GB2390171A

GB2390171A - Optical device and display

Info

Publication number: GB2390171A
Application number: GB0215058A
Authority: GB
Inventors: Adrian Marc Simon Jacobs; Martin David Tillin
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2002-06-28
Filing date: 2002-06-28
Publication date: 2003-12-31
Also published as: TW200403499A; GB0215058D0; TW594263B; AU2003238707A1; WO2004004362A1

Abstract

An optical device comprises an input polariser 4, a patterned retarder 5 and an output polariser 12. The retarder 5 has regions 8 and 9, at least one of which alters the polarisation of light from the input polariser 4. The regions supply light of different non-orthogonal polarisations 18, 19. The output polariser 7 has a transmission axis 12 such that light passing though the regions 8 and 9 of the retarder 5 and though the output polariser 7 is matched in amplitude, phase and polarisation. Such a device may be used as a switchable parallax barrier with an LCD 2 to provide a display which is switchable between an autostereoscopic 3D mode and a 2D mode.

Description

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À. 239017 1

M&C Folio No P521 ISGB | OPTICAL DEVICE AND DISPLAY

The present invention relates to an optical device which may, for example, be used in a display capable of operating in two dimensional (2D) and autostereoscopic three dimensional (3D) modes. The present invention also relates to displays incorporating such optical devices.

EP 0 829 744 discloses a display which may be operated in 2D and 3D modes. Figure 1 of the accompanying drawings illustrates the basic structure of one example of such a display in the 2D and 3D modes. In the 3D mode, the display comprises a compact extended backlight 1 disposed behind a spatial light modulator (SLM) embodied as a liquid crystal device (LCD) 2. The LCD 2 has a rear polariser 3 and a front polariser 4.

In the 3D mode, the display is of the front parallax barrier type in which the parallax barrier is formed by a patterned retarder 5 formed on a substrate 6 and a polariser 7.

In the 2D mode also illustrated in Figure 1, the polariser 7 is removed so that the parallax barrier is effectively disabled.

Figure 2 of the accompanying drawings illustrates operation in the 3D mode. The retarder 5 comprises regions such as 8, which rotate the polarisation direction of light passing therethrough by 90 , and regions such as 9, which do not alter the polarisation of light passing therethrough. The regions 8 correspond to the slits of the parallax barrier whereas the regions 9 correspond to the opaque barrier portions between the slits. In Figure 2, polarisation directions in the plane of the drawing are represented by double-headed arrows whereas polarisation directions perpendicular to the plane of the drawing are represented by filled circles. Unpolarised light from the backlight I is incident on the input polariser 3, which substantially blocks the polarisation component perpendicular to the plane of the drawing and has a transmission axis 10 which passes the polarisation component in the plane of the drawing. The LCD 2 is of a type which is controlled so as to vary the polarisation rotation through the device with 90 rotation

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M&C Folio No P5211 5GB 2 corresponding to maximum brightness. The transmission axis] 1 of the output polariser 4 is orthogonal to the transmission axis 1 () of the input polariser 3 so that the output polariser 4 transmits only light polarised perpendicular to the plane of the drawing.

Light from the output polariser 4 passing through the regions 9 has its polarization unchanged. The polariser 7 has a transmission axis 12 which is orthogonal to the transmission axis 11 of the polariser 4 so that light passing through the regions 9 is substantially blocked and the regions 9 appear dark or opaque. Light passing through the regions 8 has its polarization direction rotated by 90 so as to be parallel to the transmission axis 12 ofthe polariser 7. The polariser 7 thus transmits this light so that the combination of the patterned retarder 5 and the polariser 7 acts as a parallax barrier.

In the 2D mode of the display, the polariser 7 is moved or removed so as to be out of the light path from the display to an observer. The barrier structure is thus no longer visible and light from both the regions 8 and the regions 9 is transmitted to an observer.

EP O 833 183, EP O 887 692 and EP O 887 666 illustrate further examples of displays having an autostereoscopic mode in which a patterned retarder cooperates with a polariser to act as a parallax barrier. Some of the arrangements disclosed in these documents also have a 2D mode in which the barrier is disabled in some way. All such known displays are designed so as to optimise the 3D performance, particularly by minimising 3D crosstalk which is related to the ratio of the light transmission through the slit regions and the barrier regions of the patterned retarder. An example of this is illustrated in Figure 3 of the accompanying drawings, which shows diagrammatically an example of a display for illustrating the compromised 2D performance.

The display of Figure 3 differs from that of Figure 2 in that an additional liquid crystal retarder 15 is disposed between the patterned retarder 5 and the analysing polariser 7.

In Figure 3, the polariser transmission axes are illustrated by solid lines with filled arrow heads, the slow axes of retarders are illustrated by solid lines with open arrow heads, and light polarizations are illustrated by broken lines with solid arrow heads.

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M&C Folio No P52 11 SOB 3 In the 3D mode, the polariser 3 has its transmission axis 10 oriented at 0 , ie vertically.

The slit regions ofthe retarder 5 have slow axes 16 oriented at -22.5 whereas the barrier regions have slow axes 17 oriented at 22.5 . The polarisation direction 18 of light from the slit regions is thus rotated to -45 whereas the polarisation direction 19 of light from the barrier regions is rotated to 45 . The polarisation directions 18 and 19 are therefore orthogonal to each other. The slow axis 20 of the retarder l 5 is oriented at -

67.5 so that the polarisation directions of light from the slit and barrier regions are rotated to be as shown at 21 and 22, respectively. The transmission axis 12 of the analysing polariser 7 is oriented at 90 so that substantially 100% of light from the slit regions is transmitted whereas substantially 0% of light from the barrier regions is transmitted (in practice, slightly less than 100% of light is transmitted from the slit regions and light is not completely blocked from the barrier regions but the differences do not affect this discussion). The geometric average of light through the barrier in the 3D mode is therefore 50%.

In the 2D mode, the retarder 15 is disabled and has no effect on the polarisation of light passing through the display. The polarizations 18 and 19 are oriented at + and - 45 with respect to the transmission axis 12 of the polariscr 7 so that transmission of light is limited to a theoretical maximum of 50%. However, the attenuation is even greater for displays of the front parallax barrier reflective or transflective type. In this case, in the 2D mode, the light passes twice through the "disabled" parallax barrier structure so that the maximum light output in the reflective mode is 25%. Such relatively poor light output is disadvantageous, particularly in the case of small or battery powered devices, such as mobile telephones and personal digital assistants. For example, in the transmissive mode, the loss of light can only be made up by increasing the backlight output but this in turn requires larger batteries or reduces battery life.

According to a first aspect of the invention, there is provided an optical device comprising an input polariser for passing light having a first linear polarisation direction, a polarisation modifying element for receiving light of the first polarisation direction from the input polariser, and an output polariser for analysing light from the polarisation modifying element, the polarisation modifying element comprising at least

À c À À e. c * . À e-e M&C Folio No P521 15GH 4 first and second sets of regions, the or each region of the first set changing the polarisation of light from the input polariser to a second linear polarisation direction different from the first polarisation direction and the or each region of the second set supplying light of a third linear polarisation direction different from and non-orthogonal to the second polarisation direction.

The output polarizer may cooperate with the polarisation modifying element such that each first light path through the or each region of the first set and the output polariser has substantially the same attenuation to light from the input polariser as each second light path through the or each region of the second set and the output polariser.

The output polariser may cooperate with the polarisation modifying element such that each first light path through the or each region of the first set and the output polariser has substantially the same phase change to light from the input polariser as each second light path through the or each region of the second set and the output polariser.

The regions of the first and second sets may be interleaved and may comprise first and second parallel strips, respectively. The first steps may have a first width and the second strips may have a second width greater than the first width.

The third polarisation direction may be the same as the first polarisation direction.

The device may have an alternative mode of operation in which the output polariser is arranged to pass light from the regions of one of the first and second sets and to attenuate light from the regions of the other of the first and second sets. The one of the first and second sets may be the first set. The output polariser may be arranged substantially to block light from the other of the first and second sets in the alternative mode.

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M&C Folio No P521 1 5GB 5 The polarisation modifying element may comprise a patterned retarder. The output polariser may be arranged to transmit the same proportions of slow and fast axis components of light from the first and second sets of regions. The output polariser may be arranged to transmit only the slow axis component of light from the first and second sets of regions.

The output polariser may pass light having a polarisation direction orthogonal to the first polarisation direction.

The retarder may comprise a photo-polynerised polymer. The retarder may provide a halfwave of retardation at a visible light frequency. The slow axis of the or each region of the second set maybe oriented at 55 to the slow axis of the or each region of the first set. The slow axis of the or each region of the first set may be oriented at 27.5 to the first polarisation direction and the slow axis of the or each region of the second set may be oriented at -27.5 to the first polarisation direction.

The slow axis of the or each region of the first set may be oriented at 55 to the first polarisation direction and the slow axis of the or each region of the second set may be parallel to the first polarisation direction.

The device may comprise a further polarisation modifying element between the input and output polarisers. The further element may be a further retarder. The further retarder may provide a halfwave of retardation at a visible light frequency. The further retarder may be a liquid crystal device.

The further retarder may comprise at least one region whose slow axis is switchable between a first orientation substantially parallel to the first and second light paths and a second orientation substantially perpendicular to the first orientation. The further retarder may be a Freedericksz cell.

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À À Be À C À He M&C Folio No P52 11 SOB 6 The second orientation may be for the alternative mode and may be oriented at 62.5 to the first polarisation direction.

The further retarder may comprise at least one region whose slow axis is switchable between third and fourth orientations substantially perpendicular to the first and second light paths. The third orientation may be perpendicular to the first polarisation direction and the fourth orientation may be for the alternative mode and may be oriented at 62.5 to the first polarisation direction.

The further element may comprise a polarisation rotator. The rotator may comprise at least one region which provides a polarisation rotation of 55 . The rotator may comprise a twisted nematic liquid crystal device.

The liquid crystal device may have an alignment direction, at a liquid crystal surface nearer the input polariser, parallel to the first polarisation direction and alignment direction, at a liquid crystal surface nearer the output polariser, oriented at 55 to the first polarisation direction.

The liquid crystal device may have an alignment direction, at a liquid crystal surface nearer the input polariser, oriented at -17.5 to the first polarisation direction and an alignment direction, at a liquid crystal surface nearer the output polariser, oriented at 72.5 to the first polarisation direction.

The liquid crystal device may have an alignment direction, at a liquid crystal surface nearer the input polariser, oriented at 5 to the first polarisation direction and an alignment direction, at a liquid crystal surface nearer the output polariser, oriented at 95 to the first polarisation direction.

The polarisation rotator may be disableable for the alternative mode.

According to a second aspect of the invention, there is provided a display comprising a device according to the first aspect of the invention.

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À À M&C Folio No P5211 SGB 7 The display may comprise a spatial light modulator, such as a liquid crystal spatial light modulator. The display may have an autostereoscopic mode. The device, when in the alternative mode, may form a front or rear parallax barrier.

Throughout the present specification, positive values of angles may be either clockwise

or anticlockwise, with negative values then referring to angles in the opposite direction.

Also, all angles of polarization directions and retarder slow axes are expressed "module 180 ". Thus, each angle is equivalent to each angle (D + n. 180) , where n is any integer. However, in the cases of certain devices, because of the nature of their constructions, a value of may be preferred over the value (180 ) because of improved performance.

It is thus possible to provide an optical device which is suitable for use in displays.

When used, for example, in an autostereoscopic 3D display which also has a 2D mode of operation, it is possible to provide an increase in brightness of the display in the 2D mode. Although a reduction in brightness in the 3D mode also happens, the reduction in perceived brightness is substantially less than the increase in perceived brightness in the 2D mode so that an overall improvement in performance of such a display is obtained. Further, there is little increase in crosstalk performance in the 3D mode because extinction of light through barrier regions is not substantially changed compared with known arrangements.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross sectional diagrammatic view of a known type of display in 3D and 2D modes of operation; Figure 2 is a cross sectional diagram of the display of Figure 1 illustrating the 3D mode of operation;

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C À r À ce M&C Folio No P:52 1 I SOB 8 Figure 3 is a diagram illustrating conventional 3D and 2D modes of operation of a display with orthogonal polarizations from slit and barrier regions of a patterned retarder; Figure 4 is a diagram illustrating a display constituting a first embodiment of the invention with non-orthogonal polarizations from slit and barrier regions of a patterned retarder; Figure 5 is a graph of transmission through a linear polariser against the angle between the polarization of incident light and the transmission axis of the polariser; Figure 6 is a diagrammatical cross-sectional view illustrating an optical device formed as part of a display constituting a second embodiment of the invention; Figures 7 and 8 illustrate diagrammatically 3D and 2D modes of the optical device and display of Figure 6; Figure 9 illustrates diagrammatically an optical device and display constituting a third embodiment of the invention; Figures 10 to 12 are cross sectional diagrams illustrating different physical arrangements of the display of Figure 9; Figure 13 illustrates diagrammatically an optical device and display constituting a fourth embodiment of the invention; Figure 14 illustrates diagrammatically an optical device and display constituting a fifth embodiment of the invention; Figure 15 is a diagram illustrating an optical device and display constituting a sixth embodiment of the invention;

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À e. M&C Folio No P521 15OB 9 Figure 16 is a diagram illustrating an optical device and display constituting a seventh embodiment of the invention; Figure 17 is a diagram illustrating an optical device and display constituting an eighth embodiment of the invention; Figure 18 illustrates diagrammatically an optical device and display constituting a ninth embodiment of the invention; Figure 19 illustrates diagrammatically an optical device and display constituting a tenth embodiment of the invention; Figures 20 and 21 are diagrammatic cross-sectional views of the optical device and display shown in Figure 19; Figure 22 is a diagrammatic cross-sectional view of an optical device and display constituting an eleventh embodiment of the invention; Figure 23 is a diagram illustrating the device and display of Figure 22; and Figure 24 is a diagram illustrating an optical device and display constituting a twelfth embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

Figure 4 illustrates diagrammatically a 3D autostereoscopic display constituting an embodiment of the invention arid having a 3D autostereoscopic mode of operation and a 2D mode of operation. The display of Figure 4 differs from the comparison example display illustrated in Figure 3 in that the slow axes 16 of the slit regions of the patterned retarder are oriented at -27.5 and the slow axes 17 of the barrier regions of the retarder are oriented at 27.5 . The polarization direction 18 and 19 of light from the slit and barrier regions, respectively, are thus non-orthogonal and are oriented at - and + 55

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À #. * a, e ee. M&!C Polio No P52 1 1 SGH 10 with respect to the transmission axis l O of the polariser 3. Also, the slow axis 20 of the liquid crystal (LC) retarder 15 is oriented at -62.5 . Light from the barrier regions is thus rotated to have the polarisation direction 22 oriented at 0 and hence orthogonal to the transmission axis 12 of the analysing polariser 7. The light is therefore substantially extinguished so that substantially 0% of transmission takes place through the barrier regions. The polarization direction 18 of light from the slit regions is rotated by the retarder l 5 so as to have a polarization direction 21 which is oriented at -70 . The polarization direction 21 is thus oriented at an angle of 20 with respect to transmission axis 12 of the polariser 7 so that 88% of light through the slit regions is transmitted. Light transmission in the 3D mode is thus represented by a geometric average of 44% and is 6% less than that of the example illustrated in Figure 3.

In the 2D mode, the polarization directions 18 and 19 are oriented at angles whose magnitudes are 35 with respect to the transmission axis 12 of the polariser 7. Thus, 67% of light through the slit and barrier regions is transmitted and this represents an improvement of l 7% compared with the example shown in Figure 3.

The transmission of light through a linear polariser such as the analysing polariser 7 for an angle between the polarization direction of incident light and the transmission axis of the polariser is proportional to cost (a) and this is illustrated by the graph in Figure 5.

The values of the angle in Figure 3 and 4 are on a part of the curve illustrated in Figure 5 having a relatively large slope or gradient whereas the value of for the 3D mode in Figure 4 is at a part of the curve of Figure 5 having a much lower gradient.

Thus, a small change in angles results in a relatively large improvement in brightness in the 2D mode and a relatively small reduction in brightness in the 3D mode.

Figure 6 illustrates an optical device constituting an embodiment of the invention and forming part of an autostereoscopic display, also constituting an embodiment of the invention and having an autostereoscopic 3D mode of operation and a 2D mode of operation. The 3D mode of operation is illustrated in Figure 7 and the 2D mode of

- # c M&C Folio No P521 15GB operation is illustrated in Figure 8. In the 2D mode, the device and display of Figure 6 differ from that of Figure 4 in that the transmission axis l l of the polariser 4 is oriented at -45 to the vertical. Because liquid crystal devices are typically arranged with the transmission axis I of their output polarisers at -45 to the image vertical of the image displayed by such devices, this is the orientation which is illustrated in Figure 7 and the subsequent drawings. Thus, all angles are referred to the vertical in the drawings and hence the transmission direction 1 1 of the polariser 4 is indicated as being oriented at -

45 . The slow axes of the regions 8 are thus oriented at -17.5 whereas the slow axes of the regions 9 are oriented at -72.5 . The transmission axis 12 of the output polariser 7 is oriented at 45 .

A retarder 25 is provided in the form of an electrically switchable halfwave retarder for switching between the 2D and 3D modes of operation. The retarder 25 is switchable between a state in which it acts as a halfwavc retarder with a slow axis oriented at 17.5 (as shown in Figure 7 for the 3D autostereoscopic mode) and a state in which it provides substantially zero retardation (as shown in Figure 8 for the 2D mode). For example, in the 2D mode, the slow axis may be switched to be perpendicular to the plane of the retarder 25 and substantially parallel to the light paths through the device and the display. The switchable retarder 25 may be embodied as a liquid crystal device such as a nematic liquid crystal device of the Freedericksz configuration having anti-

parallel alignment. Devices of this type are disclosed in Liquid Crystals, 2002, Vol. 29, No. 1, "Criteria for the first order Freedericksz transistor", Jianru Shi. In such a device, when a voltage is applied across the liquid crystal layer, the liquid crystal directors, and hence the slow optic axis, lie substantially perpendicular to the plane of the device to present a uniform refractive index, and hence no birefringence, to light passing in the normal direction through the device.

The liquid crystal device may be cony gured so as to be uniform in either state, in which case the whole display is switchable as a unit between the 2D and 3D modes.

Alternatively, suitably patterned electrodes may be provided within the liquid crystal device so that different areas of the display may be configured independently of each other for 2D or 3D operation.

À À e c M&C Follo No P52 1 15GB 12 In the 3D mode, the retarders 5 and 25 may have substantially matched dispersions.

Thus, the presence of orthogonal polarisers 4 and 7 together with retarders of matched dispersions results in good extinction throughout the visible spectrum of light through the regions 9 resulting in good crosstalk performance in the 3D mode. The matched dispersions of the retarders 5 and 25 results in a bright more achromatic performance through the slit regions 8.

Figure 9 illustrates a rear parallax barrier display in which the rear parallax barrier is formed by an optical device of the same type as that illustrated in Figures 7 and 8.

However, in the arrangement shown in Figure 9, the rear polariser of the LCD becomes the output polariser 7 of the optical device and the input polariser 4 is distinct from the LCD. Also, the switching liquid crystal retarder 25 is disposed ahead of the patterned retarder 5 in the direction of light transmission through the device. This allows the patterned retarder S. which effectively defines the rear parallax barrier in the 3D mode, to be nearer the display LCD 2 as illustrated in Figure 10 so as to reduce the distance between the barrier and the display pixels. Reducing this distance allows the best viewing distance in front of the display to be reduced, for example to allow the display to be viewed in hand-held equipment such as mobile telephones and personal digital assistants. The rear parallax barrier type of display is more suitable for use in transflective displays having both kansmissive and reflective modes of operation. By disposing the parallax barrier behind the display LCD 2, attenuation in the reflective mode of light passing through a front parallax barrier twice is substantially eliminated and this allows a brighter reflective mode to be obtained.

As shown in Figure 10, the switching LCD 25, the patterned retarder and the display LCD 2 are made as individual devices which are subsequently brought together to form the complete display. Thus, the switching LCD 25 has glass substrates 40 and 41, the patterned retarder 5 is formed on a glass substrate 42, and the display LCD 2 has glass substrates 43 and 44.

r À r * 9, 1 À; r À1 M&C Follo No P52 11 SGB 13 As illustrated in Figure 1 l, the substrate 42 can be omitted by forming the patterned retarder on the substrate 41 of the switching LCD 25. A display of reduced thickness may therefore be provided and is advantageous for applications in devices which are required to be relatively thin.

Figure 12 illustrates a further reduction in thickness by eliminating the substrate 41 and sharing the substrate 44 between the switching LCD 25 and the display LCD 2. In this case, the retarder 5 and the polariser 7 are formed as internal components effectively within the LCD 25. These components, and particularly the polariser 7, must therefore be of a type which is capable of withstanding subsequent temperature and chemical processing to form transparent electrodes and alignment layers for the device 25.

Examples which are suitable for this internal application are disclosed in EP 0 887 692 and by Bobrov et al, "Lyotropic thin film polarisers", Proc. SID 2000.

Figure 13 illustrates a rear parallax barrier arrangement which differs from that shown in Figure 9 in that the slow axes of the regions 8 and 9 are oriented at 10 and -45 , the liquid crystal retarder 25 has a slow axis oriented at 72.5 in the 3D mode, and the polariser 4 has a transmission axis 11 oriented at 55 . This configuration provides a more achromatic output in the 2D mode illustrated in Figure 13 and so reduces errors in colour reproduction.

Figure 14 illustrates the 2D mode of another rear parallax barrier type of display in which the 2D mode occurs with the liquid crystal retarder 25 switched off. Such an arrangement may be preferable where the 2D mode is expected to be used primarily and power consumption is important, for example in battery powered devices.

In the display of Figure 14, the axes ofthe regions 8 and 9 are oriented at 100 and 45 , respectively. In the 2D mode with the liquid crystal retarder 25 switched off, the slow axis of the retarder is oriented at 17. 5 . The transmission axis 1 1 of the polariser 4 is orthogonal to the transmission direction 12 of the polariser 7 and is oriented at 45 .

When the liquid crystal retarder 25 is switched on, the retardation is substantially eliminated and the display functions in the autostereoscopic 3D mode.

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M&C Folio No PS2 11 5GB 14 Figure 15 illustrates another rear parallax barrier display in which the liquid crystal retarder 25 acts as a polarisation rotator to produce a 55 rotation of the polarisation direction of light from the polariser 4. The retarder 25 is a twisted nematic device having a relative angle between alignment directions 50 and 51 at the surfaces of a twisted nematic liquid crystal layer nearer the polariser 4 and the retarder 5, respectively. The alignment direction 50 is illustrated as being parallel to the transmission axis 11 and there is a twist of 55 between the alignment directions 50 and 51. However, the LCD 25 may be oriented at any angle to the transmission axis 11 and will produce a 45 of rotation of the polarisation direction of light passing therethrough.

In the 2D mode, the device 25 provides 55 of polarisation rotation. For operation in the autostereoscopic 3D mode, a voltage is applied across thetwisted nematic liquid crystal layer so that the liquid crystal directors are aligned perpendicular to the plane of the device and provide no polarisation rotation.

The display shown in Figure 16 differs from that shown in Figure 15 in that the twist of the device 25 is 90 . Such a device is "selfcompensating" and may be operated at a lower voltage. A rotation of 55 is achievable with such a device by the appropriate choice of angles and retardance. A device of this type is disclosed in our copending application filed on the same day as the present application, entitled "Polarisation Rotator, Parallax Barrier, Display and Optical Modulator", and bearing reference number P52138GB.

For linearly polarised light incident on a twisted nematic liquid crystal, linear polarisation with a polarisation azimuth of any selected value may be obtained with any device twist angle provided the twist (O. the retardation (An.d) and the orientation of the input director from the polariser (a) are correctly chosen. For rotation of linearly polarised light by 45 with respect to the incident polarisation, the following equations may be derived by considering the Stokes parameters for linearly polarised light propagating through a twisted nematic structure:

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À À À M&C Foho No PS2 11 SGB 15 tank)= An.d.. a= where is the wavelength of the incident light.

Figure 17 illustrates an arrangement which differs from that shown in Figure 16 in that the angles and retardances have been changed so as to optimise performance across the visible spectrum. When a voltage is applied to the liquid crystal layer of the device 25, the device has no optical effect on the system. The relardance and orientation may therefore be optimized for the state in which a polarization change is required such that the intensity and colour produced through the slit and barrier regions of the patterned retarder 5 are substantially identical.

Figure 18 illustrates a front parallax barrier display having a switchable retarder 25 in which the slow axis is switched between an orientation of 17.5 in the 3D mode (illustrated at the bottom left of Figure 18) and an orientation of 45 in the 2D mode (illustrated at the bottom right in Figure 18). Such a switchable retarder may be embodied as a liquid crystal device of the in-plane switching type for example a ferroelectric liquid crystal (FLC) (e.g. as disclosed in Clark N.A. and Lagarwell S.T.

1980, Appl. Phys. Lett., 36, 899), an anti ferroelectric liquid crystal (AFLC) (e.g. as disclosed in Chandani et al, 1998, Jpn. J. Appl. Phys., 28, L1261), or a bistable twisted nematic (BTN) device (e.g. as disclosed in D.W. Berreman and W.R. Heffner, J. Appl.

Phys., 52 3032, 1981). The polarizers 4 and 7 and the patterned retarder 5 are arranged as shown in Figure 7.

Figure 19 illustrates a display of a type similar to that shown in Figure 8 but in which the switching between modes is performed mechanically. Figure 20 illustrates that switching between the 2D mode and the 3D mode is performed by rotating the device 32 comprising a non-birefringent substrate 33, the polariser 7 and the retarder 25. In particular, the device 32 is rotated about a vertical axis through 180 so as to reverse the order of the individual elements in the optical path. The retarder 5 is formed on one side of a non-birefringent substrate 34.

c @ e* * C * * c :.: C * C M&C Follo No P5211 5GB 16 The 2D configuration is illustrated at the left in Figure 20 whereas the 3D configuration is illustrated at the right. In the 2D mode, the polanser 7 is disposed between the patterned retarder 5 and the uniform retarder 25 so that the uniform retarder 25 has substantially no effect and is substantially invisible to an observer. When switched to the 3D mode by rotating the device 32 through 180 about a vertical axis as illustrated by the arrow 35, the retarder 25 is disposed between the patterned retarder 5 and the polariser 7 so as to form a parallax barrier.

Figure 21 illustrates an arrangement which differs from that shown in Figure 20 in that the output polariser 7 and the uniform retarder 25 are formed on the same side of the substrate 33. Such an arrangement provides increased protection for the retarder 25 and reduces the need for "hard coating" both sides of the substrate with protective coatings.

Anti reflection coatings may be provided as necessary and are preferably substantially non-birefringent in order to avoid undesirably altering the optical effect of the device.

Figures 22 and 23 illustrate another mechanically reversible arrangement in which the 3D and 2D modes resemble the electrically switchable display of Figure 14. The display of Figures 22 and 23 may therefore be considered as a "mechanical analog" of the display of Figure 14 in which the switched liquid crystal retarder is replaced, for example, by a fixed sheet retarder.

Figure 24 illustrates a relatively simple "mechanical" embodiment which does not require any retarder 25. The slow axes of the regions 8 are oriented at 100 , the slow axes of the regions 9 are oriented at 45 , and the transmission axis 55 of the polariser is oriented at 55 for the 2D mode. In order to switch to the 3D mode, the transmission axis 11 is required to be orthogonal to the orientation illustrated in Figure 24. For example, this may be achieved by rotating the polariser 4.

The other electrically switched embodiments also have mechanical analogues, as will readily be understood by the person skilled in the art. In the case of the switched

:e;: ale.::.e.: À À À À c M&C Foho No P52] 1 5GB 17 twisted nematic arrangements, twisted fixed retarder structures may be used. For example, such structures may be used by adding a chiral dopant to a liquid crystal polymer or reactive mesogen material to produce the desired helical structure followed by polymerising.

In contrast to the liquid crystal modes described hereinbefore, out of plane switching (OPS) versions are also possible. OPS modes can either be homogeneously aligned, homeotropically aligned or hybrid aligned (HAN). The inverse operation of any homogenously aligned positive dielectric nematic LCD can be obtained (to a good approximation) by using homeotropic alignment and a negative dielectric anisotropy liquid crystal material. Therefore, by changing from one alignment to the other, the unpowered state of the display may be changed between the 2D mode and the 3D mode.

HAN LCDs may be used in place of homogeneously aligned nematic LCDs by simply making the thickness twice as large (provided that the twist is 0 ) and changing the alignment from homogenous to homeotropic. The Zenithal Bistable Nematic (ZBN) mode may also be used, and has the advantage of being truly bistable and hence has very low power consumption as power is only required to switch from one state to another. In one state, the ZBN LCD takes up the configuration of a HAN and, in the other, a homeotropically aligned LCD.

All of the optical devices described hereinbefore may be used as front or rear parallax barriers. Also, as mentioned hereinbefore, different areas of the display may simultaneously operate in 2D and 3D modes. In such arrangements, it is desirable for the brightness of the different regions to be matched, for example by adjusting the grey scale range used in software.

Claims

. e: c:.:::: À: ::: a. À À. M&C Folio No P521 15GB 18 CLAIMS:

1. An optical device comprising an input polariser for passing light having a first linear polarisation direction, a polarisation modifying element for receiving light of the first polarisation direction from the input polariser, and an output polariser for analysing light from the polarisation modifying element, the polarisation modifying element comprising at least first and second sets of regions, the or each region of the first set changing the polarisation of light from the input polariser to a second linear polarisation direction different from the first polarisation direction and the or each region of the second set supplying light of a third linear polarisation direction different from and non-

orthogonal to the second polarisation direction.

2. A device as claimed in claim 1, in which the output polariser cooperates with the polarisation modifying element such that each first light path through the or each region of the first set and the output polariser has substantially the same attenuation to light from the input polariser as each second light path through the or each region ofthe second set and the output polariser.

3. A device as claimed in claim 1 or 2, in which the output polariser cooperates with the polarisation modifying element such that each first light path through the or each region of the first set and the output polariser has substantially the same phase change to light from the input polariser as each second light path through the or each region of the second set and the output polariser.

4. A device as claimed in any one of the preceding claims, in which the regions of the first and second sets are interleaved and comprise first and second parallel strips, respectively.

5. A device as claimed in claim 4, in which the first strips have a first width and the second strips have a second width greater than the first width.

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M&C Folio No P52 11 5GB | 9

6. A device as claimed in any one of the preceding claims, in which the third polarisation direction is the same as the first polarization direction.

7. A device as claimed in any one of the preceding claims, having an alternative mode of operation in which the output polariser is arranged to pass light from the regions of one of the first and second sets and to attenuate light from the regions of the other of the first and second sets.

8. A device as claimed in claim 7, in which the one of the first and second sets is the first set.

9. A device as claimed in claim 7 or 8, in which the output polariser is arranged substantially to block light from the other of the first and second sets in the alternative mode.

10. A device as claimed in any one ofthe preceding claims, in which the polarization modifying element comprises a patterned retarder.

11. A device as claimed in any one of claims 4 to 10 when dependent on claims 2 and 3, in which the output polariser is arranged to transmit the same proportions of slow and fast axis components of light from the first and second sets of regions.

12. A device as claimed in claim 11, in which the output polariser is arranged to transmit only the slow axis component of light from the first and second sets of regions.

13. A device as claimed in any one of the preceding clams, in which the output polariser passes light having a polarization direction orthogonal to the first polarization direction.

14. A device as claimed in any one of claim 10 to 13, in which the retarder comprises a photo-polyrnerised polymer.

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15. A device as claimed in any one of claims 10 to 14, in which the retarder provides a half wave of retardation at a visible light frequency.

16. A device as claimed in claim 15, in which the slow axis of the or each region of the second set is oriented at 55 to the slow axis of the or each region of the first set.

17. A device as claimed in claim 16, in which the slow axis of the or each region of the first set is oriented at 27.5 to the first polarization direction and the slow axis of the or each region of the second set is oriented at -27.5 to the first polarization direction.

18. A device as claimed in claim 16, in which the slow axis of the or each region of the first set is oriented at 55 to the first polarization direction and the slow axis of the or each region of the second set is parallel to the first polarization direction.

19. A device as claimed in any one of the preceding claims, comprising a further polarization modifying element between the input and the output polarisers.

20. A device as claimed in claim 19, in which the further element is a further retarder.

21. A device as claimed in claim 20, in which the further retarder provides a half wave of retardation at a visible light frequency.

22. A device as claimed in claim 21, in which the further retarder is a liquid crystal device.

23. A device as claimed in claim 22, in which the further retarder comprises at least one region whose slow axis is switchable between a first orientation substantially parallel to the first and second light paths and a second orientation substantially perpendicular to the first orientation.

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À À. M&C Folio No PSZ 1 1 5GB 21

24. A device as claimed in claim 23, in which the further retarder is a Freedericksz cell.

25. A device as claimed in claim 23 or 24 when dependent on claim 9, in which the second orientation is for the alternative mode and is oriented at 62.5 to the first polarisation direction.

26. A device as claimed in claim 21 or 22, in which the further retarder comprises at least one region whose slow axis is switchable between third and fourth orientations substantially perpendicular to the first and second light paths.

27. A device as claimed in claim 26 when dependent on claim 9, in which the third orientation is perpendicular to the first polarisation direction and the fourth orientation is for the alternative mode and is oriented at 62.5 to the first polarisation direction.

28. A device as claimed in claim 19, in which the further element is a polarisation rotator.

29. A device as claimed in claim 28 when dependent on claim 13, in which the rotator comprise at least one region which provides a polarisation rotation of 55 .

30. A device as claimed in claim 29, in which the rotator comprises a twisted nematic liquid crystal device.

31. A device as claimed in claim 30, in which the liquid crystal device has an alignment direction, at a liquid crystal surface nearer the input polariser, parallel to the first polarisation direction and an alignment direction, at a liquid crystal surface nearer the output polariser, oriented at 55 to the first polarisation direction.

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M&C Folio No P521 15GB 22

32. A device as claimed in claim 3O7 in which the liquid crystal device has an alignment direction, at a liquid crystal surface nearer the input polariser, oriented at -17.5 to the first polarisation direction and an alignment direction, at a liquid crystal surface nearer the output polarser, oriented at 72.5 to the first polarization direction.

33. A device as claimed in claim 30, in which the liquid crystal device has an alignment direction at a liquid crystal surface nearer the input polariser, oriented at 5 to the first polarization direction and an alignment direction, at a liquid crystal surface nearer the output polariser, oriented at 95 to the first polarization direction.

34. A device as claimed in any one of claims 28 to 33 when dependent on claim 9, in which the polarization rotator is disableable for the alternative mode.

35. A display comprising a device as claimed in any one of the preceding claims.

36. A display as claimed in claim 35, comprising a spatial light modulator.

37. A display as claimed in claim 36, in which the modulator is a liquid crystal spatial light modulator.

38. A display as claimed in any one of claims 35 to 37, having an autostereoscopic mode.

39. A display as claimed in claim 38 when dependent on claim 9, in which the device when in the alternative mode forms a front or rear parallax barrier.