In the IPS mode, the direction of the applied voltage is parallel to the
substrate surface, which is produced from the interdigitized electrodes. As
shown in animation 1 and figure 1, the alignment layer on both substrates are rubbed at 45
with the electrodes, the linearly polarized light enters the cell with
polarization parallel to the director.
Animation 1. In-Plane-Switching
Figure 1. The
operating principle of In-Plane-Switching mode.
In the off state, LC director has a
uniform orientation throughout the cell, and no change is on the light
polarization, the exit polarizer is placed perpendicular to the front polarizer
and hence blocks the light. In the on state, the lateral electric field drives
the LC molecules to rotate in the plane the substrates, and orient along the
field direction, which is 450 from the polarizer, a phase change is placed on
the light, hence light transmits after the exit polarizer. Since the LC director
always remains in the plane of substrate, the viewing cone is very symmetric and
The transmission T of IPS can be described by equation
where ¦È(V) is the angle between polarizer and the LC
director, and it is a function of the applied voltage. ¦¤n is the birefringence
of LC , d is the cell gap, and ¦Ë is the wavelength. Usually ¦¤nd is chosen as
that the value is ~0.3, hence the second term in the equation can be maximized
for visible wavelengths.
At V = 0, LC director is parallel to the polarizer, ¦È = 00,
hence T = 0. At high voltage, most of the molecules align along the electric
field, ¦È = 450, hence T = 1.
The threshold voltage Vth for IPS is
where l is the spacing between the lateral
electrodes, K22 is the elastic constant for twist.
The IPS owns high image quality, not only the
contrast ratio larger than 10:1 in a 1700 viewing cone, but also the
color and gamma shift are very small over all viewing angles. However IPS has
drawbacks. Because of the lateral electrodes, the viewable area is smaller than
conventional LCD, hence the aperture ratio is smaller. Moreover, the peak
contrast ratio (normal incidence) is not very high because in practical
application, light leakage is present around the spacers and the formation of
the electrodes. The early product had a slow response, which was over 50ms.
A number of variation and improvement were introduced.
Hitachi developed a new structure IPS in 1998, namely Super-IPS® (S-IPS) whose
lateral electrodes have a chevron pattern, shown in figure below as comparison with
the original IPS; the S-IPS has a large improvement on the brightness, and
obtains a mega viewing angle.
Electrodes configuration comparison between conventional IPS (left) and Super-IPS
Later in 2002, Hitachi introduce a more advanced version
of IPS with a chevron patterned electrodes, called Advanced Super-IPS (AS-IPS).
A 30% enlargement of the aperture ratio is achieved, and highly improved the
brightness of the LCD. Recently, Hitachi introduced a so-called IPS-Pro® mode,
which has a different electrode structures from the S-IPS and AS-IPS, electrodes
width and inter-electrodes distance have been changed, and the inter-domain
zones also have been redesigned, figure below shows a comparison of the pixel
texture between AS-IPS and IPS-Pro.