GB2415057A

GB2415057A - Polarisation splitter with inclined non-parallel sets of interfaces and anisotropic material

Info

Publication number: GB2415057A
Application number: GB0413032A
Authority: GB
Inventors: Emma J Walton; Marina Khazova; Bronje M Musgrave; Grant Bourhill
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2004-06-11
Filing date: 2004-06-11
Publication date: 2005-12-14
Also published as: GB0413032D0

Abstract

A polarisation splitter (28) is provided for angularly separating unpolarised light incident in a first direction into light of substantially orthogonal polarisations travelling in second and third directions which are different from each other. The splitter comprises a first set of first inclined interfaces (33) between at least one first region made from a substantially optically isotropic material (32) and at least one second region made from an optically anisotropic material (31) whose optic axis is substantially non-twisted; and a second set of second inclined interfaces (36) between at least one third region made from a substantially optically isotropic material (35) and at least one fourth region made from an optically anisotropic material (34) whose optic axis is substantially non-twisted, the first inclined interfaces being non-parallel to the second inclined interfaces, and the optic axes of the second and fourth regions being substantially parallel to each other.

Description

Polarisation Splitter, Polarisation Conversion Optical System and

Projector The present invention relates to a polarisation splitter, for example for use in separating incident unpolarised or partially polarised light into two angularly separated output beams having different polarisation states. The invention also relates to a polarisation conversion optical system including such a splitter and to a projector including such a system.

Many optical systems include devices which require that they be illuminated by light that is substantially completely polarised. Where such a device is operated with light that is unpolarised or partially polarised, it is necessary for the light to be completely polarised that is, converted to a single polarisation state, before it is incident upon the device. For example, liquid crystal devices require fully polarised light.

One way of converting unpolarised light to completely polarised light is the well-known linear polariser. An idealised linear polariser transmits light that is linearly polarised in one direction without loss and completely absorbs light that is linearly polarised in an orthogonal direction, so that unpolarised light incident on the polariser is converted into light that is completely linearly polarised. While such a linear polariser is a straightforward means for producing linearly polarised light, it has the disadvantage of having a low efficiency. An ideal linear polariser, in which there is no loss of the polarisation component that is intended to be transmitted owing to absorption within the polariser and/or reflection at the surfaces of the polariser, has an efficiency of only 50%, and the efficiency of a practical linear polariser is generally within the range 40-45%.

Another known means for converting unpolarised light to polarised light is a polarisation conversion optical system (PCOS). In a polarisation conversion optical system, incident light that is already polarised in a desired polarisation state is transmitted unchanged. Light that is polarised in a polarisation state orthogonal to the desired polarisation state is converted to light of the desired polarisation state, rather than being blocked as happens if a conventional linear polariser is used.

It is known for projection displays to include a polarisation conversion optical system for receiving unpolarised light from a lamp and for converting this into uniformly polarised light. The illumination system of the projector illuminates a spatial light modulator (SLM), such as a liquid crystal device (LCD), so that, by equalising the polarisation of the illuminating light to the substantially uniform polarisation, the efficiency of the projection display is improved. Figure 1 of the accompanying drawings illustrates a typical example of this type of polarisation conversion system, for example as disclosed in US 5,978,136 . A polarisation conversion optical system generally comprises a polarisation beam splitter (PBS) that splits incident unpolarized or partially polarised light, so that light of one polarisation state is emitted from the PBS spatially separated from light having an orthogonal polarisation state. A polarisation conversion optical system also comprises a polarisation conversion element for changing the polarisation state of one of the components emitted by the PBS into the orthogonal polarisation state.

A first microlens array 3 of a PCOS 2 and homogeniser 2 receives light from a lamp 1 and forms an array of spatially distributed bright spots of unpolarised light at its focal plane. A second microlens array 4 is disposed at the focal plane of the first array 3 and the lenses of both arrays have the same focal lengths. A polarising beam splitter 5 is disposed after the second array 4 and comprises an array of elements having a pitch which is approximately half that of the microlenses of the array 4. Thus, focused light beams are incident on alternate elements or sections of the polarising beam splitter 5.

Polarisation separating films such as 8 transmit light of one polarisation, such as the P polarisation, in the "forward" direction. Light with the orthogonal polarisation, such as the S polarisation, is reflected to a reflective film such as 9 in an adjacent section of the beam splitter, where it is reflected into the forward direction. Thus, each unpolarized beam incident on the beam splitter 5 generates two spatially separated forward- propagating beams with orthogonal polarizations.

An element 6 comprises an array of individual phase plates or strips of retarder film arranged such that only beams of one polarisation are incident thereon. The retardation is chosen so that the incident polarised light emerges from the retarder with its polarisation changed to the orthogonal polarisation. Thus, all forward-propagating beams have substantially the same polarisation state.

The F-number of the microlenses of the arrays 3 and 4 is chosen to match the characteristics of the lamp 1, which generally emits slightly diverging light. In order to reduce the length of the polarisation conversion optical system 2 without substantially affecting its lightcollecting efficiency, it is necessary to reduce both the focal length and the pitch of the microlenses in order to maintain the desired F- number.

The use of microlenses with a smaller pitch for any given lamp is advantageous because the illuminating beam at a liquid crystal panel 7 can be homogenized with better uniformity. Improved homogenization is achieved because there are more lenses in the first array 3 to sample the non-uniform beam emitted from the lamp 1. In addition, the use of microlenses of smaller pitch allows light emitted by a light source having a smaller form factor to be effectively homogenized, thus allowing miniaturized "projection engines" to be provided.

A consequence of reducing the pitch of the microlenses of the arrays 3 and 4 is that the pitch of the beam splitter array 5 also has to be reduced. The polarising beam splitter is typically the most expensive element within a PCOS and reducing its pitch substantially increases the cost of manufacture because more polarisation-separating layers 8 and mirror coatings 9 are required. Also, when producing a smaller pitch element, tolerencing becomes more critical and light wastage increases. As a result of these constraints, it is not currently viable commercially to manufacture a polarisation beam splitter array with a pitch of less than about 0.5 rum so that the total length of a PCOS employing such an array cannot be made less than about 7 mm.

Figure 2 shows a further polarisation conversion optical system as disclosed in US 6 193 376. In this PCOS, the polarisation splitting element 10 is a diffractive optical element (DOE). Light in which the plane of polarisation is in the plane of the drawing is not deflected, as shown by the solid ray paths. Light polarised in a direction out of the plane of the drawing is deflected, as shown by the ray paths in broken lines. The device also comprises a first microlens array 11 for focusing light emitted by the element 10, a conventional large-size array of half wave retarder elements 12, and a second microlens array 13. The device is illuminated by light from a lamp that has been collimated by a parabolic mirror 14 and passed through a W-IR filter 15 for substantially blocking ultraviolet and infrared radiation. s

The prior art PCOS of Figure 2 has the disadvantage that it uses a diffractive optical element 10 as the polarization splitting element. Because this is a diffractive optical element it will suffer from high chromatic dispersion and will also suffer from polarization mixing owing to the overlapping of multiple diffraction orders. The high chromatic dispersion of the polarization splitting element 10 will also mean that the efficiency of the PCOS will be low.

US 6621533 discloses a PCOS as illustrated in Figure 3 of the accompanying drawings.

This system comprises microlens arrays 16 and 17 and a polarization rotating element 18, which functions similarly to the arrays 3 and 4 and the phase plate 6, respectively, shown in Figure 1. However, the PBS 5 is omitted and its function is performed by a polarization splitting element 19 comprising an array of optically isotropic microprisms such as 20 in contact with an array of optically anisotropic microprisms such as 21.

Each microprism 20, 21 has a wedge-shaped cross-section.

This arrangement allows microlenses 16 and 17 of the required F-number for collecting light efficiently to be provided with very small pitch and focal length. Consequently, the system has a minimum length of approximately 4 mm.

Perfectly collimated unpolarized input light at normal incidence on the polarization splitting element 19 encounters the inclined interfaces such as 22 between the isotropic and anisotropic prisms 20 and 21 and is angularly separated into two orthogonally polarised beams as shown in Figure 3. Such light is refracted in the designed direction, as illustrated in Figure 4a. The vertical walls 23 shown in Figure 4a, are truly vertical and do not have an effect on the incident light. In reality, however, it is very difficult to fabricate perfectly vertical walls in a polymer microprism structure. Collimated light normally incident on the splitting element 19 will therefore, in practice, encounter an inclined face 22 and a non-vertical wall 24, with some light being refracted in an ,7. 5 undesirable direction, decreasing the light efficiency of the system as illustrated in Figure 4b. When the splitting element 19 is illuminated with divergent light as occurs in real optical systems, a greater proportion of the light is incident on the nonvertical wall and the light efficiency of the system is further reduced as illustrated in Figure 4c. In practice, the greater the height of the microprism, the greater the deviation in angle of the vertical wall away from 90 .

A further disadvantage of this arrangement lies in the method of fabricating the polymer microprism structure. The polymer microprism structure is often formed by, for example, W casting. During the W casting process, refractive index gradients are often induced in the polymer microprism structure as shown schematically in Figure 5.

This has the effect of refracting light into the wrong direction, thereby increasing the source of light loss in the polarisation conversion optical system. The greater the height of the prism, the greater are the problematic refractive index gradients induced through W casting and the greater the source of light loss in the polarization conversion optical system.

The angle between the beams is determined by the angle of incidence of light on the interface 22 and the change in refractive index encountered by the light at the interface 22. Therefore, for splitting angles of 3-5 degrees required for efficient light utilization of light from a projection lamp, the liquid crystalline material used to form the birefringent micro prism 21 is required to align uniformly over the height of the microprism structure, which is the order of 60 to 100 micrometres. Poor alignment of liquid crystalline materials in thick layers increases the light loss and reduces the efficiency of such a polarization conversion system.

For a given birefringence of the optically anisotropic prisms 21, there are two ways to reduce thickness of elements of the polarization splitter 19: to reduce pitch of the micro prism or to decrease the angle of the inclined interface 22. However, a smaller pitch of the micro prisms in the polarization splitter 19 leads to increased diffraction and, as a result to increased light loss. Smaller prism angle leads to a smaller angular separation of the S and P-polarised beams by the polarization splitter 19.

To attempt to resolve this problem, US 6 621 533 discloses the use of two adjacent micro prism arrays 25 and 26, as illustrated in Fig.6 of accompanying drawings. The optical axes of the optically anisotropic material forming the birefringent prisms of the elements 25 and 26 are crossed at the interface, so that the optical axes of one of the elements 25 or 26 are twisted. Disadvantages of this approach are explained schematically in Fig.7. The thickness of the liquid crystalline layer in a micro prism 27 which comprises twisted liquid crystal varies from substantially zero at one end of the prism structure to a maximum, which is substantially equivalent to the height of the prism, at the other end of the prism structure. As a result, the polarization state is not rotated uniformly through 90 degrees across the whole prism area. This has the effect of increased light loss and spatially non-uniform efficiency across the polarization conversion system.

According to a first aspect of the present invention, there is provided a polarization splitter for angularly separating unpolarised light incident in a first direction into light of substantially orthogonal polarizations travelling in second and third directions which are different from each other, comprising a first set of first inclined interfaces between at least one first region made from a substantially optically isotropic material and at least one second region made from an optically anisotropic material whose optic axis is substantially non-twisted; and a second set of second inclined interfaces between at least one third region made from a substantially optically isotropic material and at least one fourth region made from an optically anisotropic material whose optic axis is substantially non-twisted, the first inclined interfaces being non-parallel to the second inclined interfaces and the optic axes of the second and fourth regions being substantially parallel to each other.

The first and second interfaces may be plane.

The ones of the first to fourth regions which are adjacent each other may have thicknesses which taper in the same direction.

The first to fourth regions may comprise first to fourth arrays, respectively, of microprisms.

Each microprism may have an axis of constant cross-section and extend throughout the width of the array, parallel to the axis of constant crosssection. Along the axis of constant cross-section, the cross-section has a constant shape and size.

The optic axes of the second and fourth regions may be substantially parallel to longitudinal axes of the microprisms.

The first to fourth microprism arrays may have the same pitches. At least the fust and second microprism arrays may have a varying pitch. The varying pitch may vary randomly or pseudorandomly.

The microprisms of the first and second arrays may have wedge angles which are substantially equal to each other and the microprisms of the third and fourth arrays may have wedge angles which are substantially equal to each other. The wedge angles of the microprisms of the first to fourth arrays may be substantially equal to each other.

The refractive index of the first and third regions may be different from the ordinary and extraordinary refractive indices of the second and fourth regions throughout the visible light spectrum. The refractive index may be between the ordinary and extraordinary refractive indices.

The first and third regions may comprise the same isotropic material. The first and third regions may be parts of at least one common region.

The second and fourth regions may comprise the same anisotropic material. The second and fourth regions may be parts of at least one common region.

The first and second regions may be formed as a first splitting element and the third and fourth regions may be formed as a second splitting element distinct from the first splitting element.

The material of the second and fourth regions may comprise a liquid crystal, a polymer stabilised liquid crystal or a reactive mesogen.

- Q

^''.''i According to a second aspect of the invention, there is provided a polarization conversion optical system comprising a splitter according to the first aspect of the invention and a patterned polarisation changing element.

According to a third aspect of the invention, there is provided a projector comprising a system according to the second aspect of the invention.

The use of the term "inclined" herein is intended to refer to a surface which is neither parallel nor perpendicular to a designed light propagation path through the splitter. For instance, such inclined interfaces or surfaces may be oriented at an acute angle to a plane perpendicular to the designed light propagation direction. Also, references to "tapering in the same direction" are intended to refer to structures in which the thickness in at least one direction reduces along elements in the same direction. In the case of wedge-shaped elements, for example, with small acute wedge angles, this means that the wedges point in substantially the same direction.

The expression "made from a substantially optically isotropic" material means that the material is substantially isotropic before manufacture of the splitter begins. During manufacture of the splitter, the processing applied to the material may result in its becoming anisotropic to a small extent but it is intended that this effect should be small so that any anisotropy created in this material should be much smaller than the anisotropy of the anistropic material in the splitter following manufacture.

The term "non-twisted" material means a material whose optic axis is substantially uniform and non-twisted about the light polarization direction throughout the material.

In the core of a liquid crystal material, for example, this means that the liquid crystal director is substantially non-twisted throughout the material or the bulk of the material.

It is thus possible to provide a polarization of improved performance as compared with known polarization splitters. For example, in the case of microprism arrays, the height of the individual microprisms can be reduced. This provides improved efficiency, for example because the alignment of the anisotropic material can be improved, refractive index gradients can be reduced and the angle of the vertical walls of microprisms can be l^ I' 9 better controlled. The prism angle may be reduced so that a greater percentage of incident light encounters the inclined interface as opposed to the vertical wall of microprisms. The optic axes of the anisotropic regions are parallel and there is no twist in such regions. This results in reduced complexity of fabrication, and hence reductions in cost, together with improved efficiency over arrangements in which liquid crystal material is in a twist state. Flexibility of design of elements within such polarisation splitters may also be achieved.

Such polarisation splitters may be used in a variety of applications. For example, such a splitter may be used in a PCOS in an optical engine of a liquid crystal projector and allows the size of such an optical engine to be reduced.

Figure 1 is a cross-sectional diagram illustrating a known type of projection system including a known type of polarisation conversion optical system; Figure 2 is a cross-sectional diagram illustrating another known type of polarisation conversion optical system; Figure 3 is a cross-sectional diagram illustrating a further known polarisation conversion optical system; Figure 4 is a cross-sectional diagram illustrating light loss in a polarisation splitter of the system of Figure 3 due to refraction of diverging light on non-vertical prism walls; Figure 5 schematically illustrates the mechanism of light loss in the polarisation splitter of the system of Figure 3 due to the gradient of refractive index in prismatic elements; Figure 6 is a cross-sectional diagram of a known two-element polarisation splitter; Figure 7 schematically illustrates the mechanism of light loss due to non-uniform polarisation rotation in a variable thickness cell of the splitter of Figure 6; Figure 8 is a cross-sectional diagram illustrating a polarization splitter constituting an embodiment of the invention; Figure 9 is a cross-sectional diagram illustrating a polarization splitter for a polarization conversion system constituting an embodiment of the invention; Figure 10 is a cross sectional diagram illustrating a furler embodiment of the invention; Figure 11 is a cross-sectional diagram illustrating another embodiment of the invention, in which prism angles of a first array are not equal to those of a second prism array; Figure 12 is a cross-sectional diagram illustrating a further embodiment of the invention, in which materials of isotropic and birefringent prisms of first and second arrays are not identical; Figure 13 is a cross-sectional diagram illustrating a further embodiment of the invention, in which birefringent prisms of a second array are adjacent polymer prisms of a first array; Figure 14 is a cross-sectional diagram illustrating a further embodiment of the invention, in which the pitch of a first prism array is not equal to the pitch of a second prism array; Figure 15 is a cross-sectional diagram illustrating a further embodiment of the invention, in which the pitch of at least one prism array varies randomly; Figure 16 is a cross-sectional diagram illustrating a further embodiment of the invention, in which the splitter is formed as a single element; Figure 17 is a cross-sectional diagram illustrating a polarization conversion optical system constituting a further embodiment of the invention; and --. 11 Figure 18 is a cross-sectional diagram illustrating a projector constituting a further embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

Figure 8 illustrates a polarisation splitter or splitting element 28 comprising optical elements 29 and 30. The optical element 29 comprises an array of optically anisotropic microprisms, such as 31, in contact with an array of optically isotropic microprisms, such as 32. Each of the microprisms 31 and 32 has a wedge-shaped cross-section and pairs of such microprisms meet each other at an inclined surface or interface 33.

Similarly, the optical element 30 comprises an array of optically anisotropic microprisms, such as 34, in contact with an array of optically isotropic microprisms, such as 35. The microprisms 34 and 35 have wedge-shaped cross-sections and meet each other at inclined interfaces 36. The interfaces 33 of the first optical element 29 are non- parallel to the interfaces 36 of the second optical element 30. The anisotropic microprisms 31 of the element 29 have optic axes which are parallel to each other and to the optic axes of the optically anisotropic microprisms 34 of the element 30.

Figure 9 illustrates a polarisation splitter which may be used, for example, in a polarisation conversion optical system. In use, the system receives collimated light illustrated at 37 from a light source (not shown). The light 37 from the light source is unpolarised or partially polarised and contains two linearly polarized components having orthogonal polarisation directions. One component has the plane of polarisation in the plane of the drawing. This is denoted by a double- ended arrow in Figure 9 and will be referred to as "horizontally plane- polarised". The other component has the plane of polarisation out of the plane of the drawing. This is denoted by the "dot enclosed within a circle" symbol in Figure 9 and will be referred to as "vertically plane- polarised".

The polarisation splitter 38 of Figure 9 contains two arrays of identical polarisation splitting elements 39 and 40. The splitter 38 angularly separates the two polarisation components in the incident light and deviates each polarisation component. One polarization component of the incoming light is directed in a first direction and the orthogonal polarization component is directed in a second direction which is different from the first direction. The first and second directions are each different from the direction of propagation of the incident light, for all wavelengths in the visible spectrum.

The polarization splitting element 38 comprises two identical sets of arrays 39 and 40.

Each of the sets 39, 40 comprises an array of isotropic prisms 41 or 42 and an array of birefringent prism 43 or 44. The prisms 41, 42, 43 and 44 of each array have a wedge- shaped cross-section. The wedge angle of the prisms 41 of the isotropic prism array is equal or substantially equal to the wedge angle of the prisms 43 of the birefringent prism array and the cross-sectional dimensions of the prisms 41 of the isotropic prism array are equal or substantially equal to the cross-sectional dimensions of the prisms 43 of the birefringent prism array.

Within each of the sets 39 or 40, the arrays of prisms 41 and 43 or 42 and 44 are disposed such that each prism 41 has its oblique face 41a (hypotenuse face) adjacent to the oblique face 43a of a prism 43. Since the prisms 41 have substantially the same wedge angle as the prisms 43, the base face 41b of a prism 41 is substantially parallel to the base face 43b of the corresponding prism 43. The first set 39 is adjacent to the second set 40 such that the base sides of pairs of the optically anisotropic prisms 43 and 44 are adjacent and substantially parallel.

The isotropic prisms 41 of the first array are attached to a first cover plate or substrate and the isotropic prisms 42 of the second array are attached to a second cover plate or substrate 46. The cover plates can be made of any transparent, optically isotropic material such as a glass or a plastics material. The covers plates 45, 46 preferably each have a uniform thickness, so that the front surface of the first cover plate 45 is parallel to the rear surface of the second cover plate 46. The rear face of the second cover plate 46 forms the exit face of the polarization splitting element.

The optic axes of the optically anisotropic prisms 43 of the first prism array and the optic axes of the second birefringent array prisms 44 are parallel to each other and to the wedge "grooves" or longitudinal directions.

Provided that the refractive index of the material used to form the optically isotropic wedge-shaped prisms 41 is not equal to the ordinary refractive index or to the extraordinary refractive index of the birefringent material used to form the birefringent prisms 43, both polarization components will be deviated. The refractive index of the material used to form the optically isotropic wedge-shaped prisms 41 and the ordinary and extraordinary refractive indices of the birefringent material used to form the birefringent prisms 43 may be chosen such that no < n < ne over the entire visible spectrum, where no and ne are the ordinary and extraordinary refractive indices of the birefringent material used to form the birefringent prisms 43 and n is the refractive index of the material used to form the optically isotropic prisms 41.

The arrays of birefringent prisms 43 and 44 of Figure 9 may be embodied as a liquid crystal layer, for example by using the fabrication technique disclosed by D.J. Broer in "Mol. Cryst. Liq. Cryst." Vol. 261, pp 513-523 (1995). They may alternatively be embodied using a reactive mesogen layer such as RMM34 (available from Merck) or using a polymer-stabilised liquid crystal layer such as NOA61 (available from Norland) mixed with E7 (available from Merck).

The arrays of isotropic prisms 41 and 42 may be made of a polymer material. The wedge profile of the array of prism 41 and 42 may be provided by, for example, moulding the polymer using a suitable mould, casting the polymer, or by a lithographic process.

The liquid crystal layer of the first birefringent prism 43 is disposed between a first alignment layer and a second alignment layer for controlling the alignment direction of liquid crystal molecules adjacent the alignment layers. In the embodiment illustrated in Figure 9, the first alignment layer is disposed on the cover plate 47 and the second alignment layer is disposed on the "hypotenuse side" of the array of prisms 41.

The alignment layers can be formed of any suitable material, such asrubbed polyimides. Alternatively, the alignment layers may be photoaligned so that they orient liquid crystal molecules in a desired direction. Alternatively, micro-grooves for alignment may be formed as part of a structure of polymer micro prisms during their fabrication, for example as disclosed in US2003/0137626A1 "A method of making a passive patterned retarder and retarder made by such a method" or W003/062872 "Method of making a patterned optical retarder".

The elements 39 and 40 may be attached to each other by optical adhesive. This reduces the number of surfaces and minimises Fresnel reflection at substrate/air interfaces. The elements 39 and 40 do not need to be accurately aligned in the directions along and perpendicular to the wedge "grooves".

The polarization splitter shown in Fig. 10 comprises two identical sets 48 and 49, similar to those 39, 40 illustrated in Fig.9. However, the splitter differs in that the sets 48 and 49 are assembled such that the base sides of optically isotropic prisms 50 and 51 are add acent and substantially parallel.

The polarization splitter shown in Fig. 11 comprises two sets 52 and 53 similar to the sets 39 and 40 of Fig.9. However, the wedge angle of micro prisms of sets 52 and 53 are not equal. This may give more flexibility in the design of polarization splitters for different applications using a combination of a limited number of standard elements 52 and 53.

The polarization splitter shown in Fig. 12 comprises sets 54 and 55 similar to the sets 39 and 40 of Fig.9. However, the material of isotropic prisms 56 of the first set 54 is different from the material of the isotropic prisms 58 of the second set 55. Also, the birefringent material of the anisotropic prisms 57 of the first set 54 is different from the birefringent material of the anisotropic prisms 59 of the second set 55. This arrangement has an advantage in flexibility in the design of polarization splitters for different applications using a combination of a limited number of standard elements 54 and 55. A further advantage of this arrangement is that it can provide better control over chromatic dispersion of the polarization splitter by the choice of combination of different materials for fabrication of the prisms 56, 57, 58 and 59. I, 15

Fig.13 illustrates another polarisation splitter comprising sets 60 and 61 similar to the sets 39 and 40 of Fig.9. However, the sets 60 and 61 are assembled such that the base sides of the optically isotropic prisms of the set 60 are adjacent the base sides of birefringent prisms of the set 61 and are substantially parallel. This allows an alignment layer 63 for the liquid crystal material which forms birefringent prisms of the set 61 to be disposed directly on a base side 49 of the isotropic prisms of the set 60. Additional cover plates such as that illustrated at 62 between the sets 60 and 61 may be omitted to allow fabrication of a thinner polarisation splitter.

Fig.14 illustrates a further polarisation splitter comprising sets 64 and 65 similar to the sets 39 and 40 of Fig.9. However, the pitch of prism array of the first set 64 is different from the pitch of prism array of the second set 65. This may give more flexibility in the design of polarisation splitters for different applications using a combination of a limited number of standard elements 64 and 65 and in minimising diffraction from small pitch periodic structures and moire effects between the sets 64 and 65.

Fig.15 illustrates yet a further polarisation splitter comprising sets 66 and 67 similar to the sets 39 and 40 of Fig.9. However, the pitch of the prismatic elements of the isotropic and birefringent arrays forming the set 66 is designed to be random. Such an arrangement substantially reduces diffraction of light and allows an element with much smaller features to be designed.

If the interfaces of the two optical elements within the polarisation splitter were parallel and the anisotropic regions were adjacently disposed, the incident light would be split into two orthogonally polarised beams which would emerge from the second optical element parallel to one another, i.e. no angular separation would be achieved.

Similarly, if the interfaces of the two optical elements were parallel but with the anisotropic regions of the first element adjacent the isotropic regions of the second element, incident light having two linearly polarised components with orthogonal polarisation directions would be split by the splitter into two non-parallel beams, but the splitting angle would be much smaller than that achieved by any of the above described embodiments. It is therefore preferable for the interfaces of the two optical elements to be non-parallel. The further away from parallel the interfaces are, the more efficient the polarisation splitter.

Figure 16 illustrates a further embodiment of the present invention which differs from that illustrated in Figure 8 in that the polarisation splitter 28 is formed as a single element 100 performing the function of the first and second optical elements of the preceding embodiments. Each pair of adjacent anisotropic microprisms 31, 34 is formed as a single microprism. The operation of this embodiment is the same as the previous embodiments and achieves the same angular separation. It will be appreciated that any of the other preceding embodiments may be constructed in a similar manner.

For example the isotropic regions may be disposed adjacent one another in a single splitting element, or the asymmetric arrangement of Figure 11 could be formed as a single splitting element.

Figure 17 illustrates a polarisation conversion optical system for converting unpolarized light to a desired substantially uniform polarisation. Incident light 37 comprises light polarised in orthogonal S and P polarisation states. The polarisation splitter 38 is a polarisation splitter according to any one of the above-described embodiments. Light emerging from the polarisation splitter 38 is angularly separated and incident upon a microlens array 80. The microlens array 80 focuses the incident light onto a patterned retarder 81. The pattern on the patterned retarder is such that one of the S and P polarisation states is retarded with respect to the other, resulting in the emergent light being of a single polarisation state.

Figure 18 illustrates a projector incorporating a polarisation conversion optical system as described above. A lamp 82 illuminates the polarisation conversion optical system 38 with unpolarised light. The polarisation conversion optical system converts this into polarised light. The arrangement 83 of beamsplitters and reflectors illuminates a spatial light modulator in the form of liquid crystal devices 84a to 84c, disposed adjacent a projection lens 8. 5. The efficiency of the projector is improved through having light of substantially uniform polarisation. 17

Claims

CLAIMS: 1. A polarization splitter for angularly separating unpolarised

light incident in a first direction into light of substantially orthogonal polarizations travelling in second and third directions which are different from each other, comprising: a first set of first inclined interfaces between at least one first region made from a substantially optically isotropic material and at least one second region made from an optically anisotropic material whose optic axis is substantially nontwisted; and a second set of second inclined interfaces between at least one third region made from a substantially optically isotropic material and at least one fourth region made from an optically anisotropic material whose optic axis is substantially non-twisted, the first inclined interfaces being non-parallel to the second inclined interfaces, and the optic axes of the second and fourth regions being substantially parallel to each other.
2. A splitter as claimed in claim 1, in which the first and second interfaces are plane.
3. A splitter as claimed in claim 1 or 2, in which the ones of the first to fourth regions which are adjacent each other have thicknesses which taper in the same direction.
4. A splitter as claimed in any one of the preceding claims, in which the first to fourth regions comprise first to fourth arrays, respectively, of microprisms.
5. A splitter as claimed in claim 4, in which each microprism has an axis of constant cross-section and extends throughout the width of the array, parallel to the axis of constant cross-section.
6. A splitter as claimed in claim 4 or 5, in which the optic axes of the second and fourth regions are substantially parallel to longitudinal axes of the microprisms.
7. A splitter as claimed in any one of claims 4 to 6, in which the first to fourth microprism arrays have the same pitches.
8. A splitter as claimed in any one of claims 4 to 6, in which at least the first and second microprism arrays have a varying pitch.
9. A splitter as claimed in claim 8, in which the varying pitch varies randomly or pseudorandomly.
10. A system as claimed in any one of claims 4 to 9, in which the microprisms of the first and second arrays have wedge angles which are substantially equal to each other and the microprisms of the third and fourth arrays have wedge angles which are substantially equal to each other.
11. A splitter as claimed in claim 1 0, in which the wedge angles of the microprisms of the first and fourth arrays are substantially equal to each other.
12. A splitter as claimed in any one of the preceding claims, in which the refractive index of the first and third regions is different from the ordinary and extraordinary refractive indices of the second and fourth regions throughout the visible light spectrum.
13. A splitter as claimed in claim 12, in which the refractive index is between the ordinary and extraordinary refractive indices.
14. A splitter as claimed in any one of the preceding claims, in which the first and third regions comprise the same isotropic material.
15. A splitter as claimed in claim 14, in which the first and third regions are parts of at least one common region.
16. A splitter as claimed in any one of the preceding claims, in which the second and fourth regions comprise the same anisotropic material.
17. A splitter as claimed in claim 16 where dependent on any one of claims 1 to 14, in which the second and fourth regions are parts of at least one common region.
18. A splitter as claimed in any one of claims 1 to 14 and 16, in which the first and second regions are formed as a first splitting element and the third and fourth regions are formed as a second splitting element distinct from the first splitting element. s
19. A splitter as claimed in any one of the preceding claims, in which the material of the second and fourth regions comprise a liquid crystal, a polymer stabilised liquid crystal or a reactive mesogen.
20. A polarization conversion optical system comprising a splitter as claimed in any one of the preceding claims and a patterned polarization changing element.
21. A projector comprising a system as claimed in claim 20.