Abstract:
This invention relates to a cold mirror for a head-up display apparatus and provides a cold mirror that enables suppression of a phenomenon of change in color tone of an image depending on a position from which the image is viewed. This invention also provides a head-up display apparatus including the cold mirror. In a head-up display apparatus, display light of an image displayed on a display is reflected by a cold mirror and a reflected image of the image is viewed by a viewer. A relationship between an incident wavelength and a reflection phase difference in the cold mirror is set so that the reflection phase difference falls within a range of 180±60 degrees for an entire visible wavelength range of 420 to 680 nm.

Description:
[0001]    The disclosure of Japanese Patent Application No. JP2015-224276 filed on Nov. 16, 2015 including the specification, drawings, claims and abstract is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates a cold mirror for a head-up display (hereinafter abbreviated as “HUD”) apparatus in which change in color tone of an image depending on a position from which the image is viewed is reduced. This invention also relates to a HUD apparatus including the cold mirror. 
         [0004]    2. Description of the Related Art 
         [0005]    A HUD apparatus is, for example, mounted on a vehicle for use in displaying information such as infotainment information and/or ADAS (Advanced Driver Assistance Systems) in a field of view on the front side of a driver. Generally, such in-vehicle HUD apparatus has a configuration in which an image of, e.g., drive information luminously displayed on a display such as a liquid-crystal display is reflected by a mirror to project the image onto a windshield or a combiner in front of a driver. In such in-vehicle HUD apparatus as configured above, upon sun light reversely entering and being reflected by the mirror and then entering the display, the display deteriorates as a result of being heated. Therefore, use of a cold mirror for forming the mirror has conventionally been proposed (for example, Japanese Patent Laid-Open No. 2004-347633 described below). As a result of using a cold mirror for forming the mirror, even if sun light reversely enters, infrared components included in the sun light are absorbed by the cold mirror and the display is thus prevented from being heated. 
         [0006]    A test conducted by the present inventor showed that a conventional HUD apparatus including a mirror formed of a cold mirror has a problem in that an image changes in color tone depending on a position from which the image is viewed. For example, if a white image is displayed on the entire display of the in-vehicle HUD apparatus, while the entire display looks white from a viewpoint of a driver, the display sometimes looks somewhat colored from a front passenger seat or a rear seat. 
         [0007]    This invention provides a cold mirror for a HUD apparatus, the cold mirror solving the aforementioned problem of the related art and suppressing change in color tone of an image depending on a position from which the image is viewed. This invention also provides a HUD apparatus including the cold mirror. 
       SUMMARY OF THE INVENTION 
       [0008]    This invention provides a cold mirror in a HUD apparatus, the cold mirror reflecting an image displayed on a display to make a reflected image of the image be viewed by a viewer, wherein, when a difference between an amount of variation in phase of p-polarized light reflected by the cold mirror between before and after the reflection and an amount of variation in phase of s-polarized Iight reflected by the cold mirror between before and after the reflection is defined as a reflection phase difference, a relationship between an incident wavelength and the reflection phase difference is set so that the reflection phase difference falls within a range of 180-60 degrees for an entire visible wavelength range of 420 to 680 nm. 
         [0009]    A test conducted by the present inventor showed that in a HUD apparatus including a cold mirror, a phenomenon of change in color tone of an image depending on a position from which the image is viewed occurs due to an effect of a relationship between an incident wavelength of the cold mirror and a reflection phase difference. In other words, a cold mirror for a conventional HUD apparatus is designed only in consideration of spectral reflectivity characteristic optimization (that is, achievement of a spectral reflectivity characteristic that provides a highest possible reflectivity in a visible light range and a lowest possible reflectivity in an infrared range). As a result, a spectral reflection phase difference characteristic of a cold mirror for a conventional HUD apparatus includes a wavelength band in which a reflection phase difference largely deviates from 180 degrees and a polarized light state thereby largely varies within a visible wavelength range of 420 to 680 nm. Thus, upon a position from which an image is viewed being shifted, an intensity of light in that band largely varies, and as a result, the image changes in color tone. Therefore, in this invention, a reflection phase difference of a cold mirror for a HUD apparatus is set within a range of 180±60 degrees for an entire visible wavelength range of 420 to 680 nm. As a result, there is no longer wavelength band in which a polarized light state largely varies, and change in color tone of an image depending on a position from which the image is viewed can be reduced. Consequently, a difference in color tone between an image when viewed from a driver seat and the image when viewed from a front passenger seat or a rear seat can be reduced. In general, a cold mirror for a HUD apparatus is often used in such a manner that an incident angle is set to 20 to 60 degrees relative to an optical axis of display light. A cold mirror according to this invention can be considered particularly effective when the cold mirror is used for this incident angle range. In other words, if the incident angle is small (less than 20 degrees), there are few fluctuations in spectral reflection phase difference characteristic in a visible light range due to a film design, and thus, the need of considering a spectral reflection phase difference characteristic in designing a film is not so high. On the other hand, in a large incident angle region (exceeding 60 degrees), it is difficult to make a film design so that a reflection phase difference falls within a range of 180±60 degrees for an entire visible light range. 
         [0010]    In this invention, it is possible that the reflection phase difference of the cold mirror is set within a range of 1801±30 degrees for the entire visible wavelength range of 420 to 680 nm. Consequently, change in color tone of an image depending on a position from which the image is viewed can further be reduced. 
         [0011]    In this invention, it is possible that an average reflectivity of the cold mirror for the visible wavelength range of 420 to 680 nm is set to no less than 90%. Consequently, bright display can be provided. 
         [0012]    A HUD apparatus according to this invention includes a cold mirror according to this invention. The HUD apparatus according to this invention enables reduction of change in color tone of an image depending on a position from which the image is viewed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a diagram illustrating an embodiment of a cold mirror according to this invention incorporated in a HUD apparatus in  FIG. 2 , and schematically illustrates a stack structure of the cold mirror; 
           [0014]      FIG. 2  is a schematic diagram illustrating an embodiment of an in-vehicle HUD apparatus with the cold mirror according to this invention incorporated therein, and illustrates the cold mirror according to this invention incorporated in a HUD apparatus described in Japanese Patent Laid-Open No. 2004-347633; 
           [0015]      FIG. 3  is a graph indicating respective spectral reflectivity characteristics (simulated values) of the cold mirror in  FIG. 1  and a conventional cold mirror; 
           [0016]      FIG. 4  is a graph indicating respective spectral reflection phase difference characteristics (simulated values) of the cold mirror in  FIG. 1  and a conventional cold mirror (which is the same as the conventional cold mirror mentioned with reference to  FIG. 3 ); 
           [0017]      FIG. 5  is a graph indicating relative spectral intensity characteristics of reflected images from the cold mirror in  FIG. 1  and a conventional cold mirror (which is the same as the conventional cold mirror mentioned with reference to  FIG. 3 ) and indicates characteristics when a position from which the reflected images are viewed is shifted (the characteristics are all simulated values); and 
           [0018]      FIG. 6  is a chromaticity diagram indicating chromatic coordinates according to the respective characteristics in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    An embodiment of this invention will be described.  FIG. 2  illustrates a HUD apparatus  10  with a cold mirror according to this invention incorporated therein. The HUD apparatus  10  is configured by mounting a cold mirror  12  according to this invention in a HUD apparatus described in Japanese Patent Laid-Open No. 2004-347633. The HUD apparatus  10  is disposed in a dashboard (not illustrated) of a vehicle. In a case  14  of the HUD apparatus  10 , e.g., a liquid-crystal display  16 , the cold mirror  12  and a concave mirror  18  are housed. The liquid-crystal display  16  luminously displays an image of, e.g., infotainment information and/or ADAS information. The liquid-crystal display  16  incorporates, e.g., a liquid-crystal element  13 , a lens  15  and a white LED  17 , which is a backlight, therein. The liquid-crystal element  13  has a structure in which a polarized light plate is attached to each of a front surface and a back surface of a TFT liquid-crystal panel. Light emitted from the white LED  17  is applied to the liquid-crystal element  13  via the lens  15 . Consequently, the liquid-crystal element  13  luminously displays an image. Display light  20  of the image luminously displayed on the liquid-crystal display  16  is reflected by the cold mirror  12  and the concave mirror  18  in order and projected onto a windshield or a combiner in front of a driver seat of the vehicle. Consequently, an image formed by the projected display light  20  is viewed by a driver or other occupants. The concave mirror  18  can be swung by a swing mechanism  28 , and an angle of projection of the display light  20  can be adjusted by adjustment of an angle of the swing. The cold mirror  12  is held by the case  14  via a mirror holder  22  formed of, e.g., aluminum. A radiation fin  24  is attached to a back surface of the mirror holder  22 . The radiation fin  24  is exposed outside the case  14 . Sun light  26  reversely entering from the outside is reflected by the concave mirror  18  and then falls on the cold mirror  12 . Then, infrared light contained in the sun light  26  penetrates the cold mirror  12  and is absorbed by the mirror holder  22 . Consequently, the mirror holder  22  generates heat, and the heat is dissipated to the outside by the radiation fin  24 . 
         [0020]      FIG. 1  illustrates an embodiment of the cold mirror  12 . The cold mirror  12  is configured by forming a dielectric multi-layer  32  on a front surface (light incident surface)  30   a  of a substrate  30  of, e.g., glass. The dielectric multi-layer  32  is configured as a multi-layer film in which high refractive index films  34  formed of a high refractive index material and low refractive index films  36  formed of a low refractive index material are alternately stacked. 
         [0021]    Table 1 indicates an example design of the cold mirror  12 . Table 1 also indicates an example design of a conventional cold mirror used for comparison in characteristic with the cold mirror  12  of this examprle design. “Film thickness (nm)” means physical film thickness. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Film thickness(nm) 
                   
               
             
          
           
               
                   
                   
                   
                 Present 
               
               
                 Layer 
                 Material 
                 Conventional 
                 invention 
               
               
                 Substrate 
                 Glass 
                 product 
                 product 
               
               
                   
               
             
          
           
               
                 1 
                 TiO2 
                 50.87 
                 19.20 
               
               
                 2 
                 SiO2 
                 68.11 
                 73.14 
               
               
                 3 
                 TiO2 
                 105.84 
                 33.33 
               
               
                 4 
                 SiO2 
                 87.98 
                 81.11 
               
               
                 5 
                 TiO2 
                 66.42 
                 51.27 
               
               
                 6 
                 SiO2 
                 120.75 
                 87.39 
               
               
                 7 
                 TiO2 
                 74.77 
                 51.05 
               
               
                 8 
                 SiO2 
                 109.91 
                 85.91 
               
               
                 9 
                 TiO2 
                 69.14 
                 40.92 
               
               
                 10 
                 SiO2 
                 121.72 
                 98.25 
               
               
                 11 
                 TiO2 
                 69.30 
                 50.13 
               
               
                 12 
                 SiO2 
                 118.35 
                 98.26 
               
               
                 13 
                 TiO2 
                 66.58 
                 65.28 
               
               
                 14 
                 SiO2 
                 117.98 
                 111.72 
               
               
                 15 
                 TiO2 
                 64.61 
                 43.74 
               
               
                 16 
                 SiO2 
                 103.65 
                 165.96 
               
               
                 17 
                 TiO2 
                 46.38 
                 50.39 
               
               
                 18 
                 SiO2 
                 84.60 
                 84.79 
               
               
                 19 
                 TiO2 
                 43.76 
                 104.79 
               
               
                 20 
                 SiO2 
                 93.37 
                 56.35 
               
               
                 21 
                 TiO2 
                 59.92 
                 55.99 
               
               
                 22 
                 SiO2 
                 108.04 
                 24.03 
               
               
                 23 
                 TiO2 
                 47.93 
                 24.03 
               
               
                 24 
                 SiO2 
                 92.44 
                 116.53 
               
               
                 25 
                 TiO2 
                 51.53 
                 24.91 
               
               
                 26 
                 SiO2 
                 65.75 
                 213.34 
               
               
                 27 
                 TiO2 
                 56.71 
                 21.32 
               
               
                 28 
                 SiO2 
                 209.09 
                 270.74 
               
               
                 Medium 
                 Air 
               
               
                   
               
             
          
         
       
     
         [0022]    The cold mirror  12  of this invention according to the design in Table 1 has the following configuration. The material of the substrate  30  is glass, the material of the high refractive index film  34  is TiO2, and the material of the low refractive index film  36  is SiO2. A high refractive index film  34  is formed as a first layer on a surface of the substrate  30 , and low refractive index films  36  and high refractive index films  34  are alternately formed on that high refractive index film  34 , and a low refractive index film  36  is then formed as a final 28-th layer, whereby a dielectric multi-layer  32  of a total of 28 layers is configured. A surface of the final 28-th layer is in contact with air. The design of the conventional cold mirror in Table 1 is different from the design of the cold mirror  12  of this invention in Table 1 only in film thicknesses of the high refractive index films and the low refractive index films and is the same as the design of the cold mirror  12  of this invention in the Table 1 in specifications other than the film thicknesses (e.g., materials of the substrate, the high refractive index films and the low refractive index films, the number of layers in the dielectric multi-layer and the order in which the high refractive index films and the low refractive index films are stacked). 
         [0023]    Optical characteristics of the cold mirror  12  of this invention of the design in Table 1 and optical characteristics of the conventional cold mirror of the design in Table 1 will be described with reference to  FIGS. 3 to 6 . The characteristics are all simulated values. Also, the characteristics are all those when an incident angle is 30 degrees. First,  FIG. 3  indicates spectral reflectivity characteristics. The reflectivity is an average reflectivity of p-polarized light and s-polarized light. According to  FIG. 3 , not only the conventional cold mirror but also the cold mirror of this invention has a cold mirror characteristic of transmitting infrared light and reflecting visible light. Therefore, even if sun light  26  reversely enters the HUD apparatus ( FIG. 2 ), infrared light penetrates the cold mirror  12  and thus does not enter the liquid-crystal display  16 . Consequently, deterioration of the liquid-crystal element  13  due to infrared light is prevented. The conventional cold mirror has a high average reflectivity for a visible light range (420 to 680 nm) compared to the cold mirror of this invention, but the cold mirror of this invention also has an average reflectivity of 91.8%. Therefore, the cold mirror of this invention has a visible light reflectivity that is sufficient for a cold mirror for a HUD apparatus. The reason of the high average reflectivity of the conventional cold mirror for a HUD apparatus for the visible light range is that the cold mirror is designed considering spectral reflectivity characteristic optimization only, without considering a reflection phase difference. 
         [0024]      FIG. 4  indicates spectral reflection phase difference characteristics. As the reflection phase difference is more constantly around 180 degrees, large variation in intensity of light in the visible light range when a position from which an image is viewed is shifted is suppressed, enabling suppression of a phenomenon of change in color tone of the image. According to the characteristic of the conventional cold mirror in  FIG. 4 , the reflection phase difference exceeds 240 degrees (=180+60 degrees) in a band of around 550 to 580 nm. If there is a band in which the reflection phase difference largely exceeds 180 degrees (polarized light state largely varies) in the visible light range as described above, when a position from which an image is viewed is shifted, an intensity of light in that band largely varies, resulting in change in color tone of the image. On the other hand, according to the characteristic of the cold mirror of this invention in  FIG. 4 , the reflection phase difference falls within a range of 180±60 degrees in the entire visible wavelength range of 420 to 680 nm. Therefore, even if the position from which the image is viewed is shifted, variation in intensity of light in that band is suppressed. As a result, a phenomenon of change in color tone of the image can be suppressed. In other words, while the reflectivity characteristic is sacrificed a little in the cold mirror of this invention compared to the conventional cold mirror, the reflection phase difference characteristic is greatly improved in the cold mirror of this invention compared to the conventional cold mirror. In particular, according to the characteristic of the cold mirror of this invention in  FIG. 4 , the reflection phase difference falls within a range of 180-30 degrees for the entire visible wavelength range of 420 to 680 nm, particularly enabling suppression of change in color tone of an image. Here, although the reflectivity characteristic is sacrificed a little in the cold mirror of this invention compared to the conventional cold mirror, as described above, the cold mirror of this invention has an average reflectivity of 91.8%, which is still high. 
         [0025]      FIG. 5  indicates relative spectrum intensity characteristics of reflected images from both cold mirrors when a position from which the reflected images are viewed is shifted. A white image displayed on an entire liquid-crystal display using a white LED as a backlight is assumed as a light source of each of the reflected images. The alternate long and short dash line indicates a characteristic when each of the reflected images is viewed from a front position (hereinafter “front view”), which is a characteristic common to both cold mirrors. The solid line and the dash line each indicate a characteristic when each of the reflected images is viewed from a same position shifted from the front position (hereinafter “oblique view”): the solid line indicates a characteristic of the cold mirror of this invention; and the dashed line indicates a characteristic of the conventional cold mirror. According to  FIG. 5 , there is no large variation in relative spectrum intensity characteristic of the cold mirror of this invention between the front view (alternate long and short dash line) and the oblique view (solid line). On the other hand, there is large variation in relative spectrum intensity characteristic of the conventional cold mirror between the front view (alternate long and short dash line) and the oblique view (dashed line). In other words, the relative spectrum intensity of the conventional cold mirror significantly decreases in a range of around 530 to 620 nm in the oblique view relative to the front view.  FIG. 6  indicates chromatic coordinates of the respective characteristics in  FIG. 5  on a chromaticity diagram. According to  FIG. 6 , in the case of the cold mirror of this invention, chromatic coordinates in the oblique view roughly correspond to chromatic coordinates in the front view (chromatic coordinates of white). On the other hand, in the case of the conventional cold mirror, it can be seen that chromatic coordinates in the oblique view deviate from chromatic coordinates in the front view, compared to the chromatic coordinates in the oblique view in the case of the cold mirror of this invention. 
         [0026]    The cold mirror according to this invention and the conventional cold mirror in Table 1 were fabricated, and a test of both cold mirrors was conducted. In this test, a white image was displayed on an entire display screen of a liquid-crystal display, the image was reflected by each cold mirror and a reflected image of the image was viewed, and a position from which the reflected image was shifted to check a degree of change in color tone of the reflected image. As a result, it was confirmed that the cold mirror according to the above-described embodiment can suppress change in color tone of the reflected image due to shifting of the position from which the reflected image is viewed, compared to the conventional cold mirror.