WO2012130302A1

WO2012130302A1 - Fast switching double cell liquid crystal device

Info

Publication number: WO2012130302A1
Application number: PCT/EP2011/054944
Authority: WO
Inventors: Lachezar Komitov
Original assignee: Lachezar Komitov
Priority date: 2011-03-30
Filing date: 2011-03-30
Publication date: 2012-10-04
Also published as: CN103718094A; KR20140024354A; TW201300891A; CN103718094B; JP5797323B2; JP2014509756A

Abstract

A liquid crystal device (10) comprising a first liquid crystal cell (11) and a second liquid crystal cell (12) arranged in a layered configuration such that light having passed through the first liquid crystal cell (11) hits the second liquid crystal cell (12). Each of the first liquid crystal cell (11) and the second liquid crystal cell (12) is individually controllable between a relaxed state in which the liquid crystal cell exhibits a first birefringence and a switched state in which the liquid crystal cell exhibits a second birefringence. A transition of the first liquid crystal cell (11) from the relaxed state to the switched state results in a change in a total 10 birefringence of the liquid crystal device (10) from a first total birefringence to a second total birefringence being such that the liquid crystal device (10) transitions from a first light modulation state to a second light modulation state, and a subsequent transition of the second liquid crystal cell (12) from the relaxed state to the switched state results in a change in the total birefringence of the liquid crystal device (10) from the second total birefringence to the first total birefringence whereby the liquid crystal device (10) transitions back from the second light modulation state to the first light modulation state. Hereby, both the switching from the first light -modulation state of the liquid crystal device (10) to the second light -modulation state of the liquid crystal device (10) and the switching back to the first light -modulation state are achieved using the fast process of actively reorienting the liquid crystal molecules (18, 23) in the respective liquid crystal cells (11, 12) through the application of an electric field.

Description

FAST SWITCHING DOUBLE CELL LIQUID CRYSTAL DEVICE

Field of the invention

The present invention relates to a liquid crystal device for modulating light passing through the liquid crystal device. The present invention further relates to a method for controlling such a liquid crystal device.

Background of the invention

In many applications a high switching speed between a transparent ("bright") and a non-transparent ("dark") state would be desirable.

Liquid crystal devices can, for example, be used as light shutters to achieve switching between a bright state and a dark state, but it has turned out to be difficult to achieve a sufficiently high switching speed especially with the conventional nematic liquid crystal materials, widely used at present in a variety of LCDs, and the available electronic driving techniques. In particular, the relaxation of nematic liquid crystal materials is a slow process.

Many solutions for increasing the switching speed of liquid crystal devices have been proposed and experimented, but it has turned out to be difficult to simultaneously achieve both a short switching time from dark to bright and from bright to dark. Summary of the invention

It is an object of the present invention to address the above problems of the prior art, and to provide an improved liquid crystal device being configured to enable a higher switching speed back and forth between a bright and a dark state.

According to a first aspect of the present invention, it is therefore provided a liquid crystal device for modulating light passing through the liquid crystal device, comprising: a first liquid crystal cell having a liquid crystal material comprising a plurality of liquid crystal molecules sandwiched between a first pair of substrates; and a second liquid crystal cell having a liquid crystal material comprising a plurality of liquid crystal molecules sandwiched between a second pair of substrates, the first liquid crystal cell and the second liquid crystal cell being arranged in a layered configuration such that light having passed through the first liquid crystal cell hits the second liquid crystal cell, and each of the first liquid crystal cell and the second liquid crystal cell being individually controllable between a relaxed state in which the liquid crystal cell exhibits a first birefringence and a switched state in which the liquid crystal cell exhibits a second birefringence, wherein the liquid crystal cells are arranged and configured such that: a transition of the first liquid crystal cell from the relaxed state to the switched state, while the second liquid crystal cell remains in its relaxed state, results in a change in a total birefringence of the liquid crystal device from a first total birefringence to a second total birefringence being such that the liquid crystal device transitions from a first light modulation state to a second light modulation state; and a subsequent transition of the second liquid crystal cell from the relaxed state to the switched state, while the first liquid crystal cell remains in its switched state, results in a change in the total birefringence of the liquid crystal device from the second total birefringence to the first total birefringence whereby the liquid crystal device transitions back from the second light modulation state to the first light modulation state.

A nematic liquid crystal cell with uniform alignment behaves optically as a uniaxial (birefringent) optical plate with its optic axis coinciding with the preferred direction of orientation of the liquid crystal molecules in the cell. Inserted between two crossed polarizers, the intensity of the light / transmitted through the cell and the polarizers, when the cell is oriented with its optic axis at 45° to the polarizer transmission direction, is given simply by:

/ = l₀sin²5/2 , where δ = 2π Λη/λ represents the phase retardation of the cell due to its birefringence An=n_e-n₀ {n_e and n₀ are extraordinary and ordinary refractive index of the liquid crystal material, respectively), and λ is the wavelength of the light.

When two such liquid crystal cells, with birefringence

-n₀i and An₂=n_e2-n_o2, and thickness di and d₂, respectively, are superimposed on top of each other with their respective optic axis oriented parallel to each other, the total phase retardation of the cells will be:

Stotai = δι +δ₂ = 2π (di Δπι + d₂ An₂)A However, if the optic axis of the cells are making an angle 90°, the total phase retardation of the two cells will be: totai = δι -<¾ = 2π (di Ani - d₂ An₂)A, or it can be presented also as a sum:

Stotai = Si +(-δ₂) = 2π [di An₁ + d₂ {-An₂)]A, but with An₂ having a negative sign. This is the notation used in the context of the present application.

The gap thickness of the cells in the double cell device is a permanent cell parameter, i.e. does not depend on the applied electric field. On the other hand, the birefringence of the cells is controllable through the application of an electric field. Therefore "birefringence" and "phase retardation" can be considered to be synonymous for the liquid crystal device according to the various aspects of the present invention.

The "relaxed" state may also be referred to as a "Field OFF" state in which no switching electric field exists in the liquid crystal cell, and the

"switched" state may be referred to as a "Field ON" state in which a switching electric field exists in the liquid crystal cell.

The present invention is based on the realization that the relaxation of the liquid crystal material in the liquid crystal cells (the duration of which is determined by the viscosity and the elastic constant of the liquid crystal material as well as by the thickness of the cell gap and anchoring strength) is a limiting factor for the switching speed back and forth between a bright state and a dark state, and that the relaxation process can be optically "cancelled out" by properly arranging two suitable liquid crystal cells in series. The present inventor has further realized that the total birefringence of two liquid crystal cells arranged in series can be used to switch the liquid crystal device between two light-modulation states (such as dark and bright) by actively switching the individual liquid crystal cells. The liquid crystal cells can then relax simultaneously while the liquid crystal device remains in one of its light- modulation states, after which the liquid crystal device is once again ready to be actively switched between light-modulation states.

Hereby, both the switching from the first light-modulation state of the liquid crystal device to the second light-modulation state of the liquid crystal device and the switching back to the first light-modulation state can be achieved using the fast process of actively reorienting the liquid crystal molecules in the respective liquid crystal cells through the application of an electric field.

Once the liquid crystal cells have relaxed back from their switched states to their relaxed states while in the first light-modulation state of the liquid crystal device, the liquid crystal device is ready to be switched to the second light-modulation state again, by controlling the first liquid crystal cell from its relaxed state to its switched state.

In the liquid crystal device according to various embodiments of the present invention, the liquid crystal molecules of the first and second liquid crystal cells are substantially parallel to each other in the relaxed states of the first and second liquid crystal cells. When the first liquid crystal cell is controlled, through the application of an electric field inside the liquid crystal cell, to transition to its switched state, the orientation of the liquid crystal molecules of the first liquid crystal cell will be changed so that the liquid crystal molecules are again substantially parallel to each other, but in another general direction than was the case for the relaxed state. This will result in a change in the birefringence of the first liquid crystal cell, which will in turn result in a change in the total birefringence of the liquid crystal device, since the second liquid crystal cell remains in its relaxed state.

Due to this change in total birefringence of the liquid crystal device, the intensity level of the light passing through the first and second liquid crystal cells placed between crossed polarizers, for instance, will also change, which results in the transition from the first light-modulation state to the second light- modulation state.

It should be noted that this birefringence based control of the transmitted light intensity level is quite different from the polarization control performed by a gradually changing director throughout a liquid crystal cell, as for instance in the twisted nematic cells. The use of birefringence-based control of the liquid crystal device according to various embodiments of the present invention enables designs in which it can be selected which of the above-mentioned first and second light-modulation states should be a bright/dark state. This means that it, through the design of the liquid crystal cells, can be determined, for example, if the simultaneous relaxation of the first and second liquid crystal cells should take place in the dark state or in the bright state. Accordingly, utilizing birefringence-based control of the liquid crystal cells allows for adaptation of the properties of the liquid crystal cell to various requirements on, for example, optical performance, production yield, robustness etc. Furthermore, the sensitivity to misalignment between the polarizer, the analyzer and the liquid crystal cells can be reduced.

Each of the first and second liquid crystal cells may advantageously be configured for so-called "out-of-plane" switching, which means that the liquid crystal molecules are reoriented in a plane that is perpendicular to the substrates of the liquid crystal cell, when the above-mentioned electric field is applied.

The liquid crystal material of the first and second liquid crystal cells may advantageously be in a nematic phase (for the expected operating conditions of the liquid crystal device).

It is desirable to make the simultaneous transitions of the first and second liquid crystal cells from their switched states to their relaxed states in such a way that it is not optically noticeable to a user of the liquid crystal device. To that end, the first and second liquid crystal cells may be arranged and configured such that a sum of the birefringence of the first liquid crystal cell and the birefringence of the second liquid crystal cell remains

substantially constant at the first total birefringence during simultaneous relaxation of the first liquid crystal cell from its switched state to its relaxed state and the second liquid crystal cell from its switched state to its relaxed state.

The total birefringence of the device remains constant due to the birefringence compensation effect that takes place during the relaxation process in the cells. A peculiar feature of the relaxation process in the cells is that while the birefringence of the first liquid crystal cell increases/decreases the birefringence of the second liquid crystal cell decreases/increases which results in a constant value of the total birefringence thus hiding optically the relaxation process.

The liquid crystal device could in principle be controllable through control means arranged externally to the liquid crystal device. However, the liquid crystal device may advantageously be configured such that the first liquid crystal cell comprises a first control electrode and a second control electrode arranged to enable control of the first liquid crystal cell from the relaxed state to the switched state through application of a voltage between the first control electrode and the second control electrode of the first liquid crystal cell; and the second liquid crystal cell comprises a first control electrode and a second control electrode arranged to enable control of the second liquid crystal cell from the relaxed state to the switched state through application of a voltage between the first control electrode and the second control electrode of the second liquid crystal cell.

Such control electrodes may advantageously be arranged to provide the above-mentioned out-of-plane switching of the liquid crystal cells.

According to various embodiments of the present invention, the first birefringence of the first liquid crystal cell may be non-zero and the first birefringence of the second liquid crystal cell may be substantially zero, the first and second liquid crystal cells being arranged such that the first total birefringence is non-zero. When the liquid crystal device according to these embodiments is placed between crossed polarizers, with the optic axis of at least one of the first and second liquid crystal cells making an angle φ with the transmission direction of one of the polarizers being in the interval 0°< φ<90°, preferably substantially equal to 45°, the first modulation state (when both the first liquid crystal cell and the second liquid crystal cells are in their relaxed states) will be a bright state.

Furthermore, in these embodiments the second birefringence of the first liquid crystal cell may be substantially zero, such that the second total birefringence is substantially zero. This will result in the second modulation state being a dark state when the liquid crystal device is properly arranged between crossed polarizers.

According to one embodiment, the first liquid crystal cell may be in a planar alignment configuration in the relaxed state, such that the liquid crystal molecules comprised in the first liquid crystal cell are arranged substantially in parallel with the substrates, and the second liquid crystal cell may be in a vertical alignment configuration in the relaxed state such that the liquid crystal molecules comprised in the second liquid crystal cell are arranged

substantially perpendicular to the substrates.

In the following, a liquid crystal cell that is in a planar alignment configuration in the relaxed state will be referred to as a "PA-cell", and a liquid crystal cell that is in a vertical alignment configuration in the relaxed state will be referred to as a "VA-cell". A liquid crystal device in which the first liquid crystal cell is a PA-cell and the second liquid crystal cell is a VA-cell, will be referred to as a "VA PA-double cell device".

The liquid crystal molecules comprised in the VA-cell may

advantageously exhibit negative dielectric anisotropy, and there may then advantageously be a first control electrode arranged on the first substrate of the first liquid crystal cell and a second control electrode arranged on the second substrate of the first liquid crystal cell. Hereby, the liquid crystal molecules in the VA-cell can be controlled from the vertically aligned state (with a birefringence that is substantially equal to zero) to a state with a non- zero birefringence, such as a planarly aligned state with liquid crystal molecules that are aligned in parallel to the substrates.

Alternatively, the liquid crystal molecules of the VA-cell may exhibit positive dielectric anisotropy, in which case the first and second control electrodes may be arranged on the same substrate to generate so-called in- plane electric field or fringe electric field.

A VA-cell may advantageously be configured in such a way that the liquid crystal molecules, when the VA-cell is in its switched state, are directed in substantially the same direction as the liquid crystal molecules of the PA- cell in a VA PA-double cell device, when the PA-cell is in its relaxed state.

According to various embodiments of the liquid crystal device according to the present invention, the magnitude of the first birefringence of the first cell may be substantially equal to the magnitude of the first birefringence of the second cell.

In particular, the first and second liquid crystal cells may have a substantially identical configuration in respect of liquid crystal material, cell gap, anchoring properties, electrode structure, etc. This will facilitate designing the liquid crystal device in such a way that the total birefringence of the liquid crystal device remains constant during simultaneous relaxation of the liquid crystal cells (the simultaneous transition from the switched state to the relaxed state for the first and second liquid crystal cells.)

For these embodiments of the liquid crystal device according to the present invention (where the magnitude of the first birefringence of the first cell is substantially equal to the magnitude of the first birefringence of the second cell), the first liquid crystal cell and the second liquid crystal cell may be arranged in relation to each other in such way that the total birefringence of the liquid crystal device, when the first liquid crystal cell is in its relaxed state and the second liquid crystal cell is in its relaxed state, is substantially zero. For example, the optic axis of the first cell may be substantially perpendicular to the optic axis of the second cell.

When the liquid crystal device according to these embodiments is placed between crossed polarizers, the first light modulation state (when both the first liquid crystal cell and the second liquid crystal cell are in their relaxed states) will be a dark state, since the birefringence of the liquid crystal device will be substantially zero and the incoming light will not pass through the crossed polarizers where the device is inserted in between.

Furthermore, in these embodiments (where the magnitude of the first birefringence of the first cell is substantially equal to the magnitude of the first birefringence of the second cell), the first liquid crystal cell and the second liquid crystal cell may be arranged in relation to each other in such way that the total birefringence of the liquid crystal device, when the first liquid crystal cell is in its second state and the second liquid crystal cell is in its first state, is non-zero. Accordingly, when the first liquid crystal cell is controlled to transition from its relaxed state to its switched state, the total birefringence of the liquid crystal device will be non-zero, which means that the incoming light will pass through the crossed polarizers, i.e. through the device, so that the liquid crystal device transitions to a bright state. This, of course, requires that crossed polarizers are arranged properly with respect to the in-plane direction of the liquid crystal molecules, which is well known to those skilled in the art.

According to one embodiment, each of the first and second liquid crystal cells may be in a planar alignment configuration in the relaxed state, such that the liquid crystal molecules comprised in each of the first and second liquid crystal cells are arranged substantially in parallel with the substrates. In other words, both the first and second liquid crystal cells may be PA-cells, which means that the liquid crystal device is a "PA PA-double cell device".

In the case of a PA PA-double cell device, the magnitude of the first birefringence of the first liquid crystal cell should be non-zero and

substantially equal to the magnitude of the first birefringence of the second liquid crystal cell. This means that, when the first liquid crystal cell is switched from its relaxed state to its switched state, the birefringence of the first liquid crystal cell will be changed, which means that the total birefringence of the liquid crystal device becomes non-zero, so that the liquid crystal device transitions from a dark state to a bright state.

Advantageously, each of the liquid crystal cells of the PA PA-double cell device may be configured to be switched from the parallel alignment state to a vertical alignment state in which the liquid crystal molecules comprised in the liquid crystal cell become aligned substantially perpendicular to the substrates.

Switching the second PA cell from its relaxed state to its switched state, i.e. turning the planar alignment of the second cell from planar to field- induced homeotropic alignment, while keeping the first PA cell in its switched state, the PA/PA double cell device will be switched from bright state back to dark state. Switching off the cells in this state the relaxation process in the cells will be not noticeable by an observer due to the birefringence

compensating effect. Both cells of the device now being in the relaxed state are ready to be switched again.

For a PA PA-double cell device, the liquid crystal molecules comprised in each liquid crystal cell may advantageously exhibit positive dielectric anisotropy, and each liquid crystal cell may advantageously comprise a first control electrode arranged on the first substrate and a second control electrode arranged on the second substrate. Hereby, the liquid crystal molecules in each of the liquid crystal cells can be controlled from the relaxed planarly aligned state (with a non-zero birefringence) to a switched vertically aligned state with a birefringence that is substantially equal to zero.

According to various embodiments, both the first and the second liquid crystal cells may be in a vertical alignment configuration in the relaxed state such that the liquid crystal molecules comprised in the first liquid crystal cell are arranged substantially perpendicular to the substrates. In other words, both the first and the second liquid crystal cells may be VA-cells, which means that the liquid crystal device is a "VAA/A-double cell device".

For a VAA/A-double cell device, as was the case for the VA PA-double cell device that was discussed above, the liquid crystal molecules comprised in the VA-cell may exhibit negative or positive dielectric anisotropy, and may comprise appropriately arranged control electrodes.

According to a second aspect of the present invention, there is provided a method of controlling operation of the liquid crystal device according to the first aspect, comprising the steps of: controlling the first liquid crystal cell from the relaxed state to the switched state of the first liquid crystal cell while allowing the second liquid crystal cell to remain in its relaxed state; controlling the second liquid crystal cell from the relaxed state to the switched state of the second liquid crystal cell, while maintaining the first liquid crystal cell in its switched state; and simultaneously controlling each of the first liquid crystal cell and the second liquid crystal cell to relax from its respective switched state to its respective relaxed state.

Advantageously, each of the first and second liquid crystal cells may be controlled to relax from its respective switched state to its respective relaxed state in such a way that the total birefringence of the liquid crystal device remains substantially constant during relaxation of said first and second liquid crystal cells.

Embodiments of, and effects obtained through this second aspect of the present invention are largely analogous to those described above for the first aspect of the invention.

As mentioned above, the first aspect of the invention relates to a liquid crystal device comprising a first liquid crystal cell and a second liquid crystal cell arranged in a layered configuration such that light having passed through the first liquid crystal cell hits the second liquid crystal cell. Each of the first liquid crystal cell and the second liquid crystal cell is individually controllable between a relaxed state in which the liquid crystal cell exhibits a first birefringence and a switched state in which the liquid crystal cell exhibits a second birefringence. A transition of the first liquid crystal cell from the relaxed state to the switched state, results in a change in a total birefringence of the liquid crystal device from a first total birefringence to a second total birefringence being such that the liquid crystal device transitions from a first light modulation state to a second light modulation state, and a subsequent transition of the second liquid crystal cell from the relaxed state to the switched state results in a change in the total birefringence of the liquid crystal device from the second total birefringence to the first total

birefringence whereby the liquid crystal device transitions back from the second light modulation state to the first light modulation state. Brief description of the drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:

Fig 1 is a schematic perspective view of a liquid crystal device according to various embodiments of the present invention;

Fig 2 is a diagram schematically illustrating an exemplary driving scheme for a liquid crystal device according to a first embodiment of the present invention;

Figs 3a-d schematically illustrate the state of the liquid crystal device according to the first embodiment of the invention in different stages of the driving scheme in fig 2;

Fig 4 is a diagram schematically illustrating an exemplary driving scheme for a liquid crystal device according to a second embodiment of the present invention; and

Figs 5a-d schematically illustrate the state of the liquid crystal device according to the second embodiment of the invention in different stages of the driving scheme in fig 4. Detailed description of preferred embodiments

Fig 1 is a schematic perspective view of a liquid crystal device 1 according to various embodiments of the present invention. The liquid crystal device 1 comprises a first liquid crystal cell 2 and a second liquid crystal cell 3 that are arranged in a layered configuration between crossed polarizer plates 4, 5 - a "polarizer" 4 closest to the light-source 7, and an "analyzer" 5 closest to the viewer 8. The respective polarization directions of the polarizer 4 and the analyzer 5 are indicated by the dashed lines in fig 1 . Each of the first 2 and the second 3 liquid crystal cells is individually controllable between a relaxed state in which the liquid crystal cell exhibits a first birefringence and a switched state in which the liquid crystal cell exhibits a second birefringence that is different from the first birefringence.

Light from the light-source 7 that has passed through the polarizer 4 to become linearly polarized in the direction indicated in fig 1 , encounters a total phase retardation when passing through the first 2 and second 3 liquid crystal cells that can be represented by the sum of the birefringence of the first liquid crystal cell 2 and the birefringence of the second liquid crystal cell 3. If the total birefringence is zero, no light will pass through the analyzer 5, but if the total birefringence is non-zero, at proper orientation of the optical axis of the birefringent state with respect to the transmission direction of the polarizer, the polarization state of the light will be modified so that at least some light can pass through the analyzer 5 to reach the viewer 8.

Since the above-mentioned total birefringence corresponds to the sum of the birefringence of the first liquid crystal cell 2 and the birefringence of the second liquid crystal cell 3, it follows that the total birefringence can be controlled by controlling either of the first 2 and second 3 liquid crystal cells. By proper configuration of the first 2 and second 3 liquid crystal cells, the liquid crystal device 1 can be actively switched between a dark state and a bright state, and back to the dark state, which means that the switching can be made considerably faster than using a single cell liquid crystal device, in which one of the state transitions in nematic liquid crystals takes place through elastic relaxation of the liquid crystal material, usually very slow. In the various embodiments of the liquid crystal device according to the present invention, as will be described in more detail below, the first 2 and the second 3 liquid crystal cells are configured in such way that the double cell device 1 is in the same light-modulation state (such as bright or dark) when both liquid crystal cells are switched as when both liquid crystal cells are relaxed. In this way, the liquid crystal device can be allowed to relax between switching events, so that switching between light-modulation states can always be active, that is, brought about through the application of an electric field which aligns the liquid crystal molecules in a selected direction.

Two exemplary embodiments of the liquid crystal device in fig 1 and exemplary driving schemes will be described below.

First, a VA PA-double cell device 10 and an exemplary driving scheme for a VA PA-double cell device will be described with reference to fig 2 and figs 3a-d. Figs 3a-d are exploded cross-section illustrations of embodiments of the liquid crystal device 1 in fig 1 with the section taken along the line A-A' in fig 1 .

Turning first to fig 3a, a VA PA-double cell device 10 is schematically shown without any control voltages applied. The VA PA-double cell device 10 comprises a first liquid crystal cell 1 1 and a second liquid crystal cell 12 arranged in a layered configuration between crossed polarizers 4, 5. The first liquid crystal cell 1 1 comprises a nematic liquid crystal material sandwiched between first 14 and second 15 substrates. On the sides of the substrates 14, 15 facing the liquid crystal material, first 16 and second 17 control electrodes are provided, as well as alignment layers (not shown) for aligning the liquid crystal molecules 18 of the liquid crystal material in a planar alignment as is schematically indicated in fig 3a. It should be noted that the liquid crystal molecules 18 are aligned to be angled about 45° relative the polarization direction of the incoming light defined by the polarizer 4. In this configuration, the liquid crystal material in the first liquid crystal cell 1 1 will exhibit a certain non-zero birefringence Δη_ΡΑ.

The second liquid crystal cell 12 also comprises a nematic liquid crystal material (which may be the same material as that comprised in the first liquid crystal cell 1 1 ) sandwiched between first 19 and second 20 substrates. On the sides of the substrates 19, 20 facing the liquid crystal material, first 21 and second 22 control electrodes are provided, as well as alignment layers (not shown) for aligning the liquid crystal molecules 23 of the liquid crystal material in a vertical (homeotropic) alignment as is schematically indicated in fig 3a. In this configuration, the liquid crystal material in the second liquid crystal cell 12 will exhibit a zero birefringence, An_VA = 0.

After having introduced the VA PA-double cell device 10 in figs 3a-d, an exemplary driving scheme for controlling the VA/PA-double cell 10 between different light-modulation states (dark and bright) will now be described with reference to fig 2 and figs 3a-d.

In fig 2, the voltage V_PA provided across the electrodes 16, 17 of the first liquid crystal cell, the PA-cell, 1 1 and the voltage V_VA across the electrodes 21 , 22 of the second liquid crystal cell, the VA-cell, 12 are shown in the top two diagrams as indicated in the figure. In the bottom diagrams, the birefringence of the PA-cell 1 1 (dashed line), the birefringence of the VA-cell 12 (dotted line) and the optical response of the VA PA-double cell 10 (solid line) are shown, resulting from the application of the voltages V_PA and V_VA across the PA-cell 1 1 and the VA-cell 12, respectively, indicated in fig 2.

Between the time to and the time ti, the voltage V_PA across the PA-cell 1 1 is 0 V, and the voltage V_VA across the VA-cell 12 is also 0 V, and the VA PA-double cell 10 is in the state shown in fig 3a, with the liquid crystal molecules 18 in the PA-cell 1 1 in planar alignment and the liquid crystal molecules 23 in the VA-cell 12 in vertical alignment. In this state, the total birefringence An_tot = Δη_ΡΑ + Δη_νΑ = Δη_ΡΑ≠ 0, which means that at proper orientation of the optic axis of the PA-cell 1 1 with respect to the polarization direction of the polarizer 4 the incoming light will pass through the crossed polarizers 4 and 5 where the VA PA-double cell device is inserted, thus the device will be in a bright state as is indicated in fig 2 since it will transmit the incoming light from the light source 7.

At the time ti, a voltage is applied across the PA-cell 1 1 causing the liquid crystal molecules 18 to reorient to a vertical alignment as is

schematically indicated in fig 3b. The voltage across the VA-cell 12 is still 0 V (or at least too low to cause the liquid crystal molecules 23 in the VA-cell 12 to reorient). When the liquid crystal molecules 18 in the PA-cell are switched through the application of the voltage across the PA-cell, the birefringence of the PA-cell is switched from Δη_ΡΑ≠ 0 to Δη_ΡΑ* = 0. As is schematically indicated in fig 2, this switch is relatively fast, and will result in a transition of the total birefringence so that Δη_ω = Δη_ΡΑ* + Δη_νΑ = 0. As a consequence, the light having passed the polarizer 4 in fig 3b will pass through the PA-cell 1 1 and the VA-cell without any change in its polarization state, which means that the VA/PA-double cell device 10 is switched to a dark state as is also indicated in fig 2.

At the time t₂, a voltage is applied across the VA-cell 12 causing the liquid crystal molecules 23 to reorient to a planar alignment as is

schematically indicated in fig 3c. The voltage across the PA-cell 1 1 is still applied so that the liquid crystal molecules 18 in the PA-cell 1 1 remain in the vertical alignment. When the liquid crystal molecules 23 in the VA-cell 12 are switched through the application of the voltage across the VA-cell, the birefringence of the VA-cell is switched from Δη_νΑ = 0 to Δη_νΑ* = Δη_ΡΑ≠ 0. As is schematically indicated in fig 2, this switch is relatively fast, and will result in a transition of the total birefringence of the cells so that An_tot = Δη_ΡΑ* + Δη_νΑ*= Δη_ΡΑ≠ 0 which means that light will again pass through the crossed polarizers 4 and 5 where the VA PA-double cell device is inserted thus the device will be in a bright state as is indicated in fig 2 and fig 3c.

At the time t₃, the voltages are removed across both the PA-cell 1 1 and the VA-cell 12. As a result, the liquid crystal cells 1 1 and 12 simultaneously relax back to their relaxed states. In the PA-cell 1 1 , the liquid crystal molecules 18 relax back to planar alignment and in the VA-cell 12, the liquid crystal molecules 23 relax back to vertical alignment, as is schematically indicated in fig 3d. During the relaxation process, the birefringence of the PA- cell increases from Δη_ΡΑ* = 0 to Δη_ΡΑ≠ 0, whereas the birefringence of the VA-cell 12 decreases from Δη_νΑ* = Δη_ΡΑ≠ 0 to Δη_νΑ = 0, to get back to the relaxed state shown in fig 3a after a certain relaxation time.

During the relaxation, the total birefringence An_tot will remain

substantially constant (and non-zero) due to the configuration of the PA-cell 1 1 and the VA-cell 12 and/or the control voltages applied. However, there may be a slight mismatch in the relaxation of the PA-cell 1 1 and the VA-cell, which may then give rise to a variation in the intensity of the transmitted light, as is indicated as a small "kink" 25 in fig 2. Since this variation would only occur for a duration of in the order of ms and in the bright state of the VA/PA- double cell 10 in figs 3a-d, the variation would hardly be noticeable to the viewer. Although it is expected that such a variation can be avoided or at least made negligible through proper optimization of the cell parameters and the driving scheme, it should be advantageous for some applications to have the simultaneous relaxation (taking place following the time t₃) to occur while the liquid crystal device is in the bright state. This can be achieved using the VA PA-double cell 10 of figs 3a-d.

Finally, at t , a voltage is again applied across the PA-cell 1 1 to bring the VA/PA-double cell 10 back to the dark state shown in fig 3b. In the following, a PA PA-double cell device 30 and an exemplary driving scheme for a PA PA-double cell device will be described with reference to fig 4 and figs 5a-d. Like figs 3a-d, figs 5a-d are exploded cross- section illustrations of embodiments of the liquid crystal device 1 in fig 1 with the section taken along the line A-A' in fig 1 .

Turning first to fig 5a, a PA PA-double cell device 30 is schematically shown without any control voltages applied. The PA/PA-double cell device 30 comprises a first liquid crystal cell 31 and a second liquid crystal cell 32 arranged in a layered configuration between crossed polarizers 4, 5. The first liquid crystal cell 31 comprises a nematic liquid crystal material sandwiched between first 14 and second 15 substrates. On the sides of the substrates 14, 15 facing the liquid crystal material, first 16 and second 17 control electrodes are provided, as well as alignment layers (not shown) for aligning the liquid crystal molecules 33 of the liquid crystal material in a planar alignment as is schematically indicated in fig 5a. As is schematically indicated in fig 5a, the liquid crystal molecules 33 are aligned in parallel with the substrates 14, 15 and in the plane of the paper. It should be noted that the liquid crystal molecules 33 are aligned to be angled about 45° relative the polarization direction defined by the polarizer 4. In this configuration, the liquid crystal material in the first liquid crystal cell 1 1 will exhibit a certain non-zero birefringence Δη_ΡΑι = Δη_ΡΑ.

The second liquid crystal cell 32 also comprises a nematic liquid crystal material (which may be the same material as that comprised in the first liquid crystal cell 31 ) sandwiched between first 19 and second 20 substrates. On the sides of the substrates 19, 20 facing the liquid crystal material, first 21 and second 22 control electrodes are provided, as well as alignment layers (not shown) for aligning the liquid crystal molecules 34 of the liquid crystal material in a planar alignment as is schematically indicated in fig 5a. As is

schematically indicated in fig 5a, the liquid crystal molecules 34 are aligned in parallel with the substrates 19, 20 and perpendicular to the plane of the paper. It should be noted that the liquid crystal molecules 34 are aligned to be angled about 90° relative the preferred direction of alignment of the liquid crystal molecules in liquid crystal cell 31 . In this configuration, the liquid crystal material in the second liquid crystal cell 32 will exhibit a certain nonzero birefringence Δη_ΡΑ2 = Δη_ΡΑ, which in this configuration of the cells with their optic axis making an angle of about 90° should be consider with negative sign, i.e. Δη_ΡΑ2 = - Δη_ΡΑ. After having introduced the PA/PA-double cell device 30 in figs 5a-d, an exemplary driving scheme for controlling the

PA PA-double cell 30 between different light-modulation states (dark and bright) will now be described with reference to fig 4 and figs 5a-d.

In fig 4, the voltage V_PAi provided across the electrodes 16, 17 of the first PA-cell 31 and the voltage V_PA2 across the electrodes 21 , 22 of the second PA cell 32 are shown in the top two diagrams as indicated in the figure.

In the bottom diagrams, the birefringence of the first PA-cell 31

(dashed line), the birefringence of the second PA-cell 32 (dotted line) and the optical response of the PA/PA-double cell 30 (solid line) are shown, resulting from the application of the voltages V_PAi and V_PA2 across the first PA-cell 31 and the second PA-cell 32, respectively, indicated in fig 4.

Between the time to and the time ti, the voltage V_PAi across the first PA-cell 31 is 0 V, and the voltage V_PA2 across the second PA-cell 32 is also 0 V, and the PA/PA-double cell 30 is in the state shown in fig 5a, with the liquid crystal molecules 33 in the first PA-cell 31 in planar alignment in the plane of the paper and the liquid crystal molecules 34 in the second PA-cell 32 in planar alignment in a plane perpendicular to the paper. In this state, the total birefringence Δη_ω = 0, which means that no light will pass through the crossed polarizers 4 and 5 where the PA/PA-double cell 30 device is inserted. Thus, the device will be in a dark state as is indicated in fig 4.

At the time ti, a voltage is applied across the first PA-cell 31 causing the liquid crystal molecules 33 to reorient to a vertical alignment as is schematically indicated in fig 5b. The voltage across the second PA-cell 32 is still 0 V (or at least too low to cause the liquid crystal molecules 34 in the second PA-cell 32 to reorient). When the liquid crystal molecules 33 in the first PA-cell 31 are switched through the application of the voltage across the first PA-cell, the birefringence of the PA-cell is switched from Δη_ΡΑι≠ 0 to Δη_ΡΑι* = 0. As is schematically indicated in fig 4, this switch is relatively fast, and will result in a transition of the total birefringence so that An_tot≠ 0, which means that the PA PA-double cell device 30 is switched to a bright state as is also indicated in fig 4.

At the time t₂, a voltage is applied across the second PA-cell 32 causing the liquid crystal molecules 34 to reorient to a vertical alignment as is schematically indicated in fig 5c. The voltage across the first PA-cell 31 is still applied so that the liquid crystal molecules 33 in the first PA-cell 31 remain in the vertical alignment. When the liquid crystal molecules 34 in the second PA- cell 32 are switched through the application of the voltage across the second PA-cell, the birefringence of the second PA-cell is switched from Δη_ΡΑ2 = -Δη_ΡΑ≠ 0 to Δη_ΡΑ2* = 0. As is schematically indicated in fig 4, this switch is relatively fast, and will result in a transition of the total birefringence so that An_tot = 0 which means that the PA PA-double cell 30 is switched to the dark state as is indicated in fig 4 and fig 5c.

At the time t₃, the voltages are removed across both the first PA-cell 31 and the second PA-cell 32. As a result, the liquid crystal cells 31 , 32 simultaneously relax back to their relaxed states. In the first PA-cell 31 , the liquid crystal molecules 33 relax back to the planar alignment in the plane of the paper, and in the second PA-cell 32, the liquid crystal molecules 34 relax back to the planar alignment in a plane perpendicular to the paper, as is schematically indicated in fig 5d. During the relaxation process, the

birefringence of the first PA-cell increases from Δη_ΡΑι* = 0 to Δη_ΡΑι = Δη_ΡΑ≠ 0, whereas the birefringence of the second PA-cell 32 increases from Δη_ΡΑ2* = 0 to Δη_ΡΑ2 = -Δη_ΡΑ, to get back to the relaxed state shown in fig 5a after a certain relaxation time.

During the relaxation, the total birefringence An_tot will remain

substantially constant (and substantially equal to zero) due to the

configuration of the first 31 and second 32 PA-cells (which may

advantageously be substantially identical) and/or the control voltages applied. As for the VA/PA-double cell device described above with reference to fig 2 and figs 3a-d, there may be a slight mismatch in the relaxation of the first and second PA-cells, which may then give rise to a variation in the intensity of the transmitted light, as is indicated as a small "kink" 36 in fig 4. Finally, at t , a voltage is again applied across the first PA-cell 31 to bring the PA PA-double cell 30 back to the bright state shown in fig 5b.

It should be noted that figs 3a-d and figs 5a-d are simplified and schematic illustrations that are provided to explain various aspects of the present invention, and that the proportions are not representative to a real situation. In particular, a real liquid crystal cell contains many more layers of liquid crystal molecules so that any slight deviations closest to the substrates will only have a negligible effect on the birefringence of the liquid crystal cell.

Furthermore, those skilled in the art will realize that other double cell configurations are also within the scope of the present invention as defined by the appended claims. For example, based on the detailed description provided above and his knowledge in the art, one of ordinary skill in the art would readily be able to realize a VAA/A-double cell device with similar properties as the ones of the PA/PA double cell device.

Moreover, those skilled in the art will be able to select further suitable liquid crystal materials, cell dimensions, alignment layers, more complex driving schemes, etc without undue burden.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims, for example switching of the VA cell(s) in the double cell device of VA PA or VAA/A kind, containing liquid crystal material with positive dielectric anisotropy, by in-plane electric field or fringe field.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

1 . A liquid crystal device for modulating light passing through said liquid crystal device, comprising:

a first liquid crystal cell having a liquid crystal material comprising a plurality of liquid crystal molecules sandwiched between a first pair of substrates; and

a second liquid crystal cell having a liquid crystal material comprising a plurality of liquid crystal molecules sandwiched between a second pair of substrates,

said first liquid crystal cell and said second liquid crystal cell being arranged in a layered configuration such that light having passed through said first liquid crystal cell hits said second liquid crystal cell, and

each of said first liquid crystal cell and said second liquid crystal cell being individually controllable between a relaxed state in which said liquid crystal cell exhibits a first birefringence and a switched state in which said liquid crystal cell exhibits a second birefringence, wherein said liquid crystal cells are arranged and configured such that:

a transition of said first liquid crystal cell from said relaxed state to said switched state, while said second liquid crystal cell remains in its relaxed state, results in a change in a total birefringence of the liquid crystal device from a first total birefringence to a second total birefringence being such that the liquid crystal device transitions from a first light modulation state to a second light modulation state; and

a subsequent transition of said second liquid crystal cell from said relaxed state to said switched state, while said first liquid crystal cell remains in its switched state, results in a change in said total birefringence of the liquid crystal device from said second total birefringence to said first total birefringence whereby the liquid crystal device transitions back from said second light modulation state to said first light modulation state.

2. The liquid crystal device according to claim 1 , wherein said first and second liquid crystal cells are arranged and configured such that a sum of the birefringence of the first liquid crystal cell and the birefringence of the second liquid crystal cell remains substantially constant at said first total birefringence during simultaneous relaxation of said first liquid crystal cell from its switched state to its relaxed state and said second liquid crystal cell from its switched state to its relaxed state.

3. The liquid crystal device according to claim 1 or 2, wherein:

the first liquid crystal cell comprises a first control electrode and a second control electrode arranged to enable control of the first liquid crystal cell from the relaxed state to the switched state through application of a voltage between the first control electrode and the second control electrode of the first liquid crystal cell; and

the second liquid crystal cell comprises a first control electrode and a second control electrode arranged to enable control of the second liquid crystal cell from the relaxed state to the switched state through application of a voltage between the first control electrode and the second control electrode of the second liquid crystal cell.

4. The liquid crystal device according to any one of the preceding claims, wherein the first birefringence of the first liquid crystal cell is non-zero and the first birefringence of the second liquid crystal cell is substantially zero, the first and second liquid crystal cells being arranged such that said first total birefringence is non-zero.

5. The liquid crystal device according to claim 4 wherein the second birefringence of the first liquid crystal cell is substantially zero, such that said second total birefringence is substantially zero.

6. The liquid crystal device according to claim 4 or 5, wherein: the first liquid crystal cell is in a planar alignment configuration in the relaxed state such that the liquid crystal molecules comprised in the first liquid crystal cell are arranged substantially in parallel to the substrates; and

the second liquid crystal cell is in a vertical alignment configuration in the relaxed state such that the liquid crystal molecules comprised in the second liquid crystal cell are arranged substantially perpendicular to the substrates.

7. The liquid crystal device according to claim 6, wherein the liquid crystal molecules comprised in said second liquid crystal cell exhibit negative dielectric anisotropy,

the liquid crystal device comprising a first control electrode arranged on a first substrate of the second liquid crystal cell and a second control electrode arranged on a second substrate of the second liquid crystal cell.

8. The liquid crystal device according to claim 6, wherein the liquid crystal molecules comprised in said second liquid crystal cell exhibit positive dielectric anisotropy, the liquid crystal device comprising a first and a second control electrode arranged on a first substrate of the second liquid crystal cell for generating an in-plane electric field.

9. The liquid crystal device according to any one of claims 1 to 3, wherein a magnitude of the first birefringence of the first liquid crystal cell is

substantially equal to a magnitude of the first birefringence of the second liquid crystal cell, the first liquid crystal cell and the second liquid crystal cell being arranged in relation to each other in such way that said first total birefringence is substantially zero.

10. The liquid crystal device according to claim 9 wherein the first liquid crystal cell and the second liquid crystal cell are arranged in relation to each other in such way that the total birefringence of the liquid crystal device, when the first liquid crystal cell is in its switched state and the second liquid crystal cell is in its relaxed state, is non-zero.

1 1 . The liquid crystal device according to claim 9 or 10, wherein each of the first and second liquid crystal cells are in a planar alignment configuration in the relaxed state, such that the liquid crystal molecules comprised in each of said first and second liquid crystal cells are arranged substantially in parallel with the substrates.

12. The liquid crystal device according to claim 9 or 10, wherein each of the first and second liquid crystal cells are in a vertical alignment configuration in the relaxed state, such that the liquid crystal molecules comprised in each of said first and second liquid crystal cells are arranged substantially perpendicular to the substrates.

13. A method of controlling operation of the liquid crystal device according to any one of claims 1 to 12, comprising the steps of:

controlling said first liquid crystal cell from the relaxed state to the switched state of the first liquid crystal cell while allowing said second liquid crystal cell to remain in its relaxed state;

controlling said second liquid crystal cell from the relaxed state to the switched state of the second liquid crystal cell, while maintaining the first liquid crystal cell in its switched state; and

simultaneously controlling each of said first liquid crystal cell and said second liquid crystal cell to relax from its respective switched state to its respective relaxed state.

14. The method according to claim 13, wherein each of said first and second liquid crystal cells is controlled to relax from its respective switched state to its respective relaxed state in such a way that the total birefringence of the liquid crystal device remains substantially constant during relaxation of said first and second liquid crystal cells.