Automotive displays draw much attention, and the related markets are rapidly growing [1–4]. The viewing directions of automotive displays are different from those of conventional displays. Conventional displays are generally observed at a normal angle, and optical properties such as a contrast ratio (CR) in the normal viewing direction are most important. On the other hand, automotive displays are observed from oblique viewing directions [Fig. 1(a)] [5–8]. In particular, the center information display (CID) that is widely used in current automobiles is viewed by a driver and a passenger at symmetric, oblique viewing directions. Thus an optical property’s value within the specific viewing-angle range, which is known as the automotive viewing zone (AVZ), is important for a CID [4, 5]. The horizontal viewing zone of the AVZ is between (θ, ϕ), = (−40°, 180°) and (40°, 0°), where θ and ϕ are the polar and azimuthal viewing angles, respectively [5].

Figure 1. Optical configurations for the CID. (a) Illustration of the driver’s and passenger’s symmetric viewing angles for the CID studied in this paper. (b) Optical structure of the display components for SPM operation. CID, center information display; SPM, switchable privacy mode.

Another important requirement for automotive displays is preventing dangerous situations that may be caused by the driver’s negligence in looking ahead. The switchable-privacy-mode (SPM) display was introduced to resolve this issue [6, 7]. The SPM display is not seen by the driver in the driving state, while it is seen by both the driver and passenger in the stopped state. We call the former privacy mode and the latter shared mode. The switchablility between privacy and shared modes can be achieved by stacking dual liquid-crystal-display (LCD) panels [6, 7, 9–11], a polymer-dispersed liquid crystal (PDLC) with a louverlike waveguide [12–14], or a vertically switchable electrophoretic display [15]. In our previous paper, the SPM display optimized for the passenger display (co-driver display, CDD) was reported by stacking an in-plane-switching (IPS) LCD on a twisted-nematic (TN) display [7]. A CDD is seen by the driver and the passenger from asymmetric viewing angles, while a CID is seen from symmetric viewing angles. Hence, a different optical design is required for a CID.

To resolve this issue, a different combination of LCD panels is studied in this paper. As shown in Fig. 1(b), the IPS LCD is stacked on a vertically aligned (VA) LCD. A half-wave plate (HWP) is located between the VA LCD and the polarizer of the IPS LCD. The role of the VA LCD panel is to control the angular distribution of light intensity incident upon the IPS LCD.

The angular dependence of transmitted light, i.e., the viewing-angle property of the VA LCD, is asymmetric in the field-on state, provided that the pretilt angles on both substrates are antiparallel [Fig. 2(a)]. Hence the angular dependence of the incident light upon the IPS LCD can be switched by controlling the electric field across the VA LCD. Similar to the SPM for a CDD [7], the retarder plays a crucial role in optimizing SPM performance. In this paper we investigate the effect of the optical parameters of the HWP on the optical properties of the SPM. The orientation of the slow axis, the in-plane retardation, the dispersion of retardation, and the biaxiality of the HWP are varied to optimize the SPM.

Figure 2. SPM before optimization of the retarder. (a) Transmittance of the VA panel versus polar viewing angle θ with various voltages applied, before optimization of the retarder. Azimuthal viewing angle ϕ is 0° and 180° when θ is positive and negative, respectively. Contour plots of transmittance in (b) privacy and (c) shared mode, when a positive-dispersion retarder is used. 3 and 8 V are applied across the VA LCD in (b) and (c), respectively. R_in, ϕ_R , dR_in/dλ, and NZ are 275 nm, 22.5°, −0.11, and 1, respectively. SPM, switchable privacy mode; VA, vertical alignment; LCD, liquid-crystal display.

The optical parameters used in the simulation are summarized in Table 1. The SPM display panels include top, middle, and bottom polarizers, and the transmission axes of each polarizer are at 90°, 0°, and −45°, respectively. The optical structure in Fig. 1(b) can be divided into a top panel and a bottom panel, located above and below the middle polarizer (MP), respectively. The top panel is composed of the IPS liquid crystal (LC) layer, while the bottom cell is composed of the VA LC layer and the retarder. α_LC and β_LC represent the polar and azimuthal pretilt angles of the LC molecules, respectively. The IPS LC located in the top panel is initially oriented with α_IPS = 2° and β_IPS = 0° in the zero-field state. In the bottom panel, the VA LC is oriented with α_VA = 89.5° and β_VA = 180° in the zero-field state. The VA LC molecules switch to the negative x-axis direction with increasing electric field.

TABLE 1. Simulation parameters for the IPS LC, VA LC, polarizer, and retarder.

IPS LC
Polar Pretilt Angle (αIPS) (°)		2
Azimuthal Pretilt Angle (βIPS) (°)		0
Cell Gap (idIPS) (μm)		2.8
ne (at λ = 550 nm)		1.58
no (at λ = 550 nm)		1.49

VA LC
Polar Pretilt Angle (αVA) (°)		89.5
Azimuthal Pretilt Angle (βVA) (°)		180
Cell Gap (dVA) (μm)		4.2
ne (at λ =550 nm)		1.56
no (at λ = 550 nm)		1.48

Polarizer
Polarizer Transmission Axis (°)	Top	90
	Middle	0
	Bottom	−45

Retarder
Slow Axis (φR) (°)		16–25
nx (at λ = 550 nm)		1.56
ny (at λ = 550 nm)		1.48
Thickness (dR) (μm)		3.43
Rin (at λ = 550 nm) (μm)		200–320
dRin/dλ		−0.11~0.61
NZ (at λ = 550 nm)		−0.2~1.2

IPS LC, in-plane-switching liquid crystal; VA LC, vertically aligned liquid crystal..

The backlight is polarized along the −45° direction after the bottom polarizer (BP). Then it consecutively passes through the VA LC layer, the retarder, and the MP. The in-plane retardation R_in of the retarder at the wavelength of 550 nm is varied from 200–320 nm, where R_in is defined as (n_x − n_y)d, where n_x and n_y are the projected refractive indices on the xy plane, and d is the thickness of the medium. Since the transmission axis of the MP is aligned at 0°, the slow axis of the half-wave retarder is oriented at 16–25° to maximize transmittance (TR) after the MP. The dispersion of retardation dR_in/dλ is varied from −0.11 to 0.61 to investigate the effects of positive-dispersion (PD) and negative-dispersion (ND) retarders [16–19]. The PD and ND retarders have negative and positive signs of dR_in/dλ, respectively. The NZ coefficient is varied from −0.2 to 1.2 to optimize the viewing-angle dependence of the SPM. The NZ coefficient is defined as NZ = (n_x − n_z) / (n_x − n_y) [20–25].

The optical calculations are performed using commercial software packages (Sanayi System Co., Incheon, Korea). The Poincare plot is generated with Techwiz Polar (Sanayi System Co.), the contour plots and TR value are calculated with Techwiz 2D (Sanayi System Co.), and the real-image calculation is generated with Techwiz 2D. The optical calculation is basically based on the extended-Jones-matrix method [6–9].

3.1. Dual-mode Operation before Optimization of the Retarder

Figure 2(a) is a simulation result showing the TR of the VA panel along the polar viewing angle θ with various voltages applied, before optimization of the retarder. The azimuthal viewing angle ϕ is 0° and 180° when θ is positive and negative, respectively. A PD retarder is placed above the VA panel. The MP is located above the retarder, while the IPS LCD is not included in this simulation. As reference conditions, R_in, ϕ_R, dR_in/dλ, and NZ are 275 nm, 22.5°, −0.11, and 1, respectively. Given 3 V across the VA panel, relatively low TR is obtained at the driver’s viewing angle (θ, ϕ) = (−40°, 180°) and high TR is obtained at the passenger’s viewing angle (θ, ϕ) = (40°, 0°). This corresponds to privacy mode. In contrast, when 8 V is applied a relatively symmetric TR profile is shown, corresponding to shared mode.

Figures 2(b) and 2(c) are the contour plots of TR when 3 and 8 V are applied across the VA panel, respectively. Figure 2(b) also shows that low TR is obtained at the driver’s viewing angle (θ, ϕ) = (−40°, 180°), while high TR is obtained at the passenger’s viewing angle (θ, ϕ) = (40°, 0°), resulting in privacy mode. Meanwhile, Fig. 2(c) shows that high TR is obtained at both (θ, ϕ) = (−40°, 180°) and (θ, ϕ) = (40°, 0°), resulting in shared mode. Thus, privacy and shared modes can be switched by applying 3 or 8 V across the VA panel.

The TR at the driver’s and passenger’s viewing angle was 1.6% and 40.6%, respectively, in privacy mode. Those values are 36.6% and 40.1%, respectively, in shared mode. Thus, light is not perfectly blocked at the driver’s viewing angle (θ, ϕ) = (−40°, 180°) in privacy mode. In addition, TR is slightly decreased at the passenger’s viewing angle (θ, ϕ) = (40°, 0°) in shared mode. Therefore, the TR profile along the horizontal viewing angle needs to be improved for better SPM. The optical parameters of the retarder (R_in, ϕ_R, dR_in/dλ, and NZ) are varied to reduce TR at the driver’s viewing angle in privacy mode, while holding high TR at the passenger’s viewing angle in both privacy and shared modes. For comparison of SPM performance, the extinction ratio (ER), which is the ratio of TR at the passenger’s viewing angle divided by TR at the driver’s viewing angle, is used. From Fig. 2, the ER is 23 in privacy mode before optimization of the retarder.

3.2. Effect of R_in

Figure 3(a) is the Poincare sphere showing the polarization state of light with wavelengths of 450, 550, and 650 nm that passes through the HWP in privacy mode, at the driver’s viewing angle (θ, ϕ) = (−40°, 180°). The polarization of light with wavelength of 550 nm is located close to the −S1 axis at (θ, ϕ) = (−40°, 180°) [Fig. 3(a)]. On the other hand, the polarization state of light with wavelengths of 450 and 650 nm is quite away from the −S1 axis. The dependence of polarization state on R_in is investigated [Fig. 3(b) and 3(c)].

Figure 3. Polarizaiton state vs. R_in. (a) Poincare plot of polarization state after passing through the HWP in privacy mode at (θ, ϕ) = (−40°, 180°). 3 V is applied across the VA cell. (b) Average distance l_−S1 of the final polarization state after HWP from the −S1 axis in the dark state at the driver’s viewing angle (solid), and l_+S1 from the +S1 axis in the bright state at the passenger’s viewing angle (empty) versus R_in of the HWP in privacy mode. (c) l_+S1 of the final polarization state from the +S1 axis in shared mode versus R_in of the HWP. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 22.5°, −0.11, and 1, respectively in (a), while R_in is varied with other parameters fixed in (b) and (c). Numbers in (b) and (c) correspond to the TR. HWP, half-wave plate; VA, vertical alignment; TR, transmittance.

Figure 3(b) shows the average distance between the polarization state and the −S1 (solid notation) or +S1 axis (empty notation) at the driver’s and passenger’s viewing angle, respectively, in privacy mode. The average distance means the distance that is averaged for light with wavelengths of 450, 550, and 650 nm. When R_in of the HWP is 243 nm, the average distance from the −S1 axis l_−S1. _Average at the driver’s viewing angle becomes a minimum at 0.36, which corresponds to a TR of 0.9% [Fig. 3(b)]. When R_in of the HWP is 307 nm, the average distance from the +S1 axis l_+S1. _Average at the passenger’s viewing angle becomes a minimum of 0.846. With this condition, TR at the passenger’s viewing angle is maximized. However, when R_in of the HWP is 307 nm, TR at the driver’s viewing angle increases to 3.8%, which is much greater than 0.9% when R_in of the HWP is 243 nm. The most important requirement in SPM operation is to block light toward the driver’s viewing angle in privacy mode; Thus TR at the driver’s viewing angle must be low, and it is desirable to set R_in = 243 nm.

Figure 3(c) shows l_+S1. _Average in shared mode. When R_in of the HWP is 243 nm, l_+S1. _Average at the driver’s and passenger’s viewing angle is 0.94 and 0.81, respectively, corresponding to TR of 33.3% and 41.5%, respectively. The average distance at the driver’s as well as the passenger’s viewing angle is not minimized when R_in is 243 nm in shared mode, i.e., TR at the driver’s viewing angle can be increased by increasing R_in of the HWP. However, TR at the driver’s viewing angle in privacy mode should be minimized in SPM, so R_in= 243 nm is a suitable condition. The ER value is 43 in privacy mode, which is greater than its value of 23 before optimization of the HWP.

3.3. Effect of ϕ_R

Next, we turn to the dependence on ϕ_R of the HWP (Fig. 4). ϕ_R is the angle between the x-axis and the slow axis of the HWP. Figure 4(a) shows the polarization state after the HWP at the driver’s viewing angle (θ, ϕ) = (−40°, 180°), in privacy mode. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 23°, −0.11, and 1, respectively.

Figure 4. Polarizaiton state vs. ϕ_R. (a) Poincare plot of polarization state after passing through the HWP, in privacy mode at (θ, ϕ) = (−40°, 180°). 3 V is applied across the VA cell. (b) Average distance l_−S1 of the final polarization state after the HWP from the −S1 axis in the dark state at the driver’s viewing angle (solid), and l_+S1 from the +S1 axis in the bright state at the passenger’s viewing angle (empty), versus ϕ_R of the HWP, in privacy mode. (c) l_+S1 of the final polarization state from the +S1 axis in shared mode versus ϕ_R of the HWP. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 23°, −0.11, and 1 respectively in (a), while ϕ_R is varied with other parameters fixed in (b) and (c). Numbers in (b) and (c) correspond to the TR. HWP, half-wave plate; VA, vertical alignment; TR, transmittance.

Figure 4(b) shows the average distance between the polarization state and the −S1 or +S1 axis at the driver’s and passenger’s viewing angle, in privacy mode. When ϕ_R of the HWP is 23°, l_−S1. _Average becomes a minimum of 0.36, and TR is 0.9% at the driver’s viewing angle [Fig. 4(b)]. A question may be raised: Why is the optimum ϕ_R still close to 22.5°, even though the display panel is seen at an oblique viewing angle θ = 40°? The angle of 22.5° is the optimum value when the HWP is seen from the normal viewing direction. This is related to the fact that R_in of the VA-LC layer for 3 V applied is not 275 nm but 118.1 nm, in the normal viewing direction [also see Fig. 2(a)]. The retardation is 32.0 nm at the driver’s viewing angle and 268.5 nm at the passenger’s viewing angle, in the 3-V state. Thus, the optimum retarder orientation angle does not change very much for the SPM, which is seen at oblique viewing angles.

When ϕ_R of the HWP is 18°, l_+S1. _Average at the passenger’s viewing angle shows a minimum value of 0.80, and corresponding TR of 40.3%. Although this condition can achieve maximum TR at the passenger’s viewing angle, TR increases to 2.3% at the driver’s viewing angle. As mentioned previously, blocking light at the driver’s viewing angle is crucial in SPM, so the condition of ϕ_R = 23°, which results in lower TR at the driver’s viewing angle, is more suitable for SPM operation.

Figure 4(c) shows l_+S1. _Average at the driver’s and passenger’s viewing angles, in shared mode. When ϕ_R of HWP is 23°, l_+S1. _Average at the driver’s and passenger’s viewing angles is 0.952 and 0.833, respectively, corresponding to TR of 33.0% and 41.3%, respectively. Meanwhile, the maximum TR at the driver’s and passenger’s viewing angle in shared mode is seen when ϕ_R is 18.5 and 20°, respectively. However, TR for the driver’s viewing direction in privacy mode increases to 2.1% and 1.6%, respectively, under those conditions. Therefore, the condition of ϕ_R=23° is desirable for SPM operation. The change in ER value is negligible after optimization of ϕ_R.

3.4. Effect of dR_in/dλ

Next, the effect of the retardation dispersion dR_in/dλ of the HWP on the SPM is investigated (Fig. 5). The sign of dR_in/dλ is negative and positive when the retarder has the PD and ND property, respectively [16, 17]. The ideal value of dR_in/dλ for a perfect achromatic HWP is 0.5 [14]. Figure 5(a) shows the polarization state of the light after the HWP at the driver’s viewing angle, in privacy mode. R_in, ϕ_R, dR_in/dλ, and NZ were set to 243 nm, 23°, 0.53 and 1, respectively. The polarization states of the RGB light are located near the −S1 axis.

Figure 5. Polarizaiton state vs. dR_in/dλ. (a) Poincare plot of polarization state after passing through the HWP, in privacy mode at (θ, ϕ) = (−40°, 180°). 3 V is applied across the VA cell. (b) Average distance l_−S1 of the final polarization state after the HWP from the −S1 axis in the dark state at the driver’s viewing angle (solid), and l_+S1 from the +S1 axis in the bright state at the passenger’s viewing angle (empty), versus dR_in/dλ of the HWP. (c) l_+S1 of the final polarization state from the +S1 axis in shared mode versus dR_in/dλ of the HWP. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 23°, 0.53, and 1, respectively in (a), while dR_in/dλ is varied with other parameters fixed in (b) and (c). Numbers in (b) and (c) correspond to the TR. HWP, half-wave plate; VA, vertical alignment; TR, transmittance.

Figure 5(b) shows the average distance between the polarization state and the −S1 or +S1 axis at the driver’s and passenger’s viewing angles, in privacy mode. dR_in/dλ is varied from −0.11 to 0.61. l_−S1. _Average and l_+S1. _Average for a PD HWP with dR_in/dλ = −0.11 are 0.355 and 0.878, corresponding to TR values of 0.9% and 39.2% at the driver’s and passenger’s viewing angle, respectively. l_−S1. _Average can be reduced to 0.032 by using a ND HWP with dR_in/dλ = 0.53, corresponding to TR of 0.01%. TR at the passenger’s viewing angle is 41.0% for the corresponding dR_in/dλ.

Figure 5(c) shows l_+S1. _Average at the driver’s and passenger’s viewing angles, in shared mode. l_+S1. _Average decreases with increasing dR_in/dλ. When dR_in/dλ is 0.53, the distance at the driver and passenger is 0.88 and 0.57, respectively, corresponding to TR of 33.97% and 43.4%, respectively. ER value increases dramatically to 4,100 after optimization of dR_in/dλ, implying that the ND HWP can play a crucial role in improvement of SPM performance.

3.5. Effect of NZ

As a final step, the effect of NZ of the HWP is investigated. NZ of a uniaxial a-plate is 1.0, while that of an ideal z-plate is 0.5. The ideal z-plate is known to result in the widest field of view, i.e., the least viewing-angle dependence of the retarder [18–20]. Figure 6(a) shows the polarization state at the driver’s viewing angle, in privacy mode. R_in, ϕ_R, dR_in/dλ, and NZ are set to 243 nm, 23°, 0.53 and 0.7, respectively.

Figure 6. Polarizaiton state vs. NZ. (a) Poincare plot of polarization state after passing through the HWP, in privacy mode at (θ, ϕ) = (−40°, 180°). 3 V is applied across the VA cell. (b) Average distance l_−S1 of the final polarization state after the HWP from the −S1 axis in the dark state at the driver’s viewing angle (solid), and l_+S1 from the +S1 axis in the bright state at the passenger’s viewing angle (empty), versus NZ of the HWP. (c) l_+S1 of the final polarization state from the +S1 axis in shared mode, versus NZ of the HWP. R_in, ϕ_R, and dR_in/dλ are set to 243 nm, 23°, and 0.53, respectively. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 23°, 0.53, and 0.95, respectively in (a), while the NZ is varied with other parameters fixed in (b) and (c). Numbers in (b) and (c) correspond to the TR. HWP, half-wave plate; VA, vertical alignment; TR, transmittance.

Figure 6(b) shows l_−S1. _Average and l_+S1. _Average at the driver’s and passenger’s viewing angle in privacy mode, respectively. NZ is varied from −0.2 to 1.2. l_−S1. _Average is 0.026 when NZ is 0.95, resulting in TR of 0.01% at the driver’s viewing angle. TR at the passenger’s viewing angle is 41.3% for NZ of 0.95. When NZ is 1.0, l_−S1. _Average is 0.032. The small difference in l_−S1. _Average when NZ is 0.95 or 1.0 has a negligible effect on TR.

Figure 6(c) shows l_+S1. _Average at the driver’s and passenger’s viewing angles, in shared mode. As NZ increases, the distance at the driver’s viewing angle increases, whereas that at the passenger’s viewing angle slightly decreases. When NZ is 0.7, l_+S1. _Average at the driver’s and passenger’s viewing angle is 0.786 and 0.565, respectively, corresponding to TR of 36.2% and 43.35%, respectively. However, the increase in TR is not significant, so it is desirable to set NZ to 0.95 to minimize TR at the driver’s viewing angle in privacy mode. The ER value slightly increases to 4,130.

3.6. Dual-mode Operation after Optimization of the Retarder

Figure 7 shows TR along the polar viewing angle θ, after optimization of the HWP. The simulated structure is identical to that in Fig. 1(b). When a voltage of 3 V is applied (i.e., in privacy mode), TR at the driver’s and passenger’s viewing angle is 0.01% and 41.3%, respectively, while it was 1.6% and 36.6% before optimization of the HWP. Thus the dark state as well as the bright state is improved in privacy mode. When a voltage of 8 V is applied (i.e., in shared mode), TRs at the driver’s and passenger’s viewing angle are 34.6% and 43.5%, while it was 36.6% and 40.1% before optimization of the HWP. The ER value is 23 and 4,130, before and after optimization of the HWP. Thus the SPM performance is dramatically improved by optimization of the HWP.

Figure 7. SPM after optimization of the retarder. (a) Transmittance of the VA panel versus polar viewing angle θ with various voltages applied, before optimization of the retarder. Azimuthal viewing angle ϕ is 0° and 180° when θ is positive and negative, respectively. Contour plots of transmittance in (b) privacy and (c) shared mode, after optimization of the HWP. R_in, ϕ_R, dR_in/dλ, and NZ are 243 nm, 23°, 0.53, and 0.95, respectively. SPM, switchable privacy mode.

In our previous paper [7], the IPS panel was stacked on the TN panel for SPM of the CDD. A question may be raised as to whether an IPS-TN panel can also be used for a CID. An ER value of 901 is achieved at the driver’s viewing angle by using the IPS-TN panel. In addition, TR of 40.0% and 43.9% is obtained at the passenger’s viewing angle (θ, ϕ) = (40°, 0°) in privacy and shared mode, respectively. As described above, the corresponding TR is 41.3% and 43.5% using the IPS-VA panel. Thus, the ER value at the driver’s viewing angle increases when using a IPS-VA panel, while keeping a similar TR at the passenger’s viewing angle. Therefore, the IPS-VA panel is more suitable for a CID display, while the IPS-TN panel is suitable for a CDD display.

Because the polarizers are not orthogonal in the optical structure proposed for SPM in this paper, one may raise a question about potential color shift by the VA panel. Figure 8 is a CIE 1976 color-coordinate graph, showing the colors perceived by driver and passenger. The color coordinates of the cold-cathode fluorescent lamp (CCFL) backlight are (a*, b*) = (11.6, −20.6). In privacy mode [Fig. 8(a)], the color co-ordinates at the driver’s and passenger’s viewing angle become closer to those of the CCFL after optimization of the retarder. In shared mode [Fig. 8(b)], the color coordinates at the driver become closer to those of the CCFL, while the color coordinates at the passenger are still away from those of the CCFL. Thus it is also confirmed that a biaxial ND retarder can be helpful to reduce the color shift, as well as to increase the transmittance.

Figure 8. Color property of the SPM. CIE 1976 color coordinates at the driver’s and passenger’s viewing angles in (a) privacy and (b) shared mode, after optimization of the HWP. 3 V and 8 V are applied to the bottom cell in (a) and (b), respectively. R_in, ϕ_R, dR_in/dλ, and NZ are 275 nm, 22.5°, −0.11, and 1, respectively, before optimization (empty), whereas the corresponding parameters are 243 nm, 23°, 0.53, and 0.95 after optimization (solid). SPM, switchable privacy mode.

Figure 9 shows the results of image simulation using Techwiz 2D. Figures 9(a) and 9(b) correspond to the images viewed at the driver’s and passenger’s viewing angle in privacy mode, respectively. Figures 9(c) and 9(d) are the corresponding images in shared mode, respectively. Similar to the results in Fig. 7, the light is well blocked at the driver’s viewing angle in privacy mode. It is also observed that the image is clearly seen at the passenger’s viewing angle in privacy mode. The prototype image is also clearly seen in shared mode, without significant distortion (such as grayscale inversion). Compared to other SPM modes using a TN [7, 10] or hybrid-aligned-nematic (HAN) cell [6], the approach using a VA panel plus ND retarder may be helpful for improving the ER value. On the other hand, voltage must be applied both for privacy and shared modes when a VA panel is used.

Figure 9. Demonstration of simulated images observed at the driver’s and passenger’s viewing angles, in [(a) and (b)] privacy and [(c) and (d)] shared modes. The optical structure of the simulation is the same as in Fig. 1(b). The four parameters of the retarder (R_in, ϕ_R, dR_in/dλ, and NZ) are 243 nm, 23°, 0.53, and 0.95, respectively.

In this study, we have simulated an SPM display by combining a VA LCD with an IPS LCD. The effect of the orientation, in-plane retardation, retardation dispersion, and biaxiality of the retarder were investigated for better optical quality in SPM performance. The effect of the dispersion retarder was more prominent than those of the other parameters, on the ER value of the SPM display. The ER value was 4130 for retarder parameters Rin, ϕ_R, dR_in/dλ, and NZ of 243 nm, 23°, 0.53, and 0.95, respectively.

This study was supported by a grant from the National Research Foundation (NRF) (Grant No. 2019R1A 6A1A09031717, 2023R1A2C1002767), Institute of Information & Communications Technology Planning & Evaluation (IITP) (Grant No. IITP-2024-RS-2024-00439292), and Jeonbuk National University in 2024.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All data generated or analyzed during this study are included in this published article.