e.g., thin-film diodes, TFDs, or thin-film transistors, TFTs) in order to allow the addressing of individual picture elements without crosstalk (unintended activation of non-addressed pixels).

Read more about this topic:  Twisted Nematic Field Effect

Other articles related to "description":

Essay - Forms and Styles - Descriptive
... audience, creating a dominant impression, using descriptive language, and organizing the description are the rhetorical choices to be considered when using a description ... A description is usually arranged spatially but can also be chronological or emphatic ... The focus of a description is the scene ...
Meta Element Used in Search Engine Optimization - The description Attribute
... Unlike the keywords attribute, the description attribute is supported by most major search engines, like Yahoo! and Bing, while Google will fall back on this tag when information about the page ... The description attribute provides a concise explanation of a Web page's content ... This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its ...
Universal Description Discovery And Integration
... Universal Description, Discovery and Integration (UDDI, pronounced Yu-diː) is a platform-independent, Extensible Markup Language (XML)-based registry by which businesses worldwide can list themselves on the ... by SOAP messages and to provide access to Web Services Description Language (WSDL) documents describing the protocol bindings and message formats ...
Gerald Of Wales - Natural History
... He gives a vivid and accurate description of the last colony of the European Beaver in Wales on the River Teifi, but spoils it by repeating the legend ... Likewise he gives a good description of an Osprey fishing, but adds the mythical detail that the bird has one webbed foot ... His description of Irish wildlife was harshly called "worthless" the better view perhaps is that despite its faults it gives a valuable glimpse of Irish fauna in the 1180s ...

Famous quotes containing the word description:

    Once a child has demonstrated his capacity for independent functioning in any area, his lapses into dependent behavior, even though temporary, make the mother feel that she is being taken advantage of....What only yesterday was a description of the child’s stage in life has become an indictment, a judgment.
    Elaine Heffner (20th century)

    He hath achieved a maid
    That paragons description and wild fame;
    One that excels the quirks of blazoning pens.
    William Shakespeare (1564–1616)

    Do not require a description of the countries towards which you sail. The description does not describe them to you, and to- morrow you arrive there, and know them by inhabiting them.
    Ralph Waldo Emerson (1803–1882)