Abstract:
A display device includes a translucent windscreen, particularly the windscreen of a motor vehicle, a narrow-band light source that emits light into the windscreen via an edge thereof, and a periodic structure formed in the windscreen that bends the light from the light source toward the eye of a driver.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102015001930.9, filed Feb. 13, 2015, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure pertains to a display device that is usable particularly in a motor vehicle for displaying information to the driver of the motor vehicle. 
     BACKGROUND 
     A display device is described in DE 10 2004 016 808 A1, in which the windscreen of a motor vehicle serves as the display screen. In this conventional display device, display elements are distributed over a significant fraction of the vehicle windscreen, to each of which light is directed via a waveguide associated with a light source arranged at the edge of the windscreen. To ensure that the light from such a light source actually reaches the eyes of the driver of the motor vehicle, it must be diffused in the display elements. The display elements should not obstruct the driver&#39;s vision when they are not lit. Therefore, they should include structures that are small enough to be invisible to the human eye when they are not lit. 
     However, such structures affect ambient light in the same way as they do to the light from the light source, that is to say they scatter it. Accordingly, the windscreen inevitably appears cloudy to the operator at the sites where display elements are located. The more brightly the display element is intended to appear for a given illumination strength from the light source, in other words, the more densely the light-scattering structures are positioned, the more pronounced the cloudiness appears. In order to ensure that the display element is clearly visible while minimizing the clouding of the windscreen, the density with which the light-scattering structures are positioned must be low, and the light source that illuminates the display element must emit correspondingly more light. This leads to increased energy consumption and distracting light reflections can also occur at the edges of the windscreen if a large proportion of the light is propagated through the windscreen without being diffused to the outside by a display element. 
     SUMMARY 
     The present disclosure provides a display device with a translucent windscreen that offers a high degree of visibility even with a light source having low light intensity. According to one variant of the present disclosure, a display device includes a translucent windscreen with which a light source emitting light into the windscreen via an edge thereof is designed with narrow bandwidth and a periodic structure the diffracts the light from the light source is constructed in the windscreen. With such a structure, the light from the light source can be deflected selectively and very efficiently in a direction from which it should be visible, whereas most of the broadband ambient light that passes through the windscreen is not diffused, so that the visibility of the periodic structure is in fact effectively minimized when it is not illuminated by the light source. 
     A line grid is particularly suitable for use as the periodic structure. If the periodic structure includes alternating zones with different refractive indices, light may be diffracted while minimizing the amount of light absorbed, and such a structure also favors the propagation of the light from the light source in the windscreen such that the light passes through a plurality of zones with different refractive indices. 
     In order to guide the light from the light source to the periodic structure, the windscreen may include at least one planar waveguide, which is embedded between two covering layers. The waveguide is planar not only to keep the light bundled perpendicularly to the plane of the windscreen, but at the same time to allow it to spread over the entire width of the windscreen, so that a single light source is able to illuminate a periodic structure in the windscreen extending perpendicularly to its primary radiation direction. The covering layers generally have a higher refractive index than the planar waveguide, so that the light can be guided inside the planar waveguide by total internal reflection. The periodic structure may be constructed on a surface of a covering layer adjacent to the planar waveguide, or inside the planar waveguide itself, the latter alternative being preferred in this case. Multiple periodic structures with different periods may overlap on the windscreen. 
     Each of these multiple periodic structures may be assigned to a light source. The wave-like light source is adapted to the period of the assigned periodic structure in such manner that the periodic structures bend light from the respectively assigned light sources in the same direction. In this way, it is possible for a user looking at the display device from this direction to see different colored light signals at the same location on the windscreen. 
     In addition, at least two periodic structures arranged at a distance from each other may be formed on the windscreen, so that information can also be communicated to the user based on the location where periodic structure lights up on the windscreen from the viewpoint of the user. In this case too, the at least two periodic structures should be assigned to one light source, wherein the wavelength of each light source is adapted to the period of the assigned period structure such that beams bent at the periodic structures intersect each other. When the user directs his gaze at this intersection point, the multiple periodic structures can be perceived simultaneously. 
     According to a preferred application, the windscreen is designed as a windscreen for a motor vehicle. The at least one periodic structure should then bend light in the direction of the driver&#39;s head. 
     If the at least one periodic structure is constructed symmetrically relative to a mirror image on the plane of the windscreen, light will then also be deflected to the outside with the same intensity as it is deflected in the direction of the driver&#39;s head, and will thus be wasted. With a suitable asymmetry of the periodic structure relative to a mirror image on the plane of the windscreen, the portion that is deflected to the outside can be minimized, to the benefit of the portion that is deflected toward the driver&#39;s head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
         FIG. 1  is a schematic view of a front windscreen and dashboard of a motor vehicle equipped with a display device according to the present disclosure; 
         FIG. 2  is an enlarged schematic representation of a bottom edge of the windscreen; 
         FIG. 3  is a schematic section through the windscreen according to a first variant of the present disclosure; 
         FIG. 4  is a schematic section similar that of  FIG. 3  according to a second variant of the present disclosure; and 
         FIG. 5  is a view similar to that of  FIG. 1  of a third variant of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. 
       FIG. 1  shows a schematic view of a motor vehicle dashboard  1  and windscreen  2  from the perspective of a driver. A periodic structure in the form of a line grid  3  is arranged in windscreen  2 , next to the bottom edge  4  thereof, in the driver&#39;s direct forward line of vision, on the other side of steering wheel  5 . Narrow-band monochromatic LEDs, which radiate light into windscreen  2  via bottom edge  4  are hidden in dashboard  1  below the bottom edge  4  of windscreen  2 . 
       FIG. 2  is an enlarged schematic representation of the bottom edge area of windscreen  2  with line grid  3  and a plurality of LEDs  6  arranged at intervals along edge  4  in order to emit monochromatic light into windscreen  2  via edge  4 . The distance between the lines in line grid  3  is selected such that mainly deflected light from LEDs  6  is propagated from line grid  3  toward the driver&#39;s eyes, that is to say in an X-Z plane of the motor vehicle, rising slightly toward the rear. 
     Whereas the of LEDs  6  remains bundled closely together on the way from bottom edge  4  to line grid  3  in a direction perpendicular to the windscreen surface due to reflection on boundary layers of windscreen  2 , such bundling is not provided in the plane of windscreen  2 ; as is indicated by light cone  7  represented by the dashed lines in  FIG. 2 , the light is able to propagate freely laterally in the plane of windscreen  2 , so that a limited number of LEDs  6 , arranged at a distance from each other is sufficient to illuminate the line grid  3  evenly along the entire extension thereof, and there is still room for additional LEDs  8 ,  9  between LEDs  6 , which additional LEDs emit the monochromatic light at different wavelengths from LEDs  6 . 
       FIG. 3  shows a schematic longitudinal section through windscreen  2 . The section plane extends perpendicularly to the lines of line grid  3  through one of the LEDs  6 . Line grid  3  is constructed from plastic in a planar waveguide layer  10 , which is embedded between two covering layers  11  made from mineral glass. In order to keep the light from the LEDs trapped inside waveguide layer  10 , the refractive index thereof is lower than that of the cover layers  11 . Windscreens with a sandwich structure, in which a tough plastic layer is embedded between two layers of mineral glass, are known as safety glass and are already used widely in motor vehicle construction. The plastic layer of such safety glass may be used according to the present disclosure to form line grid  3  in the interior thereof. 
     Light from LEDs  6  that is not bent at line grid  3  propagates further in waveguide layer  10  until it reaches an absorber  20  located on a top edge  19  of windscreen  2 . Absorber  20  may be formed by a black colored layer on top edge  19 , preferably arranged at the bottom of a groove, for example of a gasket  21  that surrounds the periphery of windscreen  2  to block any possible reflection from absorber  20  from dazzling the driver and other road users. 
     The lines of line grid  3  are formed by zones  12 ,  13  extending perpendicularly to the section plane, wherein the refractive index of zones  12  is different from the refractive index of the surrounding plastic material of waveguide layer  10 . Such zones  12  may be obtained for example if during curing waveguide layer  10  is exposed to two intersecting laser beams which thus create an interference pattern corresponding to the arrangement of zones  12  in the waveguide layer  10 . 
     In  FIG. 3 , each of zones  12  is created with a cross section that is asymmetrical relative to a mirror image in a plane that is parallel to the surface of windscreen  2 , and in this case is diamond-shaped. If zones  12  were symmetrical relative to the plane of propagation of the light in windscreen  2 , a deflected beam  14  that escapes from windscreen  2  would be just as intense as the beam  15  reaching the driver&#39;s eye  22  that is deflected at an angle equal to that of beam  14  opposite to the original direction of propagation of the light in waveguide layer  10 . The asymmetry makes it possible to make beam  15  more intense than beam  14  and thus make efficient use of the light from LEDs  6 . 
     Zones  12  might extend across the entire thickness of waveguide layer  10 , from one covering layer  11  to the other, to obtain an intense, deflected beam  15 . In the variant of  FIG. 3 , however, zones  12  are only limited to one surface of waveguide layer  10 , whereas zone  16 ,  17  of a line grid  18  having a shorter period than that of line grid  3 , are located on the opposite surface. The period of line grid  18  is tuned to the wavelength of LEDs  8  (see  FIG. 2 ), so that the light therefrom is bent in the same direction as beam  15 , toward the eye  22  of the driver. In this way, two signals can be displayed in different colors in the same area of windscreen  2  from the viewpoint of the driver. 
     The light does not propagate exactly parallel to the surface in waveguide layer  10 , but rather various propagation modes exist in which the light travels at individually different, small angles to the surface of waveguide layer  10 , which means that the deflected light is also not bundled exactly in the direction of beam  15 , but at all events beam  15  determines the direction of maximum intensity of the bent light; it is also visible from an eye position that is higher or lower than the position shown, though with less intensity. 
       FIG. 4  shows a development of the variant of  FIG. 3 , in which line grids with different periods are arranged in alternating manner on both surface of waveguide layer  10 . In the case under consideration here, line grids  3 ,  18 ,  23  have three different periods, which are tuned respectively to the wavelengths of LEDs  6 ,  8 ,  9  on bottom edge  4  of windscreen  2 , to bend the therefrom to the highest possible degree toward the driver&#39;s eye  22 . The line grids  3  indicated by hatching in the figure are located in alternating manner on the inside and outside of waveguide layer  10 , and they each partially overlap one line grid  18  and one line grid  23  on the opposite side of waveguide layer  10  in each case. If the various line grids  3 ,  18 ,  23  are arranged sufficiently close together, each appears to form an evenly illuminated area when seen from a distance with the eyes  22  of the driver. 
     The wavelengths of LEDs  6 ,  8 ,  9  may each be selected from the red, yellow and green spectral range to communicate a warning signal to the driver with various levels of urgency. It is also conceivable to use red, green and blue LEDs so that when all LEDs are activated together an area of white light it presented to the driver&#39;s eye  22 , or to generate light signals in various shades of color, optionally with incrementally variable intensities, by actuating different LEDs  6 ,  8 ,  9  selectively. 
       FIG. 5  shows a view similar to the  FIG. 1  according to a refinement of the present disclosure, in which a plurality of line grids  3 —optionally overlapping with line grids  18 ,  23  having different line separations as described above—are distributed across the entire width of windscreen  2 . A dash-dotted line indicates a vertical plane  24 , which passes through an LED that is concealed behind steering wheel  5  and in which the light from this LED propagates, at first inside windscreen  2  through the central line grid  3 , and after deflection at line grid  3  toward the eye  22  of the driver. The linear zones  12 ,  13  of central line grid  3  are aligned vertically on plane  24 . 
     In the same way, a plane  24 ′,  24 ″ may also be assigned to each of the non-central line grids  3 ′,  3 ″ in which plane the light from the LEDs  6  assigned to the line grids  3 ′,  3 ″ must propagate in order to reach the eye of the driver. In order to achieve efficient deflection between the eyes of the driver, all of these line grids  3 ′,  3 ″ have lines  12  that are aligned vertically on the corresponding planes  24 ′,  24 ″. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.