A LIQUID CRYSTAL DEVICE AND METHOD OF CONTROLLING
SUCH A DEVICE
The present invention relates to a liquid crystal device and a method of controlling such a device.
The present invention particularly but not exclusively relates to a liquid crystal device suitable for displaying relatively high image rates and suitable for displaying image content in a power saving mode.
Fast image rate, direct view, liquid crystal display technologies are typically based on twisted nematic devices (TN) , vertical alignment devices (VA) or in-plane switching devices (IPS) . These technologies can have particular advantages in, for example, switching speed, contrast ratio or viewing angle. Pi-cells have also been proposed for use in motion picture applications such as LCD televisions because of the relatively fast response time thereof. However, typically liquid crystal displays have insufficiently fast response times which are in the order of at least 15ms.
Another category of current display technologies includes bistable displays such as, for example, Zenithally bistable displays (ZBD) , surface stabilised cholesteric displays (SSCD) , polymer stabilised cholesteric displays (PSCD) , Bistable Twisted Nematic (BTN) displays, or non liquid crystal displays such as E-ink for example. These technologies typically require no power to keep a particular image content displayed on the screen of the device. Bistable behaviour is achieved by the occurrence of two optically different liquid crystal states that are stable without any external electrical field. However, none of these technologies is fast enough to provide frame refresh rates suitable for video application, hence their dominant applications are based on their power saving advantage in that the image displayed typically updates only relatively slowly.
Light transmitting liquid crystal displays typically use a back light emitter comprising a fluorescent tube providing white light. The light is typically controlled using three liquid crystal sub pixels each having a respective colour filter for red, green or blue. The three sub pixels are placed in close proximity to each other and form one composite liquid crystal display pixel. The colour transmitted can be controlled by controlling the intensity of light transmitted through each sub pixel. However, this type of control wastes approximately two thirds of the produced light, since each of the individual colour filters transmits only about one third of the light received on that filter.
It has also been proposed to provide a colour sequential light emitter as an alternative to a fluorescent tube. The colour mixture can be achieved, for example, by using a light emitting backlight comprising red, green and blue light emitting diodes flashing in sequential order within a given time period, whereby only one liquid crystal pixel instead of three controls the intensity of the light transmitted. Such a colour sequential system is more light efficient and can require less power compared to a fluorescent tube system.
The human eye generally perceives this sequence of flashes as a colour mixture if repeated at a frequency of approximately at least 60 Hz. For a display device, however, this means that the associated liquid crystal needs to switch on and off three times within a time period of 16 ms or, in other words, the switching time of each on and off cycle has to be faster than 5 ms. Such switching times cannot be implemented with the prior technologies used in common display applications. According to a first aspect of the invention there is provided a liquid crystal device comprising a housing provided with a layer of liquid crystal material the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material in a light transmitting mode whereby the light emitting means is selectively activated and a selected part of the layer of liquid crystal material is switched between being in a homeotropic state and being in at least one of a transient planar state, an unaligned planar state and an aligned planar state such that light is selectively transmitted from the light emitting means through the layer of liquid crystal material.
The homeotropic state occurs where the longitudinal axes of the liquid crystal molecules are, on average, mutually parallel and substantially perpendicular to the plane of the screen.
The transient planar state occurs where the longitudinal axes of the liquid crystal molecules are, on average substantially parallel to the plane of the screen, with the pitch length of the helix in which the liquid crystal molecules are arranged being approximately twice the natural helical pitch length of the liquid crystal material.
The aligned planar state is a cholesteric state which occurs where the liquid crystal molecules naturally adopt a helical structure where the helical axis is, on average normal, or close to the normal, to the plane of the layer of liquid crystal material. The longitudinal axes of the liquid crystal molecules rotate and are therefore orientated around the helical axis of the liquid crystal layer. The helical axis is substantially aligned to the normal of the liquid crystal layer and therefore the aligned planar state is capable of a distinct reflection of a selective wavelength of light directed onto the plane of the liquid crystal layer. The unaligned planar state is another cholesteric state which occurs where the liquid crystal molecules naturally adopt a helical structure where the helical axis is substantially normal to the plane of the layer of liquid crystal material but varies in a wider angle across the plane of the liquid crystal layer than in the aligned planar state. The longitudinal axes of the liquid crystal molecules are again orientated around the helical axis of the liquid crystal layer. The liquid crystal layer of the unaligned planar state is not capable of a distinct reflection of a selective wavelength of light directed onto the plane of the liquid crystal layer. It is envisaged that the device could be controlled such that the homeotropic liquid crystal state is a dark state, or is a light transmitting state.
Preferably the control means is operative such that a selected part of the layer of liquid crystal material is controlled in the light transmitting mode between being in a homeotropic state and sequentially being in each of the transient planar state, the unaligned planar state and the aligned planar state.
Preferably the control means is further operative to control the light emitting means and the layer of liquid crystal material in a second light transmitting mode whereby the layer of liquid crystal material is controlled between being in an aligned planar state and being in a focal conic state such that light is selectively transmitted from the light emitting means through the layer of liquid crystal material.
The focal conic state occurs where the liquid crystal molecules naturally adopt a helical structure where the helix axis is substantially parallel to the plane of the layer of liquid crystal material.
Preferably the device comprises at least one light polariser, the control means being operative to control the layer of liquid crystal material such that in at least one liquid crystal state, light emitted from the light emitting means and incident on the polariser is prevented from passing through the polariser.
Preferably the device comprises a pair of spaced apart polarisers, the layer of liquid crystal material being located between the polarisers, the control means being operative to control the layer of liquid crystal material such that in at least one liquid crystal state, light emitted from the light emitting means and passing through one polariser does not, in use, pass through the other polariser.
Preferably the polarisers are crossed such that the light transmission axis of one polariser is at 90° to the light transmission axis of the other polariser.
Alternatively the light transmission axis of one polariser is at an acute angle relative to the light transmission axis of the other polariser. Alignment means can be provided to realign light passing, in use, through one polariser such that the realigned light does not pass, in use, through the other polariser.
Preferably the alignment means comprises at least one passive, that is, unpowered, light retardation plate mounted in the housing.
Alternatively or additionally the alignment means comprises at least one active, that is, powered, light retardation plate.
Preferably the active retardation plate is deactivated at least when the control means is operating in the light transmitting mode. The active retardation plate may comprise a control layer of liquid crystal material adjacent to but separate from the other layer of liquid crystal material.
Preferably the control layer of liquid crystal material comprises a nematic liquid crystal material.
Preferably the device, when operating in the light transmitting mode, uses a liquid crystal control voltage of equal to or less than 7V RMS. This enables the device to further comprise a thin film transistor control matrix to control the layer of liquid crystal material.
The light emitting means preferably comprises LED means. Preferably the LED means comprises a plurality of sets of LEDs, each LED in a set emitting in use a different coloured light to each other LED in that set.
Preferably the light emitting means emits polarised light.
The light emitting means may alternatively comprise a white light emitter. In that instance, a colour filter may be provided adjacent each liquid crystal pixel.
Preferably the device further comprises a polymer lattice located within the layer of liquid crystal material, the layer of liquid crystal material being at least partially supported by the polymer lattice.
According to a second aspect of the invention there is provided a method of controlling a liquid crystal device comprising a housing provided with a layer of liquid crystal material, the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material, the method comprising steps of controlling the device in a light transmitting mode whereby the light emitting means is selectively activated and a selected part of the layer of liquid crystal material is switched between being in a homeotropic state and being in at least one of a transient planar state, an unaligned planar state and an aligned planar state such that light is selectively transmitted from the light emitting means through the layer of liquid crystal material.
According to a third aspect of the invention there is provided a liquid crystal device comprising a housing provided with a layer of liquid crystal material the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material in a light transmitting mode whereby the light emitting means is selectively activated and a selected part of the layer of liquid crystal material is switched between being in a homeotropic state and being in at least one of a transient planar state, an unaligned planar state and an aligned planar state such that light is selectively transmitted from the light emitting means through the layer of liquid crystal material, the control means being further operative to control the layer of liquid crystal material in a power saving mode whereby the layer of liquid crystal material is controlled between being in an aligned planar state and being in a focal conic state each of which is a stable state requiring no power.
In the aligned planar state the device is operative to reflect external light incident on the layer of liquid crystal material.
According to a fourth aspect of the invention there is provided a method of controlling a liquid crystal device comprising a housing provided with a layer of liquid crystal material, the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material, the method comprising steps of controlling the device in a light transmitting mode whereby the light emitting means is selectively activated and a selected part of the layer of liquid crystal material is switched between being in a homeotropic state and being in at least one of a transient planar state, an unaligned planar state and an aligned planar state such that light is selectively transmitted from the light emitting means through the layer of liquid crystal material, the method comprising further steps of controlling the device in a power saving mode wherein the layer of liquid crystal material is controlled between being in an aligned planar state and being in a focal conic state each of which is a stable state requiring no power.
According to a fifth aspect of the invention there is provided a liquid crystal device comprising a housing provided with a layer of liquid crystal material the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material in a light transmitting mode wherein the light transmission time, during which light is transmitted from the light emitting means through the layer of liquid crystal material, is controlled to determine the amount of light transmitted through the layer of liquid crystal material in a given time interval.
In one example, the light transmission time is controlled by applying a first voltage to at least part of the layer of liquid crystal material, altering that applied voltage to another voltage, and subsequently varying the set time, the set time being the sum of the light transmission time and the delay time, the delay time being the time taken for part of the layer of liquid crystal material to change state in response to the change in applied voltage. Preferably the other voltage is a substantially constant amplitude voltage. The voltage may be a DC or an AC voltage.
In another example, the light transmission time is controlled by maintaining the set time as constant, the set time being the sum of the light transmission time and the delay time, the delay time being the time taken for the layer of liquid crystal material to change state in response to a change in applied voltage, and subsequently varying the applied voltage.
Varying the applied voltage advantageously alters the delay time which consequently controls the light transmission time for a given set time.
Preferably the device comprises light colouring means to colour the light transmitted through the layer of liquid crystal material, the control means being operative to control the transmission time of light transmitted through the layer of liquid crystal material to control the colour of light transmitted.
The light colouring means may comprise a plurality of coloured light emitters, the colour of light transmitted being controlled by controlling at least one of the activation of the light emitters, the light transmission time, and the maximum intensity of light emitted.
Preferably each light emitter comprises an LED, the device comprising a plurality of sets of LEDs, each LED of a set emitting, in use, light of a different colour to that emitted by the other LEDs of that set.
Preferably the control means is operative to sequentially control the transmission time of each LED in a set within a given time cycle, the colour of light transmitted being controlled by varying the transmission time of each coloured LED within that time cycle.
The time cycle of sequentially controlling the LEDs in each set is sufficiently fast that the separate colour transmissions from each LED is perceived by the human eye as a single integral colour.
According to a sixth aspect of the invention there is provided a method of controlling a liquid crystal device comprising a housing provided with a layer of liquid crystal material, the device further comprising light emitting means, and control means operative to control the light emitting means and the layer of liquid crystal material, the method comprising steps of controlling the device in a light transmitting mode wherein the light transmission time, during which light is transmitted from the light emitting means through the layer of liquid crystal material, is controlled to determine the amount of light transmitted through the layer of liquid crystal material in a given time interval.
Other aspects of the present invention may include any combination of the features or limitations referred to herein.
The present invention may be carried into practice in various ways, but embodiments will now be described by way of example only with reference to the accompanying drawings in which:
Figures Ia and Ib are schematic sectional side views through a unit of a device in accordance with the present invention with a layer of liquid crystal material in a first two liquid crystal states; Figures 2a and 2b are schematic sectional side views through a unit of a device in accordance with the present invention with the layer of liquid crystal material in a further two liquid crystal states;
Figure 3 is a schematic view showing the layer of liquid crystal material of a unit of the device in a number of different liquid crystal states;
Figure 4 is a graph comparing light transmission with an applied liquid crystal control voltage of a unit of a device in accordance with the present invention; and
Figure 5 is a further graph comparing light transmission with an applied liquid crystal control voltage of a unit of a device in accordance with the present invention.
Referring to the figures, a liquid crystal device 1 comprises a housing (not shown) in which is mounted a plurality of light valve units. Each unit comprises light emitting means, and a layer of liquid crystal material between two parallel, spaced apart glass plates 2, 3. Light can be selectively transmitted through the uppermost plate 3 to create an image for direct view by looking at the upper plate 3, or for further transmission via another apparatus, such as, for example a projector. The plates 2, 3 could be formed from any suitable material or combination of materials including, for example, a plastics material.
The space between the plates 2, 3 is filled with the layer of liquid crystal material 4 which, in this example, comprises a relatively short pitch cholesteric liquid crystal material 4. In this example, two spaced apart polarisers 5, 7 are provided, one polariser 5 being mounted below the lowermost plate 2, and the other polariser 7 being mounted above the upper plate 3.
Light emitting means in the form of a plurality of light emitting diodes (LEDs) 9 are mounted below the lower polariser 5. The polarisers 5, 7 are crossed in that the light transmission axis of one polariser is at 90° to the light transmission axis of the other polariser such that light emitted from the LEDs 9 and passing through the lowermost polariser 5 is blocked by the upper polariser 7.
The LEDs 9 are arranged in sets, each set comprising three LEDs 9. Each set of three LEDs 9 comprises a blue LED, a green LED, and a red LED. A respective LED 9 can be provided, for example, for each liquid crystal pixel if required, the liquid crystal pixels being arranged in sets of three sub pixels, each set of sub pixels forming a composite liquid crystal master pixel whose colour can be controlled by controlling the switching of the LED 9 associated with each sub pixel.
The light emitting means, the layer of liquid crystal material, the plates 2, 3 and the polarisers 5, 7 each therefore comprise a unit of the device, the housing being provided with a plurality of units arranged in a parallel array. Each unit thus comprises a single, controllable, liquid master crystal pixel.
The device 1 is further provided with a power source and control means (not shown) which may comprise, for example an electronic data processor incorporating printed circuit boards, to control the orientation/position of liquid crystal molecules in the layer of liquid crystal material 4 within the housing, and to control the activation, i.e. the switching on and off, of each LED 9. Any other suitable type of control means can alternatively be provided.
It is envisaged that each liquid crystal pixel may be adjacent a respective thin film transistor (TFT) arranged in an active matrix formation, each TFT being independently controlled by the control means. Applying a control voltage to a TFT changes the orientation of the associated liquid crystal molecules 4 adjacent that TFT relative to the planes of the plates 2, 3 and this change in orientation alters the angle of light rays passing through the layer of liquid crystal material 4 so that the rays can or cannot pass through the upper plate 3.
Referring to Figure Ia, the layer of liquid crystal material 4 is shown in a rest state wherein the layer of liquid crystal material 4 is in a homeotropic state 21 in which the longitudinal axes of the liquid crystal molecules 4 are arranged, on average, in substantially parallel layers, the plane of each layer being, on average, perpendicular to the plane of the upper plate 3. This state can be achieved by applying a suitable liquid crystal control voltage across the layer of liquid crystal material 4. In this state, in this example, all or part of the upper plate 3 appears dark because any light transmitted from the LEDs 9 is re-orientated by the lower polariser 5 such that the light passes through the layer of liquid crystal material 4 but is not polarised in a direction to allow the light to pass through the upper polariser 7. This light path is shown by arrow 11.
Referring to Figure Ib, the layer of liquid crystal material 4 is shown in a cholesteric unaligned planar state 27 wherein the liquid crystal molecules 4 adopt a helical structure in which the helical axis is on average parallel with the plane of the layer of liquid crystal material 4. Thus the longitudinal axis of the liquid crystal molecules are rotated around the helical axis. The helical axis may not be perfectly parallel with the plane of the layer of liquid crystal material and the helix may be partially broken into smaller helical pieces. In such an unaligned planar state 27, in this example, all or part of the visible area of the upper plate 3 of each unit of the device 1 appears bright because any light emitted from the LEDs 9 passes through the lower polariser 5 when incident on the layer of liquid crystal material 4. This is because any light transmitted from the LEDs 9 is linearly polarised by the lower polariser 5 such that the light is elliptically polarised and partly depolarised as the light passes through the layer of liquid crystal material 4 so that at least a part of the light is polarised in the direction of the upper polariser to allow the light to pass through it. This light path is shown by arrow 13.
Referring to Figure 2a, the unit of the device 1 is shown in a first condition of the power saving mode wherein the liquid crystal molecules 4 are in cholesteric aligned planar state 29. This state 29 is stable in that the layer of liquid crystal material 4 does not require any power to remain in this state 29. In this aligned planar state 29, the periodic structure of the helix in which the layers of liquid crystal material 4 are arranged is such that external light passing through the upper polariser 7 is reflected by the layer of liquid crystal material 4 and subsequently passes back through the upper polariser 7. This light path is indicated by arrows 15.
Referring to Figure 2b, the device 1 is shown in a second condition of a power saving mode wherein the layer of liquid crystal material 4 is in a focal conic state 23 in which the liquid crystal molecules 4 adopt a helical structure where the helix axis is substantially parallel to the plane of the layer of liquid crystal material 4. Again, this state 23 is stable in that the layer of liquid crystal material 4 does not require any power to remain in this state 23. In this focal conic state 23, external light passing through the upper polariser 7 passes through the layer of liquid crystal material 4 and is absorbed by a dark background of the device 1 which can be provided, for example, by the LEDs 9 when deactivated. This light path is indicated by arrow 17.
The switching of the layer of liquid crystal material 4 by the control means between the different liquid crystal states 21, 23, 25, 27, 29 is indicated with reference to Figure 3.
If an alternating current (AC) field is applied to the layer of liquid crystal material 4 that is larger then the field Eu necessary to unwind the helix in which the liquid crystal molecules are arranged, the liquid crystal molecules re-orientate into the homeotropic state 21.
The reaction of the layer of liquid crystal material 4 to a reduction in the electric field strength depends on whether this reduction occurs suddenly or slowly and to what field level it is reduced.
If the electric field is reduced slowly, or to a level above a critical field E0, the layer of liquid crystal material 4 forms into the stable focal conic state 23.
However, if the electric field is reduced relatively quickly and below Ec, the layer of liquid crystal material 4 forms into a transient planar state 25. The transient planar state 25 is characterised by the helix in which the liquid crystal molecules are arranged having a helical pitch length that is approximately twice the natural helical pitch length of the liquid crystal material 4. This transient planar state 25 is not stable and the helical structure breaks up into the unaligned planar state 27 comprising small helical liquid crystal fragments having approximately the natural helical pitch length of the cholesteric liquid crystal material 4. The liquid crystal molecules 4 then slowly align to the stable aligned planar state 29 wherein the helical axes of the liquid crystal molecules are normal to plates 2, 3. If the applied electric field is then removed, the layer of liquid crystal material 4 remains in the stable aligned planar state 29 until an electric field above Eu is subsequently applied across the layer of liquid crystal material 4.
Both stable states, the aligned planar state 29 and the focal conic state 23, and the switching mechanism between them, are utilised as a power saving mode in the device 1. However, the switching speed of this mode is relatively slow (in the order of 250ms) , because it relies on the build up of the selective reflection effect of the aligned planar state 29 that in turn relies on a relatively time consuming alignment of the helical fragments of liquid crystal molecules.
It is envisaged that both stable states 23, 29 can also be switched directly between each other when a layer of dual frequency liquid crystal material 4 is used. This is achieved whereby a voltage with a low frequency is used to switch from the aligned planar state 29 to the focal conic state 23, and a higher frequency is used to switch from the focal conic state 23 back to the aligned planar state 29.
The relatively fast light transmitting mode relies on light being transmitted from the LEDs 9 during the transition of the layer of liquid crystal material 4 between the homeotropic state 21 and the unaligned planar state 27. This transition occurs via the transient planar 25 state. The formation of the transient planar state 25 is usually relatively fast (after a delay time tDclIiy) and is typically completed in less than half a millisecond. The birefringence of the chiral layer of liquid crystal material 4 in the transient planar state 25 allows light transmission through the crossed polarisers 5, 7.
The device 1 is also at least partially transparent during the subsequent nucleation process from the transient planar state 25 to the unaligned planar state 27state 27. Although the longitudinal axes of the liquid crystal molecules are not perfectly aligned when in this unaligned planar state 27, and the transmitted light is affected by the birefringence of the aligned helix, this unaligned planar state 27can still be used in a light transmitting mode.
We have discovered that a sufficiently high light transmission level can usually be achieved within less than a millisecond. In comparison, the perceived effect of a selective reflection achieved using the aligned planar state 29 , is dependent on the reorientation of the broken helical liquid crystal molecules and several hundred milliseconds are typically necessary to achieve sufficiently good light reflection.
Referring additionally to Figure 4, it can be seen that after the liquid crystal driving voltage V is rapidly reduced to a level VL from a higher level VH (VH being characterised by E0) , the layer of liquid crystal material 4 responds after a delay time tDciay. The light transmission level T may change in reality during the transition time tTrans, but this is perceived by the eye as an integral light pulse, because the set time tScl and the reset time tResot are in the order of a few milliseconds. The reset time is the time that is required by the layer of liquid crystal material 4 to switch into the homeotropic liquid crystal state 21. Moreover this characteristic of the layer of liquid crystal material 4 can be used in a method of achieving grey scale level control by varying the transmission time tTrans which determines the amount of light perceived by the eye of the observer.
A first technique comprises using the control means to alter the set time tset- The delay time tDe,ay is constant, because the lower switching voltage level VL (which could be for example zero volts) is kept constant. Hence the transmission time tTrans can be increased or decreased, which affects the amount of light transmitted from the LEDs 9, through the layer of liquid crystal material 4 and through the upper plate 3.
A further technique comprises using the control means to vary the lower voltage level VL. The lower voltage level VL has influence dominantly on the delay time tdeioy, but relatively little on the light transmission slope. This in turn also reduces the transmission time tTrans and the amount of transmitted light T, given a constant set time tSet.
The control of the light transmission time can be used to achieve grey light levels necessary for the technique of colour mixture in high quality moving image, grey scale displays. Such displays can use a white fluorescent backlight, or a white fluorescent backlight with a three liquid crystal sub-pixel system with light colouring means comprising colour filters being located on top of each liquid crystal sub-pixel. A white fluorescent back light could be used with a colour wheel that is movable to control the colour of light transmitted. Preferably, however, a time sequential LED backlight system without colour filters is used, as described above. Different shades of colour in the colour display are achieved by an alternating display of three basic colours red, green and blue, whereby the transmitted amount of light T of each colour is controlled separately.
This principle is shown in figure 5 wherein the second technique of varying the voltage level VL has been used by example to demonstrate the colour mixing. The sequential rotation of the colours has to be fast enough for the human eye to recognise the colours as a single, integral colour over the transmission periods of each LED 9 that form one full light transmitting cycle for all of the LEDs 9 in a particular set.
It can be seen that the transmission time of light from each LED 9 in a given set of LEDs 9 can be varied by altering the voltage applied to the liquid crystal pixels. If this control is conducted for each set of LEDs 9 with the activation of the three differently coloured LEDs 9 of each set being controlled cyclically, the colour and intensity of the light perceived by a viewer through the upper plate 3 can be controlled.
Under some circumstances, especially when relatively dark scenes are displayed, the brightest image areas may only need to show a brightness corresponding to a fraction of the total range available. In this case one can improve the image quality, especially the contrast, by decreasing the overall illumination intensity impinging on the display element while adjusting the transmission properties of the element containing the liquid crystal accordingly. For this purpose, the illumination intensity can be controlled in addition to the light transmission timing and the activation of the light emitters.
An alternative set of basis colours could alternatively be used. The plates 2, 3 confining the layer of liquid crystal material 4 can have either a planar or a honieotropic surface alignment which can be achieved, for example, by a thin polymer layer deposited on the surface of the plates 2, 3 that interfaces with the layer of liquid crystal 4 material.
The pitch of the helix in which the liquid crystal molecules are naturally arranged should be such that a selective light reflection in the visible optical spectrum can be achieved, if the function of the stable, light reflective mode using the aligned, planar liquid crystal state 29 is required.
The device 1 can be operated in the fast switching light transmitting mode only, if required.
The device 1 is operated between crossed linear polarisers 5, 7 whereby the light transmission axis of each polariser 5, 7 is at 90° to the light transmission axis of the other polariser 5, 7. However, in alternative implementations of the device 1, the light transmission axis of the polarisers 5, 7 can be orientated at another, acute angle in respect to each other.
One or more light retardation films can be used to control, or to further control, the polarisation of the light.
Any suitable type of polariser or combination of polarisers can alternatively be used.
The use of the polarisers and/or any additional light retardation films or other light transmission axis control means can be selected such that the homeotropic liquid crystal state 21 appears dark, or appears light. An active, switchable light retardation unit (for example a planar aligned nematic liquid crystal unit optimised to act as a λ/2-wave plate) can also be used instead of or in conjunction with the passive retardation film(s) to improve the efficiency of the light transmission through the layer of liquid crystal material 4.
Such an active light retardation unit could be positioned, for example, between the upper plate 3 and the top polariser 7. Any other suitable position could alternatively be used. The control means is operative to control the active light retardation unit and this could include switching off the light retardation function when the liquid crystal device 1 is in the fast switching light transmitting mode (for example by switching the liquid crystals of the retardation unit into a homeotropic state) .
Preferably, the independently controllable, individually coloured LEDs 9 are used as light emitting means for the device 1. However, any other suitable type of backlight can alternatively be used including for example a fluorescent white backlight - if coloured light is required, each liquid crystal pixel can be provided with a respective coloured filter, or a colour wheel could be provided.
A polymer network or lattice can be provided located within the layer of liquid crystal material 4 in order to tune and optimise the properties of the layer of liquid crystal material 4 in the device 1. These properties include but are not limited to :
• the viewing angle by the introduction of light scattering when the layer of liquid crystal material 4 is in the transient planar state 25 and the unaligned planar state 27; • the light efficiency of the fast light transmitting mode by stabilising the transient planar state 25; and,
• the increased stability of the bistable liquid crystal states 23, 29.
The additional polymer network is not shown in any of the figures but can be present in any of the described device variations described above.
Providing individually coloured LEDs 9 to provide light for the device 1 effectively reduces the required number of liquid crystal pixels to a third as compared to a device using a white backlight and filters for each liquid crystal pixel because to achieve good colour control each perceived pixel actually needs to comprise three liquid crystal pixels each provided with a differently coloured filter. Given the usual active-matrix liquid crystal driving scheme, this also reduces the number of TFTs to a third and subsequently cuts the costs of manufacturing because a much better production yield can be achieved - a problem with TFT production is that typically not all of the TFTs in a given matrix are functional which leads to portions of the liquid crystal material not being controllable .
An additional reduction in cost occurs because colour filters for the liquid crystals are not necessary. This saves material cost, reduces the number of processing steps during production, and increases the light efficiency of the device 1 which can lead to a lower overall power consumption, depending on the light efficiency of the LEDs 9.