Patent Publication Number: US-2015077820-A1

Title: Image projection apparatus

Description:
CLAIM OF PRIORITY 
     This application claims benefit of priority to Japanese Patent Application No. 2013-191640 filed on Sep. 17, 2013, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to an image projection apparatus in which laser light from a laser light source is collimated by a collimator lens, and then converted into a hologram image by a phase conversion array, and is then projected. 
     2. Description of the Related Art 
     An image display apparatus used as a so-called head-up display apparatus is disclosed in Japanese Unexamined Patent Application Publication No. 2011-90076. 
     This image display apparatus has three laser light sources that emit laser beams of wavelengths of R, G, and B. The laser beam emitted from each laser light source is given to a two-dimensional modulation element, and an image is generated. The light beam containing the image is reflected by a mirror, and is given to a hologram optical element disposed in a windshield of a vehicle. The hologram optical element is a hologram mirror, and light reflected by this hologram mirror is given to a driver. As a result, the driver can view a virtual image in front of the windshield. 
     The image display apparatus described in Japanese Unexamined Patent Application Publication No. 2011-90076 employs a small liquid crystal panel or a digital mirror device as a two-dimensional conversion element. These generate a two-dimensional image by turning on or off the transmission of laser light for each pixel. Therefore, the use efficiency of laser light is low, and there is a limit in improving the contrast of an image projected on a virtual image in front of the windshield. 
     To solve this problem, the present invention proposes to use a phase conversion array as an element for generating an image. This phase conversion array converts the phases of laser beams passing through many conversion points, makes beams passing through adjacent conversion points interfere with each other, and concentrates light in dots to many pixels for expressing an image. In this method, the lens effect of the phase conversion array can be exerted, and light shielding for each dot as in the conventional light switch array is not performed, and a light absorption component is small. Therefore, the use efficiency of laser light is high, and a projection image that is high-intensity, is easily corrected, and has an added function such as a three-dimensional image can be generated. 
     However, the laser beam needs to be incident on the phase conversion array as a collimated beam at a predetermined incident angle. If stray light at an unexpected incident angle is incident, for example, projection light is generated in a part other than pixels to be generated, the quality of the display image may thereby be decreased, and in particular, the contrast may be adversely affected. 
     On the other hand, as described in Japanese Unexamined Patent Application Publication No. 2008-216697, in an image forming apparatus, an aperture is disposed between a laser light source and a collimator lens or in front of the collimator lens, and the diameter of a beam is limited by this aperture. However, if this aperture is simply disposed between the laser light source and the phase conversion array element, a new problem arises that light diffracted by the aperture is incident on the phase conversion array as new stray light. 
     SUMMARY 
     An image projection apparatus includes a laser light source, a collimator lens configured to convert laser light emitted from the laser light source into a collimated beam, a phase conversion array configured to phase-convert the collimated beam passing through the collimator lens to generate a hologram image, and a projection portion configured to project the hologram image generated by the phase conversion array. A plurality of apertures are arranged between the laser light source and the collimator lens in a direction along the optical axis at intervals. 
     Since the image projection apparatus of the present invention is provided with the plurality of apertures between the light source unit and the collimator lens, stray light generated by diffraction of light or reflection of light due to an aperture can be shielded by the next aperture, and the probability that stray light reaches the phase conversion array can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram showing a vehicular head-up display apparatus as an embodiment of an image projection apparatus of the present invention; 
         FIG. 2  shows the configuration of an exemplary embodiment of the image projection apparatus; 
         FIG. 3  is a sectional view showing the structure on a light path from a laser light source to a collimator lens; 
         FIG. 4  is a sectional view showing the structure on a light path from the collimator lens to a phase conversion array; 
         FIG. 5  is a front view in the direction of arrow V-V of  FIG. 3 ; and 
         FIG. 6  is a sectional view showing a mode in which a shielding member is provided on the light path from the collimator lens to the phase conversion array. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     An image projection apparatus  10  of an embodiment of the present invention shown in  FIG. 1  is mounted on an automobile and is used as a so-called head-up display apparatus. 
     The image projection apparatus  10  is disposed in a dashboard  2  in the front of a cabin of the automobile  1 . A hologram image is projected from the image projection apparatus  10  onto a projection region  3   a  of a windshield  3 . This image is reflected in the projection region  3   a  toward a driver  4 . A virtual image  5  is formed in front of the windshield  3 , and the driver  4  can thereby be made to feel as if they are viewing an image in front of the windshield  3 . 
       FIG. 2  schematically shows the configuration of the image projection apparatus  10 . 
     The image projection apparatus  10  has a light source unit  11 . The light source unit  11  includes a laser light source  12 , and a collimator lens  13  located in front of the light emitting path of the laser light source  12 . A phase conversion array  14  is provided in front of the light path of the light source unit  11 . The phase conversion array  14  faces the optical axis O 2  of the collimator lens  13  at an angle of 45 degrees. A Fourier transform lens  15  and a diffuser  16  are disposed in front of the light path of the phase conversion array  14 , and a projection optical system  17  is disposed in front thereof. The Fourier transform lens  15 , the diffuser  16 , and the projection optical system  17  constitute a projection portion. 
     As shown in  FIG. 3 , a semiconductor laser chip  12   a  is disposed in a case of the laser light source  12 , and a laser beam is emitted forward. As schematically shown in  FIG. 5  (a view in the direction of arrow V-V of  FIG. 3 ), the laser beam B 0  emitted from the semiconductor laser chip  12   a  is a divergent beam that has an elliptical or oval cross-section and that gradually increases in diameter toward the front. 
     Of the laser beam B 0 , an effective beam B 1  incident in the effective diameter of the collimator lens  13  is shown in  FIG. 3 . The effective beam B 1  is converted by the collimator lens  13  into a collimated beam B 2  having an infinite focal length. As shown in  FIG. 5 , the shape of the effective diameter of the collimator lens  13  is a rectangular shape whose long side extends in the left-right direction (a direction perpendicular to the paper plane of  FIG. 3 ). For this reason, the collimated beam B 2  converted by the collimator lens  13  has a rectangular cross-section. 
     The phase conversion array  14  is of a reflective type, and a liquid crystal material is enclosed between two substrates. Intersections of electrodes provided on the two substrates are conversion points, and the conversion points are regularly arranged. Owing to an electric field applied to the conversion points, crystals of the liquid crystal layer tilt in the thickness direction of the liquid crystal layer, and the phases of laser beams passing through the conversion points are thereby converted. By the interference of laser beams of different phases passing through adjacent conversion points, laser light is collected in dots to pixels of an image desired to be displayed, and a predetermined hologram image is generated. 
     Unlike the conventional liquid crystal panel or digital mirror device, the phase conversion array  14  does not turn on or off light passing through it for each pixel but concentrates light energy in space to pixels, and is therefore light energy efficient, and a hologram image with high contrast can be generated. 
     As shown in  FIG. 2 , a converted beam B 3  reflected by the phase conversion array  14  and passing through the Fourier transform lens  15  is imaged on the diffuser  16  in a defocus state. A diffusion light B 4  passing through the diffuser  16  is image-adjusted by the projection optical system  17 , becomes an adjusted beam B 5 , and is projected onto the projection region  3   a  of the windshield  3 . 
     In the above-described embodiment, only one light source unit  11  is provided, and a laser beam of a single wavelength is incident on the phase conversion array  14 . However, for example, two types of light source units  11  that emit laser beams of different wavelengths of R (628 nm) and G (515 nm) may be arranged vertically in  FIG. 2 . In this case, laser beams emitted from respective laser light sources are converted into collimated beams having a rectangular cross-section by collimator lenses  13  provided for respective laser light source. The collimated beams of laser light of two wavelengths are incident on different regions of the phase conversion array  14 , and light beams of two wavelengths of R (red) and G (green) are each concentrated in dots to pixels constituting an image. A hologram image including an image in which two colors of R and G are mixed is generated. 
     Three light source units that emit laser beams of three wavelengths of R, G, and B may be used. 
     As shown in  FIG. 3 , in the light source unit  11 , an aperture forming member  20  is disposed between the laser light source  12  and the collimator lens  13 . The aperture forming member  20  is formed of a metal material such as stainless steel or a synthetic resin material. 
     A plurality of wall bodies are provided in the aperture forming member  20 . In this embodiment, three wall bodies: a first wall body  21 , a second wall body  22 , and a third wall body  23  are arranged from the laser light source  12  toward the collimator lens  13 . However, the number of the wall bodies is not particularly limited as long as it is two or more. 
     The inner edge of the first wall body  21  defines a first aperture  21   a,  the inner edge of the second wall body  22  defines a second aperture  22   a,  and the inner edge of the third wall body  23  defines a third aperture  23   a.  As shown in  FIG. 5 , the first, second, and third apertures  21   a,    22   a,  and  23   a  have rectangular shapes similar to the rectangular shape of the effective diameter of the collimator lens  13 . 
     In  FIG. 3 , the outer surface of the effective beam B 1  within the range from the laser light source  12  to the effective diameter of the collimator lens  13  is denoted by reference sign B 1   a.  The distance between the first aperture  21   a  and the outer surface B 1   a  is denoted by L1, the distance between the second aperture  22   a  and the outer surface B 1   a  is denoted by L2, and the distance between the third aperture  23   a  and the outer surface B 1   a  is denoted by L3. The distances L1, L2, and L3 are measured in a direction perpendicular to the optical axis O 1  of the effective beam B 1 . The distances L1, L2, and L3 gradually decrease in the order of distance L1, distance L2, and distance L3, (L1&gt;L2&gt;L3). 
     In  FIG. 3 , there are shown a first virtual surface a extending from the light emitting point of the laser light source  12  to the first aperture (inner edge)  21   a,  a second virtual surface β extending from the light emitting point of the laser light source  12  to the second aperture (inner edge)  22   a,  and a third virtual surface γ extending from the light emitting point of the laser light source  12  to the third aperture (inner edge)  23   a.  The virtual surfaces α, β, and γ each mean a divergent advancing path of a light component of the laser beam B 0  emitted from the laser light source  12 . 
     The angle between a first facing surface  21   b  of the first wall body  21  that faces the laser light source  12  and the virtual surface α is 90 degrees ±20 degrees or less. Similarly, the angle between a second facing surface  22   b  of the second wall body  22  that faces the laser light source  12  and the virtual surface β, and the angle between a third facing surface  23   b  of the third wall body  23  that faces the laser light source  12  and the virtual surface γ are also 90 degrees ±20 degrees or less. It is more preferable that these angles be 90 degrees ±10 degrees or less. It is most preferable that these angles be 90 degrees as shown in  FIG. 3 . 
     The angles θ1, θ2, and θ3 between the facing surfaces  21   b,    22   b,  and  23   b  of the wall bodies  21 ,  22 , and  23  and a plane perpendicular to the optical axis O 1  decrease in the order of θ1, θ2, and θ3 (θ1&gt;θ2&gt;θ3). 
     The facing surfaces  21   b,    22   b,  and  23   b  of the wall bodies  21 ,  22 , and  23  have a light reflection prevention structure or a light absorption structure. For example, the facing surfaces  21   b,    22   b,  and  23   b  have a light scattering surface having fine recesses and protrusions. Alternatively, a light reflection prevention film formed by laminating thin films or a light absorption film that contains carbon and absorbs light and converts into heat energy is formed on the facing surfaces  21   b,    22   b,  and  23   b.    
     In the light source unit  11  shown in  FIG. 3 , the laser beam B 0  emitted from the laser light source  12  is gradually narrowed by the plurality of apertures  21   a,    22   a,  and  23   a,  and is given to the collimator lens  13 . Therefore, a light component on the outer side of the effective beam B 1  can be prevented from entering the collimator lens  13  as stray light. 
     In particular, the distances between the outer surface B 1   a  of the effective beam B 1  and the plurality of apertures  21   a,    22   a,  and  23   a  gradually decrease in the order of L1, L2, and L3 (L1&gt;L2&gt;L3). Therefore, stray light diffracted by the first aperture  21   a  can be easily shielded by the second aperture  22   a,  and stray light diffracted by the second aperture  22   a  can be easily shielded by the third aperture  23   a.    
     A light component divergently advancing along the virtual surface α on the outer side of the effective beam B 1  is incident on a part of the first facing surface  21   b  close to the first aperture  21   a  at an angle close to 90 degrees. Therefore, light is easily absorbed by the reflection prevention film or light absorption film, and irregular reflection in directions other than the direction of return light to the laser light source  12  hardly occurs. The same holds true for the relationship between a light component divergently advancing along the virtual surface β and the second aperture  22   a,  and the relationship between a light component divergently advancing along the virtual surface γ and the third aperture  23   a.    
     Since the light source unit  11  has the above-described aperture forming member  20 , stray light incident on the collimator lens  13  at an angle different from the diverging angle of the effective beam B 1  can be limited, and a stray light component not parallel to the optical axis O 2  can be prevented from being superimposed on the collimated beam B 2  converted by the collimator lens  13 . 
     The side surfaces (four side surfaces) of the collimator lens  13  also have a light reflection prevention structure or a light absorption structure. By providing the side surfaces  13   a  with this structure, a phenomenon in which light passing through the collimator lens  13  is internally reflected by the side surfaces  13   a  can be reduced, and stray light due to this reflected light can be prevented from entering the collimated beam B 2 . 
     As shown in  FIG. 4 , a second aperture forming member  30  is provided between the collimator lens  13  and the phase conversion array  14 . 
     The second aperture forming member  30  has a plurality of wall bodies  31 ,  32 ,  33 , and  34 , and the inner edges of the wall bodies  31 ,  32 ,  33 , and  34  define apertures  31   a,    32   a,    33   a,  and  34   a,  respectively. The shapes of the apertures  31   a,    32   a,    33   a,  and  34   a  are rectangular shapes similar to the cross-sectional shape of the collimated beam B 2 . The distances La, Lb, Lc, and Ld between the apertures  31   a,    32   a,    33   a,  and  34   a  and the outer surface B 2   a  of the collimated beam B 2  in the direction of a plane perpendicular to the optical axis O 2  decrease in the order of La, Lb, Lc, and Ld (La&gt;Lb&gt;Lc&gt;Ld). 
     Facing surfaces  31   b,    32   b,    33   b,  and  34   b  of the wall bodies  31 ,  32 ,  33 , and  34  that face the collimator lens  13  have a light reflection prevention structure or a light absorption structure. 
     Since the plurality of apertures  31   a,    32   a,    33   a,  and  34   a  are provided between the collimator lens  13  and the phase conversion array  14 , and the distances between the apertures and the outer surface B 2   a  of the collimated beam B 2  are in the relationship of La&gt;Lb&gt;Lc&gt;Ld, stray light generated in the light path between the collimator lens  13  and the phase conversion array  14  can be prevented from being incident on the phase conversion array  14 . 
     The phase conversion array  14  controls the tilt of the liquid crystal material in the optical axis direction at a plurality of conversion points, and can thereby change the phases of beams passing through the conversion points. Light components of different phases passing through adjacent conversion points interfere with each other, laser light is collected in dots to pixels of an image desired to be displayed, and a predetermined hologram image is generated. Therefore, if stray light whose incident angle is significantly different from a predetermined incident angle enters light incident on each conversion point, for example, a ghost image appears in the hologram image, and the display quality is decreased. 
     In the embodiment shown in  FIG. 3  and  FIG. 4 , by providing the aperture forming members  20  and  30 , the above-described stray light can be prevented from advancing, and the display quality of a hologram image can be improved. 
     As shown in  FIG. 6 , when the second aperture forming member  30  shown in  FIG. 4  is not used, stray light can be prevented from being incident on the phase conversion array  14  by securing a relatively large space on the outer side of the collimator lens  13  and the outer surface B 2   a  of the collimated beam B 2  reaching the phase conversion array  14 . 
     In  FIG. 6 , a square tubular shielding member  40  is provided on the outer side of the collimated beam B 2 . In this case, by setting the distance S between the outer surface B 2   a  of the collimated beam B 2  and the inner surface  40   a  of the shielding member  40  in the direction of a plane perpendicular to the optical axis O 2  as follows, stray light is easily prevented from entering the collimated beam B 2 . 
     As shown in  FIG. 6 , a Gaussian intensity distribution G having a peak value P of light intensity on the outer surface B 2   a  of the collimated beam B 2  is assumed such that the center is located on the outer surface B 2   a.  In this case, the distance S is set such that the inner surface  40   a  is located at a position where light intensity is −20 dB or less of the peak value P in the Gaussian intensity distribution. 
     Since the inner surface  40   a  is located at a position where the light intensity is −20 dB or less of the light intensity on the outer surface B 2   a  of the collimated beam B 2 , the intensity of light reflected by the inner surface  40   a  can be reduced, and stray light having high intensity can be prevented from entering the phase conversion array  14 . This can be easily adapted for use in projecting a plurality of different light sources onto the phase conversion array.