Reflective liquid crystal light valve

Abstract

Reflective liquid crystal light valves are disclosed. A liquid crystal cell is also disclosed comprising a transparent electrode, a reflective electrode, and a twisted nematic liquid crystal layer interposed therebetween. A first alignment layer with a first alignment direction disposed on the transparent electrode. A second alignment layer with a second alignment direction disposed on the reflective electrode, wherein a first included angle φ is between the first and second alignment directions. A polarizing device is disposed on the exterior of the transparent electrode to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction. A relationship between the first included angle φ and the second included angle β satisfies φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.

Description

FIELD OF THE INVENTION

The invention relates to projection displays, and more particularly, to a reflective liquid crystal light valve for same.

BACKGROUND OF THE INVENTION

A reflective liquid crystal light valve is an important element in a projection display. Reflective liquid crystal light valves typically comprise a polarizing beam splitter (PBS) and a reflective liquid crystal cell. The size of each pixel of a high resolution projection display is approximately equal to a cell gap of the reflective liquid crystal cell. As such, the fringe field between adjacent pixels can interfere with and reorient the liquid crystal orientation and then degrade image contrast and reduce display brightness. Therefore, to decrease the fringe field effect, a low driving voltage is used to achieve high resolution, high contrast ratio, and high brightness in the projection display.

U.S. Pat. No. 5,490,003 to Sprang, the entirety of which is hereby incorporated by reference, discloses a reflective liquid crystal display. The reflective liquid crystal display comprises a layer of positive dielectric anisotropic liquid crystal molecules with a twist angle and a polarizer having a polarization direction at the bisector of the twist angle.

U.S. Pat. No. 5,936,697 to Yang, the entirety of which is hereby incorporated by reference, discloses a self-compensated twisted nematic (SCTN) mode reflective light valve. The reflective light valve comprises a SCTN mode reflective liquid crystal cell with negative dielectric anisotropic liquid crystal (LC) molecules, and a polarizer having a polarization direction at the bisector of the twist angle.

The conventional reflective light valve utilizing the bisector of the twist angle, however, does not take boundary layer residual phase retardation into consideration. Thus, in practice, the bisector is not in the proper polarization direction for achieving low operating voltage and high contrast ratio.

SUMMARY

According to various embodiments reflective liquid crystal light valves with a predetermined polarization direction are provided. An exemplary embodiment of a reflective liquid crystal light valve comprises a liquid crystal cell comprising a transparent electrode disposed opposite a reflective electrode with a twisted nematic (TN) mode liquid crystal layer interposed therebetween. The light valve can also include a first alignment layer with a first alignment direction disposed on the transparent electrode. A second alignment layer with a second alignment direction is disposed on the reflective electrode, wherein a first included angle φ is between the first and second alignment directions. A polarizer is disposed on the exterior of the transparent electrode to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction. A relationship between the first included angle φ and the second included angle β can satisfy φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.

According to various embodiments the optimal polarization direction of the incident beam that provides improved results is not the bisector direction between the first and second alignment directions. The relationship between the first included angle φ and the second included angle β can satisfy φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°. The reflective liquid crystal light valve can thus potentially achieve lower driving voltage and higher contrast ratio, improving display quality.

DESCRIPTION OF THE DRAWINGS

Reflective liquid crystal light valves can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:

FIG. 1A depicts an operating principle using a reflective liquid crystal cell for an embodiment of a reflective light valve, which includes an incident beam being linear polarized;

FIG. 1B depicts a schematically sectional view of the reflective liquid crystal cell shown in FIG. 1A according to various embodiments of the invention;

FIG. 2 schematically depicts a relationship between the polarization direction of the incident beam and the alignment directions according to an embodiment of a light valve;

FIG. 3A depicts a graphical plot of the relationship between the azimuthal angle of eigenmode 1 and the residual retardation for an embodiment of a left-handedness 60°-TN liquid crystal cell at uniform-twist and two-layer models according to various embodiments of the invention;

FIG. 3B depicts a graphical plot of the relationship between the azimuthal angle of eigenmode 2 and the residual retardation for an embodiment of a left-handedness 60°-TN liquid crystal cell at uniform-twist and two-layer models according to various embodiments of the invention;

FIG. 4A depicts a local enlarged view of FIG. 3A;

FIG. 4B depicts a local enlarged view of FIG. 3B;

FIG. 5 depicts a graphical plot of the relationship between the residual retardation and the applied voltage for an embodiment of a 60°-TN liquid crystal cell with a retardation value (dΔn) of 350 nm according to various embodiments of the invention;

FIG. 6A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 60°-TN liquid crystal cell according to a first test;

FIG. 6B depicts a local enlarged view of FIG. 6A in the dark state;

FIG. 7A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 57°-TN liquid crystal cell according to a second test;

FIG. 7B depicts a local enlarged view of FIG. 7A in the dark state;

FIG. 8A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 55°-TN liquid crystal cell according to a third test;

FIG. 8B depicts a local enlarged view of FIG. 8A in the dark state;

FIG. 9A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 50°-TN liquid crystal cell according to a fourth test;

FIG. 9B depicts a local enlarged view of FIG. 9A in the dark state;

FIG. 10A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 45°-TN liquid crystal cell according to a fifth test;

FIG. 10B depicts a local enlarged view of FIG. 10A in the dark state;

FIG. 11A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 40°-TN liquid crystal cell according to a sixth test;

FIG. 11B depicts a local enlarged view of FIG. 11A in the dark state;

FIG. 12A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 65°-TN liquid crystal cell according to a seventh test;

FIG. 12B depicts a local enlarged view of FIG. 12A in the dark state;

FIG. 13A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a 70°-TN liquid crystal cell according to an eighth test;

FIG. 13B depicts a local enlarged view of FIG. 13A in the dark state;

FIG. 14 depicts a schematic diagram illustrating an embodiment of a projection display apparatus, incorporating a controller according to various embodiments of the invention; and

FIG. 15 depicts a schematic diagram illustrating an electronic device incorporating an embodiment of a projection display apparatus according to various embodiments of the invention.

DETAILED DESCRIPTION

Reflective liquid crystal light valves according to various embodiments are provided. An exemplary embodiment of a reflective light valve 90, shown in FIG. 1A, comprises a reflective liquid crystal display (e.g. a reflective TN type liquid crystal cell 100) and a polarizing device (e.g. a polarizing beam splitter 7). The reflective light valve 90 is well suited for the projection display. A representative projection display is illustrated, but is not intended to limit the disclosure.

The operating principles according to various embodiments of the reflective liquid crystal light valve 90 are illustrated in FIG. 1A. A non-polarized incident light beam 6 from a light source becomes a linearly-polarized light 8 after passing through a polarizing device 7, such as a beam splitter (PBS), and being reflected 90° thereby, defining polarized light 8 as p-wave 8. It is to be understood that other polarizing devices known in the art may also be used. The linearly-polarized light 8 then impinges on a reflective TN type liquid crystal cell 100. As shown in FIG. 1A, the TN type liquid crystal cell 100 comprises a transparent front panel 1 disposed opposite a reflective rear panel 2 with a TN type liquid crystal material 5 interposed therebetween.

FIG. 1B schematically depicts a sectional view of a TN type liquid crystal cell, such as that labeled 100 in FIG. 1A. The front panel 1 comprises a transparent substrate 11, a transparent electrode 12, and a first alignment layer 13 with a first alignment direction 3 (shown in FIG. 2). The transparent substrate 11 can be glass. The transparent electrode 12, such as indium tin oxide (ITO) or indium zinc oxide (IZO), is formed on the interior of the transparent substrate 11. The first alignment layer 13 can be formed on the transparent electrode 12. The rear panel 2 comprises a substrate 21, a reflective electrode 22, and a second alignment layer 23 with a second alignment direction 4 (also shown in FIG. 2). The substrate 21 can be a silicon wafer or any other suitable semiconductor material. The reflective electrode 22, such as, for example, aluminum or silver, is formed on the substrate 21. The second alignment layer 23 is formed on the reflective electrode 22. The liquid crystal material 5 is disposed between the first and second alignment layers 13 and 23, respectively. The liquid crystal material 5 can comprise positive dielectric anisotropic (Δε>0) liquid crystal molecules. The liquid crystal molecules, near the alignment layers 13 and 23, are arranged along the alignment directions 3 and 4 shown in FIG. 2.

According to various embodiments, as depicted, for example in FIG. 1A, the TN type liquid crystal cell 100 is designed such that at or below a certain predetermined voltage defined as a threshold voltage, applied to the two electrodes 12 and 22, the incident polarized light 8 can become an s-wave 9 (or nearly s-wave) upon reflection from the liquid crystal cell 100. The s-wave 9 is a linearly polarized light with a direction of polarization perpendicular to that of the p-wave 8. The s-wave 9 is capable of passing directly through the PBS 7 to serve as a projection beam 10. The projection beam 10 is then corrected by projection lenses (not shown) for projection onto a screen (not shown) for viewing. This situation represents the bright state of the projection display.

When an external voltage is applied across the two electrodes 12 and 22 of the liquid crystal cell 100 at or above a certain voltage, defined as the saturation voltage, the liquid crystal cell 100 behaves as an optically isotropic medium. In this case, the impinging linearly polarized light 8 will be reflected from the reflective liquid crystal cell 100, preserving the same direction of polarization (a p-wave in this case). The reflected p-wave cannot directly pass through the PBS 7 and will propagate backward opposite the incident beam 6. That is, the reflected p-wave does not project onto a screen (not shown) for viewing. This situation represents the dark state of the projection display. In order to get a high contrast ratio, a perfect dark state is desired. As such, the polarization state of the incident polarized beam 8 should be an eigenmode for the reflective liquid crystal cell 100 in order to obtain the desired contrast.

For better understanding, two different models (i.e. a uniform-twist model and a two-layer model) are provided to illustrate the eigenmode of the reflective TN type liquid crystal cell 100. According to various embodiments, positive dielectric anisotropic (Δε>0) liquid crystal molecules are utilized in the liquid crystal cell 100, and the pre-tilt angle at the substrate boundary is small (3˜5°). The liquid crystal molecules undergo a uniform twist throughout the liquid crystal cell 100 when the applied voltage is below a threshold voltage. When the applied voltage is around two times higher than the threshold voltage, the liquid crystal molecules in the middle of the liquid crystal cell 100 are aligned almost parallel to the electric field between the panels 1 and 2. However, the boundary layers of molecules near the front and rear substrate interfaces can be poorly distributed due to strong surface anchoring. Therefore, the TN type liquid crystal can be defined as a uniform-twist model when the applied voltage is below the threshold voltage and as a two-layer model when the applied voltage is about two times higher than the threshold voltage.

In the uniform-twist model, there are two eigenmodes for the TN type liquid crystal cell. Both eigenmodes are linearly polarized and orthogonal. In the mentioned eigenmodes, the azimuthal angles of linear polarization are determined by “θ” in the following equation (1): $\begin{array}{cc}\tan \theta =-\frac{\cos X\pm \sqrt{1-{\left(\frac{\mathrm{\Gamma sin}X}{2X}\right)}^2}}{\phi \frac{\sin X}{X}}& \left(1\right)\end{array}$

In the above equation, Γ=2πdΔn/λ and X={square root}{square root over (φ²+(Γ/2)²)}, wherein Γ is the phase of uniformly twisted TN type liquid crystal molecules, d is the cell gap between two substrates 1 and 2, Δn is the birefringence of the liquid crystal material, λ is the wavelength of the incident beam, and φ is the twist angle of the liquid crystal molecules (i.e. the included angle between the first and second alignment directions 3 and 4). Here, the left-handedness twist angle (for example, counterclockwise direction) is defined to be positive and the right-handedness (for example, clockwise direction) twist angle as negative. Referring to FIG. 2, the positive included angle φ is between the first and second alignment directions 3 and 4. Numeral 25 denotes the bisector of the included angle φ.

In the two-layer model, each boundary layer is referred to as a non-twisted uniaxial layer with residual phase ψ=2πα/λ, wherein α is the retardation of each boundary layer. Retardation a decreases as the applied voltage increases. Similarly, there are two eigenmodes for the reflective TN type liquid crystal cell using the two-layer model. Both of the mentioned eigenmodes are linearly polarized and orthogonal. In the mentioned eigenmodes, the azimuthal angles of linear polarization are determined by “θ” in the following equation (2): $\begin{array}{cc}\tan \theta =-\frac{\cos \mathrm{\phi cos}X\pm \sqrt{{\cos }^2{\mathrm{\phi cos}}^2\psi +{\sin }^2\phi }}{\sin \phi }& \left(2\right)\end{array}$

When an intermediate voltage (the applied voltage between the threshold voltage and two times thereof) is applied, no approximation is made because of more complicated cases. Nevertheless, the azimuthal angles of the eigenmodes should be between the uniform-twist and two-layer models.

FIG. 3A is a graphical plot of the relationship between the azimuthal angle of eigenmode 1 and the residual retardation for an embodiment of a left-handedness 60°-TN (i.e. twist angle φ is 60°) liquid crystal cell at uniform-twist and two-layer models according to various embodiments. FIG. 4A is a local enlarged view of FIG. 3A. FIG. 3B is a graphical plot of the relationship between the azimuthal angle of eigenmode 2 and the residual retardation for the left-handedness 60°-TN liquid crystal cell in the uniform-twist and two-layer models according to various embodiments. FIG. 4B is a local enlarged view of FIG. 3B. Referring to FIGS. 3A, 3B, 4A and 4B, the azimuthal angles of eigenmodes 1 and 2 gradually reach the bisector 25 (i.e. φ/2=30°) of the twist angle or perpendicular to the bisector 25 (i.e. 90°+φ/2=120°). This is the reason that the cited references (U.S. Pat. Nos. 5,490,003 and 5,936,697) employ the bisector effect to achieve a dark state in simulation.

The bisector used in the conventional technology, however, does not achieve low operating voltages and/or high contrast ratios because of the poor polarization direction for achieving low operating voltage and high contrast ratio in practice. According to the conventional technology, even when the applied voltage reaches three times the threshold voltage, the residual retardation is still much greater than 0. As a result, the azimuthal angles of the two eigenmodes are not exactly parallel to the bisector or perpendicular to the bisector. One reason for the poor result is that the conventional technology does not take boundary layer residual phase retardation into consideration.

Various tests were preformed and the parameters of the liquid crystal molecules used in the tests of the specification are listed in Table 1.

TABLE 1
Parameter of LC molecules     Value
Refractive index ne           1.65
Refractive index no           1.55
Ferroelectric index εp        12.0
Ferroelectric index εv        4.0
Coefficient of elasticity k11 11.5E−12N
Coefficient of elasticity k22 6.5E−12N
Coefficient of elasticity k33 16.0E−12N
Pre-tilt angle                3°

In one test, the results of which are shown in FIG. 5, the residual retardation of an embodiment of a 60°-TN liquid crystal cell with a retardation value (dΔn) of 350 nm is plotted verses different applied voltages. Referring to FIG. 5, when the applied voltage is even at 5V_rms, the residual retardation is still about 50 nm. Referring to FIGS. 4A and 4B, the azimuthal angles of the eigenmodes 1 and 2 are about 0.5° larger than the bisector when the residual retardation is 50 nm. That is, the azimuthal angles of the eigenmodes 1 and 2 are about 30.5° and 120.5°, respectively.

In projection displays, it is desirable to decrease the driving voltage in order to minimize the fringe field effect. Because the azimuthal angles of the two eigenmodes deviate from the direction of the bisector (or the direction perpendicular to the bisector), the polarizing direction of PBS 7 can be oriented to be parallel or perpendicular to the azimuthal angles of the eigenmodes of the TN type liquid crystal cell at the desired driving voltage. An example is provided to illustrate a feature of the disclosure. Please refer to FIG. 5. A dark state with a driving voltage of 3.5V_rms has a corresponding residual retardation of about 75 nm. From FIGS. 4A and 4B, it is found that the azimuthal angles of the eigenmodes are about 1.5° greater than bisector (30°/120°) when the residual retardation is 75 nm. Thus, referring to FIG. 2, the included angle β between the polarization direction 71 of PBS 7 and the first alignment direction 3 of the alignment layer 13 is set at about φ/2+1.5° or π/2+φ/2+1.5°. In such a situation, a perfect dark state occurs at the low driving voltage of about 3.5V_rms.

From FIGS. 4A, 4B, and 5, a relationship among the applied voltage, the residual retardation, and the azimuthal angle is obtained. As the applied voltage increases, the residual retardation decreases so that the corresponding azimuthal angles of the two eigenmodes change accordingly. According to various embodiments in order to obtain a reflective liquid crystal light valve with a lower driving voltage and a higher contrast ratio, a relationship between the included angle φ and the included angle β should satisfy φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°. As such, the included angle φ is between the first and second alignment directions 3 and 4, the included angle β is between the first alignment direction 3 and the polarization direction 71 of PBS 7 and numeral 25 denotes the bisector of the included angle φ. According to various embodiments the included angle φ can be between 40° and 70°. In further embodiments, the included angle β can be φ/2+1° to about 3°, and in still further embodiments, φ/2+1.5°. That is, the polarization direction of the polarizing device 7, such as a PBS, disposed on the exterior of the transparent panel that provides improved results 1 is not the conventional bisector 25.

Note that, when the cell 100 uses right-handed TN type liquid crystal molecules, the included angle β satisfies −φ/2>β>−φ/2−30° or π/2−φ/2>β>π/2−φ/2−30°. For convenience, all angles are based on the first alignment direction 3 of the front panel 1 (i.e. the first alignment layer 13), as shown in FIG. 2, and all counterclockwise angles are defined to be positive.

The following experimental data are provided for better understanding of various embodiments of a reflective light valve having a lower driving voltage and a higher contrast ratio than that of the conventional technology.

FIG. 6A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 60°-TN liquid crystal cell with retardation (dΔn) of 350 nm at different polarization angles β, according to a first test. FIG. 6B is a local enlarged view of FIG. 6A in the dark state (i.e. the region that reflectance is about 0). In the first test, a green incident light (λ=550 nm) is used to impinge the reflective liquid crystal cell 100 shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=30.0°) in FIGS. 6A and 6B.

Because the PBS 7 has a limited extinction ratio (ER) of about 1000:1, the contrast ratio (CR) of the reflective light valve is affected by the extinction ratio of PBS as $\mathrm{CR}=\frac{1}{\left(1/\mathrm{ER}\right)+R},$
wherein R is the normalized reflectance. For example, when the normalized reflectance is R=0.00005, the contrast ratio is CR=1/(0.001+0.00005)=952.

Referring to FIG. 6B, an applied voltage of about 5V_rms provides R=0.00005 when the bisector (β=30.0°) is used as the polarization angle. According to the first test, because the boundary layers are taken into consideration, an angle β of about 31.5° provides an improved result. For example, the driving voltage of the dark state drops to 3.5V_rms, when β is about 31.5°. As shown by the first test, about 3.5V_rms results in the same CR=952. Thus, according to an embodiment, the reflective light valve has lower driving voltage than the conventional technology.

Further, using a driving voltage at 3.5V_rms of the conventional technology can only obtain a contrast ratio of CR=1/(0.001+0.0012)=455. In contrast, according to various embodiments described herein, a driving voltage of 3.5V_rms obtains a much higher contrast ratio of, for example, 952.

Accordingly, the first test verifies that a polarization angle β of φ/2+1° to about 3°, and in certain embodiments, φ/2+1.5° is advantageous. An embodiment of the reflective light valve can thus provide a high contrast ratio with a low driving voltage, thereby reducing power consumption.

FIG. 7A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 57°-TN liquid crystal cell with retardation (dΔn) of 350 nm at different polarization angles β, according to a second test. FIG. 7B is a local enlarged view of FIG. 7A in the dark state (i.e. the region that reflectance is about 0). In the second test, a green incident light (λ=550 nm) is used to impinge the reflective liquid crystal cell 100 shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=28.5°) in FIGS. 7A and 7B.

Referring to FIG. 7B, an applied voltage of about 4.8V_rms provides R=0.0001 when the bisector (β=28.5°) is used as the polarization angle. The contrast ratio of the reflective liquid crystal cell 100 at 4.8V_rms is CR=1/(0.001+0.0001)=909. According to the second test, an angle β of about 30° provides improved results. For example, the driving voltage of the dark state drops to about 3.4V_rms when β is about 30°. As shown by the second test, about 3.4V_rms results in the same CR=909. Thus, according to an embodiment, the reflective light valve as described herein has a lower driving voltage than the conventional technology.

FIG. 8A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 55°-TN liquid crystal cell with retardation (dΔn) of 350 nm at different polarization angles β, according to a third test. FIG. 8B is a local enlarged view of FIG. 8A in the dark state (i.e. the region that reflectance is about 0). In the third test, a green incident light (λ=550 nm) is used to impinge the reflective liquid crystal cell 100 shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=27.5°) in FIGS. 8A and 8B.

Referring to FIG. 8B, an applied voltage of about 4.8V_rms provides R=0.0001 when the bisector (β=27.5°) is used as the polarization angle. The contrast ratio of the reflective liquid crystal cell 100 at 4.8V_rms is CR=1/(0.001+0.0001)=909. According to the third test, an angle β of about 29° provides an improved result. For example, the driving voltage of the dark state drops to about 3.4V_rms when β is about 29°. As shown by the third test, about 3.4V_rms to reach the same CR=909. Thus, according to an embodiment of the reflective light valve has a lower driving voltage than the conventional technology.

FIG. 9A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 50°-TN liquid crystal cell with retardation (dΔn) of 350 nm at different polarization angles β, according to a fourth test. FIG. 9B is a local enlarged view of FIG. 9A in the dark state (i.e. the region with reflectance of about 0). In the fourth test, a green incident light (λ=550 nm) is used to impinge the reflective liquid crystal cell 100 shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=25°) in FIGS. 9A and 9B.

Referring to FIG. 9B, an applied voltage of about 5V_rms provides R=0.0001 when the bisector (β=25°) is used as the polarization angle. The contrast ratio of the reflective liquid crystal cell 100 at 5V_rms is CR=1/(0.001+0.0001)=909. According to the fourth test, an angle β of about 26.5° provides an improved result. For example, when β is about 26.5°, the driving voltage of the dark state drops to about 3.4V_rms. As shown by the fourth test about 3.4V_rms results in the same CR=909. Thus, according to an embodiment, the reflective light valve as described herein has a lower driving voltage than the conventional technology.

FIG. 10A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 45°-TN liquid crystal cell with retardation (dΔn) of 355 nm at different polarization angles β, according to a fifth test. FIG. 10B is a local enlarged view of FIG. 10A in the dark state (i.e. the region with reflectance of about 0). In the fifth test, a green incident light (λ=550 nm) is used to impinge the reflective liquid crystal cell 100 shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=22.5°) in FIGS. 10A and 10B.

Referring to FIG. 10B, an applied voltage of about 4.7V_rms provides R=0.0002 when the bisector (β=22.5°) is used as the polarization angle. The contrast ratio of the reflective liquid crystal cell 100 at 4.7V_rms is CR=1/(0.001+0.0002)=833. According to the fifth test, an angle β of about 24° provides an improved result. For example, when β is about 24°, the driving voltage of the dark state drops to about 3.4V_rms. As shown by the fifth test, about 3.4V_rms results in the same CR=833. Thus, according to an embodiment, the reflective light valve as described herein has a lower driving voltage than the conventional technology.

FIG. 11A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 40°-TN liquid crystal cell with retardation (dΔn) of 365 nm at different polarization angles β, according to a sixth test. FIG. 11B is a local enlarged view of FIG. 11A in the dark state (i.e. the region with reflectance of about 0). In FIGS. 11A and 11B, the solid line denotes the bisector (β=φ/2=20β). Similar to the above tests, when the polarization angle β is set at φ/2+1.5° (i.e. β=21.5°), an embodiment of the reflective light valve of the sixth test can provide a high contrast ratio with a lower driving voltage than the conventional technology.

FIG. 12A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 65°-TN liquid crystal cell with retardation (dΔn) of 345 nm at different polarization angles β, according to a seventh test. FIG. 12B is a local enlarged view of FIG. 12A in the dark state (i.e. the region with reflectance of about 0). In FIGS. 12A and 12B, the solid line denotes the bisector (β=φ/2=32.5°). Similar to the above tests, when the polarization angle β is set at φ/2+1.5° (i.e. β=34°), an embodiment of the reflective light valve of the seventh test can provide a high contrast ratio with a lower driving voltage than the conventional technology.

FIG. 13A depicts a graphical plot of the relationship between the normalized reflectance and the applied voltage for an embodiment of a left-handed 70°-TN liquid crystal cell with retardation (dΔn) of 345 nm at different polarization angles β, according to an eighth test. FIG. 13B is a local enlarged view of FIG. 13A in the dark state (i.e. the region with reflectance of about 0). In FIGS. 13A and 13B, the solid line denotes the bisector (β=φ/2=35°). Similar to the above tests, when the polarization angle β is set at φ/2+1.5° (i.e. β=36.5°), an embodiment of the reflective light valve of the eighth test can provide a high contrast ratio with a lower driving voltage than the conventional technology.

An embodiment of a reflective light valve 90 shown in FIG. 1A can be coupled to a controller 142, forming a display device 143 as shown in FIG. 14. The controller 142 can comprise source and gate driving circuits (not shown) to control the reflective light valve 90 to render images in accordance with an input. The display device 143 and associated controller 142 may be directed to a reflective projection display apparatus.

FIG. 15 depicts a schematic diagram illustrating an electronic device 151 incorporating an embodiment of the reflective light valve 90. An input device 154 is coupled to the controller 142 of the display device 143 shown in FIG. 15 to form an electronic device 151. The input device 154 can include a processor or the like, inputting data to the controller 142 to render an image. The electronic device 151 may be a portable device such as a notebook computer, tablet computer, cellular phone, or a display monitor device, or non-portable device such as a desktop computer or a projection TV.

While the invention has been described by way of example and in terms of various embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Claims

1. A reflective light valve, comprising:

a transparent substrate disposed opposite a reflective substrate with a twisted nematic type liquid crystal material interposed therebetween;

a first alignment layer with a first alignment direction disposed on the transparent substrate;

a second alignment layer with a second alignment direction disposed on the reflective substrate, wherein a first included angle φ is between the first and second alignment directions; and

a polarizing device disposed on an exterior of the transparent substrate to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction;

wherein a relationship between the first included angle φ and the second included angle β satisfies φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.

2. The reflective light valve according to claim 1, wherein the second included angle β is φ/2+1° to about 3°.

3. The reflective light valve according to claim 2, wherein the second included angle β is φ/2+1.5°.

4. The reflective light valve according to claim 1, wherein the first included angle φ is between 40° and 70°.

5. The reflective light valve according to claim 1, wherein the transparent substrate is a glass substrate comprising a transparent electrode formed thereon.

6. The reflective light valve according to claim 5, wherein the transparent electrode is an indium tin oxide (ITO) or indium zinc oxide (IZO) layer.

7. The reflective light valve according to claim 1, wherein the reflective substrate is a silicon substrate comprising a metal electrode formed thereon.

8. The reflective light valve according to claim 7, wherein the metal electrode is an aluminum layer.

9. The reflective light valve according to claim 1, wherein the twisted nematic type liquid crystal material comprises positive dielectric anisotropic liquid crystal molecules.

10. A reflective light valve, comprising:

a liquid crystal cell comprising a transparent electrode, a reflective electrode and a twisted nematic liquid crystal layer interposed therebetween, wherein a retardation value (dΔn) of the twisted nematic liquid crystal layer is about 350 nm;

a first alignment layer with a first alignment direction disposed on the transparent electrode;

a second alignment layer with a second alignment direction disposed on the reflective electrode, wherein a first included angle φ is between the first and second alignment directions; and

a polarizing device disposed on an exterior of the transparent electrode to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction;

wherein a relationship between the first included angle φ and the second included angle β satisfies φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.

11. The reflective light valve according to claim 10, wherein the second included angle β is φ/2+1° to about 3°.

12. The reflective light valve according to claim 11, wherein the second included angle β is φ/2+1.5°.

13. The reflective light valve according to claim 10, wherein the first included angle φ is between 40° and 70°.

14. The reflective light valve according to claim 10, wherein the transparent electrode is an ITO or IZO layer and the reflective electrode is an aluminum layer.

15. The reflective light valve according to claim 10, wherein the twisted nematic type liquid crystal layer comprises positive dielectric anisotropic liquid crystal molecules.

16. A reflective light valve, comprising:

a liquid crystal cell comprising a transparent electrode on a transparent substrate, a reflective electrode on a semiconductor substrate and a twisted nematic liquid crystal layer interposed therebetween;

a first alignment layer with a first alignment direction disposed on the transparent electrode;

a second alignment layer with a second alignment direction disposed on the reflective electrode, wherein a first included angle φis between the first and second alignment directions; and

a polarizing beam splitter disposed on an exterior of the transparent substrate to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction;

wherein a relationship between the first included angle φ and the second included angle β satisfies φ/2<β<φ/2+1°˜3° or 90°+φ/2<β<φ/2+91°˜93°.

17. The reflective light valve according to claim 16, wherein the second included angle β is φ/2+1.5°.

18. The reflective light valve according to claim 16, wherein the first included angle φ is between 40° and 70°.

19. The reflective light valve according to claim 16, wherein the twisted nematic type liquid crystal layer comprises positive dielectric anisotropic liquid crystal molecules.

20. An electronic device, comprising:

a reflective light valve of claim 16;

a controller coupled to the reflective light valve; and

an input device coupled to the controller to input data to the controller to render an image.