Abstract:
A polarisation control device is provided comprising in sequence an array of electrically switchable holographic lenses, a half wave plate and an electrically switchable beam deflecting holographic optical element. Said switchable holographic devices each operate on light having a first polarisation state. Light in a second orthogonal polarisation state is not affected by said switchable holographic devices. The half wave plate contains an array of apertures that overlap substantially with the focal regions formed by the holographic lenses. Light propagating through said apertures retains its polarisation state. The beam deflecting holographic optical element deflects and diffuses collimated input light. A further diffusing element may be used to apply additional diffusion to the light emerging from the beam deflecting holographic optical element. In a further embodiment of the invention the array of transmission holographic optical elements and the beam deflecting holographic optical elements each comprise a stack of red, green and blue transmitting switchable transmission holograms.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims priority to United Kingdom Patent Application No. GB 0518212.6 filed 8 Sep. 2005.  
         [0002]     This invention relates to a illumination device, and more particularly to a device that provides linearly polarized illumination from a randomly polarized light source.  
         [0003]     LCDs are now found in a wide variety of applications, including directly viewed displays, virtual image displays, where the liquid crystal device is viewed through a magnifying optical system, and projection displays. One well-known approach for providing a colour display is to illuminate a monochromatic LCD device with red, green, and blue light in sequence at a sufficient rate such that the sequential single-colour images appear to the observer as a full colour image. Colour sequential illumination is commonly used for large screen projection displays. Early sequential-sequential displays employed a rotating colour filter wheel to filter the light from a white source into sequential red, green, and blue components.  
         [0004]     One emerging illumination technology currently being considered for LCD applications is based on electrically switchable holograms. Such devices are formed by recording a volume phase grating in a polymer dispersed liquid crystal (PDLC) mixture. U.S. Pat. No. 5,942,157 and U.S. Pat. No. 5,751,452 describe monomer and liquid crystal material combinations suitable for fabricating Holographic PDLC (HPDLC) devices. A publication by Butler et al. (“Diffractive properties of highly birefringent volume gratings: investigation”, Journal of the Optical Society of America B, Volume 19 No. 2, Feb. 2002) describes analytical methods useful to design HPDLC devices and provides numerous references to prior publications describing the fabrication and application of HPDLC devices. U.S. Pat. No. 6,115,152 describes an apparatus for colour-sequential illumination of a display, which combines light from red green and blue illumination sources. The apparatus comprises a stack of electrically switchable holograms. Each switchable hologram diffracts light from one illumination source into a common direction, such that light is transmitted sequentially from each illumination source onto the display panel.  
         [0005]     HPDLC transmission gratings suffer from the problem that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that HPDLC transmission gratings efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence.  
         [0006]     Both LCDs and illuminators based on HPDLC transmission gratings require polarised illumination. The use of randomly polarised light sources therefore results in half the available illumination light being discarded. Although polarisation recycling techniques based on polarizing beams splitters and polarization retarders are well known in the field of displays they tend to be inefficient bulky and expensive for many display applications.  
         [0007]     Thus there exists a need for an improved illumination system for LCDs that can provide linearly polarized sequential-sequential illumination from a randomly polarized source in a light efficient compact configuration.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide an improved illumination system for LCDs that can provide linearly polarized sequential-sequential illumination from a randomly polarized source in a light efficient compact configuration.  
         [0009]     The objects of the invention are achieved in a first embodiment comprising an array of switchable holographic lenses, a Half Wave Plate (HWP) layer and a switchable beam deflecting Holographic Optical Element (HOE). The input light is typically provided by means of an illumination assembly comprising a set of LED sources and collimating lenses, which do not form part of the invention. The switchable holographic lens array operates on P-polarised input light. The HWP layer contains apertures through which light may propagate without polarization change. The HWPs switch the incident S-polarized light into the P-polarized state. The apertures in the HWP overlap substantially with the focal regions formed by the HOE array. The switchable beam deflecting HOE has diffusing properties such that a collimated P-polarized input beam is directed into a range of ray directions with an average direction substantially normal to the surface of the HOE. However, the P-polarized beam emerging from the holographic lens array is not deflected because it falls outside the angular bandwidth of the beam deflecting HOE.  
         [0010]     The apparatus may further comprise a diffusing layer, which applies further diffusion to the P-polarized light emerging from the beam deflecting HOE.  
         [0011]     The holographic lenses may have optical power in one plane only such that they form bar shaped focal regions. In such an embodiment of the invention the HWP layer comprises an array of bar shaped HWP elements separated by small gaps through which light may propagate without polarization change.  
         [0012]     In another embodiment of the invention the holographic lens array and the beam deflecting HOE each comprise a stack of red, green and blue transmitting switchable holograms.  
         [0013]     In a further embodiment of the invention the beam deflecting HOE is designed to deflect collimated input light without applying diffusion.  
         [0014]     In a further embodiment of the invention the holographic lens array elements may provide optical power in two orthogonal planes and the HWP layer contains a grid of circular apertures through which light may propagate without polarizations change.  
         [0015]     In a further embodiment of the invention the beam deflecting HOE is configured as an array of beam deflecting HOEs. The gaps between array elements substantially overlap the gaps between the elements of the HWP layer.  
         [0016]     In a further embodiment of the invention the diffusing element has spatially varying scattering characteristics.  
         [0017]     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings wherein like index numerals indicate like parts. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic side elevation view of a first embodiment of the invention.  
         [0019]      FIG. 2  is a schematic front view of elements of the embodiment of  FIG. 1   
         [0020]      FIG. 3  is a chart showing the illumination distribution at then output of the illuminator.  
         [0021]      FIG. 4  is a schematic side elevation view of a further embodiment of the invention.  
         [0022]      FIG. 5  is a schematic side elevation view of a further embodiment of the invention.  
         [0023]      FIG. 6  is a schematic side elevation view of a further embodiment of the invention.  
         [0024]      FIG. 7  is a schematic front view of a further embodiment of the invention.  
         [0025]      FIG. 8  is a schematic side elevation view of a further embodiment of the invention.  
         [0026]      FIG. 9  is a schematic side elevation view of a further embodiment of the invention.  
         [0027]      FIG. 10  is a schematic side elevation view of a further embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     A schematic side elevation view of a first embodiment of the invention is shown in  FIG. 1 . A polarization control device according to the principles of the invention comprises an array of electrically switchable holographic lenses  1 , a Half Wave Plate (HWP) layer  2 , an electrically switchable beam deflecting HOE  3  and a diffusing element  4 . The input light is typically provided by means of an illumination assembly comprising a set of LED sources and collimating lenses, which do not form part of the invention. Each switchable HOE comprises a HPDLC grating layer sandwiched between a pair of transparent substrates to which transparent electrode coatings have been applied.  FIG. 2A  shows a front elevation view of the switchable holographic lens array  1 .  FIG. 2B  shows a front elevation view of the HWP layer  2 .  FIG. 2C  shows a front elevation view of the switchable beam deflecting HOE  3 . The holographic lens array  1  comprises bar-shaped holographic lens elements, such as  11 . The holographic lenses have optical power in one plane only. Hence the holographic lens elements  11  are operative to form bar shaped focal regions. The switchable beam deflecting HOE  3  has diffusing properties such that a collimated input beam is directed into a range of ray directions with an average direction substantially normal to the grating. The HWP layer comprises an array of bar shaped elements such as  21 . The HWP elements are separated by small gaps such as  22 . The gaps essentially allow light to propagate without polarization change. The bar shaped apertures overlap substantially with the bar shaped focal regions.  
         [0029]     Typically, HPDLC devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates.  
         [0030]     Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerise and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied.  
         [0031]     The HWP layer may be formed by means of a mask process or by constructing the array from separate HWP elements. The HWP elements may be separated by a transparent optical medium. Alternatively, the HWP elements may be air separated. Alternatively, other methods known to those skilled in the art may be used to fabricate the HWP. The HPDLC substrates may be fabricated from glass or optical plastic.  
         [0032]     The diffuser  4  is designed to scatter incident light rays into a specified distribution of ray directions. The diffuser may be fabricated from conventional diffusing materials. Alternatively, the diffuser may be a holographic optical element such as, for example, a Light Shaping Diffuser manufactured by Precision Optical Corporation.  FIG. 3  is a chart showing typical examples of the spatial intensity distribution cross sections at a plane located beyond the diffuser  4 . The plane may correspond to the surface of an LCD device, for example. PI is a typical intensity distribution formed by the diffuser  4 . P 2  is a typical intensity distribution obtained from an element of the beam deflecting HOE  3 , which operates on the S component of the incident light after it has been converted to P polarized light. P 3  represents the resultant intensity distribution resulting from input light incident on three adjacent lens array elements in the holographic lens array  1 . The non-uniformity of the intensity distributions PI and P 2  results in ripple, which may cause unacceptable luminance variations in the display image. The ripple can be significantly reduced by controlling the diffusing characteristics of the beam deflecting HOE  3  and the diffuser  4 . The diffuser may be a Computer Generated Hologram designed to convert input light comprising separated collimated and divergent components into a uniform intensity output beam. The basic principles of the design and fabrication of CGH devices suitable for use in the present invention are discussed in references such as. “Digital Diffractive Optics: An Introduction to Planar Diffractive Optics and Related Technology” by B. Kress and P. Meyrueis, published in 2000 by John Wiley &amp; Sons Inc.  
         [0033]     The basic principles of the invention are now explained with reference to  FIG. 1 . Input monochromatic collimated light generally indicated by  1000  is incident over the aperture of the HOE array  1 . We consider the holographic lens array element  11 , which is illuminated by the portion of illumination  1100 . HPDLC transmission gratings efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Hence, the P polarized component of input light  1100  is diffracted to form the converging beam generally indicated by  1300 . Since the element  11  has lens-like properties in one plane, the converging beam  1300  forms a bar shaped focal region. Said focal region substantially overlaps the bar shaped aperture  22  in the half wave plate array  2 . The diffracted light emerges from the HWP layer as the diverging beam  1310 . The beam  1310  then passes through the beam deflecting HOE  3  without being diffracted, since the incident directions of  1310  do not satisfy the Bragg condition of HOE  3 , since said element is designed to deflect collimated light at steep incidence angles. The basic principles of Bragg diffraction will be well known to those skilled in the art of holography and are discussed in textbooks such as “Optical Holography” by R. J. Collier, C. B. Burkhardt and L. H. Lin published by Academic Press, New York (1971). The beam  1310  propagates onto the surface of the diffuser  4 . The diffuser causes the incident light  1310  to be scattered into a range of angles generally indicated by  1320 . We next consider the propagation of the S-polarized component of the incident light portion  1   100 . The S-polarized component of the input light is not diffracted by the holographic lens array  1 .  
         [0034]     The S-polarized light propagates in the zero order direction represented by  1200 . After propagation through the half wave plate array, the polarization of the beam  1200  is converted from S to P. The converted P polarized light is now diffracted by the beam deflecting HOE into a range of ray directions, generally indicated by  1210 , with an average direction substantially normal to the grating. The light  1210  is then transmitted through the diffuser layer  4 , which further modifies the diffusion profile of the light to give the diffuse output ray distribution generally indicated by  1220 . The average direction of the rays  1220  is substantially normal to the diffuser layer  4 .  
         [0035]      FIG. 4  is a schematic side elevation view of the embodiment of  FIG. 1  implemented in a projection system. The projection system further comprises relay optics  5 , a transmission flat panel display  6  and a projection lens  7 .  
         [0036]      FIG. 5  is a schematic side elevation view of an embodiment of the invention configured for colour sequential illuminations. In  FIG. 5  the switchable HOE devices  1  and  3  are replaced by the red green and blue switchable holographic lens arrays  110 , 120 , 130  and the red green and blue switchable beam deflecting HOEs  310 , 320 , 330  respectively. The combined HOEs are operative to direct red, green and blue light, in sequence towards the display panel  6  in a direction substantially normal to the surface of the display panel. To transmit red light the holographic lens arrays  120  and  130  and the beam deflecting HOEs  310  and  320  are inactive while the holographic lens layer  110  and the holographic layer  320  are activated. The red light is then transmitted through the system in accordance with the basic principles discussed above. The green and blue layers are then activated in sequence in accordance with the above procedure to provide colour sequential illumination of the display panel.  
         [0037]      FIG. 6  shows a further embodiment of the invention similar to the embodiment of  FIG. 5 . However, in  FIG. 6  the beam deflecting HOEs  340 , 350 , 360  are operative to switch light without diffusion. Hence, the incident rays emerge as the parallel rays generally indicated by  1340 .  
         [0038]     Although the invention has been described in terms of an array of bar shaped lens elements that focus the incident light into a bar shaped focal region, in further embodiments of the invention the switchable lens array may be a two dimensional array operative to form focal spots rather than bar shaped focal regions.  FIG. 7A  shows a front elevation view of the switchable holographic lens array  150 .  FIG. 7B  shows a front elevation view of the HWP layer  250 .  FIG. 7C  shows a front elevation view of the switchable beam deflecting HOE  3 . Referring to  FIG. 7  it can be seen that the holographic lens array elements  151  are configured as a two dimensional array. The lens array elements may be holographic microlenses with spherical or aspheric forms. The HWP layer now contains apertures  251  centred on the lens elements. Said apertures may be circular or of other shapes advantageously matched to the focal spot shapes of the holographic lens array elements. It will be clear to those skilled in the art that the schematic views of  FIGS. 1-6  may also be used to represent equivalent embodiments of the inventions based on two-dimensional arrays.  
         [0039]     In the embodiments discussed above the diffracted beam  1200  and the zero order beam  1300  will have appreciably different ray angles. The rays in beam  1200  will tend to have much steeper incidence angles. Hence, the rays  1310  will fall outside the angular bandwidth of the beam deflecting HOE  3  and will not be diffracted with high efficiency. However, if the lenses in the holographic lens array are designed to have a high optical power, some of the rays  1310  may fall within the angular bandwidth of the beam deflecting HOE  3 .  
         [0040]      FIG. 8  shows an alternative embodiment of the invention in which the beam deflecting HOE  300  is an array of bar shaped beam deflecting HOEs  301  each having identical properties to the beam deflecting HOE of the earlier embodiments. The HOE elements are separated by apertures such as  302 , which overlap the apertures  22  of the HWP as shown in  FIG. 8 . Alternatively,  FIG. 8  may represent an embodiment in which the beam deflecting HOE  3  comprises a two dimensional array of beam deflecting HOEs  
         [0041]      FIG. 9  shows a further embodiment of the invention similar to the embodiment of  FIG. 1 . However in  FIG. 9  the diffuser layer  4  is replaced with a diffuser layer  400  composed of an array of bar shaped diffusers such as  401 , with identical non-uniform scattering characteristics. The output rays from the diffuser element  401  are generally indicated by  1320 . The use of a diffuser array allows more precise control of the output illumination distribution. Alternatively,  FIG. 9  may represent an embodiment in which the diffuser comprises a two dimensional array of diffusing elements.  
         [0042]      FIG. 10  shows a further embodiment of the invention similar to the embodiment of  FIG. 1 . However in  FIG. 10  the diffuser layer  4  is eliminated. Incorporating suitable diffusion characteristics into the beam deflecting HOE provides output beam illumination characteristics similar to those of the embodiment of  FIG. 1 . The techniques for forming HOEs with diffusing characteristics are well known to those skilled in the art of holography.  
         [0043]     Although the invention has been discussed in terms of switchable HOEs, it will be clear from consideration of the above description that in certain applications the invention may be implemented using non switchable HOE devices to perform the functions of the lens array and the beam deflector.  
         [0044]     The basic principle of the present invention may be applied to a wide range of display applications including LED illuminators for video projectors, LCD backlights and others.  
         [0045]     To ensure efficient use of the available light and a wide colour gamut for the display, each HPDLC device should be substantially transparent when a voltage is applied and, preferably, should diffract only the intended colour without an applied voltage.  
         [0046]     It should be emphasized that FIGS.  1  to  10  are exemplary and that the dimensions have been exaggerated. For example, thicknesses of the switchable holographic elements and the HWP layer have been greatly exaggerated.  
         [0047]     Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.