SUMMARY / RELATED TOPICS

Twisted nematic field effect

The twisted nematic effect was a main technology breakthrough that made LCDs practical. Unlike earlier displays, TN-cells did not require a current to flow for operation and used low operating voltages suitable for use with batteries; the introduction of TN-effect displays led to their rapid expansion in the display field pushing out other common technologies like monolithic LEDs and CRTs for most electronics. By the 1990s, TN-effect LCDs were universal in portable electronics, although since many applications of LCDs adopted alternatives to the TN-effect such as in-plane switching or vertical alignment. Many monochrome alphanumerical displays without picture information still use TN LCDs. TN displays benefit from fast pixel response times and less smearing than other LCD display technology, but suffer from poor color reproduction and limited viewing angles in the vertical direction. Colors will shift to the point of inverting, when viewed at an angle, not perpendicular to the display; the twisted nematic effect is based on the controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field.

This is achieved at low operating voltages. The illustrations to the right show both the OFF and the ON-state of a single picture element 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 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 that twist the liquid crystal by 90° when no external field is present. If a light source with the proper polarization 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 properly polarized to pass through the second polarizer, P1, set at 90° to the first.

The light 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. 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, 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, the display will be transparent; as the voltage is increased, more of the crystals will re-align until it becomes "switched". A voltage of about 1 V is required to make the crystal align itself with the field, no current passes through the crystal itself, thus the electrical power required for that action is 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. 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; 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. For a matrix of limited resolution or for a slow-changing display on a large matrix panel, a passive grid of electrodes is sufficient to implement passive matrix-addressing, provided that there are independent electronic drivers for each row and column. A high-resolution matrix LCD with required fast response necessitates integration of additional non-linear electronic elements into each picture element of the display in order to allow active matrix-addressing of individual picture elements without crosstalk.

In 1962, Richard Williams, a physical chemist working at RCA Laboratories, started seeking new physical phenomena that might yield a display technology without vacuum tubes. Aware of the long line of research involving nematic liquid crystals, he started experimenting with the compound p-azoxyanisole which has a melting point of 115 °C. Williams set up his experiments on a heated microscope stage, placing samples between transparent tin-oxide electrodes on glass plates held at 125 °C, he discovered that a strong electrical field applied across the stack would cause striped patterns to form. These were termed "Williams domains"; the required field was about 1,000 volts per centimeter, far too high for a practical device. Realizing that development would be lengthy, he turned the research over to physicist George Heilmeier and moved on to other work. In 1964, RCA's George H. Heilmeier along with Louis Zanoni and chemist Lucian Barton discovered that certain liquid crystals could be switched between a transparent state and a scattering opaque one with the application of electric current.

The scattering was forward, into the crystal, as o

Karl-Heinz Bendert

Karl-Heinz Bendert was a German Luftwaffe ace and recipient of the Knight's Cross of the Iron Cross during World War II. Karl-Heinz Bendert claimed 55 victories in 610 missions. During his time with JG 27 in Africa he was involved in scandal with falsifying claims. Despite this, he was given credit to the victories. After a confrontation with his squadron leader, Gustav Roedel, who did not interfere with his Knight's Cross of the Iron Cross nomination, he did not score any more victories. Ehrenpokal der Luftwaffe Front Flying Clasp of the Luftwaffe Iron Cross 2nd Class 1st Class German Cross in Gold on 15 October 1942 as Oberfeldwebel in the 4./Jagdgeschwader 27 Knight's Cross of the Iron Cross on 30 December 1942 as Oberfeldwebel and pilot in the 5./Jagdgeschwader 27 World War 2 Awards.com Aces of the Luftwaffe Ritterkreuztrager 1939-1945

Clarence Seignoret

Sir Clarence Henry Augustus Seignoret was the third President of Dominica. Born in Roseau, Seignoret was educated at the Dominica Grammar School and at college in Saint Lucia before he started working as a civil servant in Dominica from 1936. From 1958 to 1960 he undertook an international public service course in Oxford University. On returning to Dominica he resumed his governmental career, acting on various occasions as first Secretary to the Cabinet and substitute to the President; the House of Assembly of Dominica elected him as President of Dominica in 1983, he was sworn in during October of that year. Re-elected to the presidency in 1988, he resigned in 1993. In the 1966 New Year Honours, Elizabeth II appointed him an Officer of the Order of the British Empire and in 1985 he was knighted with the Grand Cross of the Order of the Bath, he was Knight of Malta since 1992. Biography