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[Guest - Host (GH) mode]

In a Guest - Host system, the mixture is prepared by mixing LC and dichroic dyes. The dichroic dyes absorb the light whose E-field is along the long axis of the dye, while the negative dichroic dye absorb light whose E-field is perpendicular to the long axis of the dye. When the LC molecules change their orientation, the dye will also change along with LC molecules, consequently, the absorption axis is changing, a light transmission can be modulated.

There are three simple GH displays, which are Heilmeier type GH, the Double Layer type GH and the PDLC type GH.

Heilmeier type GH display is shown in figure 1 below, it utilizes one polarizer. In the off state (upper half), a polarized white light after the polarizer is absorbed by the pleochroic dye, but not the whole spectrum, hence the remaining spectrum exits the cell as colored image. In the on state (lower half), light passes through the cell with very small attenuation from the dye, hence a white image is obtained.

Figure 1. The Heilmeimer Guest-Host display.

Double GH devices use two orthogonally aligned GH cells which attenuate both directions of incident lights as shown in figure 2. In the off state, the director and dye align parallel to the surface alignment layer, therefore, two directions of polarized lights are absorbed. In the on state, both the director and the dye align perpendicular to the surface, therefore, the light can pass through. Since no polarizers are needed, high brightness is achieved. LC materials with ¦¤¦Å<0 can also be used, in this case, the on and off states are just the opposites to the case of LC materials with ¦¤¦Å>0.

Figure 2. Double Guest-Host display.

A PDLC GH utilize the light scattering caused by index mismatching to modulate the light transmission.

Another important GH mode is the phase change GH (PC-GH), which is named after the inventors, White-Taylor GH display. In the off state, the dye is in the cholesteric LC, and the helix axis of ChLC can be parallel or perpendicular to the surface, in both cases, the dye in oriented in all directions due to rotating directors. When the light passes through, only a small spectrum remains, hence a color image is obtained. In the on state, minimum absorption is present, hence a white image is obtained. The PC-GH does not allow grey scale because of the hysterisis in the transmission-voltage curve. The response time of PC-GH is slower than the conventional nematic display because the viscosity increases from the doping of the dyes. The viewing angle is very good, with a 900 cone of contrast ratio 10:1.

Figure 3. A White-Taylor GH-display with ¦¤¦Å>0 LC.

The GH display have found more applications in reflective type display than transmissive type. The following figure shows a scheme of PC-GH-reflective display.

Figure 4. A reflective black and white PC-GH display.

Further Readings and References:

G.H. Heilmeier and L.A. Zanoni, ¡°Guest-host interactions in nematic liquid crystals. A new electrooptic effect,¡± Appl. Phys. Lett. vol. 13, 91 (1968).

Uchida et al., "Bright Dichroic Guest-Host LCDs Without a Polarizer," Proceedings of the SID, vol. 22/1, 41 (1981).

DL White and GN Taylor, ¡°New absorptive mode reflective liquid. crystal display device,¡± J. Appl. Phys., vol. 45, 4718 (1974)


Last update: April, 2006
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