Abstract:
A head-up-display (HUD) system includes at least one scanning laser projector operative to generate laser light, and a stacked array of multiple-switchable screens arranged relative to the projector to receive laser light generated by the projector, each screen of the stacked array of multiple-switchable screens spaced apart from one another and operative to switch between a transparent state and a diffusive state. A controller is operatively coupled to the stacked array of multiple-switchable screens, the controller configured to time sequentially switch each screen of the array from a transparent state to a diffusive state, wherein only one screen is switched to the diffusive state at any given time. An output of the projector is arranged at an angle or a distance from imaging optics succeeding the array of screens to prevent a specular beam emitted by the at least one scanning laser projector from intercepting the imaging optics succeeding the array of screens when all screens are in the transparent state.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to automotive displays and, more particularly, to Head-Up-Displays (HUD) that provide enhanced driving safety. 
       BACKGROUND ART 
       [0002]    Head-Up-Display (HUD) systems enable drivers to view crucial information without the need to look away from the road. Hence HUD systems have become an increasingly important component for automotive use to enhance road safety. 
         [0003]    Augmented Reality (AR), provides a three-dimensional (3D) viewing experience. A number of technologies have been developed to provide 3D AR, the most relevant of which are as follows: 
         [0004]    U.S. Pat. No. 8,521,411 B2 (Grabowski, Aug. 27, 2013) describes a HUD system that allows a continuous depth volumetric image by rapid mechanical scanning of a lens to re-image a laser beam into floating space. “Opt. Express 14, 12760-12769 (2006)” discloses a method of showing volumetric 3D images with a rapidly rotating mirror scanner to produce multiple slices of inclined images. These mechanical systems suffer from mechanical wear over time and may easily suffer from shock damage if they are not mechanically isolated from the vehicle. Further, the former system would only provide a cable image and hence can only display very limited information to the driver. 
         [0005]    US20040164927A1 (Suyama, Aug. 26, 2004) describes a system where a liquid crystal Fresnel lens with a rapidly variable focal length is used to re-image a display panel to produce a volumetric image. The lens used for this system requires a large variation in power and a fast switching speed. For the system to work, the lens would either need to be very thick, which would compromise its switching speed, or have very small Fresnel zone size, which would compromise its image quality. For the lens to switch fast enough to display large depth variation volumetric images, a special type of “dual-frequency” liquid crystals will be required. This type of liquid crystal has not yet been utilized in mass display products, may not necessarily meet automotive standards, and may be expensive to be used in large volume production. 
         [0006]    JP2004168230A describes an automotive HUD system that uses multiple pixelated liquid crystal panels and an ordinary backlight unit to display images at different depths. Such a system will have a very low optical efficiency, meaning the system will consume significant power to achieve high brightness required by automotive displays. The system would also have a limited display contrast, which is a known limitation of liquid crystal display panels with ordinary backlights. 
         [0007]    U.S. Pat. No. 5,764,317 (Sadovnik, Jun. 9, 1988) and U.S. Pat. No. 6,100,862 (Sullivan, Aug. 8, 2000) describe systems that produce volumetric images by using a projector to sequentially project a different image onto an array of switchable screens. 
         [0008]    However, these systems are only capable of displaying images at discrete depth planes. Since the screens can never be fully transparent, haze will become noticeable if the number of screens is increased in attempt to display a pseudo-continuous volumetric image. 
         [0009]    U.S. Pat. No. 4,670,744 (Buzak, Jun. 2, 1987) describes a system that uses a stack of cholesteric liquid crystal as switchable mirrors to change the optical distance between a display panel and the observer. The switchable reflectors are highly dependent on wavelength and angle of the incident light, making it unsuitable for full-color displays. 
         [0010]    Currently there is no augmented reality technology that can achieve low haze, can be manufactured at a relatively low cost, and provide continuous volume augmented reality that uses readily-mass-manufacturable materials that fit automotive performance and safety standards. 
       SUMMARY OF INVENTION 
       [0011]    The use of three dimensional augmented reality (AR) technologies would allow information being displayed to be integrated into the background traffic. An apparatus in accordance with the present invention relates to a Head-Up-Display (HUD) system suitable for automotive use and augmented reality applications. 
         [0012]    The solution aims to solve one or more problems in the prior art, such as haze, switching speed, color performance, optical efficiency, cost to manufacture, the need for the system&#39;s materials&#39; performance to fit automotive requirements, and the need for the system&#39;s image quality and safety to automotive standard. 
         [0013]    According to one aspect of the invention, a head-up-display (HUD) system includes: at least one scanning laser projector operative to generate laser light; a stacked array of multiple-switchable screens arranged relative to the projector to receive laser light generated by the projector, each screen of the stacked array of multiple-switchable screens spaced apart from one another and operative to switch between a transparent state and a diffusive state; and a controller operatively coupled to the stacked array of multiple-switchable screens, the controller configured to time sequentially switch each screen of the array from a transparent state to a diffusive state, wherein only one screen is switched to the diffusive state at a given time. An output of the projector is arranged at an angle or a distance from imaging optics succeeding the array of screens to prevent a specular beam emitted by the at least one scanning laser projector from intercepting the imaging optics succeeding the array of screens when all screens are in the transparent state. 
         [0014]    According to another aspect of the invention, a head-up-display (HUD) system includes: a stacked array of screens, each screen of the stacked array screens spaced apart from one another and comprising transparent display panels including pixels capable of providing full image resolution; and a controller operatively coupled to the stacked array of screens, the controller configured to time sequentially display an image on each screen of the array. 
         [0015]    According to another aspect of the invention, a head-u-display includes at least one scanning laser projector operative to generate laser light; a stacked array of multiple-switchable screens arranged relative to the projector to receive laser light generated by the projector, each screen of the stacked array of multiple-switchable screens spaced apart from one another and operative to switch between a transparent state and a diffusive state; a first variable power lens arranged relative to the stacked array of multiple-switchable of screens to receive an image from the stacked array of multiple-switchable screens, the first variable power lens having a variable focal length; and a controller operatively coupled to the stacked array of multiple-switchable screens, the controller configured to time sequentially switch each screen of the array from a transparent state to a diffusive state, wherein only one screen is switched to the diffusive state at a given time. 
         [0016]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    In the annexed drawings, like references indicate like parts or features: 
           [0018]      FIG. 1  is a schematic diagram of an exemplary system in accordance with an embodiment of the invention. 
           [0019]      FIG. 2  is a schematic diagram illustrating a position of a laser beam waist within/near a stack of LC screens. 
           [0020]      FIG. 3  is a schematic diagram illustrating detailed structure of an exemplary variable power lens. 
           [0021]      FIG. 4  is an electronic signal diagram for operating an apparatus in accordance with the invention. 
           [0022]      FIGS. 5 a  and 5 b    are schematic diagrams showing a virtual image formed by an LC lens. 
           [0023]      FIGS. 6 a  and 6 b    are schematic diagrams illustrating an optical path of a specular laser coming directly from a projector in possible instances where none of the screens are fully diffusive. 
           [0024]      FIG. 7  is a schematic diagram illustrating another embodiment of an apparatus in accordance with the invention, the apparatus not having a variable power lens. Also shown in  FIG. 7  is the optical path of the specular laser emitted by the projector. 
           [0025]      FIGS. 8 a  and 8 b    are schematic diagrams illustrating an apparatus in accordance with another embodiment of the invention, where the switchable projector screen is pixelated for low temperature operation 
           [0026]      FIG. 9  is a schematic diagram showing advantages of a hybrid spatial-temporal multiplexed system. 
           [0027]      FIG. 10  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention with backscattering geometry that eliminates beam blocks. 
           [0028]      FIG. 11  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention, where the laser beam is blocked. 
           [0029]      FIG. 12  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention, where the variable power lens, instead of having a continuously variable focal length, has multiple discrete focal lengths that can be refocused to. This creates a combinatorial number of possible positions where the virtual image can be displayed. 
           [0030]      FIG. 13  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention, the apparatus using multiple transparent LCDs/OLED instead of switchable scattering screens. 
           [0031]      FIG. 14  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention, the apparatus using multiple projectors, where each projector projects different images onto one or more switchable screens. 
           [0032]      FIG. 15  is a schematic diagram illustrating an apparatus in accordance with another embodiment of the invention, the system using an interference filter to attenuate a specular laser beam. 
           [0033]      FIG. 16 a    illustrates a luminance profile of a PDLC screen in a scattering state. 
           [0034]      FIG. 16 b    illustrates a transmission profile of an interference filter at laser wavelength. 
       
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
           11 : Scanning Laser Projector according to the first embodiment 
           12 : Lens according to the first embodiment 
           13 : PDLC screens according to the first embodiment 
           13   a - d : PDLC screens at different positions. 
           14 : Mirrors 
           15 : Liquid crystal lens according to the first embodiment 
           15   a : Liquid crystal filling 
           15   b : Transparent polymer 
           15   c : Indium Tin Oxide 
           15   d : Substrate of LC lens cell 
           15   e : Polarizer 
           16 : Imaging lens 
           17 : Combiner of the HUD system 
           18 : Driver/Observer 
           19 : Laser beam block 
           20 : Volume where the continuous sets of virtual images can be created by the screens  13   a - 13   d  and the LC lens  15 . 
           21 : Virtual image according to the first embodiment 
           22 : Instantaneous cross section of laser beam according to the first embodiment. 
         a: uncollimated beam directly from the projector 
         b: collimated laser beam with beam waist focus to a position within the stack of PDLC screens. 
           23 : Electronics for the LC lens according to the first embodiment 
           24 : PDLC driver according to the first embodiment 
           25 : Graphics signal sent to the projector 
           26 : Synchronization Unit according to the first embodiment 
           30   a - d : Pixelated PDLC screens according to the third embodiment 
           31 : Lower part of the virtual image according to the third embodiment 
           32 : Upper part of the virtual image according to the third embodiment 
           40 : PSCT screens used in backscattering geometry according to the forth embodiment 
           41 : Partially transparent mirror according to the fifth embodiment 
           50 : Polarization preserving switchable screens according to the fifth embodiment 
           51 : Absorbing Polarizer according to the fifth embodiment 
           60 : A variable power lens capable of refocusing to a discrete number of focal lengths according to the sixth embodiment. 
           70   a - d : An array of transparent display panels according to the seventh embodiment 
           80   a - c : Multiple projectors, where each of them may project a different image onto one or more screens according to the eighth embodiment. 
           81   a - d : PDLC screens according to the eighth embodiment. 
           90 : Interference filters according to the ninth embodiment. 
           91 : Scattering profile of PDLC screens according to the ninth embodiment. 
           92 : Transmission profile of interference filter at laser wavelength according to the ninth embodiment. 
           93 : PDLC screen in scattering state. 
           100 : Beam waist of laser according to the first embodiment. 
           101 : Rayleigh length of laser according to the first embodiment. 
           102 : Distance between one of the virtual images within the continuously possible set of images  20  and the driver  18 . 
           103 : Distance between virtual image  21  and the driver  18 . 
           104 : Distance between virtual image  31  and the driver  18 . 
           105 : Distance between virtual image  32  and the driver  18 . 
           106 : Eye-to-eye distance of an ordinary driver 
           107   a - c : The various possible focal lengths of the lens  60   
           108 : Optical axis of the imaging optics following the switchable screens such as  15 ,  16 , and/or  17   
           109 : Offset distance of the projector from the optical axis  108   
           110 : Offset angle of the projector from the optical axis  108   
           111 : Specular laser beam paths in the case where all switchable screens are transparent. 
       
     
       DETAILED DESCRIPTION OF INVENTION 
       [0086]    An apparatus in accordance with the present invention can include a laser projector, a stacked array of electrically switchable screens, and a variable power lens. A geometry between the components of the apparatus can be arranged such that an exit pupil of the laser projector will never be visible to the driver even when all electrically switchable screens are simultaneously switched transparent to the transparent state. 
         [0087]    The laser projector can be, for example, a laser Micro-Electro-Mechanical Systems (MEMS) scanning projector where the projected image has a large depth of focus and does not require precise focusing. The projector projects an image onto a stack of several switchable diffusing screens, which can be re-imaged by a variable focal lens and other fixed optics to produce a virtual image that appears to be floating in real space at a continuously variable distance from the driver. 
         [0088]    Each of the screens in the system is capable of independently switching between a transparent and a diffusive state via an applied voltage. In order to reduce Fresnel reflections from the surface of the screens, the screens can be glued to refractive index matching material blocks or the screens&#39; surface may be anti-reflection coated. Depending on the availability, cost, image quality and switching speed requirements, the screens can be made from materials in the known arts, such as Polymer Dispersed Liquid Crystal (PDLC), Polymer Network Liquid Crystal (PNLC), Polymer Stabilized Cholesteric Texture (PSCT), or holographic switchable diffusers. 
         [0089]    The variable power lens can have a continuously variable dioptre power tuneable with electrical signals. The lens can be made of, for example, diffractive, surface refractive, gradient refractive, or reflective Fresnel lens. The lens can be immersed in nematic liquid crystals, PDLC with sub-wavelength sized domains, blue phase liquid crystal, or other known methods of electric-tuneable optical path thickness for light polarized in one or more directions. Alternatively, other known types of variable power lenses, including mechanical focusing lenses, mechanical mirrors, electro-wetting lens, and liquid lenses can also be used. 
         [0090]    During operation, each screen can be switched to a diffusive state time sequentially, and the projector forms a different image on the diffused screen. This is re-imaged by a variable focus lens and other subsequent optics such as a refractive lens and a partially transparent combiner mirror, resulting in a virtual image some distance away from the driver. 
         [0091]    The projector&#39;s image frame is synchronized with the screens&#39; sequence as well as the power of the variable lens such that the system&#39;s operation allows a different image to be displayed at a continuous choice of depths. 
         [0092]    Because the virtual image is formed by a single optical system, it exhibits all depth cues such as focus, binocular convergence, and motion parallax, making it comfortable for long term viewing. Continuously variable virtual image depth in the system allows the virtual image to be seamlessly integrated into the traffic scene. 
         [0093]    The range of positions that can be formed by the full HUD system becomes a combinatorial set of images that can be formed from each screen by the variable power lens. Since switching speed of liquid crystal cells are inversely proportional to the square of the cell gap thickness, adding extra switchable screens into the system would provide quadratic improvement to system&#39;s switching speed while still providing the full depth range of virtual images. 
         [0094]    A first embodiment of a display in accordance with the invention is shown in  FIGS. 1-6  and described in the following paragraphs. The illustrated embodiment is a head-up-display system where the overall optical components are shown in  FIG. 1 . The display includes a laser scanning projector  11  followed by a lens  12  and a stacked array of multiple-switchable screens  13 . The laser scanning projector  11  can be a Micro-ElectroMechanical Systems (MEMS) which allows a high resolution image to be displayed over a large depth of focus. A controller  10  is operatively coupled to the projector  11  and the stacked array of screens  13  and configured to operate the HUD system as described herein. 
         [0095]    Although the lens  12  in this embodiment is illustrated as a single element lens, it can also include several groups or elements. The stack of screens  13  are capable of being independently switched by the controller  10  between a transparent and a diffusive state by applying an electrical signal to each of them. The material used for the screen may be Polymer Dispersed Liquid Crystal (PDLC). The screen can also be made of other known materials that are capable of being used as switchable diffusers such as Polymer Stabilized Cholesteric Texture (PSCT), Polymer Network Liquid Crystal (PNLC), or holographic switchable diffusers. The number of screens N in the stack should be such that the amount of haze from the N-1 transparent screens would be acceptable under dark conditions such as night driving. 
         [0096]    The projector  11 , based on commands from the controller  10 , forms an image on one of the PDLC screens  13 . This is re-imaged by the subsequent optics in the system including, for example, mirrors  14 , a variable power lens  15 , other fixed optics  16 , and a combiner  17  to form a virtual image that appears to be some distance away from the driver  18 . Depending on the required image quality, the fixed optics  16  may or may not be present and could be placed before or after the variable power lens  15 . The fixed optics could include one or more spherical, aspherical, freeform, and diffractive elements in order to reduce distortion and optical aberrations in the virtual image. 
         [0097]    The combiner  17 , for example, may be a piece of curved, high reflection dielectric coated partially transparent and partially reflective optical element. However, the combiner&#39;s shape may also be flat, curved, segmented, progressive powered, segmented prism, or Fresnel lens profiled and can also be metallic or holographic coated. 
         [0098]      FIG. 2  shows that the function of the lens  12  is to improve the collimation of the specular beam  22   a  from the projector to produce a collimated scanning laser beam  22   b.  A further function of the lens  12  is to control the beam waist radius  100  to be no larger than twice the effective pixel size of the projector. The beam waist radius is the distance from the beam axis where the intensity drops to 13.5% of the maximum value. Whereas the effective pixel size of is given as the minimum of either the projector&#39;s pixel size on the screen or the resolution resolvable by the subsequent optics (e.g.,  15 ,  16 ,  17  in  FIG. 1 ). A further function of the lens  12  is to position the beam waist to be within (between the first screen and the last screen of) the PDLC stack, and to make the Rayleigh length  101  of the laser beam larger than or close to the axial span of all the PDLC screens. This allows an image with effective resolution of VGA or better to be displayed onto any screen without the need of an active focus modulator or a large physical system size. Although the lens  12  in the illustrated embodiment is a fixed lens, a second variable power lens can be used to actively focus the image onto the screen if the resolution of the image needs to be improved significantly beyond WXGA. 
         [0099]      FIG. 3  illustrates an exemplary structure of the variable power lens  15  originally shown in  FIG. 1 . The preferred lens is a liquid crystal (LC) lens with a continuously variable focal length. The component includes a Fresnel lens  15   b  moulded from a transparent polymer, which is embossed within liquid crystal  15   a . The liquid crystal  15   a  should meet the performance requirements for the automotive industry. The liquid crystal cell is sandwiched between two layers of transparent electrodes  15   c  and substrate  15   d.  The liquid crystal  15   a  is rubbed in a way such that applying a voltage to the electrodes will change the effective refractive index of the liquid crystal  15   a  as seen by incident light polarized by a polarizer  15   e.  This creates a variable difference in refractive index between the polymer of the Fresnel lens  15   b  and the liquid crystal  15   a,  meaning that the power of the variable power lens  15  can be continuously varied by adjusting the amplitude of the applied voltage. However, other known configurations of liquid crystal lenses and known variable power lenses can also be used in the system. This includes, for example, the use of liquid lenses, switchable diffractive lenses, mechanical methods of varying the power of a lens, as well as known methods of varying the optical distance between a lens and the screen. 
         [0100]    The profile of the Fresnel lens  15   b  can take the form of any shape, including spherical, aspheric, and prisms. The substrate  15   d  can be either flat or curved. However the longest linear dimension of the variable power lens  15  , after accounting for the magnification of the subsequent optics  16  and  17 , should be larger than the eye-to-eye distance  106  (See  FIG. 5 ) of an ordinary driver  18 . In this way, a virtual image with the size at least as large as the lens  15  will be simultaneously visible to both eyes. 
         [0101]      FIG. 4  shows the exemplary HUD system at the level of electronics signal. In the preferred HUD system, graphical signal  25  sent to the projector  11  is time synchronized with a switching sequence generated by a voltage controller  24  and provided to the screens  13   a - 13   d  as well as voltage  23  for controlling the focal length of the variable power lens  15 . The graphics signal  25  can be sequences of image frames, but can also be any general instructions to steer the laser beam to arbitrary positions. A synchronization unit  26 , which can be implemented at the software level using the controller  10  or other processing device, coordinates the graphics signal  25  provided to the projector  11 , the voltage controller  24  for generating the switching sequence of the screens  13   a - 13   d  and the voltage  23  for controlling a focal length of the variable power lens  15 . 
         [0102]      FIG. 5 a - b    shows the operation of an exemplary HUD system. During operation, each screen  13  is switched by the controller  10  diffusive time sequentially, but only one screen (e.g.,  13   b ) is switched diffusive at any given time, allowing a virtual image  21  to be formed in space at a distance  103  from the driver as depicted in  FIG. 5   a.    
         [0103]    Meanwhile, the controller  10  may continually sweep the power of the lens  15  from minimum to maximum back and forth (e.g., the focal length of the variable power lens may continually sweep between a first focal length and a second, different, focal length).  FIG. 5 b    shows that varying the lens&#39;s power allows the distance  102  of the virtual image  21  to be continuously varied over a limited range. This allows the image formed by each PDLC screen  13   a,    13   b,    13   c,  and  13   d  to be re-imaged to anywhere within a respective continuous volume  20   a,    20   b ,  20   c,  and  20   d.  The switching order of the screens and the voltage signal applied to the LC variable power lens  15  could also depend on the content being displayed. The image volumes  20   a - 20   d  may or may not overlap. Having a non-overlapping volume means that a weaker LC variable power lens  15  can be used hence leading to a faster switching speed; whereas having an overlapping volume may allow reduction in image flickering in cases where the switching cycle time of the screens exceeds the switching time of the LC variable power lens  15 . 
         [0104]    The combinatorial effect of the LC variable power lens  15  and the PDLC screen means that the LC variable power lens  15  would not be required to very thick while still allowing the image to be displayed within a large volume. Since the switching time of liquid crystal-filled cells scales with the square of the cell gap thickness, increasing the number of screens would improve the switching speed of the system quadratically. This significant improvement enables a full continuously volumetric system to be made from automotive grade liquid crystals—which could not be achievable in the known prior art. Secondly, the weak LC variable power lens  15  would allow the embossed Fresnel lens structure to have a smaller gradient, leading to lower fabrication tolerance requirements and costs, as well as better image quality due to reduction in liquid crystal splay, bend, and twist that arises from non-uniform cell gaps. 
         [0105]      FIG. 6 a    shows an exemplary safety feature which can help the system meet automotive standards. To prevent the specular laser beam from being visible to the driver under possible instances where none of the screens are fully diffusive, the laser projector  11  can be offset from the subsequent optics.  FIG. 6 b    shows the same optical arrangement as  FIG. 6 a    but with the fold mirrors  14  removed to show more clearly the possible arrangement of the projector  11 . The projector  11  can be offset at a distance  109  and/or an angle  110  from the optical axis of the subsequent optics (e.g.,  15  and  16 ) such that the scanning specular beam path  111  would never intercept these components (e.g.,  15  and  16 ). If all screens are simultaneously transparent, the specular laser beam will be directly incident onto a beam block  19  or onto other non-glossy elements such that none of the specular beam will exit the system through other optics such as  15  and  16 . The tilt angle of the projector should be just large enough for the specular beam not to escape from the HUD enclosure, as a tilt angle that is too large (e.g., &gt;20 degrees) may introduce haze in the image. 
         [0106]    Subsequent embodiments described below are made in reference to the first embodiment and only the differences between the subsequent embodiments and the first embodiments are discussed. 
         [0107]    A second embodiment of the system is shown in  FIG. 7 . In the case where continuously variable image depth is not required, the variable power lens  15  from the first embodiment is not needed and thus is omitted from  FIG. 7 . In this case, the projector&#39;s specular beam is still permanently hidden from the driver&#39;s access. 
         [0108]      FIGS. 8 a -8 b    illustrate a third embodiment that includes a laser projector  11 , switchable projector screens  30   a - 30   d  (collectively referred to as switchable projector screen  30 ), and variable power lens  15 . This embodiment differs from the main embodiment as the switchable projector screens  30  are pixelated or partitioned such that only a portion of the same screen can be switched diffusive at any given time (rather than the whole screen being switched uniformly diffusive or transparent). The material of the screen could be, but is not limited to, PDLC. 
         [0109]    This system enables images  31 ,  32  to be perceived by a driver  18  at different depths  104 ,  105 , where the images are displayed to the driver simultaneously without the need of temporally switching on and off different screens. 
         [0110]      FIG. 9  is the same embodiment shown in  FIGS. 8 a -8 b    and illustrates how the system can benefit from spatial multiplexing by showing a dashboard image  31  in one screen&#39;s partition  30   d  and traffic information image  32  on another screen&#39;s partition  30   b.  This would be advantageous in cold conditions where the switching speed of the screens may slow down significantly. It should be noted that, unlike JP2004168230A, the image quality in this embodiment does not depend on the size of the pixels. Instead, the image quality of this system depends solely on the projector&#39;s resolution and the laser beam waist. This means the screens can be coarsely pixelated to achieve spatial multiplexing. Since full volumetric images displayed in automotive HUDs usually contain large patches of blank space, coarsely pixelated screens would allow high resolution images to be displayed without expensive requirements in computational power. 
         [0111]      FIG. 10  illustrates a fourth embodiment where the projector  11  is arranged to be physically closer to the screen  40   d  than all other screens ( 40   a - c ), where  40   d  represent the screen that is optically closest to the driver  18 . In this embodiment, the virtual image comes from back scattered light from the switchable screens  40 . The screens  40  can be made of known materials with a satisfactory backscattering efficiency such as Polymer Stabilized Cholesteric Texture (PSCT). The last screen  40   a  (i.e., the screen optically furthest from the driver  18 ) may be replaced by a permanent diffuser. This means if the screens are anti-reflection coated, there will be no need to use a beam block in the system. 
         [0112]    In addition, one of the mirrors  14  from the first embodiment can be replaced by a partially transparent mirror or a beam splitter  41  to allow the projector&#39;s beam to reach the screens, but the mirror can also remain fully reflective if it is not obscuring the projector&#39;s light. This embodiment allows more flexibility in the folding up of the HUD system without potentially exposing the projector&#39;s exit pupil to the driver when none of the screens are fully diffusive at any instance. 
         [0113]      FIG. 11  illustrates a fifth embodiment where a stack of switchable screens  50  is used instead of the screens  13  proposed in the first embodiment. When the screens  50  are in the transparent state, the polarization state of light transmitted through the screens is preserved (i.e., the polarization of the light passing through the screen is unaltered or altered by a small amount such that a specular beam transmitted through the polarizer  51  would be safe for the driver). This allows polarized light coming from the projector  11  to be simply blocked by a polarizer  51  if all the screens are clear. Therefore, the specular laser beam is not visible to the driver and thus additional measures (e.g., off-axis geometry) to prevent such visibility are not needed. The screens  50  can be PDLC screens made of low birefringence polymers, switchable holographic diffusers, or any other known switchable screens that preserves polarization in the transparent state. 
         [0114]      FIG. 12  illustrates a sixth embodiment where, instead of the variable power lens  15  as proposed in the first embodiment, a lens  60  capable of refocusing to a discrete number of focal lengths  107   a,    107   b,    107   c  is used. The lens  60 , when combined with the stack of switchable screens, would be capable of producing a combinatorial number of virtual images. The large number of virtual images could be spaced closely enough such that their discrete depths are not distinguishable to an ordinary observer. The lens could be a diffractive type lens, birefringent lens, or other known tuneable multi-focal length lenses. An advantage of this embodiment is that it may be less costly to manufacture and have a faster switching speed compared to the first embodiment. 
         [0115]      FIG. 13  illustrates a seventh embodiment where, instead of the switchable screens  13 , a stack of transparent display panels  70  containing pixels with full image resolution are used. The transparent display panels  70  can produce a continuous image depth and/or contribute to the combinatorial number of possible virtual image depth planes. These screens  70  can be a stack of transparent liquid crystal panels, organic light emitting diode (OLED), inorganic LED arrays, wave guide displays, or other known methods of non-projector type transparent displays. 
         [0116]      FIG. 14  illustrates an eighth embodiment where, instead of using only one laser projector, two or more projectors  80   a - 80   c  are used in the system. Each of the projectors  80   a - 80   c  may be capable of simultaneously projecting different images onto one or more screens  81   a - 81   d.  The projectors can be any known types of projectors, including laser scanning MEMs projector, Digital Light Processing (DLP) projector, LCD projectors, or Liquid Crystal on Silicon (LCoS) projectors. Light from each projector could be incident onto the screens at an oblique angle to avoid unwanted scattering from other screens to reduce haze. Because multiple projectors are used, high quality images can be achieved without the need to actively refocus images onto different screens. 
         [0117]    In addition, if the screens are arranged such that they are not occluding each other from view, rapidly switching the variable power lens  15  would allow multiple volumetric images to be simultaneously visible to the driver. This could be achieved by spatially rearranging the screens, or by pixelating/partitioning the screens as described in the seventh embodiment. 
         [0118]      FIG. 15  and  FIGS. 16 a -16 b    illustrate a ninth embodiment where an interference filter  90  with low transmission in the direction of the specular beam is used to attenuate the specular beam produced by the projector  11 . As such, the projector  11  could be aligned parallel to the viewing axis of the HUD, which would provide improved optical efficiency. The interference filter  90  could also be designed such that its transmission as a function of angle from the incident laser beam  92  ( FIG. 16 b   ) is complementary to the luminance profile  91  ( FIG. 15  and  FIG. 16 a   ) of the PDLC screen  93  in the scattering state. This would allow the user to see an image with uniform brightness independent from viewing position. The interference filter  90  could be designed for a single wavelength or multiple laser wavelengths. 
         [0119]    Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application. 
       INDUSTRIAL APPLICABILITY 
       [0120]    Industrial application will be mainly for automotive head-up-display systems. Application can be used in any vehicle for traffic information display. The HUD system can be fixed into the vehicle&#39;s dashboard by the automotive manufacturer or sold as individual components that could fit into any vehicles including uses for automotive training. The key advantage of the system is visual comfort. This is because the augmented reality display demonstrates all three dimensional depth cues, allowing information to be displayed at a variable distances as seen from the driver.