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[In-Plane-Switching (IPS) mode]

The history of liquid crystal display (LCD)

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 degrees with the electrodes, the linearly polarized light enters the cell with polarization parallel to the director.

Animation 1. In-Plane-Switching LCD Mode

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 wide.

The transmission T of IPS can be described by equation below

where θ(V) is the angle between polarizer and the LC director, and it is a function of the applied voltage. Δn is the birefringence of LC , d is the cell gap, and λ is the wavelength. Usually Δnd 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, θ = 0⁰, hence T = 0. At high voltage, most of the molecules align along the electric field, θ = 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.

Figure 2. Electrodes configuration comparison between conventional IPS (left) and Super-IPS (right)

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.

Figure 3. Pixel texture comparison of AS_IPS and IPS_Pro
(Picture is from Hitachi 2005-2006 technology report)

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Further Readings and References:

R. Kiefer, B. Webber, F. Windscheid, and G. Baur, “In-plane switching of nematic liquid crystals,” Proc. Japan Displays’92, pp. 547–550 (1992).

M. Oh-e and K. Kondo, “Electro-optical characteristics and switching behavior of the in-plane switching mode,” Appl. Phys. Lett., vol. 67, pp. 3895–3897 (1995).

Z. Tajima, "IPS Technology Trends", Asia Display/IMID '04 Digest, pp. 15-18 (2004).

Willem den Boer , "Active Matrix Liquid Crystal Displays: Fundamentals and Applications", Newnes (2005).