Patent Publication Number: US-2013250381-A1

Title: Lighting optical system

Description:
This application claims the priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2012-063915 filed on Mar. 21, 2012, which is hereby incorporated in its entirety by reference. 
     TECHNICAL FIELD 
     The presently disclosed subject matter relates to lighting optical systems, and in particular, to a lighting optical system for a vehicle such as an automobile or the like. 
     BACKGROUND ART 
     Conventional lighting optical systems for use in a vehicular headlamp have been known to utilize a noncoherent system light source such as a halogen lamp, a high intensity discharge (HID) lamp, a light emitting diode (LED), and the like. Such optical systems for use in a vehicular headlamp are typically capable of forming a high-beam light distribution pattern, a low-beam light distribution pattern, and the like. The conventional lighting optical system for a vehicular headlamp can form a light distribution pattern by means of a reflector arranged around the light source, and the like to reflect the same toward a projector lens arranged in front thereof, thereby projecting the light distribution pattern forward while inverting the same. This type of lighting optical unit can be found, for example, in Japanese Patent Application Laid-Open No. 2008-152980. 
     Another lighting optical system for a vehicular headlamp can be seen in, for example, Japanese Patent Application Laid-Open No. Hei. 05-139203, in which a holographic liquid crystal element is arranged in front of a projector lens so that the illumination area by the headlamp is expanded by refraction to form a desired light distribution pattern. 
     In the former case, a light distribution control mechanism utilizing a motor, and the like mechanism should be incorporated therein. When doing so, the lighting optical system incorporating such a mechanism for forming various required light distribution patterns is difficult to be miniaturized. Furthermore, it is difficult to suppress the production costs. 
     In the technology in which the illumination area is expanded by means of a holographic liquid crystal element, the switching operation may not be achieved by an electrical means, and therefore a switching mechanism (mechanical means) for switching a normal state (not expanded) and an expanded state can be provided. Therefore, also in this case, the lighting optical system for a vehicular headlamp is difficult to be miniaturized. Furthermore, it is difficult to suppress the production costs. 
     SUMMARY 
     The presently disclosed subject matter was devised in view of these and other problems and features in association with the conventional art. According to an aspect of the presently disclosed subject matter, a lighting optical system can be electrically (instead of mechanically) controlled with regard to light distribution while the miniaturization of the optical system can be achieved with suppressed production costs. 
     According to another aspect of the presently disclosed subject matter, a lighting optical system can include: a light source configured to emit light beams; a holographic liquid crystal element configured to convert the light beams emitted from the light source to regeneration light beams forming a prescribed light distribution pattern or alternatively allow the light beams to pass therethrough as they are, in accordance with a voltage applied thereto; a wavelength converter configured to include a wavelength converting material which can be excited by the regeneration light beams from the holographic liquid crystal element and emit visible light beams; and a lens configured to project the visible light beams from the wavelength converter. 
     Herein, the wavelength converter can be a wavelength converting plate, or a phosphor plate including a phosphor as an example of the wavelength converting materials. 
     In the above lighting optical system, the holographic liquid crystal element can include a plurality of pixels separately applied with a voltage, and the plurality of pixels can convert the light beams emitted from the light source to regeneration light beams having different light distribution shapes in accordance with the applied voltage, respectively. 
     Furthermore, the lighting optical system with the above configuration can further include a reflecting member configured to reflect the light beams passing through the holographic liquid crystal element without any conversion so that the light beams is allowed to be incident on the wavelength converter. 
     Furthermore, in the lighting optical system with the above configuration, the light beams emitted from the light source can have a center wavelength of 450 nm or shorter, and the wavelength converter can absorb light beams at a wavelength range of from ultraviolet to blue light. 
     Furthermore, in the lighting optical system with the above configuration, the phosphor is included in the wavelength converter so that the phosphor is distributed therein in accordance with a luminance distribution of the regeneration light beams in terms of any one of its density of the phosphor contained in the wavelength converter and a thickness of the wavelength converter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other characteristics, features, and advantages of the presently disclosed subject matter will become clear from the following description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic cross-sectional view showing one exemplary embodiment of a lighting optical system made in accordance with principles of the presently disclosed subject matter; 
         FIG. 2  is a conceptual diagram illustrating the relationship between a holographic liquid crystal element and a phosphor plate in the lighting optical system of  FIG. 1 ; 
         FIG. 3  is a schematic cross-sectional view of the holographic liquid crystal element (refractive optical element) of  FIG. 1 , cut along its thickness direction; 
         FIG. 4A  is a conceptual diagram illustrating the light distribution state by the regeneration light beams regenerated on the basis of wavefront conversion information recorded on a wavefront conversion information recording area (pixel) of the holographic liquid crystal element of  FIG. 1 , and  FIG. 4B  is a schematic diagram illustrating one example of an optical sprit system interference exposure apparatus, both for describing how the period microstructure is formed in the wavefront conversion information recording area (pixel) of the holographic liquid crystal element (how to record the wavefront conversion information); 
         FIG. 5A  is a conceptual diagram illustrating the light distribution state by the regeneration light beams regenerated on the basis of wavefront conversion information recorded on a wavefront conversion information recording area (pixel) of the holographic liquid crystal element of  FIG. 1 , and  FIG. 5B  is a schematic diagram illustrating another example of the optical sprit system interference exposure apparatus, both for describing how the period microstructure is formed in the wavefront conversion information recording area (pixel) of the holographic liquid crystal element (how to record the wavefront conversion information); 
         FIG. 6A  is a schematic cross-sectional view of the lighting optical system illustrating the formation of the light distribution state when the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element is turned OFF (no voltage is applied) and the pixel (wavefront conversion information recording area) is turned ON (a voltage is applied), and  FIG. 6B  is a conceptual diagram showing the light distribution state achieved by the lighting optical system of  FIG. 6A ; 
         FIG. 7A  is a schematic cross-sectional view of the lighting optical system illustrating the formation of the light distribution state when the pixel (wavefront conversion information recording area) of the holographic liquid crystal element is turned ON (a voltage is applied) and the pixel (wavefront conversion information recording area) is turned OFF (no voltage is applied), and  FIG. 7B  is a conceptual diagram showing the light distribution state achieved by the lighting optical system of  FIG. 7A ; 
         FIG. 8A  is a schematic cross-sectional view of the lighting optical system illustrating the formation of the light distribution state when both the pixels (wavefront conversion information recording areas) of the holographic liquid crystal element are turned OFF (no voltage is applied), and  FIG. 8B  is a conceptual diagram showing the light distribution state achieved by the lighting optical system of  FIG. 8A ; 
         FIG. 9A  is a schematic cross-sectional view of the lighting optical system illustrating the formation of the light distribution state when both the pixels (wavefront conversion information recording areas) of the holographic liquid crystal element are turned ON (no voltage is applied), and  FIG. 9B  is a conceptual diagram showing the light distribution state achieved by the lighting optical system of  FIG. 9A ; and 
         FIG. 10  is a schematic cross-sectional view of a modification of the lighting optical system according to the present exemplary embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A description will now be made below to lighting optical systems of the presently disclosed subject matter with reference to the accompanying drawings in accordance with exemplary embodiments. 
       FIG. 1  is a schematic cross-sectional view showing a lighting optical system  100  made in accordance with the principles of the presently disclosed subject matter as one exemplary embodiment.  FIG. 2  is a conceptual diagram illustrating the relationship between a holographic liquid crystal element  3  and a phosphor plate  4  in the lighting optical system  100 . Note that every dimension, position, angle, and the like of each part illustrated in the drawings are shown for illustration purposes only and may be different from actual dimension, position, angle, and the like thereof. 
     The lighting optical system  100  can be, for example, a lighting unit for use in a projector type vehicular headlamp or the like, and can be configured to include a casing  60 , a light source  1 , a collimating lens  2 , a holographic liquid crystal element (or a refractive optical element)  3 , a phosphor plate (wavelength converter)  4 , a projector lens  5 , a mirror  7 , and the like. If necessary, a light shielding member  6  (forming a cut-off pattern) may be installed therein. (See  FIG. 10 .) The light source  1 , the collimating lens  2 , the holographic liquid crystal element  3 , the phosphor plate  4 , the projector lens  5 , and the mirror  7  can be held by the casing  60 . The holographic liquid crystal element  3  can be connected to a controller  36 . 
     The light source  1  can be a high-output laser such as a semiconductor laser diode (LD), example of which may include a blue laser diode (or deflection laser). As shown in  FIG. 1 , blue laser beam (illumination light)  10  can be emitted from the light source  1 , can pass through the collimating lens  2 , and then can be incident on the surface of the holographic liquid crystal element  3  in a direction with an angle of incidence  0 . In the present exemplary embodiment, the light source  1  can be a semiconductor laser diode emitting blue laser beams with a center wavelength of 405 nm. Note that the light source  1  may be a high-output light-emitting diode. Further, although the center wavelength is not limited to 405 nm and may be 488 nm or the like, the center wavelength can be 380 nm or higher in order to avoid any damage on the holographic liquid crystal element  3 . 
     The collimating lens  2  can collimate the laser beams  10  from the light source  1  while it can expand the beam diameter of the light beams  10 . Specifically, the collimating lens  2  can contribute to the formation of light distribution shape which is slightly wide in a horizontal direction, namely being a natural ellipse elongated in a lateral direction. 
     With reference to  FIG. 3 , the holographic liquid crystal element  3  can be a liquid crystal element having a liquid crystal layer  215  including a transparent polymer resin and a low-molecular liquid crystal material (a material capable of responding to an electric field). The liquid crystal layer  215  can include a regional distribution or a concentration distribution of the transparent polymer resin and the low-molecular liquid crystal material, so that the resulting refraction index distribution can have a planar or three-dimensional stripe structure (which will exert a wavefront conversion function). The holographic liquid crystal element  3  can further include transparent electrodes  202  and  212  arranged on arbitrary selected area (pixel) to be applied with a voltage. 
     The wavefront conversion function can be a function for converting short-wavelength laser beams (illumination light)  10  emitted from the light source  1  and having passed through the collimating lens  2  to regeneration light beams  11  having a given light distribution pattern (for example, light distribution state required for a vehicular headlamp) on the basis of wavefront conversion information. Here, the wavefront conversion information can be recorded by forming a refraction index distribution on the holographic liquid crystal element  3  as the planar or three-dimensional stripe structure. In the present exemplary embodiment different pieces of wavefront conversion information can be recorded on respective areas on the surface of the holographic liquid crystal element  3  (being the respective wavefront conversion information recording areas (pixels)  3   a  and  3   b ). Further, the respective wavefront conversion information recording areas (pixels)  3   a  and  3   b  can be separately applied with a voltage, or can be separately controlled to be turned ON/OFF. 
     In the present exemplary embodiment, not only the light distribution shape of the regeneration light beams  11  but also luminance distributions thereof can be generated. Specifically, the holographic liquid crystal element  3  can be processed on a nanoscale so that the blue laser beams can be provided with a light intensity distribution by the holographic liquid crystal element  3  in accordance with the applied voltages while a desired cut-off shape can be formed. 
     The method of producing the holographic liquid crystal element  3  will be described with reference to  FIG. 3  later. Further, the wavefront conversion information can be recorded by exposure with an optical division-type interference exposure apparatus  300  as shown in  FIG. 4B  or  5 B. The recording method will be detailed later with reference to  FIGS. 4B and 5B . 
     Note that the wavelength of laser light to be used when the wavefront conversion information is recorded to form the holographic liquid crystal element  3  can be as close to that of the laser light  10  emitted from the light source  1  as possible. Further, it is desired that the regeneration light beams  11  of the holographic liquid crystal element  3  be regenerated at a position of the phosphor plate  4  to be arranged in a subsequent stage. 
     The mirror  7  can be arranged in a stage subsequent to the holographic liquid crystal element  3  and at a position where, when the laser beam  10  having been incident on the holographic liquid crystal element  3  with the prescribed angle of incidence  0  pass through the holographic liquid crystal element  3  without any change, the laser beam  10  is incident thereon so as to be reflected to the phosphor plate  4  to be arranged in a stage subsequent to the mirror  7 . The mirror  7  can be composed of a mirror-finished mirror or one with weakly diffused property. In the present exemplary embodiment, the laser beam  10  having passed through the holographic liquid crystal element  3  “as is” can be used as the light beam for use in a vehicular headlamp, and accordingly, the mirror can be disposed at the position where the light beam reflected by the mirror  7  can be projected onto the phosphor plate arranged in the subsequent stage. However, if the lighting optical system  100  is not used for a vehicular headlamp (or the reflected light beam is not used for light for a vehicular headlamp), or if different lighting characteristics are desired, the position may not be the above-mentioned position. In addition, the mirror  7  can be replaced with a certain light absorber in accordance with the specification of the product. 
     The controller  36  can control the voltage to be applied to the holographic liquid crystal element  3 , thereby changing the wavefront conversion function of the holographic liquid crystal element  3  to be turned ON or OFF. This control can change the light distribution states. In the present exemplary embodiment, if a voltage is applied to the holographic liquid crystal element  3  (ON state), the laser beam  10  incident on the holographic liquid crystal element  3  passes the same as is, and is reflected by the mirror disposed between the holographic liquid crystal element  3  and the phosphor plate  4  to be projected onto the phosphor plate  4 . If a voltage is not applied (OFF state), the laser beam  10  incident on the holographic liquid crystal element  3  is changed to an optical image  11  with a prescribed light distribution on the basis of the wavefront conversion information recorded on the area where the laser beam  10  is incident, and then the optical image  11  is projected onto the phosphor plate  4 . 
     The phosphor plate  4  can be arranged in a subsequent stage to the holographic liquid crystal element  3  and can be disposed at or near (i.e., substantially at) the focal position of the projector lens  5 . Examples of the phosphor plate  4  may include one prepared by applying a phosphor  41  to a transparent substrate made of, for example, a resin or glass, one prepared by mixing a material for forming a transparent substrate (a resin, glass or the like) with a phosphor  41 . In the present exemplary embodiment, the phosphor plate  4  can be prepared by mixing a glass material with a phosphor  41  to be formed into a glass substrate. 
     The material for the transparent substrate of the phosphor plate  4  can be formed from a glass material or a resin material having high heat resistance and light resistance. Examples of the resin materials to be used may include acrylonitrile-butadiene-styrene (ABS) resins, silicone resins, polycarbonate resins, polystyrere resins, acrylic resins, and epoxy resins. 
     The phosphor  41  can be a material that can absorb, or can be excited by, light beams at wavelength regions ranging from UV to blue to emit (wavelength-convert to) visible light beams. Examples thereof may include a material that can absorb blue or UV light beams to emit yellow light beams or green light beams and red light beams. Examples of the material emitting yellow light may include a YAG-based phosphor material. Further examples thereof may include a silicate-based phosphor material, an aluminate-based phosphor material, a nitride-based phosphor material, a sulfide-based phosphor material, an oxysulfide-based phosphor material, a borate-based phosphor material, a phosphate-borate-based phosphor material, a phosphate-based phosphor material, and a halophosphate-based phosphor material. 
     The thickness of the phosphor plate  4  (or alternatively, the applied thickness of the phosphor when the phosphor is applied) and the density of the phosphor  41  can be appropriately optimized in accordance with the intensity of the light beams from the light source  1 . It is sometimes desirable that the color of the emitted light from the phosphor plate  4  be white. Therefore, the thickness and the density thereof can be set such that the blue light from the light source  1  is still contained in the resulting light beams. In addition, since the center area of the low-beam light distribution is brighter than the other area, the density of the phosphor at the center area can be made high or the thickness thereof at the center area can be made thick. The thickness of the phosphor plate  4  (or alternatively, the applied thickness of the phosphor when the phosphor is applied) and/or the density of the phosphor  41  can be changed in this manner in accordance with the luminance distribution of the regeneration light beams  11 , white light can be provided in all directions. 
     As shown in  FIG. 2 , the upper half of the phosphor plate  4  is composed of the phosphors  41  with white circles while the lower half of the phosphor plate  4  is composed of filled circles in order to represent the density of the phosphors  41 . Specifically, the lower half of the phosphor plate  4  corresponds to the center bright area of the low beam light distribution to represent the higher density of the phosphor  41  while the upper half thereof represents the lower density of the phosphor  41  being a darker area. Note that the lower part below the cut-off line, specifically, right lower part below the cut-off line does not include the phosphor  41 . Of course, the density of the phosphor can be appropriately changed in accordance with desired light distribution state. 
     The phosphor  41  can be illuminated with the regeneration light beams  11  being blue light beams projected onto the phosphor plate  4 , and the phosphor  41  can emit yellow light beams or green and red light beams (the light beams are mixed with remaining blue light beams of the light source  1  to generate pseudo-white light). Since the phosphor plate  4  is disposed at or near (i.e., substantially at) the focal point of the projector lens  5 , the white light image can be projected forward and inverted through the projector lens  5 . 
     The projector lens  5  can be a convex lens and can converge the regeneration light beams  11  emitted from the phosphor plate  4  (or the illumination light  10  having passed through the holographic liquid crystal element  3  as is), and then can project the light forward while inverting the same. 
     As discussed, the present exemplary embodiment with the above configuration can cause the controller  36  to control the voltage to be applied to the holographic liquid crystal element  3 , thereby converting the laser beam  10  into the prescribed regeneration light image  11  or allowing the laser beam  10  to pass therethrough as is. Thus, the different positions on the surface of the holographic liquid crystal element  3 , for example, the respective wavefront conversion information recording areas (pixels)  3   a  and  3   b  where different pieces of wavefront conversion information can be recorded, can be separately supplied with a voltage, thereby electrically changing the shape or the like of the projection image. 
       FIG. 3  is a schematic cross-sectional view of the holographic liquid crystal element (refraction optical element)  3  in the exemplary embodiment of the presently disclosed subject matter, when cut along its thickness direction. Hereinafter, the method of producing the holographic liquid crystal element  3  will be described. 
     First, a pair of glass substrates  201  and  211  which each have a transparent electrode (ITO, for example) formed thereon are prepared. Here, the glass substrates  201  and  211  can include the transparent electrodes  202  and  212  on the respective surfaces, respectively. Each of the glass substrates  201  and  211  can have a thickness of approximately 0.7 mm and can be formed from an alkali-free glass material. Each of the transparent electrodes  202  and  212  can have a thickness of approximately 150 nm and can be formed from an indium-tin oxide (ITO) with a surface patterned in a desired planar shape. For example, the transparent electrode  202  can be patterned in a line shape (strip shape) whereas the transparent electrode  212  can be formed on the entire substrate surface. 
     Before assembly, the glass substrate with ITO electrode  201 ,  211  can be washed with a washing machine. The washing method can be composed of brush washing using an alkali cleaning agent, pure water washing, air blowing, UV irradiation, and infrared ray drying in order. Another washing method can include high-pressure spraying washing, plasma washing, and the like. 
     Next, a main sealing agent containing a gap control agent in an amount of 2 wt% to 5 wt% can be applied onto one of the glass substrate (for example, the glass substrate  201 ) to form a gap control layer  216 . The formation method can be a screen printing, or an application method utilizing a dispenser. The gap control agent can be selected appropriately so that the thickness of the liquid crystal layer  215  is, for example, 10 μm. 
     Plastic balls with a diameter of 9 μm to 10 μm (produced by Sekisui House Ltd.) serving as a gap control agent  214  can be scattered onto the other one of the glass substrate (for example, the glass substrate  211 ) by a dry gap spreader. 
     Next, the glass substrates  201  and  211  are overlaid with each other with the particular surfaces facing to each other, and then they can be heat-treated while being applied with a pressure by a press, to thereby cure the main sealing agent. In the present exemplary embodiment, the heat-treatment was performed at 150° C. for 3 hours. 
     According to these processes, a blank cell can be fabricated. Note that such a blank cell can be fabricated by any suitable general method of fabricating a liquid crystal element. 
     The thus fabricated blank cell can be injected with a liquid crystal material under vacuum to form the liquid crystal layer  215 . In the present exemplary embodiment, the liquid crystal material can be prepared by mixing a polymeric resin (photocurable material) with a liquid crystal. The polymeric resin can be added with a photopolymerization initiator in a small amount, wherein the photopolymerization initiator should be reacted with the irradiation of light at a wavelength corresponding to the light emitted from the laser light source  21  (shown in  FIGS. 4B and 5B ) in order for the polymeric resin to be cured with the light of the light source  21 . Examples of the liquid crystal may include a mixed-type liquid crystal material. In this case a liquid crystal having a larger refractive anisotropy can be used. The liquid crystal material including a liquid crystal and a polymeric resin mixed or dissolved therein can be injected. The mixing ratio of the polymeric resin and the liquid crystal can be 50:50 to 60:40 where the ratio of the photocurable material can be slightly higher than that of the liquid crystal. 
     Then, the optical division-type interference exposure apparatus  300  as shown in  FIGS. 4B and 5B  can be used to form the period microstructure on the holographic liquid crystal element  3  (record the wavefront conversion information thereon). 
     In the present exemplary embodiment, the interference exposure can be performed on two locations, including the respective wavefront conversion information recording areas (pixels)  3   a  and  3   b.  The position of the wavefront conversion information recording areas  3   a  and  3   b  can be aligned with the position of the transparent electrode  202  (or  212 ) patterned in a line shape (stripe shape) in terms of position and size. By doing so, different pieces of wavefront conversion information can be recorded pixel by pixel, thereby obtaining different types of light distribution state by switching the states of the holographic liquid crystal element  3 . 
       FIG. 4A  is a conceptual diagram illustrating the light distribution state by the regeneration light beams regenerated on the basis of the wavefront conversion information recorded on the wavefront conversion information recording area (pixel)  3   a  of the holographic liquid crystal element  3 , and  FIG. 4B  is a schematic diagram illustrating one example of the optical sprit system interference exposure apparatus  300 , both for describing how the period microstructure is formed in the wavefront conversion information recording area (pixel)  3   a  of the holographic liquid crystal element  3  (how to record the wavefront conversion information). 
     The wavefront conversion information recording area  3   a  can include the information containing the low-beam light distribution state as shown in  FIG. 4A  (light distribution pattern  71   a ), for example. The light distribution pattern  71   a  shown in  FIG. 4A  is a pattern projected onto the phosphor plate  4  shown in  FIG. 1 , and accordingly, if it is used as an illumination optical system  100 , the light image in the pattern  71   a  can be inverted upside down by the projector lens  5  to be projected forward. Of course, the light distribution pattern to be recorded as information on the wavefront conversion information recording area  3   a  can be appropriately changed in accordance with the intended use. 
       FIG. 4B  is a schematic diagram illustrating one example of the optical sprit system interference exposure apparatus  300 , wherein respective arrows show the traveling directions of light beams. 
     The optical sprit system interference exposure apparatus  300  can include a laser light source  21 ; a half mirror  22 ; a reference light optical system (including a reflector  23 , and a converging lens  24 , a pinhole  25 , and a collimator lens  26  which serve together as a collimator (beam expander)  40 ); and an object optical system (including a reflecting mirror  27 , another reflecting mirror  28 , and a converging lens  29 , a pinhole  30 , and a convex lens  31  which serve together as an object light optical system  50 , and a reflector mirror  32  ( 32   a )). 
     The laser light source  21  can be a laser oscillator having almost the same emission wavelength (for example, 405 nm) as that of the light source  1  shown in  FIG. 1 . Note that the emission wavelength of the laser light source  21  can be that of the light source  1  of  FIG. 1  ±10 nm. The laser light  12  oscillated from the laser light source  21  can be incident on the half mirror  22  at an angle of incidence of 45 degrees so that the light beams can be split to light beams  13  and  14  in two travelling paths. 
     The light beams  13  can be reflected by the reflector  23  and can enter the collimator (beam expander)  40 . As shown, the collimator  40  can be composed of the converging lens  24 , the pinhole  25 , and the collimator lens  26 . 
     The light beams  13  having entered the collimator  40  can be converged by the conversing lens  24  and can pass through the pinhole  25  at which the focal point of the converging lens  24  is located, and then can be incident on the collimator lens  26 . The light beams having been incident on the collimator lens  26  can be converted into parallel light beams to become reference light  13  for producing hologram. At that time, the angle of incidence on the hologram may be adjusted by a prism or the like. 
     The reference light  13  can be incident on the surface of the wavefront conversion information recording area  3   a  of the holographic liquid crystal element  3  at an angle of incidence  0 . Herein, the angle of incidence  0  during the information recording can be the same as the angle of incidence  0  of laser beam  10  from the light source  1  as shown in  FIG. 1 . 
     On the other hand, the split light beams  14  from the half mirror  22  can be reflected by the reflecting mirrors  27  and  28  to enter the object light optical system  50 . The object light optical system  50  can be composed of the converging lens  29 , the pinhole  30 , and the convex lens  31 . 
     The light beams  14  having entered the object light optical system  50  can be converged by the conversing lens  29  and can pass through the pinhole  30  at which the focal point of the converging lens  29  is located, and then can be incident on the convex lens  31 . The light beams  14  having been incident on the convex lens  31  can be further diffused to be incident on the reflector mirror  32  ( 32   a ). 
     The reflector mirror  32  (reflector mirror for forming a low-beam light distribution  32   a ) can serve as a mirror for forming a low-beam light distribution and can reflect the light beams  14  so that the reflected light beams can become object light  15  for producing hologram. The object light beams  15  can be incident on the surface of the wavefront conversion information recording area  3   a  of the holographic liquid crystal element  3  in a normal direction. The reflector mirror  32  (reflector mirror for forming a low-beam light distribution  32   a ) can be used to form a desired light distribution state (as shown in  FIG. 4A ) of reflected light beams based on the resulting information. 
     The reference light  13  and the object light  15  can be projected onto a predetermined position on the holographic liquid crystal element  3  (wavefront conversion information recording area  3   a ) through a photo mask  37 . In the first interference exposure, the opening of the photo mask  37  can be matched to the wavefront conversion information recording area  3   a.    
     The reference light  13  and the object light  15  having been incident on the wavefront conversion information recording area  3   a  can be interfered with each other. Phase information and amplitude information contained in the reference light  13  and the object light  15  can be recorded by means of the interference fringes of these beams of light as a three-dimensional fringe structure composed of distributions of the photocurable material and the liquid crystal contained in the liquid crystal cell  215 . Specifically, the photocurable material starts curing at the antinodes of the standing wave induced by the light beams of the laser light source  21  while the liquid crystal can be concentrated at the nodes, thereby forming the fringe structure. Since the orientation direction of the liquid crystal may be limited due to the direction of the growth of the cured resin, the resulting element can show birefringence. Note that the light intensity ratio of the reference light  13  to the object light  15  to be incident on the holographic liquid crystal element  3  can be 2:1 to 10:1, and the irradiation intensity can be 5 mW/cm 2 , and the irradiation time can be 5 minutes. (The total sum of the light intensities can be 1.5 mJ/cm 2 .) 
       FIG. 5A  is a conceptual diagram illustrating the light distribution state by the regeneration light beams regenerated on the basis of the wavefront conversion information recorded on the wavefront conversion information recording area (pixel)  3   b  of the holographic liquid crystal element  3 , and  FIG. 5B  is a schematic diagram illustrating another example of the optical sprit system interference exposure apparatus  300 , both for describing how the period microstructure is formed in the wavefront conversion information recording area (pixel)  3   b  of the holographic liquid crystal element  3  (how to record the wavefront conversion information). 
     The wavefront conversion information recording area  3   b  can include the information containing the light distribution state for city driving as shown in  FIG. 5A  (light distribution pattern  71   b ), for example, for illuminating wider peripheral areas. The light distribution pattern  71   b  shown in  FIG. 5A  is a pattern projected onto the phosphor plate  4  shown in  FIG. 1 , and accordingly, if it is used as an illumination optical system  100 , the light image in the pattern  71   b  can be inverted upside down by the projector lens  5  to be projected forward. Of course, the light distribution pattern to be recorded as information on the wavefront conversion information recording area  3   b  can be appropriately changed in accordance with the intended use. 
       FIG. 5B  is a schematic diagram illustrating one example of the optical sprit system interference exposure apparatus  300 , wherein respective arrows show the traveling directions of light beams. Since the optical sprit system interference exposure apparatus  300  can have the same configuration as that in  FIG. 4B , the detailed description for the optical sprit system interference exposure apparatus  300  is omitted here. 
     In this case, the interference exposure as shown in  FIG. 4B  can first be performed, and then, the photo mask  37  can be shifted as shown in  FIG. 5B  for performing interference exposure again. In the second interference exposure, the opening of the photo mask  37  can be matched to the wavefront conversion information recording area  3   b.  In addition to this, the reflector mirror  32   a  can be replaced with another reflector mirror  32   b  for the formation of the light distribution for city driving arranged at almost the same position as that in the first interference exposure. Then, the second interference exposure can be performed. 
     In this manner, the holographic liquid crystal element  3  can be completed. The thus produced holographic liquid crystal element  3  without applying a voltage can be irradiated with laser beams at an angle of incidence  0 , to produce regeneration light beams. The regenerated light beams can form an optical image formed on the basis of the reflector mirror  32  ( 32   a,    32   b ) in a normal direction on an opposite side to the light source. For example, in the present exemplary embodiment, when laser beams are projected onto the wavefront conversion information recording area (pixel)  3   a  without applying a voltage thereto at an angle of incidence  0 , the low-beam light distribution state  71   a  as shown in  FIG. 4A  can be generated. When laser beams are projected onto the wavefront conversion information recording area (pixel)  3   b  without applying a voltage thereto at an angle of incidence  0 , the light distribution state for city driving  71   b  as shown in  FIG. 5A  can be generated. 
     In the above-described exemplary embodiment, the opening position of the photo mask  37  can be changed to perform interference exposure with the respective reflector mirrors  32   a  and  32   b  each used for forming a different light distribution pattern at respective positions with the reference light beams  13  at an angle of incidence  0 . In another exemplary embodiment, three or more types of position of the opening of the photo mask  37  and light distribution pattern of the reflector mirror  32  can be utilized to perform interference exposure at three or more times. In this case, in addition to the above two types of light distribution state, other light distribution states such as a high-beam light distribution state, a light distribution state for highway traveling, and the like can be added to a vehicular headlamp utilizing the lighting optical system as other functions. 
     Next, with reference to  FIGS. 6 to 9 , a description will be given of the switching operation of the light distribution states produced by the lighting optical system  100  in accordance with the present exemplary embodiment. In the present exemplary embodiment, the application of a voltage to the two pixels or wavefront conversion information recording areas  3   a  and  3   b  can be separately controlled to electrically change four types of light distribution state from one another as shown in  FIGS. 6B ,  7 B,  8 B, and  9 B. 
       FIG. 6A  is a schematic cross-sectional view of the lighting optical system  100  illustrating the formation of the light distribution state when the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  is turned OFF (no voltage is applied) and the pixel (wavefront conversion information recording area)  3   b  is turned ON (a voltage is applied), wherein respective arrows show the traveling directions of light beams.  FIG. 6B  is a conceptual diagram showing the light distribution state (pattern) formed on the phosphor plate  4 . In the present exemplary embodiment, if it is used as an illumination optical system  100 , the light image can be inverted upside down by the projector lens  5  to be projected forward. 
     When the illumination optical system  100  is turned on, the light source  1  can emit laser beams  10  ( 10   a  and  10   b ) through the collimator lens  2  to be projected onto the holographic liquid crystal element  3 . 
     In this case, the controller  36  can control the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  to be turned OFF (with no voltage applied) whereas the pixel  3   b  is controlled to be turned ON (with a voltage applied). Then, as shown in  FIG. 6A , the laser beams  10   a  or reference light can be incident on the pixel  3   a  to be converted on the basis of the wavefront conversion information recorded in the pixel  3   a,  thereby producing regeneration light beams as the object light  11   a  for forming the light distribution pattern  71   a  for a low-beam light distribution pattern. This pattern  71   a  can be projected on the phosphor plate  4 . Namely, the reference light beams  10   a  incident on the holographic liquid crystal element  3  at an angle of incidence  0  can be refracted by the pixel  3   a  to be directly projected onto the phosphor plate  4 . 
     On the other hand, the laser beams  10   b  incident on the pixel  3   b  can pass through the pixel  3   b  without refraction to be reflected by the mirror  7 . The reflected light beams can be slightly spread to be projected onto the phosphor plate  4 . In this case, the light distribution pattern  71   c  can be designed to become a light distribution pattern for highway driving as shown in  FIG. 6B . Therefore, the pattern  71   c  can be concentrated at the center of the phosphor plate  4  (there is no glare light to the oncoming vehicle). 
     As discussed, the controller  36  can control the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  to be turned OFF (with no voltage applied) whereas the pixel  3   b  is controlled to be turned ON (with a voltage applied). With this configuration, the synthesized optical image to be projected onto the phosphor plate  4  can be that shown in  FIG. 6B . Specifically, the light distribution pattern optical image can include a cut-off shape for low beam distribution and a bright center area above the center horizontal line where glare is not directed to oncoming vehicles. Accordingly, the light distribution state is suitable for highway driving when oncoming vehicles exist. 
       FIG. 7A  is a schematic cross-sectional view of the lighting optical system  100  illustrating the formation of the light distribution state when the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  is turned ON (a voltage is applied) and the pixel (wavefront conversion information recording area)  3   b  is turned OFF (no voltage is applied), wherein respective arrows show the traveling directions of light beams.  FIG. 7B  is a conceptual diagram showing the light distribution state (pattern) formed on the phosphor plate  4 . In the present exemplary embodiment, if it is used as an illumination optical system  100 , the light image can be inverted upside down by the projector lens  5  to be projected forward. 
     In this case, the controller  36  can control the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  to be turned ON (with a voltage applied) whereas the pixel  3   b  is controlled to be turned OFF (with no voltage applied). Then, as shown in  FIG. 7A , the laser beams  10   b  or reference light can be incident on the pixel  3   b  to be converted on the basis of the wavefront conversion information recorded in the pixel  3   b,  thereby producing regeneration light beams as the object light  11   b  for forming the light distribution pattern  71   b  for city driving. This pattern  7  lb can be projected on the phosphor plate  4 . Namely, the reference light beams  10   b  incident on the holographic liquid crystal element  3  at an angle of incidence  0  can be refracted by the pixel  3   b  to be directly projected onto the phosphor plate  4 . 
     On the other hand, the laser beams  10   a  incident on the pixel  3   a  can pass through the pixel  3   a  without refraction to be reflected by the mirror  7 . The reflected light beams can be slightly spread to be projected onto the phosphor plate  4 . In this case, the light distribution pattern  71   d  can be designed to become a light distribution pattern for a high-beam light distribution as shown in  FIG. 7B . Therefore, the pattern  71   d  can be concentrated at the center of the phosphor plate  4  (including the portion below the center). 
     As discussed, the controller  36  can control the pixel (wavefront conversion information recording area)  3   a  of the holographic liquid crystal element  3  to be turned ON (with a voltage applied) whereas the pixel  3   b  is controlled to be turned OFF (with no voltage applied). With this configuration, the synthesized optical image to be projected onto the phosphor plate  4  can be that shown in  FIG. 7B . Specifically, the synthesized light distribution pattern optical image can include a wide light distribution and a bright center area (including the lower side below the center). Accordingly, the light distribution state is suitable for city driving when no oncoming vehicles exist. 
       FIG. 8A  is a schematic cross-sectional view of the lighting optical system  100  illustrating the formation of the light distribution state when both the pixels (wavefront conversion information recording areas)  3   a  and  3   b  of the holographic liquid crystal element  3  are turned OFF (no voltage is applied), wherein respective arrows show the traveling directions of light beams.  FIG. 8B  is a conceptual diagram showing the light distribution state (pattern) formed on the phosphor plate  4 . In the present exemplary embodiment, if it is used as an illumination optical system  100 , the light image can be inverted upside down by the projector lens  5  to be projected forward. 
     In this case, as in the case described with reference to  FIG. 6A , the laser beams  10   a  or reference light can be incident on the pixel  3   a  to be converted on the basis of the wavefront conversion information recorded in the pixel  3   a,  thereby producing regeneration light beams as the object light  11   a  for forming the light distribution pattern  71   a  for a low-beam light distribution pattern. This pattern  71   a  can be projected on the phosphor plate  4 . Furthermore, as in the case described with reference to  FIG. 7A , the laser beams  10   b  or reference light can be incident on the pixel  3   b  to be converted on the basis of the wavefront conversion information recorded in the pixel  3   b,  thereby producing regeneration light beams as the object light  11   b  for forming the light distribution pattern  71   b  for city driving. This pattern  71   b  can be projected on the phosphor plate  4 . 
     Accordingly, the controller  36  can control both the pixels (wavefront conversion information recording areas)  3   a  and  3 B of the holographic liquid crystal element  3  to be turned OFF (with no voltage applied). With this configuration, the synthesized optical image to be projected onto the phosphor plate  4  can be that shown in  FIG. 8B . Specifically, the synthesized light distribution pattern optical image can include a wide light distribution pattern including the cut-off line for low-beam light distribution and being suitable for city driving when oncoming vehicles exist. 
       FIG. 9A  is a schematic cross-sectional view of the lighting optical system  100  illustrating the formation of the light distribution state when both the pixels (wavefront conversion information recording areas)  3   a  and  3   b  of the holographic liquid crystal element  3  are turned ON (a voltage is applied), wherein respective arrows show the traveling directions of light beams.  FIG. 8B  is a conceptual diagram showing the light distribution state (pattern) formed on the phosphor plate  4 . In the present exemplary embodiment, if it is used as an illumination optical system  100 , the light image can be inverted upside down by the projector lens  5  to be projected forward. 
     In this case, the laser beams  10   a  and  10   b  that are incident on the respective pixels  3   a  and  3   b  can pass through the pixels  3   a  and  3   b  as shown in  FIGS. 6A  (regarding the pixel  3   b ) and  7 A (regarding the pixel  3   a ), and can be reflected by the mirror  7 . The reflected light beams can be slightly spread to be projected onto the phosphor plate  4 . 
     Therefore, when the pixels (wavefront conversion information recording areas)  3   a  and  3   b  of the holographic liquid crystal element  3  are turned ON (with a voltage applied), the synthesized image projected onto the phosphor plate  4  can be that shown in  FIG. 9B . Specifically, the light distribution pattern optical image can include a bright center area including the area below the center horizontal line at or near (i.e., substantially at) the center of the phosphor plate  4 . Accordingly, the light distribution state is suitable for highway driving when no oncoming vehicles exist. 
     As discussed above, the lighting optical system  100  of the present exemplary embodiment can electrically switch the light distribution states from one another in accordance with the traveling modes. Thus, a driver can drive a vehicle with higher safety under various driving conditions. 
     In the present exemplary embodiment made in accordance with the principles of the presently disclosed subject matter, the laser beams  10  from the light source  1  can be converted into desired light distribution patterns by means of the holographic liquid crystal element  3 . The appropriate ON/OFF control of respective pixels of the holographic liquid crystal element  3  can cause the laser beams  10  to be converted to regeneration light beams as object light or to pass through without any conversion, thereby providing desired projected images. 
     In the above exemplary embodiment, the application of voltage can cause the element to pass the light and no application of voltage can cause the element to project its hologram image. In view of fail safe, the light distribution state to be projected when the holographic liquid crystal element  3  is not applied with a voltage can be a low-beam light distribution pattern. 
     The thickness of the phosphor plate  4  (or alternatively, the applied thickness of the phosphor when the phosphor is applied) and the density of the phosphor  41  can be appropriately changed in accordance with the intensity of the light beams from the light source  1 . The shape of the phosphor plate  4  can be changed in accordance with the focal distance of the projector lens  5 , for example, can be curved or bulged in a hemispherical shape at the center portion thereof. Furthermore, the center portion of the phosphor plate  4  may be made thick in accordance with the luminance distribution of the regeneration light beams  11  derived from the holographic liquid crystal element  3 . The phosphor plates  4  with the above-describes shapes can be formed by melting glass or a resin material by heat to be liquefied, mixing it with a phosphor material (wavelength converting material), and injecting the mixture into a mold, or by melting a phosphor material itself by heat and injecting the molten material into a mold. 
     Further, if the cut-off shape cannot be formed only with the holographic liquid crystal element  3 , a light shielding member  6  formed into a desired cut-off pattern can be provided as shown in  FIG. 10 . In this case, the light shielding member  6  can be interposed between the phosphor plate  4  and the holographic liquid crystal element  3  at an appropriate position so that part of the regeneration light beams  11   a  of the hologram image can be shielded, thereby forming the cut-off line in the light distribution pattern for low beam. 
     In the present exemplary embodiment, the holographic liquid crystal element  3  can include two independent areas (pixels) to be applied with a voltage for control, but the presently disclosed subject matter is not limited thereto. The holographic liquid crystal element  3  can include three or more independent areas (pixels) where different pieces of wavefront conversion information representing different light distribution states can be recorded, respectively. Of course, the holographic liquid crystal element  3  can include only one pixel which can be turned on/off for simply switching a low-beam light distribution and a high-beam light distribution. Also in this case, any mechanical member is not required but the electrical means can switch the light distribution states. 
     In the present exemplary embodiment, the light intensity ratio of the laser beams  10   a  and  10   b  to be projected onto the pixels  3   a  and  3   b  of the holographic liquid crystal element  3  may be changed. For example, if the pixel  3   a  is provided with a function for forming a cut-off pattern (in the OFF state) and a high-beam pattern (in the ON state), it is possible to increase the light intensity of the laser beams  10   a  to be projected onto the pixel  3   a.    
     The lighting optical system made in accordance with the principles of the presently disclosed subject matter can be applied not only to vehicular headlamps, but also to various illuminating devices such as vehicular rear lamps, vehicular fog lamps, vehicular interior/exterior illuminating devices, portable flashlight, general lighting fixtures, spotlights, stage illumination systems, and the like. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the presently disclosed subject matter without departing from the spirit or scope of the presently disclosed subject matter. Thus, it is intended that the presently disclosed subject matter cover the modifications and variations of the presently disclosed subject matter provided they come within the scope of the appended claims and their equivalents. All related art references described above are hereby incorporated in their entirety by reference.