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[Twisted Nematic (TN) mode]

The TN mode is the "workhorse" for the LC display. It was first introduced by Schadt and Helfrich, and also by Fergason in 1971.

The TN cell consists of two glass substrates coated with transparent indium-tin-oxide (ITO), alignment layers (usually polyimide) are coated on the ITO surface, then they are usually rubbed in one direction, as a result, the LC molecules orient parallel to the rubbing direction. The rubbing directions on two substrates are perpendicular to each other, which in turn, a 90 degrees twist of director from one substrate to the other is formed inside the cell, which is usually 4-10 micrometers. The two polarizers are attached in a way that its polarization is parallel to the rubbing direction of the same glass substrate. In the off state, the 90 degrees twist results in a 900 rotation of the light polarization after passing through the LC cell, in a waveguide fashion. If the condition Δnd >>0.5λ is met, where Δn is the birefringence of LC, d is the cell gap and λ is the wavelength of the light, the light emerges after the cell is linearly polarized, and therefore passes through the second polarizer which is perpendicular to the first polarizer. This is called the normally white (NW) mode, since light is transmitted without voltage is applied.

Figure 1. A normally white mode Twisted Nematic LCD

When the voltage is applied, as it increases, molecules start aligning along the electric field (the dielectric anisotropy of the LC is positive), at a sufficient high field, the 90 degrees twist is removed, and LC layer no longer rotates the light polarization, so at the second polarizer, the light is blocked, and this is the black state.

If the second polarizer is parallel to the first polarizer, then light is blocked at the relaxed state and transmitted at the field-on state, this is called the normally black (NB) mode. Since the black state of the NB mode is wavelength dependent, it is not often used.

The gray scale is achieved by applying intermediate voltages between 0 and the value at which light is completely blocked. For a LCD, each pixel is divided into three subpixels, which have red green and blue color filters. Color representation is based on the additive color principle: at regular viewing distance, the viewer will effectively perceive the mix of the colors from the three subpixels. When all three colors are transmitting, the pixel color will be perceived as white by human eyes. The exact color coordinates of the white point depend on the relative transmission and color purity of the red green and blue subpixels.

The MTN mode is evolved from the standard TN mode. The difference from TN is that in MTN mode, the polarizer is placed at non-zero angle to the rubbing directions, and the total twist inside the cell is not necessary 90 degrees. The MTN and regular TN both can be called as TN-ECB mode.

The Mixed Twisted Nematic (MTN) mode can be used in a  reflective type display, which not only doubles the brightness but also eliminates the parallax caused by using a transmissive TN cell. In the meantime, it preserves other favorable features like low operation voltage, high contrast ratio, fast response time, and broadband operation. However the usage of a specular mirror can limit the viewing angle performance, a diffusive one might be able to improve the issue.

Further Readings and References:

M. Schadt and W. Helfrich, "Voltage-dependent optical activity of a twisted nematic liquid crystal", Appl. Phys. Lett., vol. 18, pp. 127-128 (1971).

J. Fergason, "Display Devices Utilizing Liquid Crystal Light Modulation", U.S. Patent No. 3,731,986.

S. T. Wu and C.S. Wu, Mixed-mode twisted nematic liquid crystal cells for reflective displays, Appl. Phys. Lett., 68(11), pp. 1455-1457(1996).


Last update: April, 2006
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