[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).
