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[Transflective mode]

The transmissive mode finds wide applications in LC displays, for instance desktop monitor and LCD TV, because of its superior image quality. However its usage is limited to indoor or low ambient lighting. The reflective mode LCD owns the capability of outdoor using with decent image quality. However due to the fact that the backlight is lacking, it cannot be used indoor. To overcome both modes' drawbacks, transflective mode is invented. It has two working modes: transmission mode and reflection mode. Because its versatile applications, it is widely used in portable devices like PDA, GPS etc.

A simple transflective display is shown in figure 1, in which there are two regions, T and R respectively. The cell gap in two regions are different, d_T = 2*d_R. This is to maintain the reflection and transmission from two regions are the same intensity, and give same color reproduction, because in the T region, light only goes through the LC layer once, while in the R region, light passes through twice.

Figure 1. A dual-cell-gap ECB transflective display, in which the cell gap in reflective region d_R is half of that in transmission region d_T.

There are different types of transflective displays, mainly four following:

1). Absorption type - it mainly uses the Guest-Host mode to achieve the transflective mode, its working principle is shown in figure 2.

(a)

(b)

Figure 2. Scattering type transflective display based on Guest-Host system (a) nematic LC (b) cholesteric LC.

2). Reflection type - it utilizes the Bragg reflection in cholesteric liquid crystal, its working principle is shown in figure 3.

Figure 3. A reflection type transflective display based on Bragg reflection in cholesteric liquid crystals.

3). Scattering type - it relies on the scattering in polymer dispersed liquid crystal, or polymer stabilized liquid crystal, and therefore no polarizers are needed, which reduce the manufacture cost. A polarizers required transflective display based on scattering in liquid crystal gel was proposed by by Ren et al. The working principle is shown in figure 4.

Figure 4. A scattering type transflective display using liquid crystals gels.

4). Phase retardation type - this is the mostly used in current commercial transflective displays; different LC modes can be used, for example, ECB, TN, MVA, IPS etc. Because the light is modulated through changing its phase retardation, it is sensitive to the cell gap, hence dual-cell-gap is introduced by Sharp to maintain the same phase retardation both for T-region and R-region. Single cell gap mode is also proposed, as shown in figure 1. The manufacture process of dual-cell-gap is more complicated, and because of the gap difference, the response time in two regions are different. Furthermore, the viewing angle of T-region is narrow because of the polar tilt along the electric field.

Single-cell-gap transflective devices are also proposed. The advantage of single-cell-gap structure ensures the close dynamic response from both T- and R- region, resulting a more uniform display. Figure 5 shows a single-cell-gap transflective mode based on employing an image-enhanced reflector (IER). By using the IER, the aperture ratio can be greatly improved.

Figure 5. A single-cell-gap transflective LCD based on an IER.

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Further Readings and References:

J. E. Bigelow, “Transflective Liquid Crystal Display,” U.S. Patent 4 093 356, Jun. 6 (1978).

M. Okamoto, H. Hiraki, and S. Mitsui, “Liquid crystal display,” U.S. Patent 6 281 952, Aug. 28 (2001).

M. Shimizu, Y. Itoh, and M. Kubo, “Liquid crystal display device,” U.S. Patent 6 341 002, Jan. 22 (2002).

Y. P. Huang, M. J. Su, H. P. D. Shieh, and S. T. Wu, “A single cell-gap transflective color TFT-LCD by using image-enhanced reflector,” in SID Dig. Tech. Papers, pp. 86–89 (2003).

Zhu, Z. Ge, T. X. Wu, and S. -T. Wu, "Transflective Liquid Crystal Displays," J. Display Technol. 1, 15- (2005).