We design a dual-view liquid crystal display (DVLCD) which display two different images in the left and right viewing directions simultaneously. The main-pixel of the DVLCD comprises the right sub-pixels (RSPs) and the left sub-pixels (LSPs). The LCs in the RSPs and the LSPs have the opposite rotation directions, which are controlled by the inclined electric fields provided by the patterned electrodes. Addressing the RSPs and LSPs with the voltages having different polarities effectively decreases the maximum operation voltage of the DVLCD. The proposed DVLCD is free of the complicate multiple-step rubbing and shadow mask treatments, and hence has the advantages of low cost and easy fabrication.
© 2012 OSA
Multiple view display has been designed for users to see different information from different viewing directions on the same display. For example, in an automobile, the driver may wish to view the navigation while the passenger may wish to view a movie. By showing more than one image to more than one user on one display, there can be a considerable saving in space and cost. To date, only a few display modes, such as parallax barriers and lenticular lens have been proposed to achieve this function [1,2]. However, an additional optical element attached on the liquid crystal (LC) display panel is required. Recently, Chen et al. have demonstrated a dual-view liquid crystal display (DVLCD), in which the main-pixel comprises a right sub-pixel (RSP) and a left sub-pixel (LSP). The RSP and the LSP are responsible for the images in the right and the left viewing directions, respectively . Chens’ design is low cost, owing to the saving of the additional optical elements. However, the different rubbing directions between the adjacent sub-pixels require the two-step rubbing and the shadow mask treatments that complicate the fabrication processes. In this paper, we introduce a DVLCD, in which the LSPs and RSPs are discriminated by the inclined electric fields provided by the patterned electrodes. The proposed patterned electrode DVLCD (PE-DVLCD) excludes the complicate two-step rubbing and the shadow mask treatments. The electro-optical properties of the PE-DVLCD are discussed in this paper.
To analyze the design principle of the proposed PE-DVLCD, we firstly focus on the optical properties of a homogeneous LC cell, in which the LCs rotates counterclockwise when the voltage is applied to the cell, as shown in Fig. 1(a) . The azimuthal angle of the optical axis (OA) of each element is indicated in the figure. The azimuthal angle is defined as the angle measured in the x-y plane and deviated from the x-axis. Consequently, the OA of the quarter waveplate (QWP) has an azimuthal angle of 90° indicates that the OA of the QWP is in the x-y plane and perpendicular to rubbing direction of the LC cell (x-direction). The phase retardation of the LC layer is given by :
Under the crossed polarizer condition that the polarizers are located at the azimuthal angles of (45°, 135°), the transmission of the light through the LC cell becomes 
Figure 1(b) shows the tilt angle-dependent transmittance of the LC cell indicated in Fig. 1(a). The incident angle θ is 30°, the incident light is 632.8 nm and the cell gap is 6.5 μm. The LC used is ZLI-2806 (from Merck), which has a negative dielectric anisotropy and its parameters are shown in Table 1 . The cell gap is obtained by differentiating Eq. (3) with respect to dLC, setting the tilt angle α = 0° and the incident angle θ = + 30°. As shown in Fig. 1(b), when the LCs are tilted counterclockwise from the tilt angle of α = 0° to 40°, the transmittance of the cell increases monotonically. Notably, when the incident angle θ is −30°, the transmittance of the cell can be regarded to vary from the tilt angle of α = 0° to −40°, i.e., the transmittance of the cell remains in the dark state with only a tiny bounce. The obtained results indicate that the cell can provide information to the observer who stands in the right viewing direction (θ = + 30°) of the cell. Furthermore, the observer who stands in the left viewing direction (θ = −30°) of the cell cannot receive information. Similarly, the information observed in the left side of the cell can be provided by the cell in which LCs rotates clockwise when the voltage is supplied. Notably, as shown in Fig. 1(b), owing to the refraction between the air and LC layer, the transmission curve of the cell is not symmetrical at −30°, but has been shifted to ~-19°. The refractions of the optical films in between the air and the LC layer do not contribute to the shift of the symmetrical angle. Therefore, in the optical films, the actual optical path of the incident light is not shown.
Figure 2 shows the incident angle (θ)-dependent transmittance of a pair of polarizers. As shown in Fig. 2(a), when the polarizers are placed under the conventionally crossed configuration that the transmission axes of the polarizers are located at the azimuthal angles of (45°, 135°), the minimum dark state appears at the normal incident direction (θ=0°). We then rotate the polarizers to the new configuration that the transmission axes of the polarizers are located at the azimuthal angles of (43°, 137°). The obtained result indicates that the light leakages of the polarizers at θ = ± 30° are effectively suppressed. The calculated iso-luminances of the two polarizers at (45°, 135°) and (43°,137°) configurations are shown in Figs. 2(b) and 2(c), respectively. In order to achieve a low dark state in the observation directions (θ = ± 30°), we set the polarizers of the PE-DVLCD at the configuration of (43°,137°) in the following calculation.
In the proposed PE-DVLCD, the main-pixel is defined as the display unit which can display two different images in the left and right viewing directions simultaneously. In a main-pixel, the RSPs display information for the observer standing in the right viewing direction of the display; and the LSPs display information for the observer standing in the left viewing direction of the display, respectively. Figure 3 shows the main-pixel of the proposed PE-DVLCD. The azimuthal angle of the OA of each element is indicated in the figure. The main-pixel is 180-μm-wide. In the main-pixel, the slit electrode is 4-μm-wide and the gap between the slit electrodes is 26-μm-wide. The pretilt angle α of the used LCs (ZLI-2806) is 0°. The main-pixel comprises 6 RSPs and 6 LSPs. The fringe electric fields at the margins of the main-pixel will create non-uniform light leakages and then degrade the optical property of the PE-DVLCD. Therefore, on the top substrate, the 15-μm-wide black matrices located at both margins of the main-pixel are used to improve the display quality by blocking the non-uniform light leakages passing through the margins of the PE-DVLCD. As indicated in Fig. 3, the voltage difference VL between the top substrate and the bottom substrate provide inclined electric fields, which rotate the LCs in the LSPs clockwise and provides information to the observer standing in the left viewing direction of the display. Similarly, the voltage difference VR between the top substrate and the bottom substrate provide the LCs in the RSPs inclined electric fields, which rotate the LCs counterclockwise and provides information to the observer standing in the right viewing direction of the display.
4. Results and discussion
The commercially available software LCD Master 2D (Shintech, Inc.) is used to elucidate the detail viewing angle properties of the proposed PE-DVLCD. The electro-optical characteristics of the DVLCD proposed by Chen et al. are also calculated for comparison with our PE-DVLCD. The LC used in Chens’ DVLCD is ZLI-4119 (from Merck). Its parameters are also shown in Table 1. The pretilt angle α of the LCs is 1.5° and the cell gap is 5.5 μm in Chens’ DVLCD.
Figure 4 shows the calculated voltage versus transmittance (V-T) curves of a main-pixel of the PE-DVLCD, in which the RSPs are addressed but the LSPs are kept at the zero voltage. Therefore, in the main-pixel, VL is kept at zero voltage and VR is addressed from 0 to 10 V. As shown in Fig. 4, in the right viewing direction (θ= + 30°), the transmittance of the main-pixel increases monotonically when the RSPs are addressed from 0 to 10 V, and the transmittance of the main-pixel in the left viewing direction (θ= −30°) remains in dark state but still having a small bounce appears at ~5.5 V. When the supplied voltage of VR is higher than 6.5 V, the transmittance of the main-pixel in left viewing direction starts to increase. Therefore, as shown in Fig. 4, the voltage range from 0 to 6.5 V can be used as the operation range of the PE-DVLCD. It means that the RSPs provide information for the observer who stands in the right viewing direction, but also produces some light leakage to the left viewing direction. The light leakage from the RSPs will increase the dark state transmittance of the PE-DVLCD, and then slightly decreases its CR in left viewing direction. The light leakage of the sub-pixel to the un-intended viewing direction is a main factor that decreases the CR of the PE-DVLCD. In order to discuss the cross talk between the RSPs and the LSPs, we define the cross talk ratio (XTR) as the ratio between the light leakage in the un-intended viewing direction and the maximum transmittance in the intended viewing direction :
According to Eq. (4), the calculated XTR of the PE-DVLCD is 1.9%. The calculated XTR of the DVLCD proposed by Chen et al. is ~3.4%, which is slightly higher than that of our PE-DVLCD.
Figure 5 shows the calculated position-dependent transmittance of the PE-DVLCD, in which the LSPs and the RSPs are simultaneously addressed at 6.5 V. As shown in Fig. 5, the transmittance of the PE-DVLCD in the right viewing direction is contributed by the RSPs, and that of the PE-DVLCD in the left viewing direction is contributed by the LSPs. Notably, because the transmittance in Fig. 4 is an average result that includes the RSPs (addressed at 6.5 V) and the LSPs (addressed at 0 V). Therefore, in the right viewing direction, the calculated transmittance in Fig. 4 is ~50% of that in Fig. 5. The operation range restricts the maximum transmittance of the PE-DVLCD. In the proposed PE-DVLCD, the operation range is from 0 ~6.5 V and the maximum transmittance is ~20%. The transmittances of the DVLCDs (including the proposed PE-DVLCD and Chens’ DVLCD) are not as high as the conventional mono-view displays. However, our calculation reveals that the operation range and the maximum transmittance of the PE-DVLCD are affected by the employed LC materials, indicating that the maximum transmittance of the PE-DVLCD can be improved by selecting a suitable LC material. However, the more detailed investigations are necessary. Figure 6 shows the calculated viewing angle properties, i.e., the iso-transmittance and the iso-CR contours of the PE-DVLCD. For the bright state, the RSPs are addressed at 6.5 V and the LSPs are kept at zero voltage; for the dark state, both the RSPs and the LSPs are kept at zero voltage. As shown in Fig. 6, the maximum transmittance of the PE-DVLCD appears in the right viewing field of the display. Around the + 30° viewing field, the CR of the PE-DVLCD is higher than 50. The iso-CR contour of the Chens’ DVLCD is also plotted in Fig. 6(d). Comparing Figs. 6(c) and 6(d) reveals that the viewing angle (CR > 10) of the PE-DVLCD is wider than that of the Chens’ DVLCD.
The difference in the polarity of the supplied voltages VR and VL can decrease the maximum operation voltage supplied to the PE-DVLCD. We now consider a condition that the VL and VR have the same amplitude of 6.5 V. Figure 7 shows a part of the main-pixel. The supplied voltages VR and VL have the same polarity in Fig. 7(a); and have the different polarities in Fig. 7(b). As shown in Fig. 7(a), the maximum voltage supplied to the cell is 13 V, due to the same voltage polarity; however, in Fig. 7(b), the maximum voltage supplied to the cell is 6.5 V, due to the different voltage polarities. Therefore, addressing the VL and VR with different polarities effectively decreases the power consumption of the PE-DVLCD. The DC effects, such as the flexoelectric effect and the ionic charge effect may affect the electro-optical properties of the nematic LC cell [6-7]. However, by careful selection of the employed materials or using the frame inversion method, the DC effects can be overcome.
In conclusion, we have proposed a PE-DVLCD, in which the tilt directions of the LCs in the sub-pixels are controlled by the inclined electric fields. The proposed PE-DVLCD effectively simplifies the complicate fabrication processes of the DVLCD suggested by Chen et al. The calculated CR of the proposed PE-DVLCD is higher than 50 and the XTR is less than 2%. Furthermore, addressing the RSPs and LSPs with the voltages having different polarities effectively reduces the power consumption of the PE-DVLCD. For higher image quality, further studies on the elimination of the XTR and the increases in the CR and transmittance of the PE-DVLCD are under way.
This work was supported by National Science Council of Taiwan for financially supporting this research under Contract Nos. NSC 98-2112-M-018-002-MY3 and NSC100-2811-M-018-014.
References and links
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