GB2408588A

GB2408588A - Polarisation conversion optical system eg with dispersion compensation for liquid crystal projection

Info

Publication number: GB2408588A
Application number: GB0327543A
Authority: GB
Inventors: Bronje M Musgrave; Emma J Walton; Grant Bourhill; Marina Vladimirovna Khazova
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2003-11-27
Filing date: 2003-11-27
Publication date: 2005-06-01
Also published as: CN1670565A; CN100386664C; GB0327543D0; JP2005157378A

Abstract

An optical device (33) comprises a first optical element (10) for producing from unpolarised input light at least first and second sets of beams (34 and 36) having respective first and second different polarisation states and a second optical element (32) for tending to equalise the respective polarisation states of the first and second sets of beams (34 and 36) to substantially-identical resulting polarisation state. The second optical element (32) applies at least two successive changes in polarisation state to the or each of the beams of one or both of the first and second sets (34 and 36), such that the effect of dispersion introduced with one or more of the changes is at least partially compensated for by the effect of dispersion introduced with another one or more of the changes. The device may be used with first (6, Fig 3) and second (8, Fig 3) microlens arrays in a liquid crystal projector. A patterned polarisation modifying element has at least first 44 and second 46 sets of elongate regions having different polarisation modifying properties and masking strips 55 between adjacent sub-regions.

Description

lolaris;tion (conversion Optical System The present invention relates to a

polarization conversion optical system, lor use for example in a liquid crystal projector to co',vcrt un'olarised light into lolarised light to S improve the efficiency Cal the projector.

l'olarisation conversion optical systems are widely used in projection systems employing liquid crystal panels as spatial light modulators. A typical poor aft polarization conversion optical system is illustrated in Figure 1 of the accompanying lO drawings. A lamp source 2 emits a substantially collimated beam 4 of unpolarised light with a non-uniform intensity distribution. The light emitted from the lamp 2 is incident on a first microlens array 6, which forms a spatially-distributed array of bright spots of unpolarised light at its focal plane. A second microlens array 8 is placed substantially in the focal plane of the first microlens away 6 and acts as part of a beam homogeniser lS system for imaging a uniform beam on a liquid crystal (LC) panel 18. The aspect ratio of the individual microlenses in the first microlens array 6 is chosen to match the aspect ratio of the LC panel. This measure allows the beam from the lamp to be re-shaped such that the homogenised beam that illuminates the LC panel 18 has substantially the same aspect ratio as the panel 18.

A polarising beam-splitter (PBS) arTay 10 is placed after the second microlens arTay 8.

The pitch of the PBS array is approximately half that of the first microlens alTay 6, with the focussed beams from the second microlens array 8 being incident on alternate sections of the PBS arTay 10. inside each unit of the PBS array 10 that receives light from the second microlens array 8 is a polarisation-separating film 9 which transmits light of one polarization, for example P-polarised light, in the forward direction while light with the orthogonal polarization, the S-polarised light, is reflected to a reflective film 11 in an adjacent unit of the PBS array 10 where it is reflected into the forward direction. In this way, each unpolarised beam incident on the PBS affray 10 generates 3() two spatially-separatetl, for\vart.-pr-'pagatinT beams with orthogonal polarization directions. This a'langc.'nc'.t is shown and dcscribecl in More detail below with elerence to Figure ot the accompanying d awing.

Len antsy of individual half wavelength plates or strips 12 is disposeti on the back surface of the PBS array 1() such that only the beams of one ol' the two orthogonal polarizations are incident on a phase plate 12. The phase plates 12 are aligned so that the incident light emerges from a phase plate with its polarization state rotated into the orthogonal direction. Thus all the t'orward-propagating bears have substantially the same polarisation state. The l'orward-propagating beams are then condensed by a field lens 14 to illuminate the LC panel 18. The first and second microlens allays 6 and 8, the PBS array 1(), the half wave plates 12 and the field lens 14 together constitute the polarization conversion optical system 16 for the projection system shown in Figure 1.

The beam 4 from the lamp 2 is not perfectly collimated but is instead slightly divergent, and the F-nurnber of the microlens arrays 6 and 8 is chosen to collect the slightly divergent light 4 from the lamp 2. It is desirable to reduce the length of the polarization conversion optical system 16 without affecting its light-collecting efficiency, and to achieve this it is necessary to reduce both the focal length of the microlenses and their pitch in order to maintain their F-number.

The use of smaller pitch microlenses is advantageous because, for any given lamp 2, the beam at the LC panel 18 can be homogenised with better uniformity. Improved beam homogenization is achieved because there are more lenses in the first microlens array 6 to sample the non-uniform beam emitted from the lamp. Consequently, the condensed beam at the LC panel 18 is more uniform. The use of smaller pitch microlenses also means that light emitted by a lamp 2 with a smaller reflector can be homogenized effectively, allowing a miniaturized projection engine to be designed.

As a consequence of reducing the pitch of the microlens arrays 6 and 8, the pitch of the PBS array 10 and the width of the individual phase plates 12 also have to be reduced.

Because of the smaller pitch, a larger number of narrower phase plates or strips 12 are required to be attached in the correct positions on the PBS array 10, and the alignment tolerance between the PBS array 10 and each individual phase plate 12 becomes more critical. Thus the requirement for a smaller pitch PBS array 1() with attached plates or strips 12 of retarder film is difficult to achieve and the minimum sine of the PCOS is therctore limited.

A further problem with the system shown in Figure 1, which is discloscf in US- 5,978,13(', relates to (he cfl'icicncy ol2the performance in white light of the phase plates 12 which are composed of optically-anisotropic material. The ordinary and extraordinary rel'ractive indices n, and nC of optically anisotTopic materials vary independently as a function of the vacuum wavelength of' the incident light. For linearly polarised light passing through a retarder plate comprised of' an optically anisotropic material, the optical path length difference between orthogonal polarisation components resolved along the ordinary and extraordinary refractive index axes of the retarder depends on the thickness d of the optically anisotropic layer and the respective refractive indices n,() and no) experienced by the orthogonal polarisation components, as shown: A=[ne()-no()]d The corresponding phase difference A at a particular wavelength is A) = 2;z: . Add Because A) depends on wavelength in this way, a dispersion effect is introduced whereby the half wavelength phase plates 12 only act as such for a particular "peak retardance" wavelength of light, changing linearly- polarised input light to linearly polarised output light; for all other visible wavelengths, they do not act as perfect half wave plates but instead produce some degree of elliptical polarisation in the output light. This means that the polarisation conversion optical system shown in Figure 1 will not perform efficiently in white light, and it is therefore desirable to improve the performance for white light.

US-5,986,809 discloses a polarisation conversion optical system using two microlens arTays and a PBS array as described with reference to Figure l, together with a selective phase plate. In the selective phase plate, half wave phase plates arc regularly arranged and placed on selected cmergcut surfaces of the polarisation separation units of the PBS anay, with no phase plates on the remaining emergent surfaces. For cxampic, P polarised beams emerging from the polarisation separation units pass through the phase plates and arc converted into S-poiarised beams. On the other hand, S-polarised beams emerging from the reflection units of the PBS array do not pass through any phase plate and therefore pass through unchanged. No practical details arc disclosed about the fabrication of such a selective phase plate.

As shown in Figure A, I,P-A-0887667 discloses a system comprising an illumirulion source 20, a single microlens a'Tay 21, PBS array 22 and a single-layer patterned retarder 24. The unpolarised light 26 incident on the reflection units 27 of the PBS array 22 is separated into beams 18 and 19 having respectively first and second polarisation directions. I, ight 18 of the first polarisation is transmitted to a first region 28 of the patterned retarder 24, while light 19 of the second polarisation is transmitted to a second region 31 of the patterned retarder 24. The second region 31 of the patterned retarder 24 has its optical axis aligned at 45 to the polarisation direction of the light 19 from the reflection unit of the PBS array 22 while the first region 28 of the patterned retarder 24 has its optical axis aligned with the polarisation direction of the light 18 from the polarisation separating unit of the PBS array 22. The polarisation direction of the light it) is therefore rotated by the second region 31 of the patterned retarder to be substantially the same as the polarisation direction of the light 18 which passes unchanged through the first region 28 of the patterned retarder 24. The beams 29 and 32 which emerge from the first and second regions respectively of the patterned retarder 24 have substantially uniform polarisation. The system shown in Figure 2 suffers from the same problem described above in that the effect of dispersion introduced by the patterned retarder 24 means that this polarisation conversion system will not perform efficiently in white light. Furthermore, this disclosure does not make any reference to a method for homogenizing a beam for use in a projection system.

US-6,222,672 discloses multi-layer patterned retarders for illumination by the polarised light transmitted by a LC panel to form a stereoscopic display. This patent does not consider PCOS applications. Pancharatnam retarders are disclosed in Proc. Indian Acad. Sci. Vol XL1 (4), ppl30-144 (1955). 3()

According to a first aspect of the present invention there is provided an optical device comprising a first optical element for producing l'rom unpolariscd input light at least test and second sets of' beams having respective first and second different polarisation states anti a second optical clement for tending to equalise the respective polarisation states of the first and second sets ol' beams to a substantially identical resulting polarisation stale by applying at least two successive changes in polarisation state to the OT each of the beams ol' one OT troth ot' the l'irst and second sets, the effect ol'dispersion introduced with one or moth of the changes being at least partially compensated IOT by the effect of dispersion introduced with another one or more of the changes.

The second optical element may apply two successive changes in polarisation state. The first polarisation stale may be a linear polarisation state. The second polarisation state may be a linear polarisation state. The first polarisation state may be substantially orthogonal to the second polarisation state. The resulting polarisation state may be a linear polarisation state. The resulting polarisation state may be substantially the same as the first polarisation state.

The second optical element may comprise a patterned polarisation modifying element having at least first and second sets of regions for receiving light From the first and second sets of beams respectively, the or each region of the first set having polarisation modifying properties different to the or each region of the second set.

The patterned polarisation modifying element may be formed as a continuous layer of material. The patterned polarisation modifying element may be one according to the fourth aspect of the present invention.

The patterned polarisation modifying element may be a patterned retarder. In this case, the or each region of the first set may have an optical axis aligned in a first direction and the or each region of the second set may have an optical axis aligned in a second direction different to the first direction. The patterned retarder may be formed as a continuous layer of optically anisotropic material. The or each region of the first set may act as a half wave plate for a predetermined visible wavelength. The or each region of the second set may act as a hall' wave plate for a predetermined visible wavelength. The predetermined visible wavelength Nay lie in the range 470 - 570 nm. Where the first and second polarisation lo) states are linear polaisation states, the first and second directions may be respectively oriented at angles of A? 5' and + 2S to the first polarization.

The second optical element may comprise a unilonn retarder i):,r receiving light from the first and second sets oi beams and having all optical axis aligned in a third direction. The unif->nn retarde'- may act as a half wave plate for a predetermined visible wavelength. The predetermined visible wavelength may lie in the range 470 - 570 nm. The patterned retarder may be arranged between the first optical element and the uniform retarder-.

Where the first and resulting polarization states are linear polarization states, the third 1() direction may be oriented at an angle of +67.50 to the first polarization.

The unifomm retarder may be an optically anisotropic supporting member, or substrate, and the patterned retarder may form an integral unit with the optically anisotropic substrate. Or the device may further comprise a substrate made of an optically isotropic material forming an integral unit with the patterned and uniform retarders. In the latter case, the patterned and uniform retarders may be disposed on opposite faces of the isotropic substrate. An area of substrate may be left free of other elements to allow that area to be clamped without damaging those other elements. The free area may at least partially surround the patterned retarder. The integral unit may also comprise the first optical element.

The first optical element may be a polarising beam splitter array.

According to a second aspect of the present invention there is provided a polarization conversion optical system comprising a first mierolens array for receiving unpolarised input light, a second mierolens array arranged substantially in the focal plane of and optically aligned with the first microlens array, and an optical device according to the first aspect of the present invention for producing from the light from the second microlens array the first and second sets of beams having the substantially identical resulting polarization state.

A polarisation conversion optical system of the second aspect may further comprise shielding means for attenuating unwanted stray light from any microlens ot' the second anay. The shielding means Tllay comprise a ma.sl; as part of the second optical element for blocking the unwanted light. The shickling means may compose a polarisation-rotating mask t'or rotating the polarisation ol'thc unwanted light and a polariser for subsequently blocking that light.

According to a third aspect of the present invention there is provided a polarisation conversion optical system comprising a first microlens array for receiving unpolarised input light, a second rnicrolens array arranged substantially in the focal plane ot' and optically aligned with the first microlens array, an optical device comprising a first optical element for producing from light from the second microlens array at least first and second sets of beams having respective first and second different polarisation states and a second optical element for tending to equalise the respective polarisation states of the first and second sets of beams to a uniform polarisation state, the second optical element comprising a patterned polarisation modifying element having at least first and second sets of regions t'or receiving light from the first and second sets of beams respectively, the or each region of the first set having polarisation modifying properties different to the or each region of the second set, wherein the patterned polarisation modifying element is formed as a continuous layer of material.

In a polarisation conversion optical system of the third aspect, the patterned polarisation modifying element may be a patterned retarder formed as a continuous layer of birefringent material.

According to a fourth aspect of the present invention there is provided a projection system comprising a light source, a polarisation conversion optical system according to the second or third aspect of the present invention, and a spatial light modulator. The spatial light modulator may be a liquid crystal display. 3()

Accortling to a fifth aspect of the present invention there is provided a patterned polarisation modifying optical element comprising at least t'irst and second sets of elongate regions, the or each region of the first set having polar-isation modifying 'ropcrties :lillerent to the or each region of the second set and the or each region ol'thc first and second sets being notionally sub-divided at intervals along its length into sub-regions, and naslhg strips disposed respectively on the boundaries between adjacent sub-regions ol' one or both of the first and second set of' regions arid/or on the boundaries between adjacent regions The masking strips may attenuate light tolling on them The masking strips may change the polarization state of light falling on them The patterned polarization modifying optical element may be a patterned retarder. The patterned polarization modifying optical clement may further comprise a substrate on which the optical element is mounted leaving an area of substrate surrounding the optical element to allow that area to be clamped without damaging the optical element Reference will now he made, by way of example, to the accompanying drawings, in which Figure J shows a t'irst prior art polarization conversion optical system; Figure 2 shows a second prior art polarization conversion optical system; Figure 3 shows a projection system, polarization conversion optical system and optical device according to an embodiment of the present invention; Figure 4 is a schematic view showing first and second optical elements of the optical device of the Figure 3 system in more detail; Figure 5 shows a different view of the second optical element of Figure 4 for use in explaining its cont'iguration and operation; 3() Figure 6 shows parts -'f the system of Figure 3 I'omed as an into: real unit in one possible con hi Duration; Figure 7A is a pcrspcctivc view shoveling the construction of the second optical clement ol Figure 4; S Figure 7B is a perspective view showing an altenativc construction ot the second optical element; Figures 8A and 8B show alternative orderings of the layers making up the second optical element in relation to the first optical element; Figures 9A to 9D show dillerent possible configurations of the layers of the second optical element; Figure 10 shows one possible configuration of the second optical element in which two of IS the layers are separated spatially; Figure 1 I shows a pol-isation conversion optical system according to another embodiment of the present invention; and Figures 12A to 12C shows various configurations of a shielding mask applied to the second optical element.

Figure 3 shows a projection system 1 incorporating a polaisation conversion optical system 31 and optical device 33 embodying the present invention. The projection system 1 comprises a light source 2, a polarization conversion optical system 31, a field lens 14 and a liquid crystal panel 18. The polarization conversion optical system 31 comprises a first microlens arTay 6, a second microlens arTay 8 and an optical device 33 comprising a first optical element 10 in the form of a polarising beam splitter 10 and a second optical element 32 in the form of a two-layer patterned retarder 32.

The light source comprises a hig,h-pressur-e mercury lamp with a I men arc and a parabolic reflector. rl he arc is situated at or near the focus of the parabolic reflector. Since the arc has a finiec length, the light 4 emitted by the arc and reflected by the parabolic reflector is not perfectly collimatctl ('as it would be if the emission were from a point source situated at the focus -' I the paraboloid). In this embodiment, the emitted beam 4 has some divergence t3 ol'+.4".

The light 4 is incident on the f'h-st nicrolcns array 6, the microlenses of which arc designed to collimate light with an incidence angle of +2H. The microlenses have the same aspect ratio as the liquid crystal panel 18 being illuminated, with the pitch of the micTolcnses being 2 mm and the focal length being 12 mm. The first microlens array 6 forms a to spatially distributed antsy of bright spots of unpolarized light at its focal plane.

The second microlens arTay 8 is arranged substantially in the focal plane of and is optically aligned with the first microlens array 6. The number of microlenses in the second microlens array 8 is the same as that in the first microlens array 6, and the respective focal lengths and lateral dimensions of the microlcnses are also the same. Therefore in this embodiment, the second microlens allay 8 is substantially the same as the first microlens array 6, with the input apertures of the second microlens arTay 8 being placed at the source images formed by the corresponding microlenses in the first microlens array 6. The purpose of the second microlens array 8 is to make the illumination substantially telecentric.

Figure 4 shows in more detail the first and second optical elements 10 and 32 of the optical device 33. The first optical element 10 of the optical device 33 is the polaiising beam splitter (PBS) array 10 which, as described above with reference to Figure 2, produces from unpolarised input light first and second beams 34 and 36 having respective first and second different polarization states S and P. The PBS array 10 has a pitch of 1 mm, which is half that of the two microlens arrays 6 and 8. The unpolarised light from the second microlens array 8 is incident only on those alternate elements of the PBS arTay 8 having a polarization separation layer 9. Similarly to that described above with reference to Figure 3() 2, the second (P) polaisation component is transmitted in the forward direction, while the first (S) polaisation component is rel'lected to an adjacent unit of the array 1() such that it encounters a rcllecting layer l l and is reflected again into the forward direction.

Tile second optical element 32 of the optical device is the two-layer patterned retarder 32.

I'hc two-layer patterned retarder 32 is adapted so as to tend to equalisc the respective polarisation states ol' the first and second beams 34 and 36 to a unit'orm polarisation state.

Tile second optical element 32 in this embodiment ol' the present invention is different to the corresponding optical clement 12 in the prior art described above with reference to I-'igurc I in that the uniform polarisation state is achieved by applying two successive changes in polarisation state to the first and second beams 34 and 36. By applying two successive changes, rather than a single change, the effect of dispersion introduced with the first change in polarisation state can be at least partially compensated for by the effect of dispersion introduced with the second change in polarisation state. This in turn allows the optical device 33 to perform efficiently across the visible wavelength range. In the embodiment shown in Figures 3 and 4, the two successive changes in polarisation state are achieved by passing the first and second beams 34 and 36 through two separate retarder layers, as will now be described.

Tile two-layer patterned retarder 32 comprises a patterned retarder 38 and a uniform retarder 40 disposed on opposite faces of an isotropic substrate 42. In this embodiment, the patterned retarder 38 is arranged between the polarising beam splitter 10 and the uniform retarder 40.

The patterned retarder 38 comprises first regions 44 for receiving light from the first beams 34 and second regions 46 for receiving light from the second beams 36. The first regions 44 have polarisation modifying properties different to the second regions 46 and therefore the polarised beams 34 and 36 from the PBS array 10 encounter different respective polarisation state changes at the patterned retarder 38. The polarisation change at the patterned retarder 38 represents the first of the two successive changes in polarisation state for each of the first and second beams 34 and 36.

The uniform retarder 40 is formed as a single region having uniform polarisation motlifying properties across the whole retarder. 'I'he polaiscd beams 34 and 36 from the PBS away 10 therefore encounter the same optical element at the uniform retarder 40. The polarisation change fit the unif'o''m rclarcler 40 represents the second of the two successive changes in polarisation Talc for cycle of'tlle first and second beams 34 and 3(i.

As shown h1 Figure 5, jr. this embodiment the lITSt regions 44 have an optical axis alignecl at an angle of'-295" to, the polarisation direction ol' the t'iTSt beams 34;nC;dCnt On the patterned retarder 35, while the second regions 46 have an optical axis alinetl at an angle of +25 to the polarisation direction o,l' the first beams 34 incident on the patterned retarder 38 (the sign of the angle is positive when there is a clockwise rotation as viewed from the direction of the PUS array 10). The asymmetric design is optimised for average transmission efficiency of white light; it enhances the performance of the two-layer patterned retarder 32 across the broad spectral range of the lamp. The first and second regions 44 and 46 are fabricated from the same birefringent material that is chosen in this embodiment to have its half wavelength peak retardanee at 510nm. The uniform retarder layer 40 has an optical axis aligned at +67.5 to the polarisation direction of the beams 34 of the first beams and its half wavelength peak retardanee is also at 510nm.

To optimise the luminous ei'l'iciency of the two-layer patterned retarder 32, the following embodiment is suggested. In this embodiment the first regions 44 have an optical axis aligned at an angle of-22.5 to the polarisation direction of the first beams 34 incident on the patterned retarder 38, while the second regions 46 have an optical axis aligned at an angle of +22.5 to the polarisation direction of the first beams 34 incident on the patterned retarder 38. The symmetric design is optimised for average luminous efficiency of the two-layer patterned retarder 32. The first and second regions 44 and 46 are fabricated from the same birefringent material that is chosen in this embodiment to have its half wavelength peak retardance at 550nm. The uniform retarder layer 40 has an optical axis aligned at +67.5 to the polarisation direction of the beams 34 of the first beams and its half wavelength peak retardance is also at 550nm.

With the design of' the two-layer- patterned retarder 32 shown and described with reference to Figures 3 to 5, both the S- and P-polarised beams are rotated to dif'i'erent intermediate p.,laisation states on existing the patterned retarder 38. On passing through the subsequent unif'o,-m retarder 40, the polarisation states ot' both polarised beams 34 and 36 arc rotated funkier until they lie substantially in the same plane. The polarization slate of the original S-polarised beams 34 is n'Ltcd through 18()O, back into the S-polarisation state, on emerging l'rorn the two-hyer pattcncd rctar-der element 32. rl'he polansation state of the original P-polarisel beams is rotatctl substantially through 90", into the.S- polarisation state, on emerging from the two-layer patterned retarder clement 32.

1'heret'ore, beams 34 and 36 of the first and second set of beams are subject to two successive changes in polarization state, the l'irst change being brought about by the patterned retarder 38 and the second change being brought about by the uniform retarder 40. The effect of dispersion introduced with the first change in polarization state is at least partially compensated for by the effect of dispersion introduced with the second change in polarization state, so that although the two-layer paltenned retarder 32 uses retarder layers that are individually chromatic, the performance of the two-layer patterned retarder 32 can be made lo be efficient in white light.

After passing through the optical device 33 as shown in Figure 4, the forward-propagating beams are then incident on the field lens 14 as shown in Figure 3, which condenses the light at the plane of the I C panel l 8. In this way, the panel is illuminated with a beam of substantially polarised light of unii'onm brightness and substantially the same aspect ratio as the panel.

The PBS array 10 may be a single unit or it may be composed of smallerarrays mounted side-by-side. In the latter case, assembling the polarization conversion optical system 31 may be made easier if the PBS array 10 is glued to the second optical element 32 of the optical device 33. As an alternative to using anti-reflection coatings on the surfaces of the elements, two or more of the following elements may be glued together using an index- matching optical adhesive: the second microlens array 8; the PBS array 10; the second optical element 32; and the field lens 14. The adhesive may be one of the acrylic-based UV-curable adhesives made by Norland. Gluing elements together to form an integral unit allows the potential to make the elements easier to mount so that their full apertures can be 3() utilised. For example, as shown in Figure 6, the second microlens array 8, the polarising beam splitter anay l(), tlhc patterned retarder 38 and the unii'orm retarder 40 are formed as an integral unit with a substrate 48. i'hc patterned and uniform retarders 38 and 4() arc disposed on opposite ['aces of the substrate 48 and these three elements are attached to the lobs array 1() by an index-matching glue layer 52. 'I'hc PUS.u-ray 1() is in turn attached to the second microlens array 8 by another indcx-matclling glue layer 52. The isotropic substrate 48 is larger than the pattewed retarder 38 such that an area 5() ol' the substrate 48 surrounding the pattcracd retarder 38 is free ol' other elements to allow that area 5() to be clamped by a mounting element.

The patterned retarder 38 of the above-described embodiment is formed from a reactive mesogenic material, such as RMM 34, which is produced by Merck. The birefringent, liquid-crystalline material is deposited on a substrate carrying a patterned or a uniform alignment layer. Patterning of the alignment layer may be achieved by a multi-rubbing technique, as described for example in EP-A-0887667, or by use of a microstructured grating, as described for example in US 2003/0137626A I, or by use of a photo-alignment technique, as described for example in "Photofabrication of micro-patterned polarising elements for stereoscopic displays" by Matsumaga et al. in Advanced Materials, 14, pp. 1477-1480, 2002. The reactive mesogen material may be spun in solution onto the alignment layer. As the solvent evaporates, it leaves behind a layer of nematic liquid crystalline material. The optical axis of' the liquid crystal aligns with the axis of the underlying alignment layer, producing a retarder layer. The reactive mesogen is then cross-linked by exposure to UV light.

The fabrication of the patterned retarder 38 from a continuous layer of birefringent material enables the manufacture of the second optical element 32 with excellent precision and small pitch. This in turn allows the use of a PBS array 10 with a corresponding small pitch and enables the length of the polarization conversion optical system to be reduced compared to that of the prior art. A polarization conversion optical system 31 embodying the present invention is also able to provide better beam homogenization than conventional polarization conversion optical systems because smaller pitch microlens arrays can be used. A polarization conversion optical system 31 embodying the present invention is also 3() able to homogenise more efficiently light sources with small output area, allowing miniaturized projection engines to be designed.

In the ahove-desciLcd embodiment, the regions of the first and second regions 44 and 46 are l'onned so as to act as Lola wave plates for a prcdetcrmined visible wavelength ot' 51() nm, and the same applies to the unil'onn retarder 4(). It will be appreciated that changes h1 polarisation slate can also be brought about by the use of a polaT- isation-rotating layer or layers composed call' a material with a macroscopically chiral stTucturc' such as a twisted ncmatic mode liquid crystal, that is capable of rotating the plane of' polarization of transmitted light. Such chiral layers may be either patterned or uniform, and can thcrcl'orc be used in the patterned polarization modifying element 38 and/or the unicorns polarization modifying element 40. The uniform retarder 40 may be formed by a stretched plastic retarder film, such as those produced by Sumitomo, and laminated to the substrate 42.

The substrate 42 described above with reference to Figure 4 is optically isotropic, with the patterned retarder 38 and the uniform retarder 40 disposed on opposite faces of the isotropic substrate 42; this arrangement is shown in perspective view in Figure 7A.

Alternatively, a birefingent substrate 43 can be used to act both as the mounting substrate and the uniform retarder, with the patterned retarder 38 mounted to the birclringent substrate 43, as is shown in Figure 7B.

In the above-described embodiment, the patterned retarder 38 is arranged between the PBS array 10 and the uniform retarder 40. In this way, light from the PBS array 10 encounters the patterned retarder 38 before the uniform retarder 40. This arrangement is preferable, with the patterned retarder 38 placed close to the PBS array 10, because the beams 34 and 36 that emerge from the PBS array 10 are divergent, having previously been focussed by the first microlens array 6, and they should be processed by the patterned retarder 38 before the beams 34 and 36 begin to overlap. Only the patterned retarder 38 is required to be addressed to the PBS array 10, with the separation of the centres of adjacent regions 44 and 46 of the patterned retarder 38 being substantially the same as the pitch of the PBS array 10. This arrangement is shown again in Figure 8A. Nevertheless, it will be appreciated that it is also possible to reverse the order of the patterned and uniform retarders 38 and 40 such that beams 34 and 36 from the PBS array 1() encounter the unil'om retarder 4() before the patterned retarder 38, and this case is illustrated in Figure 8B. 1(,

Various other possible arrangemcnt.s of the pattcned retarder 38, the uniform retarder 4() and the substrate 4? arc illustrated in l igues'9A total). Figure 9A shows the case where the patterned retarder 38 is dislloscd fin the uniform retarder 40 which is disposed on the substrate 42, with light lrom the PINS array I () encountering these elements in the following order: (i) patterned retarder 38; (ii) uni f one retarder 40; and (iii) substrate 42.

Figure 9B shows an arrangement which is similar to that shown in Figure 9A, but with the order of the patterned and uniform retarder layers 38 and 40 being reversed. Figure 9C lO shows an arrangement of these elements that is the same as that shown in Figures 4 and 8A. Figure 9D illustrates that two separate substrates 42 can be employed, with the patterned retarder 38 disposed on the front surface of one of the substrates 42 and the uniform retarder 40 disposed on the front surface of the other substrate 42. The two substrates respectively carrying the patterned and retarder layers 38 and 40 may be placed immediately after one another or they may he separated by some distance within the projection engine. Indeed, one or more of the substrates may be other optical elements within the projection system. For example, Figure 10 shows an arrangement in which the retarders 38 and 40 are separated spatially and where the uniform retarder 40 is disposed on the plane surface of the piano-convex field lens 14. Separation of layers is advantageous if the layer or layers displaced from the PBS array 10 do not have good endurance properties in hot or bright conditions. Such a layer or layers may be placed at a greater distance from the hot lamp and from the beams focussed by the first microlens array 6.

In the embodiment described above, the S- and P-polarised beams are rotated into a substantially S-polarised state. The second optical element 32 may alternatively be designed to rotate both S- and P- polarised beams into the P-polarisation state.

Alternatively, some other linear polarization state could be produced. It would also be possible to produce efficiently other polarization states in white light, such as circularly 3() polarised light or elliptically-polarised light. if these were preferable tor an application.

It is also not necessary that both the first and second beams 34 and 3(' from the PBS array 1() arc subject to two or more successive changes in polarization slate by the second optical clement 39. For example, the second optical element 32 could he patterned and adapted in a manner that allows the second beams 36 to pass through it with their polarization state S unchanged, while applying at least two successive changes in polarization state to the first beams 34. 'Io achieve this, the second optical element 32 would require at least two patterned retarde. layers that are patterned and arranged such that the second beams 36 only pass through regions of the patterned layers having no retardation eilect, while the first beams 34 pass through regions of the patterned layers which apply appropriate changes in polarization state.

Although the second optical element 32 in the above embodiment applies two successive changes in polarization state to the beams 34 and 36, it will be appreciated that the second optical element 32 can be adapted to apply more than two successive changes in polarization state one or both beams 34 and 36, such that the effect of dispersion introduced with one or more of the changes is at least partially compensated for by the effect of dispersion introduced with another one or more of the changes.

To achieve the advantages associated with an embodiment of the present invention associated with dispersion reduction, it is not essential that the patterned retarder 38 is formed as a continuous layer of optically anisotropic material. The same advantages will be achieved if the regions 44 and 46 of the patterned retarder 38 are formed by laying down individual plates or strips onto the substrate 42 or even onto the PBS array 10 as in the prior art described above with reference to Figure 1. Likewise, the advantages associated with an embodiment of the present invention relating to the reduction in pitch of the PBS array 10 and the second optical element 32 can be achieved without the associated advantages of dispersion reduction by employing only a single retarder layer in the second optical element 32, the single layer being a patterned retarder layer formed as a continuous layer of birefringcnt material so as to enable precision fabrication of reduced-sized regions.

Such a single-layer retarder 32' is shown in a polarization conversion optical system 31' embodying the present invention in leisure 11.

The patterned retarder layer 38 hang fir-et and second regions 44 and 46 coccal not be formed as a single layer, but may instead be tonged from two patterned layers, with one ol the layer-e incopor-ating the first legions 44 and the other layer incorporating the second regions 46. I hose two layers could be disposed atijacent to each olher or even on opposite S sides ol the substrate 42, or in some oilcr arrangement.

To improve the performance of the projection system, the patterned retarder 38, OT some other suitable optical clement of the projection system such as the PBS array 1(), may comprise a masking layer for attenuating unwanted stray light.

Three designs for such a masking layer are shown in Figures 12A to 12C, which each show a patterned polarization modifying optical element comprising first and second sets of elongate regions 44, 46, the or each region of the first set having polarization modifying properties different to the or each region of the second set. Each region of the first and second sets can be considered notionally to be sub-divided at intervals along its length into sub-regions. The masking layer 55 shown in Figure 12A comprises masking strips disposed respectively on the boundaries between adjacent sub-regions of both the first and second set of regions and on the boundaries between adjacent regions. The masking layer shown in Figure 1 2B comprises masking strips disposed respectively on the boundaries adjacent sub-regions of both the first and second set of regions. The masking layer 55 shown in Figure 12C comprises masking strips disposed respectively on the boundaries between adjacent sub- regions of only the first set of regions 44. The masking layer 55 could be made of a reflective material, such as aluminium, which may be sputtered through a mask onto the patterned retarder 38. Alternatively, the masking layer 55 could be fabricated from an absorbing material or from a polarising material, which may be web- coated or laminated onto the patterned retarder 38. Alternatively, the design of the patterned retarder 38 may include areas acting as the masking layer 55 that rotate incident unwanted polarised light into a polarization state that is blocked by a clean-up polariser at, for example, the IT panel plane 18.

The purpose of using a masking layer S5 is to prevent stray light from propagating through the projection engine. Either stray light is blocked at the masking layer 55 or it is blocked 1') at a clean-up polariser. The apcnurcs in the masking layer 55 should he laryc enough to allow the useful light through tail the retarder layer 38, while blocking any stray light. The inclusion of' a light-hlocking masking layer 55 can aid with the aligrnmcnt of the p-'larisation conversion optical system in the final assembly since very little light will be transmitted by the system unless it is well-aligned.

Although in the above-described embodiment the pitch of the microlens arrays 6 and 8 is 2 mm, it will be appreciated that other pitches are possible, for example any pitch within the range 0.5 mm to 20 mm, with a corresponding PBS array pitch in the range 0.25 mm to to mm. It is also possible that the microlenscs in the second microlens array 8 have dit'ferent lateral dimensions to those in the first microlens array 6, for example as described in US- 6,260,972. The layers within the second optical element 32 can be fabricated from the same material, or from dii'fcrcnt materials, and it is also possible that the first and second sets of regions 44 and 46 are t:abricated from different materials or from the same material but with different thicknesses.

An embodiment of the present invention has many practical applications, for example in projection optical systems t'or data or video projection, front or rear projection systems, desktop projection systems, and in various other home and business applications.

Claims

CLA IMS: 1. An optical device comprising a first optical eiewent for

producing from unpolarised input light at least first and second sets of beams having respective first and second different polarisation states and a second optical element l-or tending to equalise the respective polaTisation states of the first and secoTld sets ol beams to a substantially identical resulting polarisation state by applying at least two successive changes in polarisation state to the or each of the beams of one or both of the first and second sets, the effect of dispersion introdueeti with one or more of the changes being at least partially 1() compensated for by the effect of dispersion introduced with another one or more of the changes.

2. A device as claimed in claim 1, wherein the second optical element applies two successive changes in polarisation state.

3. A device as claimed in claim I or 2, wherein the first polarisation state is a linear polarisation state.

4. A device as claimed in claim 1, 2 or 3, wherein the second polarisation state is a linear polarisation state.

5. A device as claimed in any preceding claim, wherein the first polarisation state is substantially orthogonal to the second polarisation state.

6. A device as claimed in any preceding claim, wherein the resulting polarisation state is a linear polarisation state.

7. A device as claimed in any preceding claim, wherein the resulting polarisation state is substantially the same as the first polarisation state.

8. A device as claimed in any preceding claim, wherein the second optical element comprises a palteTleti polarisation motlilying element having fit icasl first anti second sets oi regions for TCCCiVing light Ir->rn the first and second sets ol heaths respectively, the or

-

cach region of the first set has ing polarization modifying properties dil ferent to the or each region of the scconcl set.

t3 A device as claimed in claim 8, wherein the pattcTned polarization modifying clement is formed as a continuous layer of material.

1(). A device as claimed in claim 8 or 9, wherein the patterned polarisati:'n modifying element is one as claimed in claim 39.

11. A device as claimed in claim 8, 9 or IO, wherein the patterned polarization modifying element is a patterned retarder.

12. A device as claimed in claim 11, wherein the or each region of the first set has an optical axis aligned in a first direction and the or each region of the second set has an optical axis aligned in a second direction different to the first direction.

13. A device as claimed in claim 11 or 12, when dependent on claim 9, wherein the patterned retarder is loomed as a continuous layer of optically anisotropic material.

14. A device as claimed in claim 11, 12 or 13, wherein the or each region ol the first set acts as a half wave plate for a predetermined visible wavelength.

15. A device as claimed in any one of claims 11 to 14, wherein the or each region of the second set acts as a half wave plate for a predetermined visible wavelength.

16. A device as claimed in claim 14 or 15, wherein the predetermined visible wavelength lies in the range 470 - 570 nm.

17. A device as claimed in any one of claims 11 to 16, when dependent on claims 3 and 4, wherein the first and second directions are respectively oriented at angles ol -22.5 and +25' to the first polari.sation direction.

18. device as claimed in any one of claims I I to 16, when dependent on claims 3 and 4, wherein the first and second directions are respectively oriented at angles of -22.5 and +22.5" lo tile first polarization direction.

19. A device as clahncd in any preceding claim, wherein the second optical element comprises a unitonn retarder for receiving light from the first and second sets ot beams and having an optical axis aligned in a third direction.

20. A device as claimed in claim 1'3, wherein the uniform retarder acts as a half wave plate for a predetermined visible wavelength.

21. A device as claimed in claim 20, wherein the predetermined visible wavelength lies in the range 470 - 570 nm.

22. A device as claimed in claim 19, 20 or 21, when dependent on claim 11, wherein the patterned retarder is arranged between the first optical element and the uniform retarder.

23. A device as claimed in any one of claims 19 to 22, when dependent on claims 3 and 6, wherein the third direction is oriented at an angle of + 67.5 to the first polarization direction.

24. A device as claimed in any one of claims 19 to 23, when dependent on claim 11, wherein the uniform retarder is made of an optically anisotropic material as a supporting member for the patterned retarder and the patterned retarder forms an integral unit with the supporting member.

25. A device as claimed in any one of claims 19 to 23, when dependent on claim 11, further comprising a supporting member made of an optically isotropic material, tonging an integral unit with the pattcncd and unifi''m retarders.

26. A device as claimed in claim 25, wherein the patcrucd and uniform retarders are lisposcd on opposite faces of the supporting member.

27. device as claimer in claim 24, 25 or 26, wherein an area of the supporting member is free ol other elements to Blew that area to he clamped without damaging those other elements.

28. A device as claimed in claim 27, wherein the 1rec area at least partially surrounds the patterned retarder.

29. A device as claimed in any one of claims 24 to 28, wherein the integral unit also comprises the first optical element.

30. A device as claimed in any preceding claim, wherein the first optical element is a polarising beam splitter array.

31. A polarization conversion optical system comprising a first microlens array for receiving unpolarised input light, a second microlens array arranged substantially in the focal plane of and optically aligned with the first microlens array, and a device as claimed in any preceding claim for producing from the light from the second microlens array the first and second sets of beams having the substantially identical polarization state.

32. A system as claimed in claim 31, further comprising shielding means for attenuating stray light.

33. A system as claimed in claim 32, wherein the shielding means comprise a mask as part of the second optical element for blocking stray light.

34. A system as claimed in claim 32, wherein the shielding means comprise a p-'larisation-rotating mask 1or rotating the polarization of the stray light and a polariser for subscluently blocking that light.

Ad. A pol;T-s;iiion conel-siiTT optical system conlrisilT,I til-sl 1lic'olcns;n-.\ roT reCejVjT1g iTr](jl;! j jSeLI jT1I)UI lit,TTl, .L SeCOT]d TTTiCiolCiTS litter aTTaTTPCCI.itthSt;)TTli;lily ii] to focal plane ol;IT] CI optically aligned with;hc first microlcns aT-ray art OI/ljCLII deVjCe COTTlpriSiTlg a list ollic.i elerT,eTlt fit'- p'-oclucinj - Il-OfTT II(TI] l l'-orTT talc SCC)TTti TTiiCToiCTiS aTl-:ly at least tit-St and second self OI hcaTns TlaViiTg T1.SI:)CC.tjVe f-h-st anti seconct diff-ercnl polarisatiin st.lei anti a secoTlcl optical elerT'cTlt for tenciinj<- to clualise the rcspcctivc polaT-is;'tioTT slates of talc first and second sets of beams to a substanliail> identical polaisation stale. the second optical element comprising a patterned polarisation modifying clement having al least first and second sets of regions for receiving light from 1() the first and second sets of beams respectively, the oi each region of tile first set having polarization modifying properties different to the or each region of the second set, wherein the patterned polaisation modifying element is fommed as a continuous layer of material.

36. An optical system as claimed in claim 35, wherein the patterned polarization modifying element is a patterned retarder formed as a continuous layer of birefringent material.

37. A projection system comprising a light source, an optical system as claimed in any one of claims 31 lo 36, and a spatial light modulator.

38. A projection system as claimed in claim 37, wherein the spatial light modulator is a liquid crystal display.

39. A patterned polarization modifying optical element comprising at least first and second sets of elongate regions, the or each region of the first set having polaisation modifying properties different to the or each region of the second set and the or each region of the first and second sets being notionally sub-divided at intervals along its length into sub-regions, and masking strips disposed respectively on the boundaries bctwccn adjacent sub-regions ol one or troth of the first and second set of regions and/or on the boundaries between adjacent regions.

40. An optical element as claimed in claim 39 wherein the masking strips attenuate light falling on them.

41. An optical element as claimed in claim 39 or 40, wherein the masking strip!) change the p-,larisation state of light falling on them.

42. An optical element as claimed in cl.iiTn 39, 40 or 41, being a patterned retarder.

43. An optical element as claimed in any one of claims 39 to 42, Iurther comprising a substrate on which the optical element is mounted leaving an area of substrate surrounding the optical element to allow that area to be clamped without damaging the optical element.

44. An optical device substantially as hereinbefore described with reference to Figures 3 to 12C of the accompanying drawings.

45. A polarization conversion optical system substantially as hereinbefore described with reference to Figures 3 to 1 2C of the accompanying drawings.

46. A patterned polarization modifying optical element substantially as hereinbefore described with reference to Figures 3 to 12C of the accompanying drawings.

47. A projection system substantially as hereinbefore described with reference to Figures 3 to 12C of the accompanying drawings.