Abstract:
Multiple pairs of substrate-guided wave-based holograms (SGWHs) are laminated to a common thin substrate to form a transparent substrate-guided wave-based holographic CHMSL (SGWHC) that diffracts playback LED illumination over a wide angular range. This device is made pursuant to a technique that includes the steps of recording a first set of SGWHs with one setup, that upon playback, will couple and guide the diffracted light inside the substrate, and a second set of SGWHs recorded with another setup, that will diffract and couple the guided light out.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject invention related generally to holograms for center-high mounted stop lights (CHMSL) for vehicles, and more particularly to holograms that efficiently meet the required transparency, brightness and angular coverage and to exposure techniques for recording such holograms. 
         [0002]    In the United States, Federal regulations require CHMSLs on most motor vehicles in addition to the standard “tail light” stoplights on the rear of the vehicle. CHMSLs are intended to maximize the visibility of the “brake lights” to drivers behind the CHMSL-equipped vehicle, and are usually mounted on or above the rear window of said vehicle. 
         [0003]    Pre-CHMSL high-mounted stoplights found in the prior art comprise a standard lenticular lens and an incandescent bulb or light emitting diode (LED) mounted in a housing that is usually secured near the top or bottom of a motor vehicle&#39;s rear window. However, such stop light units have bulky housings that partially obscure the rearward vision of the vehicle&#39;s driver. Additionally, limitations on the aesthetic and functional design of the automobile are imposed. Some other center high-mounted stoplights found in prior art have been also integrated into automobile body features such as trunks, airfoils, and roofs, which, to a limited degree, substantially reduce or remove the driver&#39;s vision problem. However, such high-mounted stoplights are unnecessarily complex and may impose further limitations on the functional and cosmetic design of vehicles. 
         [0004]    There were attempts to make transparent CHMSL based on transmission holograms. The illumination lamp was bulky and there was probability that the light beam could be blocked from inside the car. A holographic CHMSL based on a single hologram is known. Such a holographic CHMSL, couples out the light trapped in the substrate by edge lit. This geometry has a well-known disadvantage of significant aberrations and dispersions intrinsic to this type of holographic optical system, and resulted in a non-uniform output beam. 
         [0005]    One benefit of the subject invention is high light throughput. 
         [0006]    Another benefit of the subject invention is high transmission of implemented holograms that don&#39;t create obscuration of the driver&#39;s rear view. 
         [0007]    Also, flexibility in terms of direction and shaping the beam emitting from the CHMSL is yet another benefit. 
         [0008]    Another benefit of the subject invention is low aberration and distortions that allow images to be included in CHMSL. 
         [0009]    Another benefit of the subject invention is more uniform light coupled out to the viewer. 
         [0010]    Another benefit of the subject invention is flexibility in recording and using either reflection or transmission SGWH. 
         [0011]    Still another benefit of the subject invention is adjustability to the different LED, robustness and environmental stability. 
       SUMMARY OF THE INVENTION 
       [0012]    A substrate-guided wave-based transparent holographic CHMSL (SGWHC) with multiple pairs of holograms is disclosed herein. Two holograms are capable of compensating for aberrations in each individual hologram, so the emitted light from the present invention is more uniform, and its beam quality is more predictable. 
     
    
     
       CONCISE DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other benefits of the subject invention, together with additional features contributory thereto and advantages occurring therefrom will be apparent from the following description of several embodiments of the invention when read in conjunction with the accompanying drawings. 
           [0014]      FIG. 1  shows beam probation in a substrate with a pair of holograms. 
           [0015]      FIG. 2  is a schematic for recording hologram H1. 
           [0016]      FIG. 3  is a schematic for recording hologram H2 which is retrieved by hologram H1 guided beam as one of the recording beams. 
           [0017]      FIG. 4  shows a second embodiment for recording hologram H2. 
           [0018]      FIGS. 5  ( a ), ( b ) and ( c ) show schematics of a substrate-guided wave based stop light. 
           [0019]      FIG. 6  ( a ) shows a coupled out beam without an image. 
           [0020]      FIG. 6  ( b ) shows a coupled out beam with an image (arrow). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  demonstrates the beams propagation in operating one pair of reflection substrate-guided wave-based holograms (SGWHs)  10  and  11 . The substrate  19  is acrylic, polycarbonate, glass or any transparent material with the index of refraction close to the hologram index of refraction. A tiny 200 mW red LED  12  with required beam divergency (˜120°) and peak wavelength −635 nm, illuminates reflection substrate-guided wave-based hologram (SGWH) H1  11  with the size −20 mm×20 mm (¾″×¾″) at Bragg conditions to maximize the diffraction efficiency (DE) of the beam coupled due to the diffraction at TIR (total internal reflection) inside the transparent substrate. The substrate  19  can be of any width and length. In the embodiment of  FIG. 1 , the substrate  19  is −6 mm thick, 200 mm long and 350 mm wide (¼″×8″×14″) and positioned at a distance −3 mm (⅛″) from the PCB (printed circuit board) with at least one LED  12  illuminating hologram H1  10 . Coupled beams propagate along the substrate  19  at total internal reflection experiencing multiple bounces from the walls and expanding eventually to the size sufficient to illuminate a 50 mm×50 mm (2″×2″) reflection hologram H2  11  that couples the beam out at the required divergency and direction. 
         [0022]    Schematics of setups for recording reflection SGWHs H1  10  and H2  11  are shown in  FIGS. 2 and 3 . 
         [0023]      FIG. 2  shows H1  10  being recorded by two beams. Holographic recording thin film material  13  of size −25 mm×25 mm (1″×1″) is laminated to the acrylic substrate  19 . One recording beam created by the beam expander  18  is coupled inside the acrylic substrate at total internal reflection using glass block  20 . Before this beam is coupled inside the substrate at total internal reflection, it passes through the holographic recording thin film material  13 . This thin film material can be a wide variety of materials and can be chosen to support the specific application. A low birefringence polycarbonate or polyester such as polyethylene terephthalate or a polyethylene naphthalate film are materials in common use as substrates for holographic polymers. Examples of holographic materials that can be implemented are holographic polymers manufactured by Bayer (Bayfol®) and DuPont (Melinex®, Teijin®). The divergency of this beam is determined to illuminate the area 50 mm×50 mm (2″×2″) at a certain distance (as shown in  FIG. 5 , this distance is 6.5″ or ˜165 mm) after experiencing several required internal reflections from the substrate walls. Another recording beam is created by the beam expander  17  with input beam  27  ( FIG. 3 ) and is coming from air normal to the substrate. The divergency of this beam should match the divergency of the LED beam to maximize the beam  26  ( FIG. 2 ) coupling and light throughput. 
         [0024]    A guided beam  14  retrieved by the hologram H1  10  by illuminating it with beam expander  17  is used as guided beam for recording on holographic recording thin film material  13  which is 50 mm×50 mm (2″×2″) hologram H2  11  that is laminated to the substrate  19  as is shown in  FIG. 3 . Referring now to the beam expanding—means  15  ( FIGS. 3 ),  17  and  18  ( FIG. 2 ) are typically in the form of a microscope objective lens, and a pinhole spatial filter of a few micrometers aperture. The purpose of the beam expanding means  15 ,  17  and  18  is to expand beams and render them spatially homogenous. Recording beam  25  ( FIG. 3 ) is divergent impinging the holographic recording thin film material from air with the required beam divergency and incident angle to the surface. This beam  25  divergency as well as the incident angle to the surface can be of any required value in the horizontal and vertical directions. In the example on  FIG. 5  the beam  25  is shown normal to the substrate surface with divergent angle 20°. To homogenize the divergent beam coupled out by the hologram H2  11  as is shown in  FIG. 1 , the elliptical light shaping diffuser  23  ( FIG. 3 ) with crossed lenticular lenses  16  are stacked and positioned in the front of the hologram H2  11  as is shown in  FIG. 3 . 
         [0025]    In another embodiment ( FIG. 4 ), the recording setup for SGWH H2  11  is as shown in  FIG. 4 . Retrieved by the hologram H1  10 , beam  14  used in  FIG. 3  as one of recording beams for the hologram H2  11  is replaced by a beam  21  guided using glass block  24  with the same divergency and direction created by the beam expander  22 . This recording setup gives more flexibility in terms of laser energy used for recording, doesn&#39;t require precise alignment of H1  10  to direct the recording beam to the recording area, and doesn&#39;t depend on the quality of the beam retrieved by H1  10  and quality of the substrate material that is not of high optical quality tolerant to the LED  12  light, but sensitive to the coherent laser light. 
         [0026]    This design is flexible in terms of the width of the rear-mounted stop lights. In the configuration of  FIG. 5 , there are several pairs of holograms HOE 1  and HOE 2  that together create the emitted beam of 50 mm (2 inches) in height and width of 14 inches, thereby creating the possibility of including signs, letters, arrows etc. in the hologram HOE 2 . 
         [0027]    The light throughput is determined by the diffraction equivalency of holograms (theoretically &gt;90%) and efficiency of coupling the LED  12  light in the substrate  19  (theoretically up to 100%). Because the hologram H1  10  can be recorded with any beams, the recording beam angles can be chosen to match the LED  12  beam divergency and wavelength bandwidth to maximize the beam coupling. The central beam wavelength and the bandwidth can be adjusted by recording the hologram at an appropriate angle using existent red lasers (632.8 nm or 647 nm) and choosing the type of hologram (reflection that has narrower bandwidth ˜20 nm, or transmission with wider wavelength selectivity&gt;20 nm). 
         [0028]    Photos of played back hologram samples having two holograms are shown in  FIGS. 6  ( a ) and ( b ), where  FIG. 6(   a ) shows a coupled out beam without an image, and  FIG. 6(   b ) shows a coupled out beam with an arrow as the image. 
         [0029]    It will be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific forms shown or described herein. Various modifications may be made in the design, arrangement, and type of elements disclosed herein, as well as the steps of making and using the invention without departing from the scope of the invention as expressed in the appended claims.