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Illumination & Displays

Mode switching of liquid crystals improves display performance

A novel LCD technology yields both high reflection and transmission.
9 March 2008, SPIE Newsroom. DOI: 10.1117/2.1200802.0971

Mobile display devices such as portable media players, satellite navigators, and laptop computers demand good readability in both indoor and outdoor settings. These stringent constraints make transflective LCDs (displays that are readable under both bright sunlight and low-light situations) a promising technology. In transflective LCDs, each pixel is divided into two parts, one transmissive and the other reflective. The limited suitability of LCDs due to their narrow viewing angles can be overcome by introducing in-plane (IPS) and fringe-field horizontal switching (FFS) modes. However, manufacturing horizontal-mode-switching LCDs requires complicated and expensive fabrication processes.1,2

Transflective LCDs operating in a single mode and manufactured with a so-called single-cell-gap structure are very desirable to reduce production costs. But they require in-cell retardation layers to prevent image inversion between the reflective and transmissive modes.1,2 Moreover, these retarders, too, have a number of significant problems, including increased operating voltage and unreliability of the materials used for their fabrication. A new optical configuration for a transflective IPS LCD was recently reported,3 which employs a twisted-nematic liquid-crystal (TNLC) cell with a single-cell-gap structure and without in-cell retarders. Although the transmissive-mode viewing angle performance is significantly improved, the resulting reflective-mode display capability is poor, and both modes exhibit different electro-optic characteristics.

Figure 1. Optical configuration of the proposed transflective LCD. HWP: Half-wave plate. QWP: Quarter-wave plate. LC: Liquid Crystal. TN: Twisted nematic.

Figure 2. Color prototype transflective FFS panel (∼5×5cm).

Here, we propose an optical TNLC cell design for a single-cell-gap transflective display in FFS mode (see Figure 1). A systematic manufacturing procedure simultaneously achieves optimal display performance of both reflection and transmission. The wavelength dispersion of the TNLC cell is suppressed effectively by introducing a half-wave plate (HWP), the best conditions for which are found using the Muller-matrix method. High reflectance and transmittance can both be achieved by applying bidirectional electric fields for either mode. The configuration can be used to produce a high-contrast transflective display with a single-cell gap and without in-cell retardation layers

The combination of the front polarizer, an HWP, and a TNLC layer plays the role of a circular polarizer in the reflective mode (see Figure 1). The addition of an orthogonal circular polarizer behind the mirror can provide cross-polarization in the transmissive mode, resulting in the dark state. To obtain the bright state, a horizontal electric field is applied to align the LCs to −30° in transmission and to +15° in reflection.

Employing different electric-field directions in either mode, both high reflection and transmission, and single gamma can be realized simultaneously. We have developed a working color prototype panel employing the proposed design (see Figure 2). Efforts in this area are still ongoing. Application of the technology will require solving a number of practical problems. In particular, the reflective component of the device will require a bumpy reflector and a hole-type color filter for wide viewing angle, high reflectance, and color reproduction.

This research was supported by LG. Philips LCD and by grant F0004130-2007-23 from the Information Display Research and Development Center, one of the 21st Century Frontier R&D Programs funded by the Korean Ministry of Commerce, Industry, and Energy.

Tae-Hoon Yoon, Gak Seok Lee, Jeong Hyun Lee, Dong Han Song, Jae Chang Kim
Pusan National University
Busan, Korea
Dae Lim Park, Seong Soo Hwang, Dae Hyun Kim, Sung II Park
LG. Philips LCD
Kumi City, Korea