1. Introduction

Privacy protection is important in daily life. People do not want their personal information been noticed by others in various fields, which certainly include information displays [1–12]. Liquid crystal displays (LCDs) are currently the most common information displays in the world. People need to read information, see footage/movie, play video game, or work by LCDs. LCDs usually have wide viewing-angles. However, in case that private information (PI) must be shown to some users through such LCDs and the PI does not want to be noticed by people nearby the users, so controllable viewing-angle LCDs are developed [6–12]. Controllable viewing-angle LCDs can be switched between narrow viewing angles and wide ones. Placing commercial filters on common LCDs can also limit viewing angles to prevent people around the LCDs from peeping PI. Such commercial filters are usually made of considerable micro louvers [12,13].

Although an LCD can be switched from a wide viewing-angle to a narrow one using the above methods, people standing within the narrow viewing-angle range can still see PI displayed on the screen. Accordingly, a method to prevent people around an LCD from peeping PI displayed on the screen at any viewing angles should be developed. The angle between the transmissive axes of a top linear polarizer (LP) and a bottom LP of a common LCD is usually 90°. A currently available way to reach the goal is to remove the top LP of a common LCD, such an LCD is named as top LP-removed LCD [14]. One can see the image shown by the top-LP removed LCD through a portable LP. However, the angle (α) between the transmissive axis of the portable LP and that of the bottom LP inside the top-LP removed LCD should be kept at 90° because the image brightness/contrast displayed on the top-LP removed LCD is a function of α according to Malus’ law.

Two methods to prevent people nearby LCDs from peeping PI displayed on the screens are reported. The first one is given as follows. When a user does not want PI displayed on a phone screen to be seen by someone else, the user can make the phone generate a blurred graphic output. The user wearing a special eyewear could clearly perceive the blurred graphic output, while people without wearing the eyewear could not clearly perceive it [15]. However, people without wearing the eyewear may know that PI is hidden because they see blurred graphic output. The second one (smart spying prevention) to protect privacy uses facial identification. When a cell phone detects faces of others, notifications/PI displayed on phone screens will be hided [16]. Such identification may not be 100% accuracy under all situations. The key feature of the study is to use optical manners to realize privacy protection in an LCD, and people without wearing a special eyewear around the LCD cannot notice that PI is hidden in images.

Circular polarizers (CPs) have been widely studied and are key components of transflective LCDs and 3D LCDs. Lin et al. and Ge et al. designed wide-view and broadband CPs (WVBCPs) for transflective LCDs to reduce off-axis light leakage to increase the contrast ratio [17,18]. Kang et al. also developed WVB quarter waveplates (WVBQWPs) for 3D LCDs to display high-quality images [19].

The technique of hiding PI in PI protection LCDs (PIPLCDs) using periodical waveplates and pixel quaternity (pixel_4in1) is reported. Waveplates used herein are WVBQWPs, and pixel quaternity means that four pixels are merged to be one pixel. The operation mechanism and structure of the PIPLCD to realize the goal are elucidated. Users employing WVBCPs can see PI hidden in images displayed on the PIPLCD in PIP mode (PIPLCD_PIP), while people without using the WVBCPs cannot see the PI and only see the displayed images. The issues of the PIPLCD_PIP are reported; possible methods to fix them are discussed. Possible ways to fabricate the periodical waveplates are also reported.

2. PIPLCD structure

Figure 1(a) schematically shows a PIPLCD consisting of a common LCD [color filters and backlight unit are not drawn] and a checkerboard-pattern-(CBP-) like WVBQWP array, which is composed of periodically arranged WVBQWPs#1 and WVBQWPs#2 and placed on top of the top LP [17–20]. The transmissive axes of top and bottom LPs are orthogonal. A single WVBQWP#1/WVBQWP#2 aligns to a single pixel of the thin-film transistor (TFT) array. A light after normally passing through the top LP, whose transmissive axis is along the x-axis, is linearly polarized; the linearly polarized light then transforms to a circularly polarized light#1 (circularly polarized light#2) after passing through a WVBQWP#1 (WVBQWP#2). The handedness of circularly polarized light#1 and that of the circularly polarized lights#2 are opposite. The structure drawn in Fig. 6(a) in Ref. [18] can be used to realize the WVBQWP#1/WVBQWP#2 and will be further discussed using Fig. 3. Figure 1(b) schematically presents the cross section of the PIPLCD structure on the xz-plane (the red plane) in Fig. 1(a).

 

Fig. 1. (a) A PIPLCD comprising a common LCD with a CBP-like WVBQWP array. (b) Cross section of the PIPLCD structure on the xz-plane (the red plane) in (a).

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2.1 Method to hide PI using the PIPLCD_PIP

According to Fig. 1(a), Fig. 2(a) schematically draws top view of a partial area of a normally black PIPLCD in 4 K resolution mode (PIPLCD_4K) under a microscopic perspective. A square represents a pixel comprising red (R), green (G), and blue (B) subpixels [20]; a R/G/B subpixel can display 256 R/G/B gray levels (GLs). To display a white image, Fig. 2(a) shows that a R/G/B subpixel in each pixel of the PIPLCD_4K display the 255^th GL. Such a 4 K LCD, i.e. PIPLCD_4K, can be used in a private space to display ultrahigh definition images. For the scenario where users are in a public space, when the PIPLCD_4K is switched to the PIPLCD_PIP, as illustrated in Fig. 2(b), the inset in Fig. 2(b) indicates that the adjacent four pixels are combined to form a single pixel, such a four-in-one pixel is called pixel_4in1 (pixel quaternity); pixel#1/#4 (pixel#2/#3) in each pixel_4in1 aligns to a WVBQWP#2 (WVBQWP#1). The inset in Fig. 2(b) indicates that when the PIPLCD_PIP is working, pixel#2 and pixel#3 in each pixel_4in1 are not operational; only pixel#1 and pixel#4 in each pixel_4in1 are operational; pixel#1 and pixel#4 in a pixel_4in1 display the same color. Figure 2(b) shows the condition that the PIPLCD_PIP still displays a white image when there is no PI to be hidden, so R/G/B subpixels of pixel#1 and pixel#4 in each pixel_4in1 displays the 255^th GL; R/G/B subpixels of pixel#2 and pixel#3 in each pixel_4in1 display the 0^th GL. Given that only two of the four pixels in each pixel_4in1 in Fig. 2(b) are operational, the brightness of the white image of Fig. 2(b) is lower than that of Fig. 2(a) under the same backlight intensity. The issue can be solved by increasing backlight intensity when the PIPLCD_4K is switched to the PIPLCD_PIP; the issue will be further discussed in section 4 (§ 4 PIPLCD_PIP issues). The red “NH” character frames in Fig. 2(b) indicate that the information “NH” is going to be hidden.

 

Fig. 2. (a) Top view of partial area of a normally black PIPLCD_4K displays a white image under a microscopic perspective according to Fig. 1(a). (b) Condition that the PIPLCD_PIP displays a white image; the inset in (b) indicates that R/G/B subpixels of pixel#1 and pixel#4 [pixel#2 and pixel#3] in each pixel_4in1 of the PIPLCD_PIP display the 255^th [0^th] GL. The red “NH” character frames in (b) means that the PI “NH” that is going to be hidden. (c) To hide “NH” and display a white image at the same time, the original display method of each pixel_4in1 within the red “NH” character frames in (b) should be changed. The inset in (c) shows that the PIPLCD_PIP in (b) needs to hide “NH”, so R/G/B subpixels of pixel#2 and pixel#3 [pixel#1 and pixel#4] in each pixel_4in1 within the red “NH” character frames display the 255^th [0^th] GL. (d) The expected observation result which a user employing a WVBCP through the condition drawn in the inset (i) in (d) can see; the user can see “NH” hidden in a white image displayed on the PIPLCD_PIP in (c). The insets (ii) and (iii) in (d) reveal that the WVBCP comprises a WVBQWP#3 and a LP_WVBCP. The insets (ii) and (iii) show that the WVBCP can let circularly polarized light#1 pass through and blocks circularly polarized light#2, respectively.

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To hide “NH” and display a white image at the same time, the original display method of each pixel_4in1 within the red “NH” character frames in Fig. 2(b) should be changed. The inset in Fig. 2(c) indicates that in each pixel_4in1 inside the red “NH” character frames, pixel#1 and pixel#4 [see the pixel_4in1 within the green square] are not operational; only pixel#2 and pixel#3 are operational; pixel#2 and pixel#3 in a pixel_4in1 display the same color. Accordingly, Fig. 2(c) shows that R/G/B subpixels of pixel#2 and pixel#3 [pixel#1 and pixel#4] in each pixel_4in1 inside the red “NH” character frames displays the 255^th [0^th] GL, whereas R/G/B subpixels of pixel#1 and pixel#4 [pixel#2 and pixel#3] in each pixel_4in1 outside the red “NH” character frames still displays the 255^th [0^th] GL. For the same R/G/B light exiting the top LP, assuming that the loss of it passing through WVBQWP#1 and that of it passing through WVBQWP#2 are close, we deduce that the brightness of a pixel_4in1 inside the red “NH” character frames and that of a pixel_4in1 outside the red “NH” character frames are close (the brightness difference between them is basically indistinguishable). A user employing a WVBCP through the condition drawn in the inset (i) in Fig. 2(d) can see the “NH” hidden in a white image displayed on the PIPLCD_PIP drawn in Fig. 2(c), and Fig. 2(d) is the expected observation result which the user can see. The inset (ii)/(iii) in Fig. 2(d) shows that the WVBCP comprises a WVBQWP#3 and a LP_WVBCP (to avoid confusion, the LP used in the WVBCP is called LP_WVBCP). The inset (ii) and (iii) in Fig. 2(d) present that WVBCP can let circularly polarized light#1 pass through and block circularly polarized light#2, respectively [17–19]. The structures of WVBCP, WVBQWP#1, and WVBQWP#2 are elucidated in the next paragraph.

Figure 3 presents the WVBCP, WVBQWP#1, and WVBQWP#2 structures on the basis of the WVBCP structure of Fig. 6(a) in Ref. [18]; the symbols ($\varphi $) with the + A/-A plates in Fig. 6(a) in Ref. [18] are consistent with those in Fig. 3. To connect Fig. 6(a) in Ref. [18] to Fig. 3 in this study, we set that the transmissive axes of the top and bottom LPs of Fig. 6(a) in Ref. [18] are along the y- and x-axes in Fig. 1(a), respectively. Referring to Fig. 3, the WVBQWP#1 comprises a λ/4 +A plate ($\varphi _{\frac{1}{4}\lambda }^{top} ={-} {75^o}$) and λ/2 -A plate ($\varphi _{\frac{1}{2}\lambda }^{top} = {75^o}$); the WVBQWP#2 consists of a λ/4 -A plate ($\varphi _{\frac{1}{4}\lambda }^{btm} ={-} {75^o}$) and λ/2 +A plate ($\varphi _{\frac{1}{2}\lambda }^{btm} = {75^o}$); the WVBCP comprises an LP_WVBCP ($\varphi _P^{top} = {90^o}$), λ/2 -A plate ($\varphi _{\frac{1}{2}\lambda }^{top} = {75^o}$), and λ/4 +A plate ($\varphi _{\frac{1}{4}\lambda }^{top} ={-} {75^o}$). Figures 3(a) and 3(b) correspond to the insets (iii) and (ii) in Fig. 2(d), respectively; the top LPs in Figs. 3 and 1(a) are the same. Figure 8(b) in Ref. [18] indicates that the light leakages viewed at an azimuthal angle around 0°/180° [90°/270°] and a polar angle from 0° to 80° under the configuration in Fig. 3(a) is lower than about 1%, and the light leakage viewed almost over the entire viewing cone under the configuration in Fig. 3(a) is lower than about 3.5%. The result indicates that the WVBCP [inset (iii) in Fig. 2(d)] can effectively block the circularly polarized light#2 almost over the entire viewing cone. To further evaluate the performance of the configuration in Fig. 3(a)/3(b), Figs. 3(c) and 3(d) show the simulation angular light leakage and transmission pictures at wavelength of 550 nm using the configurations in Figs. 3(a) and 3(b) by a commercial software,1D-DIMOS, respectively. The refractive indices of the + A/-A plate used in Ref. [18] and the default LP in 1D-DIMOS are used in the simulations; the λ/2 (λ/4) +A and λ/2 (λ/4) -A plates used in the simulations are designed for 550 nm. Depending on the results in Figs. 3(c) and 3(d), PI with enough contrast ratio (> 5:1) [20] can be realized over the viewing cone with azimuthal [polar] angle from 0° to 360° [0° to 81°] at wavelength of 550 nm. According to the above discussions, we deduce that a user who uses the WVBCP can see “NH” with enough contrast ratio hidden in a white image displayed on the PIPLCD_PIP [see discussion of Fig. 2(d)] over an enough wide viewing cone for practical use. However, a person who does not use the WVBCP nearby the PIPLCD_PIP cannot see “NH” and still see a white image almost over the entire viewing cone. The advantage of using the WVBCP is that the brightness/contrast of PI displayed on the PIPLCD_PIP theoretically does not vary when users rotate the WVBCP on the xy-plane [see inset (i) in Fig. 2(d)]. The deduction is reasonable because the transmissive axis of the LP_WVBCP and the optical axis of the λ/2 -A plate ($\varphi _{\frac{1}{2}\lambda }^{top} = {75^o}$) is fixed [14]. The WVBQWP#3 of the WVBCP can let circularly polarized light#1 transform to a linearly polarized light with its polarization direction, which is only parallel to the transmissive axis of the LP_WVBCP. Compared with the top-LP removed LCD, image brightness/contrast varies when users rotate the portable LP on the xy-plane.

 

Fig. 3. Structures of the (a) WVBCP/WVBQWP#2 and (b) WVBCP/WVBQWP#1 to realize the PIPLCD_PIP based on the WVBCP structure of Fig. 6(a) in Ref. [18]. The WVBCPs in (a) and (b) are the same. Simulations of (c) leakage and (d) transmission of light (λ=550 nm) through the configurations plotted in (a) and (b) at various viewing angles (azimuthal and polar angles), respectively.

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The main issue of the display method in Fig. 2(b) is that “NH” could be seen by people who do not use the WVBCPs because the display method of the pixels_4in1 inside the red “NH” character frames [inset of Fig. 2(c)] and that outside the red “NH” character frames [inset of Fig. 2(b)] are different. The issues will be further discussed in section 4 of the paper (§ 4 PIPLCD_PIP issues). The method to hide PI in colorful images displayed by the PIPLCD_PIP depends on the same method discussed in Figs. 2(a)–(d).

2.2 Alternative way to hide PI

The concept in § 2.1 can also be realized by replacing the CBP-like WVBQWP array in Fig. 1(a) with a CBP-like WVB half waveplate (WVBHWP) array, which is composed of periodically arranged WVBHWPs and isotropic transparent plates (ITPs), as illustrated in Fig. 4(a) [18]. Figures 1(a) and 4(a) are the same, except that the CBP-like WVBQWP array is replaced with a CBP-like WVBHWP array. To avoid confusion, the PIPLCD with a CBP-like WVBHWP array is named as PIPLCD*; PIPLCD* in PIP (4 K) mode is named as PIPLCD*_PIP (PIPLCD*₄K). An individual WVBHWP/ITP aligns to a pixel of TFT array. Figure 4(b) schematically presents the cross section of the PIPLCD* structure on the xz-plane (the red plane) in Fig. 4(a). The linear polarization direction of a linearly polarized light after passing through a WVBHWP is designed to be 90° rotated, so the polarization directions of linearly polarized light#2 and linearly polarized light#1 are orthogonal. Replacing the PIPLCD_4K and PIPLCD_PIP with the PIPLCD*₄K and PIPLCD*_PIP, respectively, replacing Fig. 1(a) with Fig. 4(a), and replacing the WVBQWP#2 (WVBQWP#1) with ITP (WVBHWP), the text about Fig. 2(a) [2(b)] {2(c)} in the first two paragraphs in § 2.1 and Fig. 2(a) [2(b)] {2(c)} can be used to elucidate the operation of the PIPLCD*₄K displaying a white image [the operation of the PIPLCD*_PIP displaying a white image] {the method to hide “NH” in a white image displayed on the PIPLCD*_PIP}. Figure 4(c) shows that a portable LP can let linearly polarized light#1 pass through and block linearly polarized light#2 when the transmissive axes of the portable LP and the top LP are perpendicular. Figure 4(d) shows that when a user sees PI hidden in a white image displayed on the PIPLCD*_PIP [see Fig. 2(c)] through a portable LP, and Fig. 2(d) is also the expected observation result that the user sees. The user can see “NH” hidden in a white image because they only can see linearly polarized light#1 [Fig. 4(c)]. A person who does not use the portable LP cannot see “NH” and only see a white image. Referring to Fig. 4(d), when a user faces the PIPLCD*_PIP and rotates the portable LP, PI brightness/contrast changes [14]. As previously mentioned, the change of PI brightness/contrast issue theoretically does not occur in PIPLCD_PIP by rotating the WVBCP [Fig. 3] on the xy-plane. Thus, the PIPLCD [Fig. 1(a)] is preferred to realize PIP in this study.

 

Fig. 4. (a) A PIPLCD* comprising a common LCD with a CBP-like WVBHWP array. (b) Cross section of the PIPLCD* on the xz-plane (the red plane) in (a). (c) A portable LP can let linearly polarized light#1 pass through and block linearly polarized light#2. (d) Observation configuration of a user seeing the PI hidden in a white image displayed on the PIPLCD*_PIP using a portable LP.

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3. Expected results using the PIPLCD_PIP

Figure 5(a) schematically shows that the PIPLCD_PIP is displaying a colorful image. Figure 5(b) shows the expected image seen by a person who does not use the WVBCP; the images displayed on Figs. 5(a) and 5(b) are the same. Each lens of a common eyewear in Fig. 5(c) is added with the WVBCP, so Fig. 5(c) schematically presents that the PI [“All we…given us.” is quoted from the movie “The Lord of the Rings: The Fellowship of the Ring”] can be seen by a user who wears the eyewear with the WVBCPs.

 

Fig. 5. (a) A PIPLCD_PIP displays a colorful image. (b) Expected image seen by a person who does not use the WVBCP. (c) Expected image seen by a user who wears a common eyewear with the WVBCPs. The image is captured by the author (C.-K. Liu).

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The images in Fig. 6 are schematically used to further discuss the expected results of Fig. 5(c). Figures 6(a), 6(b), 6(c), and 6(d) show the images displayed on the PIPLCD_PIP, which are consistent with the images seen by people without using the WVBCPs. The images of Figs. 6(a), 6(b), 6(c), and 6(d) seen by users employing the WVBCPs are illustrated in Figs. 6(e), 6(f), 6(g), and 6(h), respectively. We deduce that users can see the same PI even if the images are different, and the contrast of the PI depends on the colors/contents of the background images.

 

Fig. 6. (a), (b), (c), and (d) are images displayed on the PIPLCD_PIP and are consistent with the images seen by people without using the WVBCPs. Pictures in (a), (b), (c), and (d) seen by users employing the WVBCPs are shown in (e), (f), (g), and (h), respectively. Images/sketches are taken/drawn by the author (C.-K. Liu).

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4. PIPLCD_PIP issues

We state that backlight intensity should be enhanced when the PIPLCD_4K is switched to PIPLCD_PIP [see discussion in Fig. 2(b)]. However, the light-intensity enhanced backlight spectrum of the PIPLCD_PIP and the original backlight spectrum of the PIPLCD_4K could be different. Accordingly, the color gamut of PIPLCD_4K and that of PIPLCD_PIP in a chromaticity space are different, so the colors of an image displayed by the PIPLCD_4K and that by the PIPLCD_PIP are different. This case is not an issue if the colors of an image displayed by the PIPLCD_PIP is still acceptable by users. Moreover, hidden PI should have suitable contrast for reading. Referring to Fig. 6(e), the contrast of the words “All we” is much higher than that of “what to do,” so a user should select a bright image to read hidden PI. Another issue is that colors of hidden PI users see using the WVBCPs at normal and off-axis viewing-angles could be different [18,20]. Such an issue can be neglectable if the users can still understand the content of the hidden PI at off-axis viewing angle.

Referring to Fig. 2(c), “NH” could be seen by people who do not use the WVBCPs because the display method of the pixels_4in1 inside the red “NH” [inset of Fig. 2(c)] and that outside the red “NH” [inset of Fig. 2(b)] are different. By zooming out Fig. 7(a) [or see Fig. 7(b)], people who do not use the WVBCPs could notice “NH” if they are close enough to the PIPLCD_PIP screen; Figs. 7(a) and 2(c) are the same, except that Fig. 7(a) does not have red “NH” character frames.

 

Fig. 7. Image (a) and Fig. 2(c) are the same, except that which does not have red “NH” character frames. (b) Image is obtained by zooming out (a).

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The above issue can also occur in the images in Figs. 6(a)/6(b)/6(c)/6(d). Two conditions are qualitatively discussed to solve the issue. Referring to Fig. 6, although no actual demonstrations are provided, we deduce that if people who do not use the WVBCPs are not too close to the PIPLCD_PIP screen, they cannot notice the PI hidden in images because of the reason discussed in Fig. 7. Even when people who do not use the WVBCPs are close to the PIPLCD_PIP screen, we deduce that they have difficulty in noticing “what to do with the time” in Fig. 6(c) because the words are hidden in the groves/plants and buildings; however, they could notice “what to do with the time” in Fig. 6(b) because the words are hidden in a bright area, and the color across the area is similar.

Fabrications of the CBP-like WVBQWP array in Fig. 3 are conceptually and briefly elucidated as follows. The slow axis direction and retardance, determined by LC polymer film thicknesses, of λ/4 +A plate ($\varphi _{\frac{1}{4}\lambda }^{top} ={-} {75^o}$) [or λ/2 +A plate ($\varphi _{\frac{1}{2}\lambda }^{btm} = {75^o}$)] can be controlled by photoalignment of SD1 and spin-coating speeds, respectively {see Fig. 6 in Ref. [21]}; the slow axis direction and retardance of the λ/4 -A plate ($\varphi _{\frac{1}{4}\lambda }^{btm} ={-} {75^o}$) [or λ/2 -A plate ($\varphi _{\frac{1}{2}\lambda }^{top} = {75^o}$)] can be controlled by the polarization and parameters {e.g. pulse energy and scanning speed} of a femtosecond laser, respectively, during the writing process [22,23]. The challenge is to use the above methods to fabricate the CBP-like WVBQWP array in Fig. 1(a). Using other WVBQWP#1 and/or WVBQWP#2 structures could simplify the CBP-like WVBQWP array fabrication [18]. Moreover, the R/G/B-light-loss assumption in § 2.1 can be realized by optimizing WVBQWP#1/#2 parameters. With the development of CBP-like WVBQWP array, its structures and a TFT array can be aligned precisely by an industry method [24].

5. Conclusions

In this study, we propose a two-mode switchable LCD; PIPLCD_4K can be used in a private space to display ultrahigh resolution images, and can be switched to PIPLCD_PIP to display hidden PI in a public space. Moreover, the methods to hide PI in images using the PIPLCD_PIP and PIPLCD*_PIP are conceptually reported, and the PIPLCD_PIP (with a CBP-like WVBQWP array) is preferred to realize the goal. A driving circuit/scheme should be developed to demonstrate actual images of Figs. 6(e)–6(h). The PIPLCD_PIP issues, including resolution, brightness, color, and CBP-like WVBQWP array fabrication are discussed. Users wearing WVBCPs should select suitable images to read PI with high contrast. Overall, people around the PIPLCD_PIP cannot see PI hidden in images from almost over the entire viewing cone if they do not use WVBCPs, and such features increase the potential of the PIPLCD_PIP to be applied in daily life for PIP.