Abstract:
A display panel assembly comprises a transflective holographic screen, i.e., a transparent screen that reflects light from a projection system, comprising at least a volume hologram, a first protective element and a second protective element, each arranged in contact with the volume hologram such that the volume hologram is sandwiched between the first protective element and the second protective element. The display panel assembly further comprises a projection system focusing an image on the volume hologram comprising at least projection optics, mounting means arranged to fixedly mount the projection system relatively to the transflective holographic screen. The volume hologram comprises a plurality of diffractive patterns disposed in sequence across the volume hologram, each of the plurality of diffractive patterns being configured to diffuse the light rays from the projection system in a determined direction corresponding to the specific diffractive pattern and oriented towards a position of an intended eye of a user wearing the display panel assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to augmented reality displays, in particular, those systems that give the possibility to superimpose virtual images to normal vision, i.e. see-through displays such as head-up displays (HUD&#39;s) found for example in the automotive industry or head worn displays (HWD&#39;s) placed near to the eye. 
       BACKGROUND 
       [0002]    The adoption of mobile devices such as PDA&#39;s and more recently smartphones for consumer use offers new possibilities to interact with our environment, to obtain instantaneous information and to connect with people. Next generation mobile devices are expected to provide information by displaying it in a different manner than the current hand portable display screen. Advances in projection display technologies are enabling near the eye displays, such as a pair of see through glasses. 
         [0003]    See-through displays have been used for decades for defence applications. For example, jet fighter pilots have been using head mounted displays on the fighter helmets to provide navigational and other critical information to the pilot in his/her field of view. While projection technology is advancing, there is still currently a trade-off between field of view and footprint in see-through HWD. A wide field of view (&gt;30-40 degrees) requires bulky optics. A field of view of the order of 120 degrees laterally and 60-70 degrees vertically would give the user the feeling of total immersion into the virtual world and thus vastly improving the range of applications in augmented reality. This current trade-off makes HWD&#39;s non-aesthetically pleasing for large field of views and non-appealing for everyday use. Thus, there is a need for a small footprint, aesthetically pleasing see-through HWD with a large field of view. 
         [0004]    A way to obtain HWD&#39;s with both large field of view and small footprint is to integrate optical components within a contact lens. This particular contact lens for HWD is described by Sprague—METHOD AND APPARATUS TO PROCESS DISPLAY AND NON-DISPLAY INFORMATION, U.S. patent application Ser. No. 12/204,567, Pub. No. US 2012/0053030 A1. A small focusing lens is placed at the centre of the contact lens to assist the eye to focus on the screen. The small lens of the contact lens collimates the light diffracted by the screen prior to entering the HWD wearer&#39;s eye. 
         [0005]    A key part of a contact lens based HWD system is a transflective screen that redirects each displayed pixels towards the eye, while providing undisturbed see-through vision. Sprague et al. described a method providing such a transflective screen with a buried microlens array (MLA)—BURRIED NUMERICAL APERTURE EXPANDER HAVING TRANSPARENT PROPERTIES, U.S. patent application Ser. No. 11/852,628, publication No. US 2009/0067057 A1. In this invention, an increase in display reflection efficiency inevitably induces a reduction in the display transmission and reversely. Contrary to Sprague&#39;s display screen, the screen presented in the current patent provides high diffraction efficiency (up to 100%) and high transparency to the ambient light (up to 95%) because the reflection bandpass of the holographic screen is small (˜15 nm) compared to the bandwidth of visible light (300 nm). 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect the invention provides a display panel assembly comprising a transflective holographic screen, i.e., a transparent screen that reflects light from a projection system, comprising at least a volume hologram, a first protective element and a second protective element, each arranged in contact with the volume hologram such that the volume hologram is sandwiched between the first protective element and the second protective element. The display panel assembly further comprises a projection system focusing an image on the volume hologram comprising at least projection optics, mounting means arranged to fixedly mount the projection system relatively to the transflective holographic screen. The volume hologram comprises a plurality of diffractive patterns disposed in sequence across the volume hologram, each of the plurality of diffractive patterns being configured to diffuse the light rays from the projection system in a determined direction corresponding to the specific diffractive pattern and oriented towards a position of an intended eye of a user wearing the display panel assembly. 
         [0007]    In a first preferred embodiment the display panel assembly is used as a Head-Up Display (HUD). 
         [0008]    In a second preferred embodiment the display panel assembly is further arranged to be used as a near to the eye Head-Worn Display (HWD). 
         [0009]    In a third preferred embodiment the display panel assembly further comprises a bi-focal contact lens comprising a centre part which is arranged relative to the transflective holographic screen to collimate the light diffracted by the volume hologram prior entering the intended eye of the user thereby enabling the intended eye of the user to focus onto the transflective holographic screen, and an outerpart, which surrounds the centre part and is intended to allow an image of a view through the transflective holographic screen to be seen. 
         [0010]    In a fourth preferred embodiment the projection system is a scanner projection system that is used to display information on the transflective holographic screen. 
         [0011]    In a second aspect the invention provides a method for fabricating the volume hologram of the inventive display panel assembly. The method comprises interfering a reference beam and an object light beam on the photosensitive holographic material, the light beams having similar wavelengths than the light used within the projection system of the display panel, by means of a recording holographic setup. The step of interfering comprises directing the reference beam to impinge on the photosensitive holographic material with the properties of the projection system, i.e., whereby the properties are indicative at which angle of incidence and with which numerical aperture the reference beam is projected on the photosensitive holographic material, and directing the object beam to impinge on an opposite side of the photosensitive holographic material as compared to the reference beam thereby producing a reflection hologram. 
         [0012]    In a fifth preferred embodiment the method comprises providing the photosensitive holographic material as a film laminated onto a transparent substrate. 
         [0013]    In a sixth preferred embodiment the method further comprises providing the photosensitive holographic material as a liquid photopolymer by coating any surface shape in contact with the photosensitive holographic material. 
         [0014]    In a seventh preferred embodiment the method further comprises shaping the any surface in contact with the photosensitive holographic material according to one of the following list of shapes: flat, cylindrical, spherical. 
         [0015]    In an eighth preferred embodiment the method further comprises recording the volume hologram either simultaneously or sequentially with several wavelengths to produce a colour screen. 
         [0016]    In a ninth preferred embodiment the method further comprises transmitting the object beam through a structure that diffuses light within a given angular spread so that upon use, the then obtained volume hologram enabling the transflective holographic screen to direct the projected light toward the intended eye of the user within a certain angular spread. 
         [0017]    In a tenth preferred embodiment the method further comprises providing for the structure that diffuses light a microlens array (MLA) whose lenses&#39; numerical aperture defines an angular spread of each pixel and a pitch of the microlens array defines a minimum pixel size of the screen. 
         [0018]    In an eleventh preferred embodiment of the inventive method, a fill factor of the microlens arrays (MLA) is larger than 90%. 
         [0019]    In a twelfth preferred embodiment the method further comprises replicating the microlens array (MLA) on a curved surface with at least the following fabrication steps: replicating a negative replica of the microlens array (MLA) in, but not limited to, an elastomer, and dispensing a drop of, but not limited to, curable polymer on a concave side of the curved surface. The microlens array (MLA) negative replica acts as a mold and a flexibility of the elastomer enables the negative replica to conform to the curved surface. Further the fabrication steps comprise curing the polymer with a UV curing treatment, and removing the mold to release the microlens array (MLA) on a curved surface. 
         [0020]    In a thirteenth preferred embodiment the method further comprises using a condenser lens in combination to the structure that diffuses light such that light is directed toward the intended eye of the user, thus increasing the field of view and tolerance to rotation of the eye. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will be better understood in view of the description of preferred example embodiments and in reference to the figures, wherein 
           [0022]      FIG. 1  is a schematic showing the basic principle of the see-through display; 
           [0023]      FIG. 2  is a schematic of the holographic recording principle to fabricate the transflective screen; 
           [0024]      FIG. 3  is a schematic of the optical properties of a condenser lens ( 203 ) and MLA ( 204 ) combination showing that each beamlet transmitted through each lenslet reaches the eye pupil entrance of the HWD user even when the eye rotates from  FIG. 3A  to  FIG. 3B ; 
           [0025]      FIG. 4  is a schematic of the hologram readout; 
           [0026]      FIG. 5  is a schematic showing the basic principle of the wide field of view HWD; 
           [0027]      FIG. 6  shows the process flow of the MLA replication on a curved surface; 
           [0028]      FIG. 7  is a schematic of a particular setup that can be used to record a holographic screen; 
           [0029]      FIG. 8(   a ) shows an image of a setup to demonstrate the proof of concept of the wide angle see-through display with the holographic transflective screen; 
           [0030]      FIG. 8  ( b ) and ( c ) are images of a contact lens taken under linearly polarized light for two different angle of rotation; and 
           [0031]      FIG. 9  ( a ) and ( b ) show see-through image without and with virtual image respectively. (c) Same as (b) with see through vision partially blocked. 
       
    
    
       [0032]    Same reference numbers will be used throughout the description to refer to the same or similar element(s). 
       DETAILED DESCRIPTION 
       [0033]    In the following paragraphs a more detailed description of selected figures is given. 
         [0034]      FIG. 1  is a schematic showing the basic principle of the see-through display. A volume hologram  111  sandwiched between transparent protective films  112  and  113  and constituting a transflective holographic screen  114  diffracts the incident light  104  projected from a system  107  toward the eye  110 . Light  106  providing normal see-through vision is transmitted through the transflective screen  114 . The light projected onto the transflective screen is diffracted toward the eye into light  105  within a certain angular spread. 
         [0035]      FIG. 2  is a schematic of the holographic recording principle to fabricate the transflective screen. A condenser lens  203  followed by an MLA  204  is placed in the object beam  202 . Coherent light is split into a reference beam  201  and an object beam  202  that interfere at the volume hologram  111 , thus modifying locally the refractive index of the volume hologram  111  and recording the waveform created by the MLA and condenser lens combination. 
         [0036]      FIG. 4  is a schematic of the hologram readout. The beam  104  incident on the volume hologram  111  at the same angle and wavelength used to record the volume hologram  111  is diffracted toward the eye  110  within an angular spread given by the MLA used during recording. 
         [0037]      FIG. 5  is a schematic showing the basic principle of the wide field of view HWD. A bi-focal length contact lens  501  allows the eye  110  to simultaneously focus the image displayed on the spectacle surface and diffracted toward the eye by the transflective screen  111  and forming an image through the glasses (normal see-through vision). The light  104  projected onto the transflective screen  111  is diffracted toward the eye  110  within a certain angular spread. 
         [0038]      FIG. 6  shows the process flow of the MLA replication on a curved surface. Commercially available MLA&#39;s typically provided on flat substrates  601  are replicated in an elastomer  602 . A drop of curable polymer  604  is placed on the concave side of the curved surface  603 . The MLA negative replica  602  acts as a mold. The flexibility of the elastomer allows the negative replica  602  to conform to the curved surface  603 . After UV curing, the mold  602  can be removed and an MLA  204  is present on the curved surface  603 . 
         [0039]      FIG. 7  is a schematic of a particular setup that can be used to record a holographic screen. 
         [0040]      FIG. 8(   a ) shows an image of a setup to demonstrate the proof of concept of the wide-angle see-through display with the holographic transflective screen. A micro-projector together with a lens is used to project images on the holographic screen. A camera together with a camera lens is used as an artificial eye. A contact lens is placed on this artificial eye. Both light diffracted by the screen and transmitted through the screen is then focused onto the camera sensor. 
         [0041]    The present invention is a system that uses a number of elements, the combination of which provides a large field of view see-through display. The elements comprise
       1. a volume holographic optical element used as a “transflective” screen for see-through HWD&#39;s. The near to the eye screen allows light from the surrounding environment to be transmitted through the screen while light from a projection system impinges on the screen which manipulates light by diffraction to re-direct it toward the centre of the wearer&#39;s eye;   2. a scanning projection system that uses a micromirror to scan a near collimated output to form an image by raster scanning; and   3. a bi-focal contact lens whose centre part (focal  1 ) forms an image of the spectacle glass placed near the eye and whose outerpart (focal  2 ) forms an image of the through-view.       
 
         [0045]    The present invention is not limited to HWD&#39;s. The transflective screen can also be used in HUD&#39;s, i.e. in systems which do not need the display to be placed near the eye, and consequently which do not need a bi-focal contact lens. 
         [0046]    In at least one embodiment, the transflective screen could be fabricated by use of a reflective holographic technique. In at least one embodiment, the fabrication of the holographic screen is obtained from a recording holographic setup where two coherent beams of similar intensity interfere. One of the two beams of the recording holographic setup, called reference beam, should impinge on the holographic material with the properties of the HWD projection system, i.e. at which angle of incidence and numerical aperture light is projected on the screen. The second beam, called object beam, should impinge on the opposite side of the holographic material as compared to the reference beam so that to produce a reflection hologram. 
         [0047]    In yet another embodiment, the object beam should be transmitted through a structure that diffuses light within a given angular spread so that upon use, the then obtained transflective screen directs the projected light toward the wearer eye within a certain angular spread. In another embodiment, a condenser lens could be used in combination to the diffusing structure such that light is directed toward the eye, thus increasing the field of view and tolerance to rotation of the eye. In another embodiment, the diffusing structure consists of a microlens array (MLA) whose lenses numerical aperture defines the angular spread of each pixel and the pitch or the array defines the minimum pixel size of the screen. The fill factor of the MLA should be as high as possible in order to have good display homogeneity and low diffraction upon watching at a bright scene. The fill factor should be larger than 90%. 
         [0048]    A bi-focal contact lens placed on the eye of the HWD user allows light diffracted by the transflective screen to be collimated prior to entering the wearer&#39;s eye. The user eye focuses then the light coming from the display onto the wearer&#39;s retina thus mimicking an image coming from infinity. Light from the surrounding environment remains unperturbed by the contact lens, thus allowing images from both the display and the wearer&#39;s surrounding environment to superimpose. 
         [0049]    In yet another embodiment, the transflective screen could be fabricated by any technique allowing structures similar to the ones obtained by the holographic technique to be reproduced. 
         [0050]    The techniques, apparatus, materials and systems as described in this specification can be used to fabricate a transflective screen. 
         [0051]    Described is a transflective screen to be used, but not limited to, close to the eye in HWD systems. Such transflective screen could be similarly used in other devices such as Head-Up Displays like those used in the automobile industry. Light from the environment is largely transmitted through the screen whereas light emitted from the projection system of the HWD is directed toward the human visual system. The described invention leads to a large displayed field of view together with a small footprint of the device. 
         [0052]    The screen principle is illustrated in  FIG. 1 . A collimated light beam  103  is incident on a projection system consisting of a 2-dimensional scanning mirror  107  and projection optics (not shown) to produce an exiting light ray  104 , which is focused on a holographic film  111 . Protective elements  112  and  113  sandwich the film  111 . The film  111  produces a diffracted cone beam  105  whose chief ray is diffracted toward the eye. The protective elements  112 ,  113  and the film  111  form the transflective holographic screen  114 . In at least one embodiment, information consists of scanned points which form an image on the film  111  which in turn diffracts every said points to form a light beam of a certain angular spread such that any illuminated portion of the film  111  can be seen for a given rotation range of eye  110 . The pupil  109  stops part of the light cone  105  and part of it is transmitted through the crystalline lens  108 . The screen plays the role of a light combiner as well as an eye pupil expander. Light  106  from the outside environment remains essentially undistorted after being transmitted through the screen. 
         [0053]    The present invention suggests fabricating such a screen by means of a holographic technique, more precisely by fabricating a reflection hologram. A reflection hologram is fabricated by interfering two coherent light sources located on both sides of a holographic film, as illustrated in  FIG. 2 . In the present case, one beam called reference beam  201  possesses comparable wavelengths, angle of incidence and numerical aperture as the projection system  107  that can be for example, but not limited to, mounted on the side of the eyewear. The second beam called objet beam  202  determines how the screen diffracts incident light. 
         [0054]    In at least one embodiment illustrated by  FIG. 2 , an MLA  204  containing individual lenslets  205  together with a condenser lens  203  is used in the object beam of the holographic fabrication setup to tailor the screen diffraction properties. The role of the condenser lens  203  is to redirect light from each point on the screen toward the eye pupil entrance. The condenser lens  203  is placed in close proximity to the holographic film  111  such that the focal point of the condenser lens corresponds to the position of the centre of rotation of the user&#39;s eye in the HWD. Each lenslet  205  of the MLA  204  can be considered the equivalent of a transflective screen pixel, which diffuses light over a solid angle determined by the numerical aperture of the lenslets. As illustrated in  FIG. 3  where the eye is rotated from  FIG. 3(   a ) to  FIG. 3(   b ), the light from every lenslet  205  enters the pupil of the eye  110  for any eye rotation within a range governed by the lenslet numerical aperture. 
         [0055]    As the reflection hologram records the optical properties of the MLA  204 , each area on the film  111  is observable for any eye rotation within a range governed by the lenslet  205  numerical aperture. 
         [0056]    The optical characteristics of the holographic film fabricated according to the description above are illustrated in  FIG. 4 . A collimated beam  103  is deflected in two dimensions by a micromirror  107  producing a collimated beam  104  that is focused on the holographic film  111  at similar angle, numerical aperture and wavelength as the beam  201  used in the recording setup. Beam  104  is diffracted by the film  111  producing a cone beam directed toward the eye  110  as if the beam is coming from the object beam  202  (not shown in  FIG. 4 ). Such a configuration provides a display with a wide field of view given by the combination of the numerical aperture of the lenslets  205  and the direction of the chief-rays from each lenslets converging to the centre of the eye  110 . 
         [0057]    In the case where the screen  114  is placed too close to the eye  110 , it is not possible or rather effort demanding to focus on the screen. In at least one embodiment, the HWD user can focus on the near-to-the-eye screen with the help of a special contact lens  501  illustrated in  FIG. 5 . A small focusing lens  503  is placed at the centre of the contact lens  501  to assist the eye  110  to focus on the screen  114 . The small lens  503  of the contact lens  501  collimates the light  105  diffracted by the screen  114  prior to entering the HWD wearer&#39;s eye  110 . 
         [0058]    A band pass filter  505  is placed behind or before the small lens  503  to block light  106  from the outside environment. A notch filter  504  is placed on the outer region  502  of the contact lens  501  to block light  105  coming from the display and allow light  106  from the outside environment to be transmitted. 
         [0059]    In another design, a polarization filter  505  is placed behind or before the small lens  503  to block light  106  from the outside environment. A polarization filter  504 , with polarization orthogonal to the filter  505  placed behind or before the small lens  503 , is placed on the outer region  502  of the contact lens  501  to block light  105  coming from the display and allow light  106  from the outside environment to be transmitted. 
         [0060]    The eye  110  can then focus simultaneously light  105  and  106  from the display and the outside environment respectively, onto the retina. 
       Device Fabrication 
       [0061]    Commercially available MLAs are typically provided on flat substrates. In at least one embodiment, MLA  601  can be replicated on curved surfaces  603 , e.g. either cylindrical or spherical surfaces, with the process shown in  FIG. 6 . A negative replica  602  of the MLA  601  is replicated in, but not limited to, an elastomer. A drop of, but not limited to, curable polymer  604  is then dispensed on the concave side of the curved surface  603 . The MLA negative replica  602  acts then as a mold. The flexibility of the elastomer enables the negative replica  602  to conform to the curved surface  603 . After UV curing, the mold  602  can be removed yielding a MLA  204  on a curved surface. 
         [0062]    Any efficient holographic material can be used to fabricate the holographic screen  114 . In order to produce a colour screen, a holographic material presenting a polychromatic sensitivity could be used. For example, the holographic film is sensitive to red, green and blue light. It is then possible to obtain holographic screens diffracting efficiently at several wavelengths by recording either sequentially or simultaneously the hologram with different wavelengths. Another method is to record each holographic film with one wavelength and subsequently place the films on top of each other. In this last case, different holographic materials with different spectral sensitivity could be used. 
         [0063]    A holographic film can be laminated onto transparent substrates having either flat or cylindrical surfaces. In the case of spherical surfaces, a liquid photopolymer is necessary as a flat sheet is not compliant onto such surface. 
         [0064]    It is preferable to place the holographic film  111  in close proximity to the MLA  204  during the holographic recording such that multiple interferences from different lenslets  205  are avoided at the holographic film  111  plan. To achieve this, for the case of a curved screen, the radius of curvature of the holographic screen needs to be similar to the radius of curvature of the replicated MLA  204 . 
         [0065]    In order to obtain the optimal diffraction efficiency, the intensity of the reference  201  and object  202  beams, on the holographic film  111 , should be nearly equal to generate high interference fringes contrast. 
       Proof of Principle Demonstration and Measurements 
       [0066]    As a proof of principle, colour holographic screens have been fabricated on cylindrical surfaces using the setup illustrated in  FIG. 7 . A red  643  nm laser diode, green 532 nm DPSS laser and blue 458 nm Argon laser are used to record colour holograms. The red and green beams are combined with the dichroic mirror DM 1  prior being spatially filtered by the microscope objective OBJ 1 , a single mode optical fibre and a 40 mm collimating lens (L 1 ). In a similar way, the 458 nm beam is spatially filtered by the microscope objective OBJ 2 , a single mode optical fibre and a 40 mm collimating lens (L 2 ). Red, green and blue beams are combined with the dichroic mirror DM 2 . Each beam can be controlled with shutters separately. The polarizing beam splitter PBS splits the incoming beam light into reference and object beams. The intensity ratio between object and reference beam and intensity ratio between wavelengths can be adjusted using the half wave plates HWP 1 , HWP 2  and HWP 3  and modifying the coupling inside the optical fibres. Both object and reference beams are expanded by lenses L 3 , L 4 , L 6  and L 7 . The reference beam is transmitted through the lens L 5  before it is incident on the centre of the holographic film at an angle of 45° with a numerical aperture of 0.3 so as to mimic the illumination conditions of a laser projection system (picoprojector) mounted on the side of the eyewear. The object beam is transmitted through a 60 mm focal length condenser lens L 8  and the MLA replicated on a curved surface. 
         [0067]    The holographic film used is a BAYFOL® HX photopolymer provided by Bayer. The film consists of a 16 μm thick photopolymer with polychromatic sensitivity sandwiched in between a 40 μm thick protective cover film and a 175 μm thick substrate. The photopolymer surface was then laminated on the convex side of a 2.5 mm thick cylindrical surface cut from a DURAN® tube having an outer radius of curvature similar to the radius of curvature described by the position of the top of each lenses within the MLA. 
         [0068]      FIG. 8  illustrates the setup to demonstrate the capabilities of the fabricated hologram to act as a transflective screen. A SHOWWX+™ Laser Pico Projector from MicroVision is used to display information on the holographic screen. As the commercial projector provides sharp images from 500 mm onwards, a 50 mm focal length lens is placed at the output of the projector to obtain an image size on the holographic screen corresponding to the field of view of our imaging system. A 1/2.5″ board CMOS colour camera together with a 7.5 mm focal length Sunex camera lens is used as an artificial eye. This camera system provides a 55° field of view. 
         [0069]    A contact lens (as described above) is placed in front of the camera lens. At its centre, the contact lens has a 1 mm diameter lens of focal length 29 mm. The central part of the contact lens collimates the light coming from the holographic screen placed at 29 mm from the contact lens while light transmitted by the outer part remains unaltered. A polarizer is placed at the centre of the contact lens to allow only the polarized light from the projector to be transmitted at the centre of the lens. A polarizer oriented perpendicular to the polarizer placed at the centre of the contact lens blocks display light on the outer part of the contact lens. This is illustrated in  FIGS. 8(   b ) and  8 ( c ) where the contact lens is imaged under linearly polarized light for two different angle of rotation. Light is blocked and transmitted respectively by the central and outer part of the contact lens in  FIG. 8(   b ), while the reverse is observed on rotating the contact lens by 90° in  FIG. 8(   c ). 
         [0070]      FIG. 9  shows images taken from the display system.  FIG. 9   a ) is taken without any projected image on the holographic screen, showing that there is virtually no parasitic effect in the see-through vision.  FIG. 9   b ) is obtained with an image projected on the holographic screen. It can be seen that both the see-through vision and added information are both in focus.  FIG. 9   c ) is obtained with an image projected on the holographic screen and the see-through vision is partially blocked. Large contrast, good brightness homogeneity and vision over the 55° field of view of the imaging system are observed.