Abstract:
The invention is directed to an arrangement for the homogeneous illumination of an image plane, preferably for application in a head-up display in a motor vehicle, comprising illumination optics having an array of emitters with a broad emitting characteristic, for example, an arrangement of luminescent diodes (LEDs, OLEDs), an integrator array, and an image-generating element, and the optical axis of an emitter is associated with the mechanical axis of an integrator of the integrator array. According to the invention, at least two microlens arrays are provided for the purpose of achieving an angular homogeneity of the rays exiting from the integrator array on the illuminated area of the image-generating element at the light outlet of the integrator array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of German Application No. 10 2005 013 950.7, filed Mar. 26, 2005, the complete disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     a) Field of the Invention  
         [0003]     The invention is directed to an arrangement for the homogeneous illumination of an image plane, preferably for application in a head-up display (HUD) in a motor vehicle, comprising illumination optics having an array of emitters with a broad emitting characteristic, for example, an arrangement of luminescent diodes (LEDs, OLEDs), an integrator array, and an image-generating element, and the optical axis of an emitter is associated with the mechanical axis of an integrator of the integrator array.  
         [0004]     b) Description of the Related Art  
         [0005]     Head-up displays are known and are increasingly offered as accessories in particular vehicle models. A virtual image of an object to be displayed is generated and, for example, is reflected into the windshield of the vehicle. To the observer, the image appears virtually in front of the vehicle on the road. Because of the optical imaging system that is required for this purpose, the observer can only discern the image when at least one of the observer&#39;s eyes is situated in the illuminated observation field. The image-generating elements are usually LCDs (liquid crystal displays) which require a very bright light source that can be dimmed in a dynamic range of 3000:1. Illumination sources of this kind are high-output luminescent diodes (LEDs) which are being used to an increasing extent.  
         [0006]     In order to see a uniformly illuminated image in every position of the observation field, the light of the LEDs must be “shaped” in such a way that it illuminates the surface of the image-generating element as well as a defined angular area homogeneously.  
         [0007]     Known arrangements comprise an array of hollow integrators. However, they have the disadvantage that neither the angular area nor the surface of the image-generating element is illuminated in a completely homogeneous manner. This is further complicated by the fact that hollow integrators are not particularly efficient because of the inner reflecting surfaces. Further, it is very difficult in terms of technique to coat hollow integrators having lengths over 10 mm from the inside. Yet, the requirement for homogeneous illumination of surfaces and angles necessarily leads to longer integrators.  
         [0008]     An illumination arrangement of the kind mentioned above for image projection which comprises an illumination source array and an array of funnel-shaped hollow integrators is described, for example, in U.S. Pat. No. 6,318,863. The funnel shape of the hollow integrators has the advantage that the light radiation proceeding from the illumination source is distributed in a homogenized manner on a larger surface when the numerical aperture is reduced. This is important precisely when using light sources with relatively large radiating angles in order to avoid complicated, bulky collecting optics. In systems in which the light emergence angles vary in x-direction and y-direction and in which high efficiency is demanded, it is very complicated to achieve a homogeneous, well-defined angular distribution.  
         [0009]     The disadvantage in the use of integrator arrays consists in the high manufacturing cost required to achieve a high positioning accuracy of the individual integrators, so that components of this type are very cost-intensive.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0010]     Proceeding from the above-described disadvantages of the prior art, it is the primary object of the invention to further develop an arrangement for the homogeneous illumination of an image plane in such a way that it is possible to improve the delimitation of the angular distribution at the light outlet and to improve the homogeneous illumination of the image plane by reducing technological costs with respect to the arrangement combined with a cost reduction for the illumination unit in its entirety.  
         [0011]     This object is met, according to the invention, by an arrangement of the type described in the above in that at least two microlens arrays are provided for the purpose of achieving an angular homogeneity of the rays exiting from the integrator array on the illuminated area of the image-generating element at the light outlet of the integrator array, wherein the rays exiting from the integrator array impinge on the microlenses of the first microlens array.  
         [0012]     The light proceeding from the illumination optics, that is, from the LED light sources, is initially collected by the associated integrators of the integrator array. Because of the multiple reflections in the integrators, the light components are homogenized when passing through the integrators, wherein only the light outlet surfaces are homogeneously illuminated. The areas of the radiating angles are homogenized through the subsequent arrangement of the microlens arrays in such a way that the light on the image-generating element uniformly illuminates a sharply delimited area, that is, in such a way that the angular distribution of the light after the microlens array is homogeneous in the aperture of the microlenses.  
         [0013]     It is advantageous when two microlens arrays which are arranged one behind the other are characterized by identically constructed, regular arrangements of microlenses, which arrangements lie parallel to one another and in a mirror-inverted manner relative to one another, wherein the microlenses whose optical axes lie parallel to the optical axis of the illumination optics have raised functional surfaces.  
         [0014]     In another conceivable constructional variant of the microlens arrays, the latter comprise two identically constructed arrangements of microlenses that are arranged one behind the other and the microlenses whose optical axes lie parallel to the optical axis of the illumination optics have raised functional surfaces which are oriented in the same direction.  
         [0015]     For the purpose of an efficient field homogenization of the light exiting from the integrators, the distance between the microlens arrays that are arranged one behind the other should preferably be only ≦10 mm.  
         [0016]     The paraxial focal length of the first microlens array should be in the vicinity of the output surface of the second microlens array. Since this focal length is rarely greater than 10 mm, the distance between the microlens arrays which are arranged one behind the other should preferably be only ≦10 mm.  
         [0017]     An advantageous further development of the arrangement consists in that the two microlens arrays are made from one component (twofold microlens array). This reduces the quantity of individual elements and, therefore, also the assembly cost.  
         [0018]     In order to achieve a high efficiency with respect to the homogeneous illumination of the image-generating element, the radii of curvature of the microlenses preferably deviate from one another by only ≦20%.  
         [0019]     In another embodiment form, the microlenses are constructed so as to be cylindrical and rectangular, and the microlenses of the first array are arranged so as to be oriented at a 90-degree offset (crosswise) to the microlenses of the second array. In this way, the light is homogenized in the x-direction and y-direction. The substantial advantage of this variant consists in the reduced manufacturing cost as a result of less strict tolerances in the alignment and centering of the microlens arrays.  
         [0020]     Further, it can be advisable to arrange the arrays in such a way that the microlenses of neighboring rows of microlenses are arranged so as to be displaced by one half of their length. This improves field homogeneities particularly in relatively large lenses.  
         [0021]     The integrator array comprises identically constructed solid integrators or hollow integrators which are arranged directly next to one another and are produced from plastic or glass. It is better to use solid integrators because they can be produced more easily. It is disadvantageous that the structural lengths of solid integrators must be about 1.5-times larger than the structural lengths of hollow integrators.  
         [0022]     The integrators are advantageously funnel-shaped and every light inlet surface is smaller than the light outlet surface. Every funnel-shaped integrator advisably comprises at least two step segments. The light outlet surface of a first step segment is adapted to the light inlet surface of a second step segment, and the angles between the reflecting beam guiding surfaces and the centering axes of the integrators of adjacent step segments are unequal.  
         [0023]     As a result of the multiple-step arrangement of the integrators, virtually the totality of light proceeding from the illumination optics is transported to the light outlet surfaces when there is a change in the radiating angle. Through the dimensioning of the segments, which become progressively smaller from the light entrance surface to the light outlet surface, the respective light entrance angle and light emergence angle can be adapted in such a way that a homogeneous beam bundle that meets requirements with respect to the beam field and radiating angle occurs at the end of a multiple-step integrator.  
         [0024]     The cross-sectional areas of the individual segments of the integrators are advantageously rectangular because these shapes bring about an exactly homogeneous field and a well-defined elliptic angular distribution. Up to 80 percent of the light entering an integrator reaches the required angular area (acceptance angle) so that the light drops off very sharply outside this acceptance angle.  
         [0025]     It is also conceivable to obtain different efficiencies of the light transmission through different cross-sectional shapes between the light inlet surfaces and the light outlet surfaces of the segments of the integrators in order to adapt the intensity distribution in the field and in the radiating angle area to the illumination arrangement in accordance with requirements.  
         [0026]     Also conceivable are arrays of the type mentioned above in which the inner segments of an integrator have different light inlet surfaces and light outlet surfaces (cross section) so that the lateral surfaces of an integrator are constructed in an irregular manner.  
         [0027]     The inventive arrangement of the illumination optics in an array, a subsequent integrator array, and the lens arrangements having at least two arrays obviate the need for collecting optics for homogenization and intensity profiling, so that the arrangement is more economical and compact compared to the solutions of the prior art. Moreover, by adapting the illumination angles to the acceptance angles of the subsequent system, the efficiency of the system is increased and, by reducing stray light components, contrast is increased.  
         [0028]     In an advantageous constructional variant of the arrangement using solid integrators made of plastic, the integrator array comprises at least two array portions which are manufactured by injection molding, each array portion having a base plate on which the integrators are shaped in multiple rows in such a way that they communicate with one another by the corners of their light outlet surface corners, while openings are provided between the integrators of an array portion for receiving the integrators of the second array portion.  
         [0029]     The two array portions are produced by an injection molding process. Subsequently, by inserting one array portion into the other array portion, a closed integrator array is formed in which the individual integrators lie directly against one another by their walls.  
         [0030]     The two-part arrangement of the arrays comprising solid integrators is a very economical variant, especially since a monolithic construction of an array in which the integrators contact one another directly could only be produced by a very costly manufacturing technique. This could not be realized by means of injection molding because an injection molding die must have a minimum wall thickness (distance between the integrators to be molded) of about 0.8 mm.  
         [0031]     The arrangement according to the invention will be described more fully in the following by way of example with reference to the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     In the drawings:  
         [0033]      FIG. 1  is a schematic view of the arrangement according to the invention with two microlens arrays;  
         [0034]      FIG. 2  is a schematic view of the arrangement according to the invention with a twofold microlens array;  
         [0035]      FIG. 3  shows the radiating angle at the light outlet surface of an integrator;  
         [0036]      FIG. 4  shows the acceptance angle at the light outlet of the microlens array;  
         [0037]      FIG. 5  shows an embodiment form of a microlens array with uniformly arranged lenses;  
         [0038]      FIG. 6  shows an embodiment form of a microlens array with lenses that are arranged so as to be offset;  
         [0039]      FIG. 7  shows an embodiment form with two crossed microlens arrays;  
         [0040]      FIG. 8  shows an embodiment form of an individual integrator;  
         [0041]      FIG. 9  is a schematic view of a first array portion comprising solid integrators;  
         [0042]      FIG. 10  is a schematic view of a second array portion comprising solid integrators;  
         [0043]      FIG. 11  shows the positioning of the array portions according to  FIGS. 7 and 8  before their assembly; and  
         [0044]      FIG. 12  shows an assembly position of the two-part solid integrator array. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]      FIG. 1  is a schematic view of the arrangement according to the invention with illumination optics  1  comprising an LED array, an integrator array  2 , two microlens arrays  3  and  4 , and an image-generating element  5 .  
         [0046]     The light emitted by the individual LEDs of the illumination optics  1  first reaches the associated integrators of the integrator array  2 . Through the multiple reflections in the integrators, the light components are homogenized when passing through the integrator array  2  and the light outlet surfaces are illuminated homogeneously. Due to the arrangement of the microlens arrays  3  and  4  downstream, these microlens arrays  3  and  4  comprising two identically constructed, regular arrangements of identical lenses with raised surfaces, which arrangements are parallel to one another and mirror-inverted with respect to one another, the optical axes A 1 , A 2 , A 3  and A 4  of the lenses extending parallel to the optical axis AB of the illumination optics  1 , the areas of the radiating angles α 1  are homogenized in such a way that the light  6  on the image-generating element  5  uniformly illuminates a sharply delimited area  7 .  
         [0047]     In a modification of  FIG. 1 ,  FIG. 2  shows an illumination arrangement in which only one component  8  with a twofold microlens array  3   a  and  4   a  is provided instead of the two microlens arrays  3  and  4  arranged one behind the other. The arrangement of the microlenses relative to one another was carried out analogous to the variant shown in  FIG. 1 . The advantage of this variant is that the quantity of individual elements and, therefore, the assembly cost is reduced.  
         [0048]      FIGS. 3 and 4  show the radiating angle α 1  between the light flux  6  at the light outlet surface of an integrator and the optical axis A 1  of the integrator ( FIG. 3 ) and the acceptance angle α 2  ( FIG. 4 ) at the light outlet from the microlens array  4  between the light flux  6  and the optical axis A 1  of a lens of the microlens array, where 
 α1=α2.  
         [0049]      FIG. 5  shows the detailed embodiment form of a twofold microlens array  3   a  and  4   a  according to  FIG. 2 . This twofold microlens array  3   a  and  4   a  comprises two identically constructed, parallel and mirror-inverted regular arrangements of identical lenses  9  and  10  with raised surfaces  11  and  12  whose radii do not differ from one another by more than 20%. The focal points f 11  of the raised surfaces  11  lie virtually on the oppositely located surface  12 .  
         [0050]     Two microlens arrays which are required for homogenization and are arranged one behind the other at the distance of the focal point f can be dispensed with through the construction of the surfaces  11  and  12 , that is, due to the distances d between these surfaces  11  and  12 . Only one component  8  with the twofold microlens array  3   a  and  4   a  is required. The absolute value of the distance d between the surfaces  10  and  111  of the microlenses  9  and  10  is defined by the equation 
 
 d=n·f,  
 
 where f is the focal length of the microlens in air and n is the index of refraction of the medium from which the microlens was produced. 
 
         [0051]     The length L of a microlens  9  or  10  is given by the required maximum angle α of the light exit from the microlens array  3   a  or  4   a  (acceptance angle) and the focal length f according to the following equation: 
 
 L= 2 ·f ·tan α. 
 
         [0052]     When using a microlens array  3   a  and  4   a  of the type mentioned above, the homogeneous light field which exits from the microlens array  4   a  facing the image-generating element  5  reaches the image-generating element  5  (see  FIG. 1 ) at the defined angular distribution, a field point being associated with each lens of the microlens array  3   a  and  4   a . When the microlenses  9  and  10  are very small, these field points can no longer be resolved, so that the illuminated surface  7  on the image-generating element  5 , or the observation field, is homogeneously illuminated.  
         [0053]     When cylindrical, rectangular lenses are used, for example, the areas in the x-direction and y-direction of the homogeneous illumination are different so that stripes can occur in a coordinate direction. In order to prevent this effect, the rows of microlenses lying next to one another on the microlens arrays are arranged so as to be offset relative to one another by one half of a microlens  9  or  10 . An arrangement of this kind with an offset V is shown in  FIG. 6 . With rectangular microlenses, e.g., made of BK 7 , with the following dimensions:  
         [0000]     length L=420 μm,  
         [0000]     width B=320 μm,  
         [0000]     radius of curvature R=0.98 mm,  
         [0000]     an optimal illumination of the image-generating element  5  without stripes is generated at an offset V of the lenses of adjacent rows of 210 μm.  
         [0054]      FIG. 7  shows an arrangement with two crossed microlens arrays  13  and  14  which can be applied when a rectangular observation field (eye box) is needed. In this connection, each of the microlens arrays  13  and  14  takes over the angle homogenization in the x-direction and y-direction. In order to achieve a good field homogenization, the distance  1  between the microlens arrays  13  and  14  should be ≦10 mm.  
         [0055]      FIG. 8  shows an individual integrator  15  of an integrator array  2 , according to  FIGS. 1 and 2 , which comprises six different, assembled segments S 1 , S 2 , S 3 , S 4 , S 5  and S 6  having rectangular cross sections. Segments S 1 , S 2 , S 3 , S 4 , S 5  and S 6  are shaped in such a way that the lateral surfaces are formed as plane surfaces and the light inlet surfaces are smaller than the light outlet surfaces. Every segment S 1 , S 2 , S 3 , S 4 , S 5  and S 6  is defined by the lengths H 0  (light inlet), H 1 , H 2 , H 3 , H 4 , H 5  and H 4  and the semiaxes in the x-direction ry 0 , rx 1 , rx 2 , rx 3 , rx 4 , rx 5  and rx 6  and the semiaxes in the y-direction ry 0 , ry 1 , ry 2 , ry 3 , ry 4 , ry 5  and ry 4 .  
         [0056]     A homogenization efficiency of approximately 70.5% is achieved with the following parameters:  
                                                                                         Semiaxis   Semiaxis               Length H0 to H6   rx0 to rx6   ry0 to ry6           Segment   [mm]   [mm]   [mm]                                        Light inlet   0.000   0.450   0.450           S1   2.565   0.950   0.850           S2   2.5625   1.248   1.050           S3   5.125   1.800   1.440           S4   10.250   2.400   1.890           S5   10.250   2.900   2.230           S6   10.250   3.300   2.500                      
 
         [0057]      FIGS. 9, 10 ,  11  and  12  show the embodiment form of an arrangement of solid integrators  18  and  19  which comprises two array portions  16  and  17 . Array portion  16  is shown in  FIG. 9  and array portion  17  is shown in  FIG. 10 . The two array portions  16  and  17 , which are produced from plastic by injection molding, have nine solid integrators  18  and  19  that are formed on the base plates  20  and  21 . The base plate  21  of array portion  16  is constructed in such a way that there are openings  22  between the solid integrators  18 . The openings  22  serve to receive the solid integrators  19  and are shaped and dimensioned in such a way that the solid integrators  19  of array portion  17  completely fill the openings  22  of array portion  16  in the assembled state of the array portions  16  and  17 .  
         [0058]      FIG. 11  shows the positioning of the array portions  16  and  17  before they are assembled, according to  FIG. 12 , to form the complete integrator array.  
         [0059]     After the solid integrators  19  are fully inserted into the openings  22 , that is, when the base plate  20  of array portion  17  contacts the base plate  21  of array portion  16 , the integrator array, in which adjacent solid integrators  18  and  19  contact one another directly, is formed in a relatively simple manner.  
         [0060]     While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.  
       Reference Numbers  
       [0000]    
       
           1  illumination optics (LED)  
           2  integrator array  
           3 ,  3   a ,  4 ,  4   a ,  13 ,  14  microlens array  
           5  image-generating element  
           6  light flux  
           7  image-generator area  
           8  optical element  
           9 ,  10  microlens  
           11 ,  12  raised surface  
           15  integrator  
           16 ,  17  individual element  
           18 ,  19  solid integrator  
           20 ,  21  base plate  
           22  opening  
          D,  1  distance  
          f focal point  
          α acceptance angle  
          A 1 -A 4 , AB optical axis  
          L lens length  
          B lens width  
          R radius of curvature  
          V offset  
          S 1 -S 6  integrator segment  
          H 0  light inlet  
          H 1 -H 6  integrator segment length  
          X, Y coordinate direction  
          rx 0 -rx 6  semiaxis, x-direction  
          ry 0 -ry 6  semiaxis, y-direction  
          α 1  radiating angle  
          α 2  acceptance angle