Abstract:
A mixing optic combines a variety of features to enhance the brightness and uniformity of the light emitted from a combination of different color light emitting sources. The mixing optic may include a parabolic reflector that redirects low-angle emissions, a reflective waveguide that mixes the light spatially, and a diffuser plate that mixes the light angularly. In an embodiment, the output of the mixing optic exhibits a substantially Lambertian output pattern of a substantially uniform mix of colors, exhibiting, for example, the light output pattern of a single white light source.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of light emitting devices, and in particular to an optical element that efficiently mixes the colors from multiple light sources to produce a substantially uniform programmable color output with Lambertian characteristics. 
       BACKGROUND OF THE INVENTION 
       [0002]    Optics for combining the light output from multiple light sources are common in the art, particularly for directional illumination, as illustrated in  FIGS. 1A and 1B . 
         [0003]      FIG. 1A  illustrates the use of a compound parabolic reflector  110  that receives the light output from multiple light sources  101  via a light input surface  120  and emits the composite light from a light emitting surface  130 . The light sources  101  may be mounted on a submount  105 . 
         [0004]    The side surfaces  115  of the parabolic reflector  110  may be coated with a reflective coating, or the light guide  110  may be encased in a second reflective material, or a material with an index of refraction that facilitates reflection by total internal reflection. 
         [0005]    The light output patterns of conventional light sources  101  generally exhibit a Lambertian light output pattern, emitting substantially the same amount of light in each angular direction from the light emitting surface. Light that is emitted at near-orthogonal angles (not illustrated) from the light emitting surfaces of the light sources  101  may be emitted from the light exit surface  130  of the light guide directly, without reflection within the light guide  110 . A substantial portion of the light emitted from the light emitting surfaces, on the other hand, will be emitted from the light exit surface  130  after one or more reflections at the side surfaces  115  of the light guide  110 , as illustrated. 
         [0006]    Because the surfaces  115  of the tapered light guide  110  are at an angle that is not orthogonal to the light emitting surfaces of the light sources  101 , the angle at which the light is reflected from the surface, relative to the surfaces of the light sources  101 , is closer to the orthogonal than the angle at which the light was emitted from the light sources  101 . With each reflection from the sloped surfaces  115 , the angle of reflection, relative to the light emitting surfaces, continues to become closer to the orthogonal. Accordingly, the light output from the exit surface  130  is more collimated than the light emitted from the surfaces of the light sources  101 . 
         [0007]    The curvature of the surfaces  115  is designed such that light at any angle coming from a point source at the center of the light guide  110  will be reflected in the orthogonal direction relative to the light emitting surface. That is, the tangent to the curve at each point on the surface  115  is at an angle relative to the surface of 90-A/2 degrees, where A is the angle from the point source to that point. If the light source were, in fact, a point source at the center of the light guide  110 , all of the light reflected from the surfaces  115  would be reflected in the same direction, orthogonal to the surface, producing a highly directional light output from the light guide  110 . 
         [0008]    In an actual embodiment of a light source such as a semiconductor light emitting device, the light is emitted from the light emitting surface area of the light source, rather than an ideal single point in the center of the light guide  110 . Light that is emitted from locations on the surfaces of the light sources  101  that are not at the center of the light guide will not strike the surface  115  at the proper point for being reflected orthogonal to the surface, resulting in a light output pattern that is less collimated than the light output from a single point in the center of the light guide  110 . 
         [0009]    Accordingly, a design goal for applications requiring a highly directional light output is to minimize the surface area of the light source to more closely resemble a point source. However, the amount of light that can be emitted by a semiconductor light emitting device is dependent upon the light emitting surface area of the device, and, in general, the greater the light emitting surface area, the greater the intensity (or brightness) of the light output. To achieve a light output that is very bright, and suitable for directional lighting, multiple light emitting devices are densely situated in the center of the light guide that serves to collimate the light output. 
         [0010]    In some embodiments, the curvature of the surfaces  115  is modified so as not to ‘favor’ light that is emitted from the center of the light guide  110 . That is, the curvature may be such that light emitted from the center of the light guide  110  is reflected at an angle that is not orthogonal, while light emitted from off-center locations are reflected at a more orthogonal angle. However, regardless of the modifications to the light guide, the light output from a light guide that receives light from a surface area will be less collimated than the light output from a light guide that receives light from a point source. 
         [0011]    In most situations, particularly when the maximum deviation from the center of the light guide  110  is small, the lack of perfect collimation does not introduce adverse effects, other than producing a light output pattern having a wider beamwidth than the ideal. Consider, however, the effects when the surface area of the combined light sources  101  is large, and when the different color light sources  101  are not randomly distributed on the substrate  105 . 
         [0012]      FIG. 1B  illustrates an example light guide  160  that is designed to accommodate a large number of light sources,  101 R,  101 G, and  101 B, representing red, green, and blue light sources, respectively. For a variety of manufacturing reasons, arrays of multicolor light emitting devices are typically arranged in banks of each color on a substrate  105 . In this example, a 9×9 array of light sources is arranged with a 3×9 bank of red light sources  101 R, a 3×9 bank of green light sources  101 G, and a 3×9 bank of blue light sources  101 B. 
         [0013]    Three light beams  180 R,  180 G, and  180 B are illustrated as being emitted from a red light source  101 R, a green light source  101 G, and a blue light source  101 B. Each of these beams  180 R,  180 G,  180 B are emitted at the same angle relative to the surfaces of the light sources  101 R,  101 G, and  101 B. In this example, the curvature of the side surface  165 A of the light guide  160  is such that the light  180 B is reflected at a near-orthogonal angle, relative to the exit surface  190 . However, the light  180 G, striking the surface  165  at a higher elevation, having a steeper slope, is emitted at an angle that farther from the orthogonal of exit surface  190  than the light  180 B; and the light  180 R, striking at an even higher elevation, is emitted at an angle that is even further from the same orthogonal. 
         [0014]    On the side surface  165 B, on the other hand, the opposite effect is produced. Light  185 R is reflected at a near-orthogonal angle, while light  185 B is reflected far off the orthogonal. 
         [0015]    One of skill in the art will recognize that curvature of the sides  165 A,  165 B may be shaped differently, and a different optical effect will be produced. For example, the sides  165 A,  165 B may be shaped such that the light  180 G,  185 G from the center of the array is reflected at an orthogonal, or near orthogonal angle, rather than the light  180 B striking side  165 A and  185 R striking side  165 B, but this adjustment will only cause the light  180 B and  185 R to be reflected at an angle that is significantly off the orthogonal. In practice, the curvature (parabolic characteristics) are selected such that this non-uniform (different angles of reflection for different colors) is least noticeable, or least objectionable. 
         [0016]    The overall effect of the non-uniform reflection pattern of  FIG. 1B  is that on the left side of the light emitting surface  190 , very little light  185 B from the blue light source  101 B will exit the surface  190  at an orthogonal angle, while substantially more light  185 R from the red light source  101 R will exit the surface  190  at an orthogonal angle. On the right side of the light emitting surface  190 , very little light  180 R from the red light source  101 R will exit at an orthogonal angle, while substantially more light  180 B from the blue light source  101 B will exit the surface  190  at an orthogonal angle. Light from the green light source  101 G, on the other hand, will be symmetrically distributed across the left and right sides of the exit surface  190 , albeit not orthogonal in this example. 
         [0017]    This non-uniform distribution of the different colors and different emission angles of the colors presents a number of drawbacks, particularly in systems that are designed to provide a directional light output with a uniform mix of the colors, such as a system that produces a directional white light output. Viewed from an orthogonal direction, the right side of the surface  190  is likely to appear more blue than the left side, and the left side of the surface  190  is likely to appear more red than the right side. As the viewing angle changes, the intensity of the off-orthogonal light on one side will appear to increase while the intensity of the off-orthogonal light on the other side will appear to decrease. 
         [0018]    It is significant to note that this non-uniform distribution of colors and emission angles is primarily caused by the optics  160  used to provide a directional, collimated light output. In an application that provides non-directional lighting, such as a retrofit light bulb having a wide field of illumination, the Lambertian light output from each of the light sources of different color naturally overlap each other, and will provide a similarly perceived output regardless of the angle of view. 
         [0019]    In like manner, if each of the light sources were of the same color, or the different color sources were sufficiently randomly distributed on the substrate, or the different colors struck the input surface of the light guide  160  in a random manner, the different patterns of reflection on each side  165 A,  165 B of the light guide  160  would be immaterial, other than being a cause of an increased beamwidth compared to a truly collimated light output. 
         [0020]    USP 2012/0069547 published for Gielen et al. on 22 Mar. 2012, provides a color-mixing optical element  250  situated between the light sources  101  and the parabolic reflector  210 , as illustrated in  FIG. 2 . Light that is emitted from the light sources  101  at angles substantially off from orthogonal is reflected from the walls  255  of the color mixing element  250 , increasing the likelihood that the light emitted from any particular light source  101  will exit the surface  220  and strike the walls  215  of the reflector  210  in a more randomly distributed pattern, thus producing a more uniform light output from the light emitting surface  230 . Although light from the left and right sides of the surface  220  will be reflected differently from the walls  255  than light from the center of the surface  220 , the particular color or pattern of the light from each side is more random, producing less of a color-specific non-uniformity on the surface  230 . 
       SUMMARY OF THE INVENTION 
       [0021]    It would be advantageous to provide a high brightness light source comprising a mix of different color light outputs. It would also be advantageous to provide a high efficiency color mixing optic to provide this high brightness light source. 
         [0022]    To better address one or more of these concerns, in an embodiment of this invention, a mixing optic is provided that combines a variety of features to enhance the brightness and uniformity of the light emitted from a combination of different color light emitting sources. The mixing optic may include a parabolic reflector that redirects low-angle emissions, a reflective waveguide that mixes the light spatially, and a diffuser plate that mixes the light angularly. In an embodiment, the output of the mixing optic exhibits a substantially Lambertian output pattern of a substantially uniform mix of colors, exhibiting, for example, the light output pattern of a single white light source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
           [0024]      FIGS. 1A and 1B  illustrate a conventional parabolic reflector used for providing collimated light from a mix of different color light emitting sources. 
           [0025]      FIG. 2  illustrates a convention color mixing element situated between the light emitting sources and the parabolic reflector. 
           [0026]      FIG. 3  illustrates an example color mixing optic comprising a parabolic reflector and a light guide. 
           [0027]      FIGS. 4A-4C  illustrate an example embodiment of an example white light source having a substantially Lambertian output pattern of uniform color. 
           [0028]      FIGS. 5A-5B  illustrate an example embodiment of an example white light source with a loft element that converts a rectangular light output pattern to a circular light output pattern. 
       
    
    
       [0029]    Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
       DETAILED DESCRIPTION 
       [0030]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
         [0031]    In the following description, the term ‘white’ light output is used to define a desired combination of the multi-color light sources, because white light is typically the most commonly desired composite color combination. One of skill in the art will recognize that if a different composite color combination is desired, the principles of this invention will apply. That is, the choice of particular colors for the light sources, the ratios of intensities among the light sources, and so on, may be determined independent of this invention, and this invention is substantially independent of such a choice. 
         [0032]    A disadvantage of the prior-art color mixing element illustrated in  FIG. 2  is that light that is emitted at shallow angles relative to the surface of the light emitting elements  101  is either trapped within the mixing element  250 , or experiences a large number of reflections before it strikes the surface  220 . The likelihood of optic loss increases with each reflection, thereby diminishing the light output from the mixing element  250  for shallow angle light. Additionally, the rectilinear shape of the mixing element  250  introduces a symmetry such that shallow angled light will likely strike the surface  220  at the same shallow angle and be totally internally reflected at the surface  220 . Assuming that the emissions from the light emitting sources  101  are Lambertian in nature, even the loss of only the shallowest 10% of the emitted light (e.g. light emitted between 81 and 90 degrees) will result in a 10% loss of light output, and the aforementioned losses due to repeated reflections will further increase this loss. 
         [0033]      FIG. 3  illustrates an example color mixing optic  300  comprising a parabolic reflector  310  and a light guide  320 , the interface between the two being indicated by the dot-dashed line, corresponding to an exit surface  318  of the parabolic reflector  310 . 
         [0034]    A lower portion of the mixing optic  300  is shaped to provide sidewalls  315  that are parabolic, forming the compound parabolic reflector  310 . In an example embodiment, the parabolic reflector  310  has an input angle of 75°, and an output angle of 90°, and serves to reflect and redirect light that is emitted at shallow angles relative to the surface of the light sources  101 . The input angle may range between 65° and 75°, and the output angle may range between 80° and 90°, although other ranges may be used, depending upon the amount of redirection desired. 
         [0035]    By increasing the angle of the reflected light relative to the surface of the light emitting elements  101 , the number of reflections of the light emitted at shallow angles is substantially reduced, and the likelihood of the light being totally internally reflected at the exit surface  330  is also reduced. 
         [0036]    The overall size of the optic  300  may be dependent upon the size of the array of light sources  101 , which is typically arranged in a square pattern. The size of the input surface  317  of the parabolic reflector  310  is configured to accommodate this array without being much larger than the array of light sources  101  (e.g. see  FIGS. 4B, 4C ). Nominally, the height h 1   311  of the parabolic reflector may be about a quarter of the width w  312  of the input surface  317 , depending upon the desired amount of redirection. The height h 1   311  may range from 0.15 to 0.40 of the width w  312 , although shorter and taller heights may be used. 
         [0037]    An upper portion of the mixing optic  300  is shaped as a light guide  320 , with sidewalls  325  that are substantially orthogonal to the exit surface  318  of the parabolic reflector  310 . Because the walls are substantially orthogonal to the exit surface  318 , and thus orthogonal to the surface of the light sources  101 , the angle of the light that is reflected from the sidewalls  325  will generally equal the angle of the light emitted from the light sources  101 . Accordingly, the light that strikes the surface  330  will exhibit a substantially Lambertian pattern, with the exception of the shallow angled light from the light sources  101  that is redirected by the parabolic reflector  310 . 
         [0038]    It should be noted that, for ease of presentation and understanding, the term ‘walls’ as used herein may include a single wall, such as the continuous wall of a cylindrical structure extending between the ‘top’ and ‘bottom’ of the cylinder. Also, the term ‘surface’ as used herein may include a discrete interface between adjoined elements, or an imaginary interface between identified components of a common structure. For example, if differently shaped components of a structure are formed using a common material, such as by molding the composite structure, the plane of material between the differently shaped components forms a ‘surface’ between these elements. 
         [0039]    The amount of mixing that occurs, i.e. the range of emission angles that will cause a reflection from the sidewalls  325 , will be dependent upon the height h 2   321  of the waveguide  320  relative to the width w  312  of the input surface  317  of the parabolic reflector  310 , assuming that the size of the array of light sources  101  is approximately the size of the input surface  317 . A height h 2   321  of 0.75-3 times the width  312  of the input surface  317  is generally sufficient to provide a uniform mix of color and incident angles on the surface  330 , although other heights may be used, depending upon the particular uniformity requirements and the particular distribution of colors within the array of light sources  101  on the substrate  105 . 
         [0040]    Another parabolic reflector  210  is illustrated in dashed lines in  FIG. 3 . This parabolic reflector  210  receives the composite light output from the surface  330 . Because the light output from the surface  330  is substantially Lambertian, the reflector  210  may be designed to optimize the collimation of the light from the reflector  210 . 
         [0041]    Although the light that is emitted from the different points on the surface  330  will be reflected by the reflector  210  differently from light emitted from the center of the surface  330 , the substantially Lambertian pattern of the light from the surface  330  allows for the reflector  210  to be designed to provide a substantially uniform light output intensity. Additionally, the color mixing provided by the light guide  320  may substantially eliminate discernible color-specific patterns across the surface  330 . 
         [0042]    Effectively, from the perspective of the parabolic reflector  210 , the optic  300  appears as a white light source having a substantially Lambertian output pattern. That is, the combination of the optic  300  and the light sources  101  provide an output pattern that might be produced by a light emitting element that emits white light directly. 
         [0043]      FIGS. 4A-4C  illustrate an example embodiment of an example white light source having a substantially Lambertian output pattern of uniform color.  FIG. 4A  illustrates a profile cross-section of a light source  400 , and  FIGS. 4B and 4C  illustrate the top views of differently shaped structures of the light source  400 . 
         [0044]    In this embodiment, the mixing optic  300  is packaged within a structure  410  to form an integral white light source  400  that can be incorporated within lighting devices such as spotlights, camera flashes, backlights, and so on. To facilitate handling, the structure  410  may be rectilinear. 
         [0045]    A substrate  405  may include conductive strips (not illustrated) that interconnect the light sources  101  and/or facilitate coupling of the device  400  to a source of external power. In the example illustrated, the substrate  405  extends laterally beyond the device  400 , and may include contacts to the conductive strips upon the upper and/or lower surface of the substrate  405  in this extended portion. Alternatively, the substrate  405  may be the same width as the device  400 , with contacts to the conductive strips on the bottom of the substrate  405 , below the device  400 . Such an arrangement facilitates the formation of an array of immediately adjacent devices  400 . 
         [0046]    The mixing optic  300  may be a discrete block of transparent material, or it may merely be a hollow cavity in the structure  410 . The parabolic reflector  310  and lightguide  320  of the mixing optic  300  may be formed as a single composite, or as discrete elements that are bonded together. One or both of the parabolic reflector  310  and the lightguide  320  may be a hollow cavity within the structure  410 . 
         [0047]    The exterior of the mixing optic  300  may be coated with a reflective material, and/or the structure  410  may be reflective, and/or the indices of refraction of the material of the optic  300  and the structure  410  may be selected to provide total internal reflection across a wide range of incidence angles. For ease of reference, the phrase ‘reflective surface’ is used herein to identify a surface from which most or all of the light emitted from the light sources  101  is reflected, regardless of the particular scheme(s) used to achieve this reflection. 
         [0048]    The choice of material for the mixing optic  300  may be dependent upon the material of the lighting device in which the device  400  will be used, and the respective indices of refraction and other characteristics. In some embodiments, the material for the mixing optic may be the same as the material that will be used in the lighting device, or it may be an interface material that facilitates the efficient coupling of light from the light sources  101  to the material in the lighting device. If the material of the optic  300  is other than air, the structure  410  may be used as a mold to form the optic  300 ; if the optic  300  is pre-formed, the structure  410  may be molded around it. 
         [0049]    To further enhance the Lambertian characteristics of the optic  300 , a diffusion layer  440  may be included. When a light ray strikes the diffusion layer  440 , a plurality of light rays may be emitted by the diffusion layer  440 , with varying angles of emission, depending upon the particular characteristics of the diffusion layer  440 . Although there may be some back-scattering produced by the diffusion layer  440 , a substantial majority of the light striking the diffusion layer  440  is likely to be emitted from the exit surface  430 . 
         [0050]    The optic  300  may have any of a variety of shapes,  FIGS. 4B and 4C  illustrating two potential shapes as viewed from the top of the device  400 . 
         [0051]    In  FIG. 4B , the walls  325 A of the optic  300  may be orthogonal to each other, providing for a rectangular light output surface  430 A. The parabolic reflector  310  (not illustrated in  FIG. 4B ) in this example may be formed as a complex reflector with a rectangular perimeter that narrows in the direction toward the light emitting sources  110  and a smooth transition at each vertex of the light guide formed by walls  325 A and the array of light emitting elements  101 , or it may be formed so as to produce a compound-miter-like edge at each vertex. 
         [0052]    In  FIG. 4C , the walls  325 B of the lightguide  320  form a cylindrical structure with a light output surface  430 B centered on the array of light sources  101 . In this example, the optic  300  may be bullet-shaped, with the parabolic reflector  310  being formed as a cylinder with decreasing diameter in a direction toward the light sources  101 . 
         [0053]      FIGS. 5A-5B  illustrate an example embodiment of an example white light source  500  with a loft element  550  above a light source  400 . The loft  550  converts a rectangular light output pattern of the light source  400  (as illustrated in  FIG. 4A-4B ) to a circular light output pattern. 
         [0054]    A rectangular light output surface  430 A, such as illustrated in  FIG. 4B , may be the easier (less costly) pattern to provide than a circular light output pattern, or it may be the more frequently desired light output pattern. The loft element  550  is provided to allow for the production of a mixing optic  400  with a rectangular output surface  430 A, while also satisfying a demand for a mixing optic  500  that provides a circular output surface  570 . 
         [0055]    The loft element  550  includes an input aperture  560  that receives the light provided by the rectangular light output surface  430 A of the device  400 . The input aperture  560  may be at least as great as the light output surface  430 A, so that all the light from the optic  400  is provided to the loft  550 . To provide a continuous wall surface, the light input aperture  560  may be the same size as the light output surface  430 A. 
         [0056]    The light emitting surface  570  of the loft  550  may be circular, with an area that is equal to the area of the light output surface  440 . The area of the circle may be smaller or larger, in the event that a more concentrated or more dispersed (respectively) output pattern is desired; however, optical losses are usually minimized when the area of the light emitting surface is equal to the area of the light output surface  440 . 
         [0057]    The loft  550  may be formed using a continuous transition from the rectangular shape of the light output surface  440  to the circular light output surface  440 , akin to “morphing” a rectangle into a circle in three dimensions. A slow transition from rectangular to circular light output provides for minimal optical loss. In an example embodiment, the height of the loft  550  may be between half and twice the width of the array of light sources  101 , although other heights may be used, depending upon the acceptable degree of loss in the transition from a rectangular to circular beam pattern. 
         [0058]    Although the light source  500  is illustrated with the diffusion element  440  at the transition between the mixing optic  400  and the loft  550 , one of skill in the art will recognize that the diffusion element  440  may be a circular diffusion element at the light emitting surface  570  of the loft  550 . 
         [0059]    Because the light emitted from the light output surface of the optic  400  is substantially Lambertian, even without the diffusion element  440 , the output from the loft  550  of the device  500  can be expected to be Lambertian, with minimal color-specific characteristics, thereby providing an output that is comparable to the light output pattern of a surface of a light source that provides a white output directly. 
         [0060]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0061]    For example, it is possible to operate the invention in an embodiment wherein one or more wavelength conversion elements are included in the mixing optic. Because the light emission pattern of a conventional wavelength conversion element provides a Lambertian output of the wavelength converted light on both surfaces of the wavelength conversion element, it may be situated at any elevation within the mixing optic including at the aperture. A dichroic filter may be used to reflect the ‘downward’ emissions from the wavelength conversion element upward. 
         [0062]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.