Schadt-Helfrich cell

from Wikipedia, the free encyclopedia

The Schadt-Helfrich cell is a specific type of liquid crystal display , also known as a TN cell .

history

Martin Schadt (2007)

On December 4, 1970, Martin Schadt and Wolfgang Helfrich , then working in the Central Research Laboratory of Hoffmann-La Roche , registered the first patent for the “ nematic rotating cell” (TN cell, Schadt-Helfrich cell) in Switzerland. In Germany, the patent office refused to grant patents due to lack of inventive step, but the patent was granted in 21 other countries.

Layout and function

In the Schadt-Helfrich cell, the liquid crystal molecules in the de-energized state form a continuous screw connection of 90 °, which can follow the polarization direction of the linearly polarized light when entering the cell. If the second polarizer is arranged perpendicular to the first, the light can pass through the cell in the de-energized state, the cell is transparent. This operating mode is known as normally white mode .

As the voltage across the liquid crystal layer increases gradually, the liquid crystal molecules rotate in such a way that they align themselves increasingly parallel to the electric field. The molecules on the substrate plates, on the other hand, retain their alignment even with high voltages applied. With increasing alignment parallel to the electric field, the rotation of the polarization direction decreases and there is an increasing absorption of the light in the second (perpendicular to the first) polarizer. This means that the transparency of the cell continuously decreases with increasing voltage.

Due to the low control voltage (in the range of a few volts), and the almost powerless control (no current flow is necessary for operation), the Schadt-Helfrich cell forms the basis for the use of LCDs in portable battery-operated devices, such as. B. Calculators and watches. These applications in turn formed the basis for the widespread use of these first LCDs and their rapidly advancing technical development up to today's LCD television screens.

Basic representation of the structure and function of a picture element (pixel) of a Schadt-Helfrich cell. Without voltage, OFF state (left diagram), with voltage V (a few volts), ON state (right diagram). (Graphics courtesy of M. Schadt)

Worth mentioning

It should be mentioned that in addition to the TN cell controlled by thin-film transistors (TFTs), two other technologies have established themselves for modern, large-area liquid crystal flat screens with reduced dependence on viewing directions:

  • VA displays (Vertical Aligned)
  • IPS displays (In-Plane Switching)

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

swell

  • Kristallmanufaktur - Steps on the way to a direction-independent 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 that 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 fell out of the liquid crystal race - The fate of Roche and BBC developments in ten sections. In: Neue Zürcher Zeitung No. 141 56 / B12, June 20, 2005
  • Werner Becker and Hans-Juergen Lemp: 100 years of Commercial Liquid Crystal Materials. In: Information Display 2, 2004
  • 100 years of Liquid Crystals at Merck: The history of the future. Merck KGaA, Corporate Communications, 2004
  • Werner Becker (editor): 100 years Liquid Crystals. In: Liquid Crystal Newsletter No. 19, 2004
  • Michael Heckmeier, among others: Liquid Crystals for Active Matrix Displays.
  • Liquid Crystals: Merck Makes Bits & Bytes Visible. Merck KGaA, Corporate Communications

Individual evidence

  1. Swiss Patent No. 532 261
  2. Details from Gerhard H. Buntz : Twisted Nematic Liquid Crystal Displays (TNLCDs) - An invention from Basel goes around the world.