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, overlapping the illumination surfaces of adjacent light-guide 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 INVENTION 
       [0002]    This invention relates to illumination systems, and in particular to systems involving adjacent light-guide elements. 
       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. Light propagates from the in-coupling region to an “out-coupling” region of the panel, where it is emitted through an illumination surface to provide illumination. In general, the light is emitted substantially uniformly across the illumination surface. 
         [0004]    In an array configuration, light-guide elements can be arranged such that only the illumination surfaces of the light-guide elements (except those at one border of the array) are visible from a location above the array. In this configuration, the in-coupling region of a light-guide element is positioned below the out-coupling region of an adjacent light-guide element. Thus, a viewer sees only the illumination surfaces of the light-guide elements in the array (except, once again, those at one border of the array). 
         [0005]    The resulting discontinuity between adjacent light-guide elements may result in “stitches”—i.e., visible discontinuities in light intensity—in the array. These artifacts are visible in both the longitudinal and lateral directions. 
         [0006]    One approach to minimizing stitch artifacts is to overlap the out-coupling regions of adjacent light-guide elements in an array. As a result, gaps between the illumination surfaces of adjacent light-guide elements are substantially eliminated. As the light-guide elements contract or expand due to a change in temperature, the borders of the out-coupling region of a light-guide element overlapping another light-guide element may shift to different locations, potentially creating stitch artifacts. 
         [0007]    Indeed, stitch artifacts can be seen in an array of light-guide elements having overlapping out-coupling regions even without temperature changes. This can occur, for example, due to the extra light transmitted from the underlying illumination surface through the overlying light-guide element in the overlap region. Further contributing to visible stitch artifacts are internally reflecting end walls, which do not emit light and therefore appear dark, with the degree of visibility depending on the thickness of the end walls and the angle of view with respect thereto. 
       SUMMARY OF THE INVENTION 
       [0008]    Illumination devices according to the present invention can eliminate or at least reduce the “stitch” effect. As a result, light of substantially uniform intensity is emitted across the entire slim illumination system. Stitch artifacts arising from the spacing between overlapping illumination surfaces can be substantially eliminated or reduced by decreasing the thickness of the out-coupling region of a light-guide element overlapping another light-guide element, in the region of overlap. Stitch artifacts can also be substantially eliminated or at least mitigated by configuring the walls of the out-coupling regions of light-guide elements such that they emit some light, and thus compensate (at least partially) for the light discontinuity between overlapping out-coupling regions. Additionally, the walls of the out-coupling regions of light-guide elements may have mirrors that reflect the light emitted from the illumination surface of a light-guide element positioned below the wall. The reflected light may also compensate at least partially for the discontinuity, and may thus eliminate or at least mitigate stitch artifacts. 
         [0009]    In one aspect, embodiments of the invention relate to an illumination device that includes first and second light-guide elements. The first light-guide element comprises a first in-coupling region, where a light source injects light into the light-guide element, and a first out-coupling region that emits the light through an illumination surface. The second light-guide element comprises a second in-coupling region and a second out-coupling region. The two light-guide elements are configured such that a portion of the first light-guide element overlaps a portion of the second light-guide element. In particular, at least a portion of the first out-coupling region overlaps the second in-coupling region but overlaps only a portion of the second out-coupling region. The light-guide elements are in slidable contact to permit relative movement with, for example, changes in temperature. In this arrangement, the second in-coupling region and a small portion of the second out-coupling region are hidden under the overlying first out-coupling region. Therefore, to a viewer positioned above the light-guide elements, only the illumination surfaces of the two out-coupling regions may be visible. To avoid artifacts arising from the addition of light through the first light-guide element from the underlying out-coupling region of the second light-guide element, an absorber may be provided between the elements in the region of overlap. 
         [0010]    In some embodiments of the illumination device, the out-coupling regions each have a flat, planar bottom surface and an opposed illumination surface. Thus, the illumination surface and the bottom surface may be substantially parallel to each other. In some embodiments, however, the illumination surfaces of the out-coupling regions can have a thickness that diminishes along at least a portion of the light-guide element. Thus, the thickness of the first light-guide element may be at a maximum at the end near the in-coupling region, and at a minimum at the other end where the first out-coupling region overlaps the second out-coupling region. For example, the out-coupling regions may be smoothly angled relative to the bottom surfaces of the out-coupling regions 
         [0011]    The first out-coupling region of the illumination device, in some embodiments, may have a partly reflective end wall opposite and facing the first in-coupling region. Some amount of the light propagated from the in-coupling region to the out-coupling region may be reflected back into the out-coupling region by the partly reflective wall, while some amount of light may be emitted from the partly reflective wall. The partly reflective end wall may be homogeneous or may comprise a pattern of reflective coating, with denser areas of the pattern reflecting more light. 
         [0012]    In some embodiments of the illumination device, the first out-coupling region can have an externally reflective end wall opposite the first in-coupling region. Some amount of the light emitted from the illumination surface of the second out-coupling region, positioned below the first out-coupling region, may be incident upon the externally reflective wall, and subsequently, may be reflected by that wall. 
     
    
     
       LIST OF FIGURES 
         [0013]    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: 
           [0014]      FIG. 1  is a plan view of light-guide elements arranged in an array to form an illumination area. 
           [0015]      FIGS. 2A and 2B  are plan and elevational views, respectively, of a single illumination element. 
           [0016]      FIGS. 3A and 3B  are sectional elevations of an illumination device. 
           [0017]      FIG. 4  is a sectional elevation of an illumination device in which the distal end of the out-coupling region is thinner than the light-guide thickness at the in-coupling region. 
           [0018]      FIG. 5  is a sectional elevation of an illumination device that has an end wall having an partially reflective internal surface. 
           [0019]      FIG. 6  is a sectional elevation of an illumination device that has an end wall having an externally reflective surface. 
           [0020]      FIGS. 7A and 7B  is a sectional elevations of two illumination devices, respectively, in each of which the light-guide elements are spaced apart. The illumination device of  FIG. 7B  has an absorber positioned in between the two light-guide elements. 
       
    
    
     DESCRIPTION 
       [0021]    With reference to  FIG. 1 , an illumination surface  100  is formed by arranging a plurality of non-overlapping, adjacent light-guide elements  110  in an array. In the surface  100 , 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 and dimensions 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 in one dimension can be 0.1 mm per 25° C. 
         [0022]    Positioning the light-guide elements so they overlap in one or both directions eliminates the need to reserve a gap for thermal expansion in that direction, because one light-guide element can slide over the other as it expands. It is desirable to position the light-guide elements so that each overlaps not only the out-coupling region of the neighboring light-guide element but also a portion of the neighboring light-guide element&#39;s out-coupling region. This ensures that over an expected range of expansion, the unilluminated surface of the in-coupling region will not be exposed. 
         [0023]    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 out-coupling region  215  having illumination surface  214 ; and opposite to 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 . 
         [0024]    The problem of stitch artifacts arising from overlapping light-guide elements is illustrated in  FIGS. 3A and 3B . A light-guide element  301  has an in-coupling region  303  and an out-coupling region  305 . The out-coupling region  305  has an illumination surface  306  and an opposed bottom surface  308 . The illumination surface  306  has out-coupling features (not shown) which can influence the angle with respect to Z axis at which rays are emitted from the illumination surface  306 . In  FIG. 3B , the angle between Z axis and ray  331  is denoted as θ. The out-coupling region  305  has an end wall  309  opposed to the in-coupling region  303 . In  FIG. 3A , the height of end wall  309  (i.e. the thickness of light-guide element  301 ) is denoted as t. Similarly, a light-guide element  311  has an in-coupling region  313  and an out-coupling region  315 . The out-coupling region  315  has an illumination surface  316  and an opposed bottom surface  318 . Out-coupling region  315  has out-coupling features (not shown), which may be, for example, along the bottom surface of region  315  or dispersed (as in the case of scattering particles) through the thickness thereof. The out-coupling region  315  has an end wall  319  opposed to the in-coupling region  313 . 
         [0025]    As shown in  FIG. 3A , the in-coupling region  313  of light-guide element  311  is positioned under the out-coupling region  305  of light-guide element  301 . A portion of the out-coupling region  315  of light-guide element  311  is positioned under light-guide element  301 . The in-coupling region  313  of light-guide element  311  is in contact with the bottom surface  308  of the out-coupling region  305  of light-guide element  301 . Thus, there may be substantially no vertical gap (i.e., along the Z axis) between the out-coupling region  305  of light-guide element  301  and the in-coupling region  313  of light-guide element  311 . It should be understood that this configuration is illustrative only, and that a configuration of overlapping out-coupling regions having a spacing between such out-coupling regions (e.g., due to mechanical assembly requirements or limitations, or to permit introduction of an absorber as described below) is within the scope of this invention. 
         [0026]    When temperature changes, light-guide elements  301  and  311  may expand or contract along their lengths (i.e., along the X axis) or along their widths (i.e., along Y axis). Light-guide elements  301  and  311  do not expand or contract substantially, however, along their height dimensions (i.e., along the Z axis). Therefore, even when the temperature changes, the out-coupling region  305  of light-guide element  301  remains in contact with the in-coupling region  313  of light-guide element  311 . This characteristic of light-guide elements can be useful in eliminating or substantially reducing stitch artifacts resulting from a change in temperature, as described below. 
         [0027]    The rays denoted  331  are emitted from the illumination surfaces  306 ,  316  of light-guide elements  301 ,  311 , respectively, in a forward direction, denoted F. Additionally, the rays denoted  332  are emitted from the illumination surfaces  306 ,  316  in a backward direction, denoted B. Rays may not be emitted through the end wall  309  of light-guide element  301 , however. Because end wall  309  emits no light, a dark stitch artifact  342  occurs. A stitch occurs even if the end face is unreflective, however, because in that case, too much light will be emitted through the end face. As a result, the stitch will be bright instead of dark. 
         [0028]    The width of the stitch artifact is given by the expression W stitch =t tan (θ), where t is the thickness of end wall  309  and θ is the angle between rays  331  and the Z axis. It should be understood that θ represents the smallest angle between rays  331  and Z axis that can reach the illuminated plane  340 . This is because rays from illumination surface  306  emitted at angles greater than θ are not visible and therefore do not affect the stitch artifact. The rays actually observed depend on the relative position of the observer and on the illumination system geometry. In some applications there are additional optical means that may limit or filter the rays that can reach the illuminated plane  340  as done by brightness enhancement foils in backlight unit for LCD. 
         [0029]    According to the expression above, the width of stitch  342  increases as the thickness of end wall  309  increases. Stitch  342  also appears wider if angle θ is large, i.e. if rays  331  are emitted at an angle close to the X axis. As explained above, if the temperature changes, the length and width of a light-guide element may change, but there may no substantial change in a light-guide element&#39;s thickness. Similarly, the angles at which rays are emitted through the out-coupling features in an illumination surface also generally does not change with temperature. Therefore, the width of a stitch arising due to the configuration shown in  FIG. 3A  may not change substantially in response to temperature changes. 
         [0030]    We now describe various embodiments of an illumination device that address stitch artifacts caused by the end wall of an overlapping a light-guide element. With reference to  FIG. 4 , an illumination device  400  includes a first light-guide element  401 , which itself has an illumination surface  406 . The thickness of light-guide element  401  diminishes to t− at the end wall  409 , which is less than the thickness t+ at the in-coupling region  403 . For example t− can be 0.5 mm and t+ equal to 1 mm. As a result, the surface  406  may not be parallel to the bottom surface  408  of light-guide element  401 , but instead follows an angle with respect thereto. According to the expression set forth above, the reduction in t to t− produces a narrower stitch  442  in the illuminated plane  440 . This is also the case with respect to the second light-guide element  411 , with which light-guide element  401  overlaps. 
         [0031]    A commensurate reduction in the stitch effect can also be achieved by using a light-guide element having a uniformly small thickness t−. Such a light-guide element may be structurally weak, however, compared to light-guide element  401 ,  411  and it may not emit light of a desired intensity. In a typical light-guide element, the intensity of light emitted from the illumination surface is directly related to the number of rays reflected by the bottom surface toward the illumination surface. The latter number depends on the distance between the illumination and bottom surfaces. Thus, if the thickness of a light-guide element is uniformly small, its illumination and bottom surfaces may be too close to each to achieve the desired light output. Therefore, a light-guide element having uniformly small thickness may be unsuitable for applications that require high brightness. 
         [0032]    Illumination device  400  exhibits adequate strength because the thinnest portion overlaps the adjacent element, and because the thickness of the element diminishes only gradually to t−, the bottom surfaces  408 ,  418  can reflect a substantial amount of light within the respective elements. 
         [0033]      FIG. 5  shows another embodiment  500  of an illumination device according to the present invention. A first light-guide element  501  is positioned above a second light-guide element  511  such that the out-coupling region  505  of light-guide element  501  overlaps the in-coupling region  513  and a portion of the out-coupling region  515  of light-guide element  511 . The out-coupling region  505  does not overlap a significant portion of the out-coupling region  515 . 
         [0034]    The inside surface of end wall  509  (i.e., the surface facing the in-coupling region  503 ) has a partially reflecting mirror  551 . By “partly reflecting” is meant that the mirror  551  reflects at least 5% of the incident light, and preferably at least 30%, but no more than 95%. A partly reflecting mirror can be fabricated, for example, by applying a partly reflective coating on an end wall or by patterning a coated end wall with small openings through which some of the light is emitted. 
         [0035]    Mirror  551  reflects a portion of light incident upon it to the out-coupling region  505 , and allows a portion of light to be emitted from end wall  509  as rays  533 ,  534 ,  535 . At least a portion of the light emitted from end wall  509  may reach the illuminated plane  540  as rays  533 . The reflectivity of mirror  551  is selected such that the amount of light emitted from end wall  509  (in particular, the number of rays  533 ) is substantially the same as the amount of light that would reach plane  540  in the absence of an end wall  509 . As a result, the stitch artifact that would occur due to the end wall  509  is substantially eliminated or at least reduced. 
         [0036]    In another embodiment, illustrated in  FIG. 6 , the end wall  609  of light-guide element  601  has a mirror  662  on its outer surface (i.e., the surface facing the space above the illumination surface  616  of light-guide element  611 ). The out-coupling regions  605 ,  615  of light-guide elements  601 ,  611 , respectively, have out-coupling features  660  such as printing dots (shown schematically). The out-coupling features  660  are selected such that the distribution of light emitted from illumination surfaces  606 ,  616  in backward and forward directions (denoted B and F, respectively) is symmetrical. A symmetrical light distribution in backward and forward direction means that the angle of rays  632  with respect to the Z axis and the angle of rays  631  with respect to the Z axis are substantially the same in magnitude but opposite in direction. 
         [0037]    A backward ray  652  emitted from illumination surface  616  has substantially the same angle with respect to the Z axis as rays  632 . Ray  652  is incident upon mirror  662  of end wall  609 . Mirror  662  reflects ray  652  as ray  651 . Because rays  631 ,  632  have a symmetrical distribution, the reflected ray  651  is emitted substantially at the same angle with respect to the Z axis as rays  631 . Thus, the light that would not have reached an illuminated plane  640  had the end wall  609  been without mirror  662  is replaced by rays  651 . As a result, the stitch artifact may be substantially eliminated or reduced. As described above, the distribution of light from out-coupling features  660  does not change with temperature, so the efficacy of stitch correction does not vary with temperature changes. 
         [0038]    It should be noted that the embodiment shown in  FIG. 5  is particularly useful in connection with configurations where most of the light is out-coupled in the forward direction. The embodiment shown in  FIG. 6  is particularly useful in connection with configurations where the out-coupled light is distributed evenly between the forward and backward directions (e.g., a Lambertian distribution). 
         [0039]    In some embodiments, light-guide elements can overlap one another, in part, without making contact. Such an illumination device is shown in  FIGS. 7A and 7B . In the illustrated embodiments, a gap separates the overlapping portions of the light-guide elements  701 ,  703  and light from the portion of out-coupling region  705  (of light-guide element  703 ) that underlies the gap  708  is emitted therefrom. Light trapped in gap  708  can propagates to the right and escapes at the end of light-guide element  701 . This light can create a bright stitch artifact due its different light distribution. As shown in  FIG. 7B , the bottom of the upper light-guide element  701  may be coated with an absorber  715  for absorbing this stray light. 
         [0040]    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.