1. Introduction

With the extension of the three-dimensional (3D) display market, many 3D technologies having different methods have been studied and reported [1–9]. These methods can be classified into two groups—autostereoscopy and stereoscopy—depending on whether the method requires the use of 3D glasses. Autostereoscopy is divided into lenticular lens [1,2] and parallax barrier [3,4]. In this group, the viewer does not need to wear 3D glasses because the left- and right-eye images are separated by geometric devices which are incorporated on a two-dimensional (2D) panel. Thus, the viewer can distinguish the left and right images without the inconvenience of wearing glasses. However, limited viewing angles and viewing distances are the major drawbacks of this method. On the other hand, in stereoscopy, the viewer needs to wear 3D glasses and this leads to some inconvenience. However, the left and right images are perceived from any angle or distance. Hence, many viewers can view excellent 3D images without any spatial restriction.

The stereoscopy-type devices for 3D displays are also classified into three categories; shutter glasses (SGs) [5,6], patterned retarders (PRs) [7], and polarization switching (PS) 3D displays [8,9]. The drawbacks of SGs include weight and flicker issues and the need for batteries of glasses. PRs, on the other hand, have resolution issues because their PR film is divided into two spatial parts to generate a 3D image. A PS-3D display does not have the aforementioned issues. In addition, light 3D glasses are required since only two polarizers are needed. The polarization states of the right and left eye images from a 2D display are changed alternately via another panel which is called PS panel. This PS panels are generally designed as an active $\lambda /2$ retarder. However, this $\lambda /2$ PS panel in the PS-3D display arises an unanticipated weak point such as an asymmetric CR on the left- and right-hand side of 3D glasses owing to the wavelength-dependent phase dispersion of the $\lambda /2$ PS panel.

Herein, we propose an optical configuration for achieving a symmetric and high CR in a PS-3D display. This optical configuration is accomplished using three retardation films and a general active $\lambda /2$ retardation panel. To overcome the drawback, we designed a novel optical configuration, which is appropriate for the active $\lambda /2$ retardation panel, and then ascertained its possibility, by applying our optical configuration to the electrically controlled birefringence (ECB) cell.

2. Optical configuration and analysis in the Poincare sphere representation

Figure 1 shows the proposed optical configuration for symmetric and high CR in the PS-3D display. This configuration consists of three retardation films and a $\lambda /2$ PS panel. The $\lambda /2$ PS panel can be switchable by a vertical electric field applied to the liquid crystal (LC) cell. When electric field is applied to the PS panel, the LC molecules are transited from the initial homogeneous state having the $\lambda /2$ retardation to the homeotropic state having the residual retardation. The three retardation films are needed to compensate the wavelength-dependent phase dispersion. The specific conditions of the three retardation films can be obtained by the Jones matrices method [10].

 

Fig. 1 Optical configuration for the development of symmetric and high CR PS-3D display.

Download Full Size | PDF

The feasibility of our optical configuration for the application mentioned above was confirmed with the Poincare sphere representation. Figure 2 shows the traces for three wavelengths, 450 (blue), 550 (green) and 650 nm (red), in the voltage-off state of compensated PS panel. The three wavelengths were assumed to be emitted from the front polarizer on the 2D display and to start at S1 ($0°$ linear polarization state, see Fig. 2(a)). After passing through the $\lambda /2$ retardation LC cell with $45°$ rubbing direction, the three wavelengths experience wavelength-dependent phase dispersion and arrive near -S1 ($90°$ linear polarization state). As can be seen from Fig. 2(b), the three wavelengths are not congregated at same point. After passing through the film1 with $74°$ slow axis, three wavelengths go to S2. (Fig. 2(c)) And then passing through the film2 with $173°$ slow axis, they turn and rotate around -S1 as shown in Fig. 2(d). Finally, they arrive near -S1 due to a film3 with $17°$ slow axis. (Fig. 2(e)) It is noted that wavelength-dependent phase dispersion can be alleviated by our optical compensation. As a result, the initial $0°$ linear polarization state from the front polarizer on the 2D display can pass through the perpendicular polarizer condition between the front polarizer of the 2D display and rear polarizer of the 3D glasses. However, the initial $0°$ linear polarization state cannot pass through the parallel polarizer condition between the front polarizer of the 2D display and rear polarizer of the 3D glasses. The operation scheme in the voltage-off state is summarized in Fig. 2(f).

 

Fig. 2 Poincare sphere representation of the polarization path (a) from front polarizer on the 2D display, after passing through (b) a general $\lambda /2$ retardation LC cell, (c) film 1, (d) film 2, (e) film 3, and (f) summarized operation scheme in the voltage-off state.

Download Full Size | PDF

Figure 3 shows the behavior of the quoted wavelengths in the voltage-on state of compensated PS panel. The three colors passed through a front polarizer on the 2D panel. The initial $0°$ linear polarization state (S1) passes through the $\lambda /2$ retardation LC cell and its position changes slightly because of the residual retardation in homeotropic LC state. (Fig. 3(a)) Then, three wavelengths head to -S2 due to a film1 with $74°$ slow axis as shown in Fig. 3(b). The three wavelengths turn and rotate around S1 due to a film2 with $173°$ slow axis as illustrated in Fig. 3(c). Finally, the three colors arrive near S1 due to the film3 with $17°$ slow axis as shown in Fig. 3(d). Consequently, the initial $0°$ linear polarization state from the front polarizer in the 2D display can pass through parallel polarizer condition, but not the perpendicular polarizer condition. The operation scheme in the voltage-on state is summarized in Fig. 3(e).

 

Fig. 3 Poincare sphere representation of the polarization path after passing through (a) a general $\lambda /2$ retardation LC cell, (b) film 1, (c) film 2, (d) film 3, and (e) summarized operation scheme in the voltage-on state.

Download Full Size | PDF

3. Calculated and measured device performances

The feasibility of our optical configuration was further analyzed using an ECB cell which plays role as active $\lambda /2$ retarder. The used ECB cell conditions are as follows; the pre-tilt angle and the LC considered for the calculation were, respectively, $1°$ and ML-0905 ($\Delta \epsilon =10.3$, $\Delta n=0.1024$). The cell gap was considered to be 2.8 $\mu m$. Our calculations for non-compensated ECB and compensated ECB cell are shown, respectively, in Figs. 4(a) and 4(b). The left ones show the transmittance as a function of wavelength in the perpendicular polarizer condition and the right ones show the corresponding transmittance in the parallel polarizer condition. As shown in Fig. 4(a), the dark and bright states show asymmetric transmittances between the perpendicular polarizer and parallel polarizer condition in the case of PS-3D display adapting non-compensated ECB cell. However, the PS-3D display adapting compensated ECB cell exhibits similar transmittance shape in both the perpendicular polarizer and parallel polarizer condition as shown in Fig. 4(b). In addition, according to the calculated results, transmittance performance at short and long wavelength range can be anticipated to improve. In calculated result, CR for the perpendicular polarizer and parallel polarizer condition in the case of PS-3D display adapting non-compensated ECB cell are 430:1 and 60:1, respectively. On the other hand, the CR for the perpendicular polarizer and parallel polarizer condition in the case of PS-3D display adapting compensated ECB cell are 250:1 and 240:1, respectively. Namely, an asymmetric CR on the left- and right-hand side of 3D glasses due to the wavelength-dependent phase dispersion of the $\lambda /2$ PS panel can be drastically improved by means of our optical configuration.

 

Fig. 4 Calculated transmittances as a function of wavelength for (a) non-compensated ECB and (b) compensated ECB cell. The left ones show the in the perpendicular polarizer condition and the right ones show the corresponding transmittance in the parallel polarizer condition.

Download Full Size | PDF

Figure 5 shows the actually measured transmittance curves as a function of wavelength in PS-3D display adapting compensated ECB cell fabricated in this work. The transmittance curves in the perpendicular polarizer condition and parallel polarizer condition are, respectively, shown in Fig. 5(a) and 5(b). The curves were detected by MCPD-3000. (Photal) The observed trends were corresponded to the calculated ones. The measured CR over the entire visible range was obtained using following equations:

 

Fig. 5 Measured transmittance as a function of wavelength in the (a) perpendicular polarizer condition and (b) parallel polarizer condition.

Download Full Size | PDF

4. Conclusion

In conclusion, we propose an optical configuration suitable for the development of a 3D PS display with symmetric and high CR. Our configuration consists of three retardation films and an active $\lambda /2$ retardation switching panel. An ECB cell was tested with the proposed configuration. The calculated and measured CRs suggest that ECB cells with symmetric and high CRs can be achieved. Thus, the optical configuration proposed here is suitable for 3D PS panels with active $\lambda /2$ retarder.