Twisted nematic field effect

The Schadt -Helfrich cell is a specific type liquid crystal display, also known as TN cell (twisted nematic cell).

History

On December 4, 1970 Martin Schadt and Wolfgang Helfrich reported, then the Central Research Laboratory of Hoffmann- La Roche active, the first patent on the " twisted nematic cell" ( TN cell, Schadt -Helfrich cell) in Switzerland ( Swiss Pat. 532 261 ). In Germany, the Patent Office rejected then the grant of a patent for lack of inventive step down, but in 21 other countries, the patent was granted.

Design and function

In the Schadt- Helfrich cell the liquid crystal molecules forming the stress-free state a continuous screw of 90 ° of which can follow the direction of polarization in the cell, the linearly polarized entering light ( see the law of Malus, often used each having a small angle). If the second polarizer disposed perpendicular to the first, the light can pass through the cell in a voltage- free state, the cell is transparent. This mode is called the normally white mode.

With a gradual increase in the voltage across the liquid crystal layer, the liquid crystal molecules rotate so that they are increasingly align parallel to the electric field. In contrast, the molecules on the substrate plates to maintain their orientation at even at high applied voltages. With increasing orientation parallel to the electric field, the rotation of the polarizing direction decreases and there is an increasing absorption of the light in the second (perpendicular to the first aligned ) polarizer. This increases the transparency of the cell voltage increases continuously.

Due to the low driving voltage ( in the range of a few volts), and the virtually powerless control ( there is no current flow required for operation ), the Schadt -Helfrich cell forms the basis for the use of LCDs in portable battery-powered devices, such as Calculators and wristwatches. These applications in turn formed the basis for the widespread use of this first LCDs and their rapidly progressing technological development up to the present LCD TV screen.

Worth mentioning

It should be noted that in addition to the controlled by thin film transistors ( TFTs) TN cell have not yet set two other technologies for modern large-area liquid crystal flat panel displays with reduced Sehrichtungsabhängigkeit:

  • VA display ( vertically aligned )

Both technologies differ significantly from the developed by Schadt and Helfrich TN cell.

Swell

  • Crystal manufacturer - steps on the way to sehrichtungsunabhängigen LC - screen. c't 22 (2005) p 222
  • M. Schadt: Milestones in the History of Field - Effect Liquid Crystal Displays and Materials. In: Jpn J Appl Phys. 48, 2009, pp. 1-9.
  • Gerhard H. Buntz: Twisted Nematic Liquid Crystal Displays (TN - LCDs), an invention from Basel with global effects, information No. . 118, October 2005
  • David A. Dunmur and Horst Stegemeyer Crystals did Flow: Classic papers from the history of liquid crystals. Taylor and Francis, 2004, ISBN 0-415-25789-1 History of Liquid Crystals Homepage
  • Rolf Bucher: How Swiss companies from the liquid-crystal -races - The fate of Roche and BBC developments in ten sections. In: Neue Zürcher Zeitung No.141 56 / B12 20 June, 2005
  • Werner Becker (Editor): 100 years Liquid Crystals. In: Liquid Crystal Newsletter No.. 19, 2004
  • Michael Heck Meier, including: Liquid Crystals for Active Matrix Displays.
  • Liquid Crystals: Merck Makes Bits & Bytes Visible. Merck KGaA, Corporate Communications
Polarizer#Malus.27 law and other properties
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