Abstract:
Illumination surfaces according to the present invention eliminate or at least reduce linear “stitch” artifacts at edges between tiled illumination devices. As a result, light of substantially uniform intensity is emitted across the entire illumination system. This is achieved, in various embodiments, by reflecting, through the gaps between adjacent light-guide elements, light directed through the bottom surfaces of the elements.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application Nos. 61/151,347 and 61/151,351, filed on Feb. 10, 2009, the entire disclosures of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]    This invention relates to illumination systems, and in particular to systems involving adjacent lighting panels. 
       BACKGROUND 
       [0003]    Slim illumination systems are desirable for many illumination applications, and particularly for low-profile back-illuminated displays. A slim illumination system can be assembled by arranging many small lighting elements in an array. Each lighting element may be, for example, a light-guide panel having a light source that injects light into an “in-coupling” region of the panel and an illumination region where light is “out-coupled” from the light-guide element to provide illumination. In general, the light is emitted substantially uniformly across the illumination region. 
         [0004]    In a typical array configuration, light-guide elements are arranged adjacently in longitudinal and lateral directions. Even if the light-guide elements are butted tightly together, gaps will remain between adjacent elements. Indeed, gaps are often provided intentionally to allow the light-guide elements to expand and contract as the ambient temperature varies without damaging the overall array configuration. Unfortunately, the intensity of light at or near a gap will typically differ from that emitted from the illumination regions. Therefore, the gaps may appear as “stitches”—i.e., relatively dark or light linear discontinuities—in the array. These artifacts are visible in both the longitudinal and lateral directions. 
       SUMMARY OF THE INVENTION 
       [0005]    Illumination devices according to the present invention eliminate or at least reduce the “stitch” effect. As a result, light of substantially uniform intensity is emitted across the entire slim illumination system. This can be achieved by reflecting, through the gaps between adjacent light-guide elements, light directed through the bottom surfaces of the elements. One or more mirrors may be disposed below the light-guide elements, and by adjusting the distance between the bottom surfaces and the mirror(s), the intensity of light reflected through the gap can blend unnoticeably with the light emitted from the illumination surfaces of the light-guide elements. The mirror-to-surface spacing may be adjustable to compensate, for example, for temperature changes, which can cause the light-guide elements to expand or contract and thereby change the gap width. 
         [0006]    In a first aspect, embodiments of the invention relate to an illumination device that comprises a first light-guide element and a second light-guide element. Each light-guide element may include an illumination surface from which light is emitted, and a bottom surface opposite the illumination surface. The first and second light-guide elements are positioned such that there is a gap between the two light-guide elements, i.e., the light-guide elements may not be in contact with each other. One or more mirrors are positioned below the bottom surfaces of the light-guide elements and below the gap between them. The light-guide elements have externally reflective side walls (perpendicular to the illumination and bottom surfaces) that reflect light back into the gap. 
         [0007]    In some embodiments of the illumination device, one or more mirrors are spaced apart from the bottom surfaces of the light-guide elements. One or more mirrors can be specular and one or more mirrors can be diffusive. The illumination device may also include two mirrors positioned such that a portion of one mirror overlaps a portion of the second mirror under the gap. 
         [0008]    In another aspect, the invention relates to a planar illumination device comprising first and second light-guide elements each comprising an illumination surface and an opposed bottom surface, where the first and second light-guide elements are separated by a gap; at least one mirror in opposition to the bottom surfaces of the light-guide elements and underlying the gap; and a position changer for changing a position of the at least one mirror relative to the bottom surfaces of the light guide elements. This facilitates responsiveness to changes in the width of the gap. A position changer may, for example, respond to a change in temperature, e.g., by moving a mirror closer to the bottom surfaces when the temperature increases, and moving a mirror away from the bottom surfaces when the temperature decreases. The position changer can include an expandable element positioned below the mirror. The expandable element may expand when the temperature increases, thereby pushing the mirror towards the bottom surfaces of light-guide elements, and contract when temperature decreases, pulling the away from the bottom surfaces. Alternatively, the position changer may include one or more expandable elements and one or more fulcrums, positioned above the mirror. 
         [0009]    In some embodiments, the first and second light-guide elements may each have a mirrored (i.e., externally reflecting) side wall facing the gap. The reflective side wall can be formed using a partially reflecting mirror, and the reflectivity of the partially reflecting mirror may vary along the length of the side wall. 
         [0010]    One or more mirrors in the illumination device can be positioned at an angle with respect to the bottom surfaces, and the angle may be along a light-guiding direction i.e. an end of the mirror near the in-coupling region may be close to the bottom surfaces and the opposite end of the mirror, near the end wall of the light-guide element opposite to the in-coupling region, may be relatively at a greater distance from the bottom surfaces. Alternatively, the end of the mirror near the in-coupling region may be far from the bottom surfaces and the opposite end near the end wall may be at a relatively shorter distance from the bottom surfaces. 
         [0011]    In some embodiments, the illumination device may include two or more mirrors positioned below the bottom surfaces of light-guide elements. One or more of these mirrors can be positioned substantially in parallel to the bottom surfaces, and one or more of these mirrors may be positioned at an angle with respect to the bottom surfaces. Alternatively, one or more of these mirrors may be positioned substantially in parallel to the bottom surface of the first light-guide element, and one or more mirrors can be positioned at an angle with respect to that bottom surface. The latter configuration can be employed when the bottom surfaces of the two light-guide elements may themselves be at an angle with respect to one another. 
         [0012]    The bottom surfaces of the light-guide elements can have out-coupling features, which can influence the distribution of light from the bottom surface. For example, an out-coupling feature can vary the number of rays transmitted through the bottom surface and may also vary the angle at which such rays are transmitted. The out-coupling features can be bumps and/or grooves. 
         [0013]    In a second aspect, embodiments of the invention relate to an illumination device that comprises a first light-guide element and a second light-guide element. Each light-guide element may include an illumination surface from which light is typically emitted, and a bottom surface opposite to the illumination surface. The first and second light-guide elements are positioned such that there is a gap between the two light-guide elements, i.e., the light-guide elements may not be in contact with each other. The first and second light-guide elements may each have a mirrored side wall facing the gap. The mirrored side wall can be formed using a partially reflecting mirror, and the reflectivity of the partially reflecting mirror may vary along the length of the side wall. 
     
    
     
       LIST OF FIGURES 
         [0014]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0015]      FIG. 1  is a plan view of light-guide elements arranged in an array to form an illumination area. 
           [0016]      FIGS. 2A and 2B  are plan and elevational views, respectively, of a single illumination element. 
           [0017]      FIG. 3A  is a sectional elevation of a portion of a light-guide element having convex bumps as bottom-surface out-coupling features. 
           [0018]      FIG. 3B  is a sectional elevation of a portion of a light-guide element having concave features as bottom-surface out-coupling features. 
           [0019]      FIG. 4A  is a sectional elevation schematically illustrating the behavior of light in connection with the embodiments shown in  FIGS. 3A and 3B , using a single underlying mirror. 
           [0020]      FIG. 4B  is a sectional elevation schematically illustrating the behavior of light in connection with the embodiments shown in  FIGS. 3A and 3B , using a pair of underlying mirrors that overlap beneath the gap between light-guide elements. 
           [0021]      FIGS. 5A and 5B  are partial sectional elevations schematically illustrating two temperature-responsive embodiments of the present invention. 
           [0022]      FIG. 6  is a partial sectional elevation schematically illustrating an embodiment involving a tilted or angled mirror. 
           [0023]      FIGS. 7A and 7B  are plan and partial sectional elevations, respectively, of an embodiment involving blurring of stitch artifacts. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    With reference to  FIG. 1 , an illumination surface  100  is formed by arranging a plurality of light-guide elements  110  in an array. In the surface  100 , a plurality of gaps  115  occur between adjacent light guide elements  110 . With changes in temperature, light-guide elements  110  can contract or expand, thereby changing the widths of the gaps  115  (which may be intentionally created to accommodate temperature-induced changes in the sizes of the light-guide elements  110 ). The dimensional response of the light-guide elements  110  to temperature depends on the material of the light-guide element, as well as the mechanical harness used to create the array  100 . For polymer-based light-guide elements, the change along one dimension can be 0.1 mm per 25° C. 
         [0025]    As shown in  FIGS. 2A and 2B , an individual light-guide element  210  includes an in-coupling region  212 , which receives light from a source such as a light-emitting diode (LED) (not shown); an illumination region  214 ; and opposite the illumination surface  214 , a bottom surface  216 . The light-guide element  210  also has side walls  218  and an end wall  220  distal to the in-coupling region  212 . Light is generally emitted from the illumination surface  214 . End wall  220  has a reflective coating so that light does not penetrate it; instead, it is retained within the light-guide element  210 . 
         [0026]    An embodiment of the present invention is shown in  FIG. 3A . A light-guide element  310  has an illumination surface  314  and an opposed bottom surface  316  (as well as the other features, not illustrated here, that are shown in  FIGS. 2A and 2B ). A mirror  325  is positioned below the bottom surface  316 . The bottom surface  316  has a series of bumps  317  as out-coupling features; that is, these features direct light traveling within the body of light-guide  310  out the bottom surface  316 . Without the out-coupling bumps  317 , light would not be emitted through bottom surface  316 . 
         [0027]    A ray of light  330  in the light-guide element  310  incident on a bump  317  may be reflected as ray  332  toward the illumination surface  314 , in which case it may be emitted as ray  334  from the illumination surface  314 . Alternatively, a ray  330  incident on bump  317  may be directed through the bottom surface as ray  336 . Upon exiting the bottom surface  316 , ray  336  may be reflected back into light-guide element  310  (i.e., through bottom surface  316 ) by mirror  325 . 
         [0028]    The light reflected by mirror  325  may be emitted subsequently from a gap between adjacent light-guide elements. In order for the intensity of light emitted from a gap to approximate the intensity of light emitted from the illumination surface  314 , a certain amount of light (i.e., the number of light rays  336 ) must be directed through the bottom surface  316  toward mirror  325 . In the light-guide element  310 , bumps  317  on the bottom surface  316  may direct approximately 90% of light incident upon them through bottom surface  316 . 
         [0029]    An alternative structure is shown in  FIG. 3B . In this embodiment, the bottom surface  316  has dents  319  as bottom-surface out-coupling features. Dents  319  may direct approximately 50% of light incident upon them through bottom surface  316 . 
         [0030]      FIG. 4A  illustrates the manner in which the embodiments shown in  FIGS. 3A and 3B  direct light through a gap between light-guide elements to hide stitch artifacts. Two light-guide elements  402 ,  404  are positioned adjacent each other. The light-guide element  402  has an illumination surfaces  410  and an opposed bottom surface  412 . Similarly, light-guide element  404  has an illumination surface  414  and an opposed bottom surface  416 .  FIG. 4A  schematically shows out-coupling features  418  generically (i.e., they can be bumps, grooves or both) on bottom surfaces  412  and  416 . The light-guide elements  402 ,  404  are separated by a gap  420 . A mirror  425  is positioned below the bottom surfaces  412 ,  416  and gap  420 . 
         [0031]    A ray of light reflected by mirror  425  through light-guide element  402  may be emitted as ray (b). On the other hand, a light ray reflected by mirror  425  through light-guide element  402  may be retained within the element  402  by total internal reflection, i.e., as ray (c). At least some of the rays striking mirror  425  due to out-coupling features  418  will be reflected into the gap  420  and emerge therefrom, as exemplified as ray (a). Because most of the light reflected into gap  420  will emerge as visible light, whereas only a portion of the light reflected into the light-guide elements  402 ,  404  is actually emitted through respective surfaces  410 ,  414  (the remainder being confined with one of the elements), the “extra” light through gap  420  can serve to hide or at least reduce the stitch artifact. 
         [0032]    Thus, to achieve substantially uniform intensity of light across illumination surfaces  410 ,  414  and gap  420 , the quantity of reflected light retained within elements  402 ,  404  (ray (c)) versus the quantity of reflected light emitted from elements  402 ,  404  (ray (b)), as well as the amount of light entering gap  420 , may be adjusted by varying the distance d between mirror  425  and the bottom element surfaces  416 ,  418 . If mirror  425  were placed in contact with bottom surfaces  412  and  416 , relatively little light would be reflected by mirror  425  into gap  420 , causing the gap to appear dark relative to illumination surfaces  410  and  414 . If mirror  425  were situated too far from bottom surfaces  412 ,  416 , too much light would be reflected by mirror  425  into gap  420 , causing gap  420  to appear brighter than illumination surfaces  410 ,  414 . By optimizing d, the light through gap  420  substantially matches the light emitted through illumination surfaces  410 ,  414 . 
         [0033]    As shown in  FIG. 4B , two mirrors  427 ,  429  may be positioned below the bottom surfaces  412 ,  416 , respectively, and overlap beneath gap  420 . Specifically, a portion  443  of mirror  429  is positioned below a portion  441  of mirror  427 . As mirrors  427  and  429  can be thin, the distance of mirror  427  from the bottom surface  412  can be substantially the same as the distance of mirror  429  from the bottom surface  416 . Accordingly, the intensity of light emitted from gap  420  may be substantially the same as the intensity of light emitted from the illumination surfaces  410  and  414 , eliminating or at least reducing the stitch artifact at gap  420 . 
         [0034]    One limitation of these configurations is that they do not compensate for temperature-induced changes in the width of the gap. If the gap width changes, the amount of light emitted from the gap will also change unless the amount of light reflected into the gap is altered. While this may not be noticeable in some applications, it may well be in others. Two embodiments adapted to alter the amount of light reflected through the gap in a temperature-responsive manner are shown in  FIGS. 5A and 5B , respectively. 
         [0035]    In an embodiment shown in  FIG. 5A , the light-guide elements  502 ,  504  have a gap  520  between them. A mirror  525  (e.g., a polished aluminum plate) is positioned below the bottom surfaces  512 ,  516  of light-guide elements  502 ,  504  and gap  520 . An expansion element  540 , which expands when temperature increases and contracts when temperature decreases, is positioned below and in contact with the underside of mirror  525 . When the temperature increases, causing light-guide elements  502 ,  504  to expand, gap  520  narrows. But at the same time, expansion element  540  expands, pushing mirror  525  toward the bottom surfaces  512 ,  516  (the degree of mirror displacement depending on the temperature change). As explained above, as the distance between mirror  525  and bottom surfaces  512 ,  516  decreases, the amount of reflected light transmitted through gap  520  also decreases. But because gap  520  has become narrower, decreasing the “extra” light emitted through the gap has the effect of preventing overcorrection (and retaining a substantially similar light output across the entire illumination surface). 
         [0036]    Conversely, when the temperature decreases, causing light-guide elements  502 ,  504  to contract, gap  520  widens. Contraction of expansion element  540  pulls mirror  525  away from the bottom surfaces  512 ,  516 , increasing the amount of reflected light through the now-wider gap  520  to prevent undercorrection. Thus, both in the case of increased and decreased temperature, the amount of light emitted from gap  520  remains substantially the same as that obtained without the change in temperature. 
         [0037]    Another approach to temperature correction is shown in  FIG. 5B . A pair of expansion elements  542 ,  544  and a pair of fulcrums  546 ,  548  are positioned above mirror  525 . As the temperature increases, expansion elements  542 ,  544  expand, pushing portions  527 ,  528  of mirror  525  away from the bottom surfaces  512 ,  516 , respectively. As a result, a portion  529  of mirror  525  is pushed toward the bottom surfaces  512 ,  516 , thereby decreasing the amount of light transmitted to gap  520 . Conversely, when the temperature decreases, expansion elements  542 ,  544  contract, pulling portions  527 ,  528  of mirror  525  toward the bottom surfaces  512 ,  516 , respectively, while pushing portion  529  of mirror  525  away from the bottom surfaces  512 ,  516 . The effect of these movements is to increase the amount light reflected through gap  520 . It should be noted that only portions of light-guide elements  502 ,  504  are shown in the figure; in general, mirror  525  will not extend beyond the boundaries of the light-guide elements. 
         [0038]    In some embodiments, the visibility of a stitch is reduced or eliminated by blurring the light emitted through the gap. With reference to  FIG. 6 , a mirror  610  is positioned below the bottom surface  604  of a light-guide element  600  at an angle relative to the bottom surface  604 . Importantly, if the mirror passes beneath the gap, the angle underlies the width of the gap (i.e., the illustrated dimension) but there is no angle along the length of the gap (i.e., the dimension into the page); that is, the distance between the mirror and the plane defined by the bottom surfaces of the light-guide elements varies across, but not along, the gap. The angled position of mirror  610  can be achieved using fasteners or a transparent wedge (both not shown). Moreover, the illustrated embodiment involving one long wedge per light-guide element can be replaced by a “multi-wedge” structure in which multiple wedges, arranged along the width of the light-guide element, so that the light-to-dark variation occurs more than once along the light-guide element. 
         [0039]    As described above, the amount of light transmitted to a gap (not shown) between adjacent light-guide elements increases or decreases as the distance between mirror  610  and the bottom surface  604  increases or decreases, respectively. Consequently, the amount of light reflected back into the light-guide element  600 , and subsequently emitted from the illumination surface  602  of the light-guide element  600 , changes in inverse relation to the distance between mirror  610  and the bottom surface  604 . 
         [0040]    Because mirror  610  is positioned at an angle relative to the bottom surface  604 , its distance from the bottom surface  604  varies along the length of the bottom surface  604 . This causes the amount of light reflected by mirror  610  into the light-guide element  600 , and subsequently emitted through illumination surface  602 , to vary along the length of the illumination surface  602 . As a result, the “extra” light from mirror  610  emitted through the illumination surface  602  is not uniform, but varies gradually from relatively low in region  611  (where the distance between mirror  610  and bottom surface  604  is relatively small) to relatively high in region  613  (where the distance between mirror  610  and bottom surface  604  is relatively large). It should be noted that the in-coupling region of light-guide element  600  is at or beyond (i.e., to the right of) region  611 . 
         [0041]    As the ambient temperature changes, the gap width may change, as explained above. Because the position of mirror  610  is not altered in response to a temperature change in this embodiment, the intensity of light emitted from the gap may also change. But because the intensity of light emitted near the gap varies gradually, the stitch artifact may be less visible. 
         [0042]    Another embodiment in which the visibility of stitch artifacts can be reduced by blurring is shown in  FIGS. 7A and 7B . In this embodiment, the light-guide elements  701 ,  703  are separated by a gap  720 , and have in-coupling regions  704 ,  706 , respectively. A source of light (not shown) injects light into each in-coupling region. A side wall  731  of light-guide element  701 , facing gap  720 , is coated with a partially reflective mirror  741 , and the opposed side wall  733  of light-guide element  703 , facing gap  720 , is also coated with a partially reflective mirror  743 . A partially reflective coating can be formed, for example, by using a mirror coating having varying reflectivity, by introducing openings in the mirror, by varying the sizes of the openings, or by a combination of these techniques. 
         [0043]    As illustrated in  FIG. 7B , a light ray  742  transmitted through side wall  731  is reflected by the partially reflective mirror  743  and emitted from gap  720  as ray  744 . By appropriately selecting the reflectivity of partially reflective mirrors  741 ,  743 , the number of rays  742  transmitted to gap  720  and the number of rays  744  emitted from gap  720  can be adjusted. Accordingly, light emitted from gap  720  can be made substantially similar in intensity to light emitted from illumination surfaces  705 ,  707  of light-guide elements  701 ,  703 . Thus, a stitch artifact near gap  720  can be reduced or eliminated. In this embodiment, the reflected light emerging through number of rays  742  transmitted to gap  720  does not change as gap width changes due to a change in temperature. Therefore, a stitch artifact may appear as a line along the length of gap  720  as temperature changes. 
         [0044]    The artifact can be mitigated, however, by blurring the stitch line. To achieve this, the reflectivity of the partially reflective mirrors  741 ,  743  is varied along the length of gap  720 . As shown in  FIG. 7A , portions  751 ,  752  of mirrors  741 ,  743 , respectively, have high reflectivity. Accordingly, the intensity of light emitted from gap  720  near the in-coupling regions  704 ,  706  is high, causing the gap  720  near the in-coupling regions  704 ,  706  to appear relatively bright. Conversely, portions  754 ,  755  of mirrors  741 ,  743 , respectively, have low reflectivity. Accordingly, the intensity of light emitted from gap  720  near the end of the light-guide elements  701 ,  703  opposite the respective in-coupling regions  704 ,  706  is low, causing the gap  720  near these ends to appear relatively dark. Because the intensity of light emitted along the length of gap  720  is non-uniform, a stitch artifact does not appear as a line; instead it is blurred, thereby reducing its visibility. 
         [0045]    Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.