This invention relates to wide viewing angle liquid crystal displays and methods of increasing the viewing angles of liquid crystal displays with multi-film compensation.
Liquid crystal displays (LCDs) are widely used in information displays. Due to the intrinsic optical anisotropy of liquid crystal materials, the incident light “sees” different effective birefringence when viewed from different directions. For this reason, the viewing angle of conventional LCDs is not as wide as the viewing angle for self-luminescence displays, such as the cathode-ray tube (CRT), organic light-emitting diode and the plasma display panel. In an effort to widen the viewing angle, several display modes have been disclosed using a lateral electric field to activate the liquid crystal (LC) molecules. In-plane switching (IPS) mode is disclosed in M. Oh-e, et al., “Principles and characteristics of electro-optical behavior with in-plane switching mode”, Asia Display, 95, pp. 577-580 (1995) and U.S. Pat. No. 5,600,464, issued to Oh-e in 1997, and fringe field switching mode (FFS) is disclosed in S. H. Lee et al., “Electro-optic characteristics and switching principles of a nematic liquid crystal cell controlled by fringe-field switching”, Appl. Phys. Lett., Vol. 73, pp 2881-2883 (1998) and U.S. Pat. No. 5,886,762, issued to Lee in 1999.
In both IPS and FFS modes, the LC molecules at voltage-off state are basically homogeneously aligned on glass or plastic substrates that are coated with a thin indium-tin-oxide (ITO) layer and then overcoated with a polyimide alignment layer. The surfaces of polyimide layers are rubbed in parallel or anti-parallel directions to create a homogeneous alignment. The display panel is sandwiched between two crossed polarizers, and the long axis of LC molecules is either parallel or perpendicular to the transmission direction of their adjacent polarizers at off-state. At on-state, the lateral electric field generated from the comb-shaped electrodes cause the molecules to twist within the plane parallel to the supporting substrates. Therefore, from the opposite direction of the display panel, the incident light experiences almost the same birefringence and a relatively wide and symmetric angle is achieved.
However, the two orthogonally crossed polarizers are no longer perpendicular to each other when viewed from the oblique off-axis direction, especially from the bisector of the crossed polarizers.
Compensation methods have been disclosed for solving the light leakage problem associated with crossed polarizers. In Chen et al., “Optimum film compensation modes for TN and VA LCDs”, SID 1998 Digest, pp 315-318 (1998) and J. E. Anderson and P. J. Bos, “Methods and concerns of compensating in-plane switching liquid crystal displays”, Jpn. J. Appl. Phys., Vol. 39, pp 6388-6392 (2000), a method is disclosed for using a positive birefringence C-film (nx=ny<nz) plus a positive birefringence A-film (nx>ny=nz), where the z-axis is along the film surface normal direction, i.e. the film thickness direction and x axis is parallel to the optical axis direction. An alternative method using a single biaxial film (nx>ny>nz) to compensate for the light leakage of crossed polarizers is disclosed in Y. Saitoh et al., “Optimum film compensation of viewing angle of contract in in-plane-switching-mode liquid crystal display”, Jpn. J. Appl. Phys. Part 1, Vol. 37, pp 4822-4828 (1998). In addition, a design using two biaxial films to compensate light leakage in a large wavelength range is disclosed in T. Ishinable et al., “A wide viewing angle polarizer and a quarter-wave plate with a wide wavelength range for extremely high quality LCDs”, IDW' 01, pp 485-488 (2001) and T. Ishinable et al., “A wide viewing angle polarizer with a large wavelength range”, Jpn. Appl. Phys. Part 1, Vol. 41, pp. 4553-4558 (2002). However, the cost associated with a C-film and a biaxial film is much higher than the cost of A-film. Additionally, use of a combination of a C-film and an A-film or a single biaxial film does not achieve the desired symmetric viewing angle.
The present invention advances the art by providing a method and apparatus using a positive uniaxial A-film and a negative uniaxial A-film to compensate the dark state light leakage of the liquid crystal display, in which the liquid crystal molecules are homogeneously aligned at inactive state when no voltage is applied to liquid crystal layer and are driven by a substantially lateral electric field. After compensation, the dark state light leakage is greatly decreased and the contrast ratio at oblique viewing polar angle is greatly enhanced, as a result, more than 100:1 contras ratio is achieved in all viewing angles. During the analysis process of dark state light leakage, the Poincaré sphere presentation is used to illuminate the polarization state change in liquid crystal panel.
A primary objective of the present invention is to provide a novel liquid crystal display with a wide viewing angle, for use as large screen high definition televisions (HDTV) and computer monitors.
A secondary objective of the present invention is to provide a novel method for decreasing light leakage of crossed polarizers at voltage-off state to obtain a wide viewing angle polarizer.
A third objective of the present invention is to provide a compensation method for liquid crystal displays having a liquid crystal layer that is homogenously aligned at voltage off-state, such as IPS mode and FFS mode liquid crystal displays.
A fourth objective of the present invention is to provide a compensation method to keep the normal view dark state of liquid crystal displays relatively unchanged.
A liquid crystal display that includes a first substrate with alignment film having a first polarizer laminated on an outside surface, wherein the first polarizer faces a light source, a second substrate with alignment film having a second polarizer laminated on an outside surface, wherein the second polarizer faces an observer, a liquid crystal layer sandwiched between the first substrate and the second substrate, a positive birefringence uniaxial A-film with its optical axis parallel to the positive birefringence uniaxial A-film surface plane, and a negative birefringence uniaxial A-film with its optical axis parallel to the negative birefringence uniaxial A-film surface plane, wherein the negative birefringence uniaxial A-film is adjacent to the positive birefringence uniaxial A-film, wherein the positive birefringence uniaxial A-film and the negative birefringence uniaxial A-film are located between the first polarizer and the second polarizer, and wherein the optical axis of the positive birefringence uniaxial A-film is approximately perpendicular to the optical axis of the negative birefringence uniaxial A-film.
The angle between an absorption axis of the first polarizer and an absorption axis of the second polarizer is in a range between approximately 85° and approximately 95°, and preferably in the range of between approximately 88° and approximately 92°.
The angle between an absorption axis of the first polarizer and an alignment direction of the liquid crystal layer is in a range between approximately −5° and approximately +5°, and preferably in the range between approximately −2° and approximately +2°.
The angle between an absorption axis of the second polarizer and an alignment direction of the liquid crystal layer is in a range between approximately −5° and approximately +5°, and preferably in the range between approximately −2° and approximately +2°.
The angle between the optical axis of the positive birefringence uniaxial A-film and the absorption axis of the first polarizer is in a range between approximately 85° and approximately 95°, and preferably in the range between approximately 88° and approximately 92°.
The angle between the optical axis of the positive birefringence uniaxial A-film and the absorption axis of the second polarizer is in a range between approximately 85° and approximately 95°, and preferably in the range between approximately 88° and approximately 92°.
The angle between the optical axis of the negative birefringence uniaxial A-film and the absorption axis of the second polarizer is in a range between approximately 85° and approximately 95°, and preferably in the range between approximately 88° and approximately 92°.
The angle between the optical axis of the negative birefringence uniaxial A-film and the absorption axis of the first polarizer is in a range between approximately 85° and approximately 95°, and preferably in the range between approximately 88° and approximately 92°.
The positive birefringence uniaxial A-film and the negative birefringence uniaxial A-film can be located between the liquid crystal layer and the first polarizer.
The positive birefringence uniaxial A-film and the negative birefringence uniaxial A-film can be located between the liquid crystal layer and the second polarizer.
The positive birefringence uniaxial A-film has a retardation value dΔn/λ between approximately 0.05 and approximately 0.25, where λ is the wavelength of incident light, d is the thickness of the positive birefringence uniaxial A-film and Δn=ne−no is birefringence of the positive birefringence uniaxial A-film.
The negative birefringence uniaxial A-film has a retardation value dΔn/λ between approximately −0.25 and approximately −0.05, where λ is the wavelength of incident light, d is the thickness of the negative birefringence uniaxial A-film and Δn=ne−no is birefringence of the negative birefringence uniaxial A-film.
The liquid crystal layer can be substantially homogeneously aligned between the first substrate and the second substrate at voltage-off state and is driven by a substantially lateral electrical field. The liquid crystal display can be an in-plane switching mode liquid crystal display. The liquid crystal display can be a fringe field switching mode liquid crystal display.
A method for increasing the viewing angle of a liquid crystal display, can include the steps of applying a positive birefringence uniaxial A-film, having its optical axis parallel to the positive birefringence uniaxial A-film surface plane, between a liquid crystal layer and one of a first polarizer and a second polarizer, and applying a negative birefringence uniaxial A-film, having its optical axis parallel to the negative birefringence uniaxial A-film surface plane, adjacent to the positive birefringence uniaxial A-film.
The method can further include integrating the positive birefringence uniaxial A-film and the negative birefringence uniaxial A-film with a liquid crystal display having a homogenously aligned liquid crystal layer at voltage-off state, wherein the liquid crystal layer is driven by a substantially lateral electrical field.
The method can further include locating the positive birefringence uniaxial A-film, the negative birefringence uniaxial A-film and the liquid crystal layer between two orthogonally crossed polarizers.
Further objects and advantages of the invention will be apparent from the following preferred embodiments which are illustrated schematically in the accompanying drawings.
Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its applications to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.
Many LCDs use orthogonally crossed polarizers because they achieve a good dark state from a normal viewing angle. The conventional polarizer used for LCDs are produced by stretching a polymeric film, such as polyvinyl alcohol (PVA) doped with iodine or organic dye. The optical property of a polarizer is equivalent to a uniaxial absorption film with its absorption axis parallel to the optical axis of the polarizer and its transmission direction perpendicular to the optical axis of the polarizer; therefore, the absorption direction and the transmission direction of a polarizer is perpendicular with each other.
Under normal viewing, the absorption axes (directions) of orthogonally crossed polarizers are perpendicular to each other, as illustrated in
However, under oblique view from the off-axis direction, the absorption axis 52 of the bottom polarizer and absorption axis 54 of the top polarizer are not perpendicular, especially from the oblique direction of the bisector of the crossed polarizers as shown in the schematic view of
As a result, light leakage occurs as previously illustrated in
The negative A-film 67 is a uniaxial birefringence film with its optical axis 68 parallel to its film surface plane and has a birefringence Δn=ne−no<0. The angle between the optical axis 68 of the negative A-film 67 and the absorption axis 64 of the top polarizer 63 is less than approximately 95° and greater than approximately 85°. In a preferred embodiment, the angle is less than approximately 92° and greater than approximately 88°. The negative A-film 67 has a phase retardation in the range of −0.25≦dΔn/λ≦−0.05, where λ is the wavelength of incident light, d is the thickness of the negative A-film 67 and Δn=ne−no is the birefringence of the negative A-film 67.
The following is an explanation of the film compensation principle used in the method and apparatus of the present invention. As illustrated in
Alternatively, the positions of the positive A-film and the negative A-film can be exchanged as illustrated in the schematic view of
To extend the compensation method of the present invention to LCDs with liquid crystal layer initially homogenous alignment, such as IPS mode LCD and FFS mode LCD, the alignment direction (rubbing direction) of the liquid crystal layer, the optical axis directions of both positive and negative A-films, the absorption axis directions of both polarizers should be properly set up and the film retardation values of both positive and negative A-films need to be optimized. Table 1 lists the liquid crystal parameters that were used in the following computer simulation.
In a first embodiment,
The LC layer 103 is substantially homogeneously aligned at off-state when no voltage is applied to the LC layer 103 and forms a twist profile when driven by the lateral electric field generated from the comb-shaped electrodes. While
The incident light becomes linearly polarized (point POL(T)) after passing through the bottom polarizer 101. Since the alignment direction of the LC layer 103 is along the absorption direction 102 of the bottom polarizer 101, the linearly polarized light does not change its polarization state after it passes through the LC layer 103. Therefore, the same compensation method used with the pure crossed polarizers can be used with the IPS-mode and FFS-mode LCDs. When the linearly polarized light then passes through the positive A-film 105, point POL(T) moves to point B. After the light passes through the negative A-film 107, point B moves to point ANA(A), which is the absorption direction of the top polarizer 109. Therefore, the light is totally absorbed by the top polarizer 109 and very little light leakage occurs at other oblique angles. Table 2 provides a sample list of the optimized film parameters for the positive A-film and the negative A-film.
In a second embodiment, the position of the positive A-film 105 and the negative A-film 107 are exchanged as illustrated in the schematic structure of
The compensation principle for the second embodiment is illustrated by the Poincaré sphere in
Previous examples and embodiments have illustrated the method and apparatus of the present invention wherein the positive A-film 105 and the negative A-film 107 are laminated to the top substrate of the liquid crystal layer 103. However, the compensation films 105, 107 may also be laminated to the bottom substrate of the liquid crystal layer 103.
The compensation principle corresponding to this third embodiment is illustrated in the Poincaré sphere in
As previously disclosed in embodiment two, the position of the positive A-film 105 and the negative A-film 107 can be exchanged.
The compensation results for the fourth embodiment are illustrated in
For the purpose of comparison,
The optical axis of the positive C-film 204 is perpendicular the surface plane of the positive C-film 204. That means the optical axis of the positive C-film 204 is along the surface normal direction of the positive C-film 204.
The resulting dark state, bright state transmission, and contrast ratio of the prior art are plotted in
The method and apparatus of the present invention laminates one of a top substrate and a bottom substrate of the liquid crystal layer with a positive A-film and a negative A-film. The multi-film compensated liquid crystal displays of the present invention increases the contrast ratio for a larger viewing angle at a reduced cost.
While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.
Number | Name | Date | Kind |
---|---|---|---|
5138474 | Arakawa | Aug 1992 | A |
5343317 | Wada et al. | Aug 1994 | A |
5557434 | Winker et al. | Sep 1996 | A |
5570214 | Abileah et al. | Oct 1996 | A |
5583679 | Ito et al. | Dec 1996 | A |
5600464 | Ohe | Feb 1997 | A |
5886762 | Lee | Mar 1999 | A |
6741311 | Hong et al. | May 2004 | B1 |
20020167620 | Anderson et al. | Nov 2002 | A1 |
20040114080 | Miyachi | Jun 2004 | A1 |
20040135949 | Maeda | Jul 2004 | A1 |
20060292372 | Paukshto et al. | Dec 2006 | A1 |