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A viewing angle switching (VAS) panel used for the conversion of wide and narrow viewing angle has been demonstrated. Unlike the previous VAS panels, this one features a pair of same-handed circular polarizers and it is capable of switching the viewing angle omni-directionally from wide to narrow or vice versa by electrically controlling the birefringence effect of the LC layer which is sandwiched in between. To explain how it works, the design principle as well as the optimization of both wide and narrow viewing modes will be elaborated hereinafter.
©2007 Optical Society of America
1. Introduction
The viewing angle is often viewed as a very important factor in evaluating the performance of displays or devices alike, and it is such a hot topic that a lot of studies and technical improvements had been done to make it as wide as possible [1–2]. Usually, it is true that most people do not bother with the contents of display. However, for those who have to deal with personal privacy or confidential data, like extracting cash from the automated teller machines (ATMs), or sending e-mail with sensitive contents on laptops at public places, the concerns about display security come to the fore as a big problem. To find a solution, the viewing angle under the said situations should be narrowed as much as only the user himself in front of the monitor can clearly see the image or text on screen. When the ATMs are not in use, instead playing commercials, or the user wants to share some non-private information on his monitor, a wide viewing angle mode would be favored again. Ultimately, an optical device with switchable viewing angle settings depending on the different situations is needed.
The core technologies of viewing angle control are mainly based on two means, i.e. to reduce contrast ratio [3–7], the ratio of transmittance of bright and dark states, and to reduce outcome transmittance [8–15] at off-normal direction. The former, in practice, is not quite effective because, with a poor contrast, the colors and gray levels can be incorrectly displayed, but the image is still being out there and more or less readable. In a different way, the latter makes the entire screen appear darkened so as to be indistinguishable. Technically, reducing the transmittance is better than merely reducing the contrast ratio to achieve narrow viewing mode and this specific function generally requires an additional panel, identified as viewing angle switching (VAS) panel, to be added to a thin-film-transistor (TFT)-liquid crystal display (LCD) which is in charge of the gray-level control. The so far developed VAS panels have been found such as those utilizing the electrophoresis of pigments [10] and many others utilizing the birefringence of liquid crystal (LC). Since the pigments may inevitably result in a palpable decrease in light transmittance, the LC is preferable in practical application. Unfortunately, the LC-based VAS panels made from the ferroelectric liquid crystal (FLC) cell [11], electrically controlled birefringence (ECB) cell [12], hybrid aligned nematic (HAN) cell [13], and vertically aligned (VA) cell [14] fail to switch the viewing angle from wide to narrow along the certain directions where the narrow viewing angle characteristics still behave wide along and nearby the absorption axis of the polarizer. Motivated from this issue, we are going to propose an omni-directional VAS panel that differs from the previous designs on account of the presence of circular polarizers rather than linear polarizers. The greatest merit of employing circular polarizers is the elimination of the angular dependence on the transmittance which will be explained in what follows.
2. Linear polarizer vs. circular polarizer
Referring to Figs. 1(a) and 1(b), consider the light passing through the arrangement of two parallelly located linear polarizers sandwiching a layer of liquid crystal (Fig. 1(a)), and two parallelly located linear polarizers with two crossed quarter wave plates (QWPs), also known as same-handed circular polarizers, sandwiching a layer of LC (Fig. 1(b)). The transmission axes (TAs) of both polarizers are set along the X-axis and the light propagation direction is along the Z-axis. For the first case (Fig. 1(a)), the transmitted light intensity, Iout, through the system of linear polarizers is
where Iin is the intensity of the incident light of a wavelength λ, ϕ is the azimuthal angle of the LC director and Γ is the optical retardation of the LC medium, that is 2πΔnd/λ. We see that the Iout should be determined by two factors, the optical retardation Γ and azimuthal angle ϕ. When ϕ is 0 or integer multiples of π/2, Iout=Iin, irrespective of the value of Γ. As a result, the transmittance of light is impossible to be modulated by the birefringence of the middle LC layer whose azimuthal angle is at those specific directions, which well explains that, for the VAS panels using linear polarizers, the viewing angle at some direction can not be switched no matter how.
For the second case, the transmitted light intensity through the system of same-handed circular polarizers (Fig. 1(b)) is
It is noticed in Eq. (2) that the term containing the azimuthal angle vanishes, which means the transmitted intensity no longer depends on the azimuthal angle, ϕ, of LC director and it is merely a function of the retardation Γ. The transmittance of light can be controlled by the birefringence of LC layer at every direction from the maximum Iin occurring at the retardation Γ of 0 to the minimum zero at π or a half wavelength. Even though the above mentioned azimuthal angle actually means the rubbing direction of LC director, we could also expect a similar relationship between the transmittance and the azimuthal angle of incidence, as proved in previous literatures [16–17]. It therefore makes sense to replace the commonly adopted linear polarizers by circular polarizers in the VAS panels for an omni-directional control of viewing angle.
Fig. 1. Optical arrangements of (a) parallel polarizers sandwiching an LC layer, and (b) same-handed circular polarizers an LC layer
3. Design of viewing angle switching (VAS) panel
The prototype structure of our proposed VAS panel, among other things, is schematically depicted in the Fig. 2 in which optical axis of each component is indicated. At the two end sides are two circular polarizers with the same handedness and a layer of LC, whose birefringence effect can be altered by an application of electric filed, is sandwiched in between. Beneath the VAS panel is the main TFT-LCD panel which implements the gray-level control; alternatively, this VAS panel can be placed at the bottom of the TFT-LCD as well. It is necessary to be aware of that there are as many as three polarizers in this whole system.
The design principle follows from a method, named parameter space diagram (PSD), which is derived from the Jones matrix [18]. For an LC cell with uniform twist, zero tilt and the input director aligned along the X-axis, the Jones matrix is given by
where,
In Eqs. (4)–(7), ϕ and d are the twist angle and cell thickness, respectively, and
Then, the transmittance of VAS panel of the foregoing structure is formulated as
where the column vector (1-i)T and row vector (1 i) can be regarded as the reduced forms for the right-handed input and output circular polarizers, respectively. Fixing the reference wavelength λ at 550 nm, the transmittance, T, is calculated against the twist angle, ϕ and retardation dΔn as the free parameters. Figure 3 shows a contour map of T, where a plurality of peaks can be obtained with different combinations of ϕ and dΔn. In other words, those are the potential choices to guarantee a full transmittance at the normal direction. However, the PSD does not give a description about the transmittance at oblique viewing angle and wavelengths other than 550 nm.
To take into account the viewing angle, we performed the optical simulation on the commercial software “DIMOS” which employs the algorithm of 2×2 extended Jones matrix method [19]. The LC material adopted in the VAS panel is MLC-6204-000 (Merck Co.) having the dielectric anisotropy, Δε=35.3 (ε‖=44.8, ε⊥=9.5), refractive anisotropy, Δn=0.1478 (no=1.5039, ne=1.6517), and surface pretilt angle of 2°. Twist angle and retardation will be set as independent variables upon which different viewing angle characteristics are to be investigated hereafter.
4. Wide viewing (WV) mode
The WV mode is designed at the voltage-off state or the initial state of LC layer. According to the PSD achieved before, we replotted the transmittance for polychromatic wavelengths ranging from 400 nm to 700 nm in Fig. 4 with respect to the retardation, dΔn, for some discrete values of twist angle, and the incident light is assumed as unpolarized light so that the maximum transmittance is 0.5 or 50% of the incident light. Since the reflection induced at the interface of air (n=1) and polarizer (n=1.5) has also been calculated, the maximum transmittance obtained at normal is, as a matter of fact, 0.4608, a little bit lower than 0.5. For each curve, there are several peaks of which the first peak always corresponds to the zero retardation and the second peak has a relatively lower transmittance but still maintains a high level over 90% of the first one, which is legitimate since the transmittance (see Eq. (2)) is wavelength dependent. In practical usage, a positive LC with zero retardation or a very low retardation can not be switched; even so, little birefringence effect can barely affect the viewing angle. Thus, we are more concerned about the second peaks from which optimal parameters for twist angle and retardation should be selected. As a result, in all, we chose six combinations of twist angle and retardation listed as follows: (90°, 473.6 nm), (0°, 562.4 nm), (280°, 710.4 nm), (240°, 814.0 nm), (180°, 947.2 nm), (150°, 1036 nm), and compared their viewing angles, as shown in Figs. 5(a)–(f) where red, purple and blue lines represent the iso-transmittance levels of 0.1, 0.25 and 0.4, respectively. Among those viewing angles, not every of them demonstrates wide viewing angle characteristics, but only (90°, 473.6 nm) of Fig. 5(a) and (240°, 814.0 nm) of Fig. 5(d) have wide viewing angles analogous to those of the counterparts [12–14]. Incidentally, the reason why the viewing angle along the horizontal direction always appears rather wider than that along the vertical direction especially for the contour line of 0.4 can be understood that the horizontal direction coincides with the TA of polarizer that attenuates the light wave less than the vertical direction, or the absorption direction, and this is the typical nature of the commonly used O-type polarizer [19].
Fig. 4. Transmittance calculated with respect to the retardation at variations of twist angle of LC layer
In order to better illustrate how the WV mode is achieved by the LC configuration, we reproduce the change of polarization state on the Poincare sphere using the 4×4 Muller matrix method [19] as the polar angle increases from 0–90° in steps of 10° and the azimuthal angle of observation is fixed at 0°. Comparing between the cases of (0°, 562.4 nm) and (240°, 814.0 nm), the points of polarization state in the former case (Fig. 6(a)), deviate far away from the south pole of Poincare sphere, which corresponds to the right-handed circular polarization, and apparently, the closer are these points to the south pole, the greater is the outcome transmittance. In contrast, for the latter case (Fig. 6(b)), those points gather closer to the south pole, thereby preserving a high transmittance for the oblique incident light.
Apart from the optimization of the twist angle and retardation of LC layer for the WV mode of the VAS panel, further improvements can be made to enhance the achromaticity or the wide-band characteristics, as we recall that there is, more or less, some loss of transmittance around the second peaks that is due to the wavelength dispersion discussed above. One thing is to examine the influence of the azimuthal angle of input LC director on the transmittance, which has not been dealt in the PSD method where it is specified at 0°. We take (240°, 814.0 nm) as a paradigm and alter its azimuthal angle from 0–180°, as shown in Fig. 7. The range of 180–360° is omitted here considering the symmetry of the LC director. The total transmittance fluctuates with respect to the azimuthal angle of input director and reaches a maximum of 0.4561 at 150°, 2% higher than that at 0°. As we plotted the wavelength-dependent transmittance in Fig. 9, it is very interesting to notice that a wide-band characteristic is acquired for the long wavelengths, while the short wavelength still needs more refinement. Another thing is to modify the wavelength dispersion properties of the QWP, i.e. to adjust its extraordinary refractive index (ne). Figure 8 reveals the influence of ne on the transmittance for three different wavelengths (400, 550 and 700 nm). The default parameters for QWP before optimization are no=1.5, ne=1.51 for all wavelengths and a thickness of 13.75 um and the parameters after optimization are ne(400 nm)=1.515, ne(550 nm)=1.51, and ne(700 nm)=1.509. Fortunately, this would be technically feasible with the copolycarbonate films whose wavelength dispersion of birefringence can be adjusted [20]. By virtue of this modified QWP, achromaticity is achieved with full transmittance over the entire wavelength regime, as shown in Fig. 9.
Fig. 9. Transmittance calculated by adjusting the azimuthal angle of input LC director and modified ne of QWP
5. Narrow viewing (NV) mode
NV mode is designed at the voltage-on state of LC layer and the voltage applied across the LC cell is set at 15 V such that almost every LC director except for those near the boundary with a strong anchoring force is tilted homeotropically to the substrate. Under this case, twist effect on the polarization of light can be ignored; therefore, the birefringence is simply determined by the retardation of the LC layer itself, and not too rigorously, this can be estimated by using the expression for the retardation of a C-plate [21],
where no and ne are the ordinary and extraordinary refractive index, respectively, d is the cell thickness and θ stands for the polar angle. When θ=0, Γ=0 and by substituting it back to Eq. (2), we will have a full transmittance. When θ becomes larger, Γ increases in amount too. Meanwhile, the transmittance, Iout, will decrease to its first minimum of 0 and then bounces up again like a sinusoidal function. Most importantly, this first minimum can be designed at a specific angle. For example, if we would like to let the first minimum occur at a desired angle θ, say 45°, then d should be 8.64 um. Moreover, it is crucial to recognize that, so far, because of the irrelevance of the azimuthal angle of incidence on the transmittance in our calculation, we would expect a symmetric or the so-called omni-directional narrow viewing angle.
Seen from Figs. 10(a)–(e), the border of iso-transmittance level of 0.1 becomes shrunk in every direction according to the gradually increased retardation of LC layer. Nonetheless, too much retardation as in Fig. 10(f) will cause a reversion of level of 0.1 at some area, which has been predicted by the Eq. (11). A slight asymmetry versus the azimuthal angle can be attributed to the strong anchoring force on the LC directors near the boundary so that the homeotropic alignment of LC mostly holds at the bulk volume. By the way, for NV mode, the issue of achromaticity needs no consideration any more because the maximum transmittance is obtained at almost zero retardation of the homeotropic LC. Once again, the change of polarization state of the cases of (0°, 562.4 nm) and (240°, 814.0 nm) is compared on the Poincare sphere to explain the narrow viewing angle effect, as shown in the Fig. 11. In terms of the retardation, the former one is relatively small. As a result, the points of polarization state at polar angle of even up to 90° are at a distance to the north pole that represents the lowest transmittance. On the contrary, for the latter one, the points of the polarization state at large polar angle are located around the north pole. So a better narrow viewing angle is realized. Besides, we shall mention that although an increase in the retardation might shrink the area of low transmittance, a reversion of the high transmittance would be resulted in at large polar angle, referring to the Fig. 10(f). To have an optimized VAS design, the adequate parameters are required to embrace both wide and narrow angle characteristics.
6. Conclusion
We have proposed a viewing angle switching panel that is added to a TFT-LCD panel to electrically control the viewing angle from wide to narrow or vice versa. Unlike the previous VAS panels, this one features a pair of same-handed circular polarizers and it is capable of switching the viewing angle omni-directionally, thus, delivering a safer NV mode. The design principle has been elaborated in detail where the optimizations on the WV mode and NV mode are carried out respectively. WV mode is designed with the voltage-off state of LC layer whereas NV mode with the voltage-on state of LC layer. As representatives, six different combinations of parameters are given to make a comparison among them, yet there must be more viable parameters to select from. The case of (240°, 814.0 nm) is seen as an example to have not just wide but narrow viewing angle characteristics. Therefore, in practical application, the optimal VAS design could be a twisted nematic LC cell if an appropriate amount of chiral dopant is mixed. Weak anchoring rubbing process is also suggested to reduce the driving voltage as low as possible because high extra power consumption would not be accepted especially for the mobile phone manufacturers.
Acknowledgements
This research was supported by Brain Korea (BK) 21 Program of the Ministry of Education & Human Resources Development of Korea, and the Samsung Electronics.
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