This work presents a simple compensation method for widening the viewing angle of transflective liquid-crystal displays (TR-LCDs). For an off-axis light, the slow axis of a biaxial film shifts linearly as the Nz factor is varied. By using this optical characteristic of a biaxial film, the broadband condition of broadband circular polarizers exactly holds over a full 80° viewing cone, thus eliminating the off-axis light leakage to widen the viewing angle of TR-LCDs. Based on the proposed compensation method, the TR-LCDs theoretically have a wide spectral bandwidth and a viewing angle of 80° for contrast-ratio (CR) >100:1 and >30:1 in transmissive and reflective modes, respectively. Experiments also show that the proposed TR-LCD has a viewing angle of over the entire 80° and 65° viewing cone in T-mode and R-mode, respectively, for CR>10:1. The proposed TR-LCD is highly promising for mobile display applications.
©2010 Optical Society of America
Transflective liquid-crystal displays (TR-LCDs) have been extensively used for mobile displays owing to outdoor legibility and low power consumption . The pixel structure of a TR-LCD comprises a transmissive mode (T-mode) and a reflective mode (R-mode). The T-mode uses a backlight unit, while the R-mode uses an ambient light to perceive the displayed images. The image of TR-LCDs can thus be perceived clearly in both indoor and outdoor environments. Most TR-LCDs use a double cell-gap structure [2,3], in which different cell gaps are embedded in a single pixel to compensate for the optical phase difference between T-mode and R-mode. Recently, TR-LCDs based on a single cell-gap structure have received considerable attention due to their simple fabrication process [4–6]. However, regardless of whether having a double cell-gap or a single cell-gap structure, broadband circular polarizers are required for TR-LCDs to obtain a good dark state for the R-mode. A broadband circular polarizer consists of a linear polarizer, a λ/2 plate and a λ/4 plate. Once the broadband circular polarizers satisfy a broadband condition 2ϕλ/4-4ϕλ/2=±π/2 , where ϕλ/4 is the azimuthal angle between the optic axis of the λ/4 plate and the transmission axis of the linear polarizer and ϕλ/2 is the azimuthal angle between the optic axis of the λ/2 plate and the transmission axis of the linear polarizer, the TR-LCDs can achieve a good dark state over a wide spectral bandwidth in a normal direction. However, the broadband condition of the broadband circular polarizer does not exist anymore for an off-axis direction because of the shift of optic axes of the λ/4 plate, the λ/2 plate and the linear polarizer [8,9], subsequently incurring serious off-axis light leakage and eventually degrading the viewing angle of TR-LCDs. Although optimizing the broadband circular polarizer by the combination of A-plates and C-plates significantly reduces the off-axis light leakage of the broadband circular polarizer to widen the viewing angle of TR-LCDs [6,10], its polarizer configuration is complex. A simple compensation method, which combines positive and negative λ/2 plates, as well as positive and negative λ/4 plates, has been proposed to widen the viewing angle of TR-LCDs . This method significantly improves the color dispersion and viewing angle of T-mode. However, the viewing angle and color dispersion of R-mode still require further improvement to satisfy future mobile multimedia applications. Although in-plane switching (IPS) TR-LCDs with an in-cell phase retarder have been developed to obtain a good dark state in R-mode with the use of linear polarizers , fabricating an in-cell phase retarder still remains a technical challenge. Therefore, a practical compensation method for TR-LCDs must be developed for future mobile applications.
This work presents a simple compensation method that uses biaxial films for TR-LCDs to obtain an extraordinarily wide viewing angle. The slow axis of a biaxial film shifts linearly as the Nz factor is varied for an off-axis light, where Nz=(nx-nz)/(nx-ny) with the principal refractive indices of a biaxial film nx, ny and nz . Based on this unique optical characteristic of a biaxial film, each broadband circular polarizer of the proposed compensation method satisfies the broadband condition 2ϕλ/4-4ϕλ/2=±π/2 over the entire 80° viewing cone by using biaxial films, thus eliminating the off-axis light leakage of TR-LCDs. By using the proposed broadband circular polarizers, both the multi-domain vertical alignment (MVA) and the IPS TR-LCDs exhibit a viewing angle of 80° viewing cone for CR>100:1 and 30:1 in T-mode and R-mode, respectively. A low color dispersion over the entire 80° viewing cone is also demonstrated in both the T-mode and R-mode of the compensated TR-LCDs.
2. Design of wide-view broadband circular polarizers for transflective liquid crystal displays
Figure 1 depicts a compensation method to widen the viewing angle of TR-LCDs. Each broadband circular polarizer of the proposed compensation method comprises a linear polarizer, a biaxial λ/2 plate of Nz =0.5 and a biaxial λ/4 plate of Nz =0.5 with their orientation satisfying the broadband condition 2ϕλ/4-4ϕλ/2=±π/2 in a normal direction. Additionally, a biaxial λ/2 plate with Nz=0.25 is inserted between the linear polarizer and the biaxial λ/2 plate of Nz=0.5.
In Fig. 1, the viewing angle of TR-LCDs is widened using biaxial films. For an optically biaxial medium, by solving the eigenstates of the impermeability tensor [8,14], the orientation of slow axis for off-axis light as a function of the Nz factor can be derived asEq. (1), the shift of a slow axis in azimuthal angle Δψ(=ψ(θo)-ψ(θo=0)) for off-axis light at viewing angle θ=80°, which corresponds to θo~40° in Eq. (1), under different Nz values is calculated, as shown in Fig. 2(a) . According to this figure, Δψ depends linearly on Nz values. Moreover, Nz=1 and 0 have the largest Δψ with the same magnitude, but different shift directions. Thus, the magnitude of Δψ of Nz=0.25 is almost half that of Nz=0 and the slow axis of Nz=0.5 is nearly fixed, i.e. Δψ=0, for an off-axis light, as shown in Fig. 2(a).
Based on the unique optical behaviors of a biaxial film in Fig. 2(a), a simple compensation method is proposed for widening the viewing angle of TR-LCDs. In the proposed compensation method, the slow axes of the biaxial λ/2 plates of Nz=0.25 are parallel to the transmission axes of corresponding linear polarizers, as shown in Fig. 1. For a linear polarizer, whose transmission axis is perpendicular to the absorption axis, the shift angle Δψ of its transmission axis can be represented by a biaxial film of Nz=0. In Fig. 2(b), while the direction of incident light changes from a normal direction to an off-axis direction, as shown in the inset of Fig. 2(b), the transmission axis of the linear polarizer changes from to and the slow axis of the biaxial λ/2 plate of Nz=0.25 changes from to . According to Fig. 2(a), the shift angle Δψ of Nz=0.25 is almost half that of Δψ of Nz=0 in magnitude for an off-axis light, indicating that ∠BOB’=1/2∠AOA’ in Fig. 2(b). Since is a λ/2 plate, the orientation of the polarization state of off-axis incident light then shifts from back to after passing through in Fig. 2(b). Meanwhile, due to Δψ ~0 with Nz=0.5 in Fig. 2(a), both the slow axes of the λ/2 plate of Nz=0.5 and the λ/4 plate of Nz=0.5 in the proposed design are fixed for an off-axis incident light. This indicates that = and = for off-axis light as shown in Fig. 2(b). Finally, the orientations of the optic axes of the optical films of the proposed broadband circular polarizer in an off-axis direction are exactly the same as those at a normal direction. Similarly, the same result can be obtained for an off-axis incident light with the opposite direction. Thus, the broadband condition 2ϕλ/4-4ϕλ/2=±π/2 still holds for off-axis light, subsequently eliminating the off-axis light leakage to achieve a wide viewing angle and a wide spectral bandwidth in both the T-mode and R-mode of the proposed TR-LCDs.
3. Simulation and experimental results
Based on the above method, the following refractive indices and film thicknesses of the biaxial λ/2 plate of Nz=0.5, the biaxial λ/2 plate of Nz=0.25, and the biaxial λ/4 plate of Nz=0.5 are used in the proposed structure: nx λ/2_0.5=1.511, ny λ/2_0.5=1.5095 and nz λ/2_0.5=1.51025 with d λ/2_0.5=184μm, nx λ/2_0.25=1.511, ny λ/2_0.25=1.5095 and nz λ/2_0.25=1.510625 with d λ/2_0.25=184μm, and nx λ/4_0.5=1.511, ny λ/4_0.5=1.5095 and nz λ/4_0.5=1.51025 with d λ/4_0.5=92μm. The refractive indices and the cell gap of the LC material used in the calculation are ne=1.589 and no=1.487 (ne=1.549 and no=1.476) with cell gaps of 4.0μm (3.7μm) and 2.0μm (1.85μm) in T-mode and R-mode, respectively, for MVA (IPS) TR-LCDs. For brevity, in our calculations, (ϕλ/2, ϕλ/4) in the proposed TR-LCD are chosen to be (−75°, −15°) and (15°,75°) for the top and the bottom broadband circular polarizers, respectively, which have been reported to exhibit minimal color dispersion . A commercial simulation program, LCD master (Shintech Japan), is used for the calculations in our study. The opticalproperties are calculated based on the 2×2 extended Jones matrix method . In our calculations, two identical negative C-plates (A-plates), laminated next to the corresponding substrates, are needed for MVA (IPS) TR-LCDs to cancel the off-axis light leakage results from LC residual phase .
Figures 3(a) and 3(b) show the iso-contrast ratio curves of the compensated MVA and the compensated IPS TR-LCDs, respectively. Since the proposed design maintains the broadband condition of the broadband circular polarizers over the full 80° viewing cone by using biaxial films, which subsequently reduces off-axis light leakage in the dark states, both the compensated MVA and the compensated IPS TR-LCDs have a viewing angle over the entire 80° viewing cone for CR>100:1 and CR>30:1 in T-mode and R-mode, respectively, as shown in Figs. 3(a) and 3(b). Figures 4(a) and 4(b) show spectral light leakage of the compensated TR-LCDs in T-mode and R-mode, respectively, for normal and off-axis viewing directions (θ=80°). In Fig. 4, the spectral light leakage of the compensated TR-LCD of θ=80° is almost the same as that of normal direction for both T-mode and R-mode. Both T-mode and R-mode of the compensated TR-LCDs exhibit a wide spectral bandwidth in normal and off-axis directions with the maximal off-axis light leakage <1.86×10−3 and <1.59×10−2 in T-mode and R-mode, respectively, over 450nm~650nm spectral range. Since the proposed TR-LCD has a wide spectral range, as shown in Fig. 4, the T-mode and the R-mode can have a viewing angle over the entire 80° for CR>75:1 and >20:1, respectively, by using a white light source as shown in Fig. 5 . The extraordinarily wide viewing angle and the wide spectral range of theproposed TR-LCDs are the most desired features for a high performance mobile display.
The quality of the bright state is important for LCDs. Figures 6(a) and 6(b) show the calculated iso-luminance curves of the bright states for the conventional and the compensated MVA TR-LCDs, respectively. Since the broadband condition of the proposed broadband circular polarizers holds over the entire viewing cone, the compensated MVA TR-LCD has more symmetrical iso-luminance curves as shown in Fig. 6(b). The spectra of the bright states of the compensated TR-LCD in T-mode and R-mode are also calculated in Fig. 7 . From Fig. 7, the bright states of the T-mode (R-mode) have a lower transmittance (reflectance) in the short wavelength region in an off-axis direction for the conventional TR-LCD and the proposed TR-LCD. This indicates that the bright states of T-mode and R-mode both exhibit slightly yellowish in an off-axis direction for the conventional TR-LCD and the proposed TR-LCD.
In the experiment, the proposed TR-LCD is measured to have a viewing angle of over the entire 80° and 65° (diffusive light source) viewing cone in T-mode and R-mode, respectively, for CR>10:1. In the real panel, the actual CR is lowered because the ideal parameters of the cell and the optical films may not be precisely controlled. Furthermore, the light leakage near the spacers, the protrusions and the pixel edges also reduce the CR. In R-mode, the diffusive light source, the LC alignment distortion on the relief structure of the reflector surface, and the reflections from the thin-film transistor (TFT) array and the air-polarizer interface also degrade the CR. To achieve the extremely wide viewing angle of the proposed TR-LCDs, the improvements of the pixel structure are needed. In practice, the pixel structure with a light-shielding layer that is formed during the TFT fabrication process can effectively prevent the light leakage to improve the contrast ratio. Figure 8 shows the photographs of a MVA TR-LCD with the conventional and the proposed wide-view broadband circular polarizers from different viewing directions. As is expected, the bright states of T-mode and R-mode both exhibit slightly yellowish for the conventional TR-LCD and the compensated TR-LCD in Fig. 8. The yellowish bright states result from the fact that a large LC retardation of ~410nm (~205nm), which is usually used in the MVA-LCDs of mobile applications, is used for T-mode (R-mode) in our study. Calculations reveal that the yellowish bright states of the proposed TR-LCD in an off-axis direction can be improved by decreasing the LC retardation. But the trade off is the lower transmittance (reflectance). Furthermore, the dark states of T-mode and R-mode of the compensated TR-LCD have a significantly lower off-axis light leakage than those of the conventional TR-LCD in Figs. 8(a) and 8(b), thus exhibiting a wide viewing angle in both T-mode and R-mode. The low light leakage in the dark state of R-mode, shown in Fig. 8(b), indicates that the broadband circular polarizer of the proposed TR-LCD is also suitable for anti-reflection applications of various optoelectronic devices.
This study demonstrates a wide-view TR-LCD using biaxial compensation films. The broadband condition of the broadband circular polarizer exactly holds over a full 80° viewing cone by using the biaxial films, thus eliminating the off-axis light leakage to widen the viewing angle of TR-LCDs. Based on the proposed design, the TR-LCDs have an extraordinarily wide viewing angle and a wide spectral bandwidth over the entire 80° viewing cone in both the T-mode and R-mode. The proposed compensated TR-LCD with wide-view and wide spectral range in both the T-mode and R-mode is appropriate for future mobile applications. We also believe that the wide-view broadband circular polarizer in the proposed compensated method is suitable not only for widening the viewing angle of TR-LCDs but also for anti-reflection applications of various optoelectronic devices.
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract Nos. NSC 98-2112-M-110-006-MY3 and the technical support of the TPO displays corporation.
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