Twisted Nematic Effect

A big breakthrough in LCD technology came about in late 1970 when Martin Schadt and Wolfgang Helfrich constructed an LCD based on the twisted nematic effect (Buntz 2). This method employs liquid crystal in the nematic phase that exhibits an angle of 900 in its molecular alignment. The LC-configuration is twisted in a continuous rotation. When the light passes through the LC-cell it is rotated by 900, this allows the light to pass through a second polarizer that is crossed with the first. When a voltage is applied to the LC cell from the electrodes, the liquid crystal molecules align themselves with the field, causing transparency to decrease as the light is blocked when the liquid crystal does not reorient its polarization (Yeh 4). The electrodes applied to the LC-cell are generally made from transparent materials with good electrical conductivity, ITO (indium tin oxide) has been used often in conjunction with LCD screens (Yeh 4).

Super Twisted Nematic Displays came along in the 1980s and differ from TN-LCDs in twist angle and polarizer angle. Instead, STN-LCDs rotate from 1800-2700, while the polarizer angle—instead of 00 as with TN-LCDs—is 450.  The STN-LCDs allowed for more complex pictures. Color Super Twisted Nematic (CSTN) displays use red, green, and blue color filters to create a colored display. Double STN displays stack two STN films with opposite twist in order to achieve a better black/white display. When the color filters are added, the DSTN-LCD has a much wider range of colors than the STN-LCD.

I will be focusing on TN-LCDs as I am particularly interested in the “origin story” of LCD technology and the discovery of twisted nematic effect certainly provided a path for modern LCD improvements.

References:

Buntz, Gerard H. (Patent Attorney, European Patent Attorney, Physicist, Basel). “Twisted Nematic Liquid Crystal Displays (TN-LCDs), an invention from Basel with global effects,” Information No. 118 (October 2005): issued by Internationale Treuhand AG, Basel, Genf, Zurich.

Yeh, Pochi and Claire Gu. Optics of Liquid Crystal Displays. Canada: John Wiley & Sons, Inc., 1999.

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One thought on “Twisted Nematic Effect

  1. joandrade

    Your diagram looks very nice, and also somewhat familiar: Does the twisting of the liquid crystal between the two polarizers by exactly 90 degrees allow for the further polarization to occur? Is it the applied voltage that changes the amount by which the LC twists, and thus, the amount by which the light is polarized?

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