Abstract:
Embodiments of the present invention relate to a compact optical assembly which improves collimation of light produced by multiple LED light sources in a light engine. A shaped primary reflector located over the light engine reflects the light toward a larger shaped secondary reflector. The shapes of the reflectors are selected to cooperatively produce a highly collimated light beam. Color mixing may be improved by providing a plurality of facets on the reflective surfaces of at least one of the primary reflector or the secondary reflector.

Description:
This application claims priority from U.S. Provisional Patent Application No. 60/885,224, the entire content of which is hereby incorporated by reference in its entirety. 
     Numerous references including various publications may be cited and discussed in the description of this invention. The citation and/or discussion of such references is provided merely to clarify the description of the present invention and is not an admission that any such reference is “prior art” to the present invention. All references cited and discussed in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to optical devices. More specifically, embodiments of the present invention relate to a folded optical system with compact free form reflectors, which collimate a multi-cavity LED light engine to a narrow beam. 
     BACKGROUND OF THE INVENTION 
     Applications for high intensity, high efficiency narrow beams are prevalent through the lighting industry. Certain industries, for instance the entertainment, architectural or theater industries, have applications for specialized lighting which can benefit from an apparatus or system which is able to collimate and control the direction at which the light is projected. In addition, there is a need to “throw” or project a selected color for a relatively long distance while maintaining an acceptable level of illumination and color uniformity. A long throw distance requires a narrow beam at a high intensity, while minimizing the intensity dispersion. 
     A directed light beam is light emitted in a preferred direction, and can be characterized by beam angle and dispersion. Beam width refers to the full-beam dispersion angle at which the off-axis luminous intensity of the light is one-half of the maximum on-axis luminous intensity (measured in candela), and field width refers to the full-angle at which the off-axis luminous intensity of the light has fallen to 10% of the on-axis luminous intensity. Dispersion is a measure of the distribution of the luminous intensity over the beam angle. The throw distance is increased when the emitted light is concentrated into a small beam angle with a small dispersion. 
     Conventional LED arrays produce light emissions having a relatively wide Lambertian beam angle of, e.g., 120°. The conventional LED arrays can be coupled with primary optics, thereby capable of forming, for example, an LED light engine in a 1.5×1.5 inch package and producing a light beam having an intensity of 1,000 lumens over a still wide beam angle of 60°, such as the Lamina Lighting Titan™, suitable for some residential, stage, architectural, and commercial lighting applications. Such light engines typically include multiple emitters and cavities to produce a light beam having an acceptable intensity, however this increases the apparent source size to be much larger than the apparent source size from a single emitter light engine, thereby making it more difficult to collimate the light into a beam having a low level of intensity dispersion. 
     Conventional collimation solutions which are tall (e.g., ≧5″) or wide (e.g., ≧6″) are too costly to manufacture, ship, and install when the light source itself is already two to three times the cost of energy-inefficient incandescent and halogen sources that it replaces. A compact, low-cost collimation design is preferable for applications where space or cost considerations dominate. Therefore, a need exists to provide a compact, low-cost optics assembly which can optimize the collimation and throw distance of a light beam produced by a light engine. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an optical assembly produces a narrow beam by replacing a single tall reflector with a compact optical system including two revolved spline reflectors, e.g., ≦2″ tall and ≦5″ wide. Micro-facets on the reflectors improve the uniformity of the beam with a minimal degradation in intensity dispersion. Light emitted by the LEDs passes through an optical assembly including the optical features of a primary reflector and a secondary reflector, the reflectors having predetermined shapes which cooperatively match in order to produce a light beam having a desired amount of collimation. 
     A device in accordance with an embodiment of the present invention preferably includes one or more of the following assembly design features or functions: 
     1) a light engine mounted on a substantially planar substrate, the light engine having multiple light emitters, each light emitter situated within a cavity, in which the light produced by the light engine is produced having a predetermined beam angle, and directed along a predetermined direction; 
     2) a secondary reflector having a substantially concave shape, oriented having a central axis substantially perpendicular to the substrate, having an entrance aperture, and an exit aperture at the top of the concave shape, the entrance aperture may be co-planar with the substrate and enclosing the light engine, and one or more mounting locations adjacent to the exterior of the exit aperture; 
     3) a plurality of support struts, each having a lower end attached to the substrate, and an upper end attached to a mounting portion of the secondary reflector; 
     4) one or more support spars, each support spar having an outer end attached to a mounting portion of the secondary reflector, and having an inner end extending toward the central axis of the secondary reflector; 
     5) a primary reflector attached to the inner end of a support spar, the primary reflector having a reflective surface facing the light engine, the support spar suspending the primary reflector within the predetermined beam angle of the light engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a top view of a light engine, showing exemplary dimensions and LED placement locations within the light engine; 
         FIG. 2  is a side view of a light engine, showing exemplary dimensions and locations of the substrate and mounting fasteners supporting the light engine; 
         FIG. 3  is a perspective view of the folded optic of an embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the folded optic of an embodiment of the present invention; 
         FIG. 5  is a raytrace diagram of light produced by an embodiment of the present invention; 
         FIG. 6  is a plot of the luminous intensity distribution for the light engine of  FIGS. 1-2 , showing a beam angle of approximately 60°; 
         FIG. 7  is a top view comparison of the folded optic of an embodiment of the present invention, overlaid on the footprint of conventional narrow collimation optics; 
         FIG. 8  is a side view comparison of the folded optic of an embodiment of the present invention, overlaid on the profile of conventional narrow collimation optics; 
         FIG. 9  is a wire-frame view of the primary reflector and secondary reflector of an embodiment of the present invention, without facets on the reflector surfaces; 
         FIG. 10  is a wire-frame view of a first embodiment of the primary reflector and secondary reflector of an embodiment of the present invention, having 37 facets on the reflector surfaces; 
         FIG. 11  is a wire-frame view of a second embodiment of the primary reflector and secondary reflector of an embodiment of the present invention, having 126 facets on the reflector surfaces; 
         FIG. 12  is a wire-frame view of a third embodiment of the primary reflector and secondary reflector of an embodiment of the present invention, having 368 facets on the reflector surfaces; 
         FIG. 13  is a wire-frame view of a fourth embodiment of the primary reflector and secondary reflector of an embodiment of the present invention, having 2835 facets on the reflector surfaces; 
         FIG. 14  is a wire-frame view of a fifth embodiment of the primary reflector and secondary reflector of an embodiment of the present invention, having 7,150 facets on the reflector surfaces; 
         FIG. 15  is a plot of the relative intensity distribution for the various faceting patterns shown in  FIGS. 10-14 ; 
         FIG. 16  is a comparative plot of the beam angle of conventional optics, compared to the beam angle for an embodiment of the present invention; 
         FIG. 17  is a table of supporting data showing an approximately 2.1-fold improvement in illuminance at a predetermined distance from the light engine, compared to the prior art; 
         FIG. 18  is a relief plot of the intensity in the X-Y plane of a light beam produced by an embodiment of the present invention; 
         FIG. 19  is a photograph of the spatial illuminance in the X-Y plane of a light beam produced by an embodiment of the present invention; 
         FIG. 20  is a raytrace plot of a light beam produced by an embodiment of the present invention; and 
         FIG. 21  is an approximate polynomial fit of the bezier splines which form an embodiment of the cross-sectional profiles of both the primary reflector and the secondary reflector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a top view of LED placement locations within a plurality of light engine cavities in accordance with an embodiment of the present invention. Preferably, the cavities have a reflective inner surface, and each cavity has a light extraction lens encapsulating the LED. The individual LEDs  1   a  may be a blue excitation emitter with wavelength 440-495 nm, a direct emission red, orange, or amber excitation emitter with wavelength range 575-680 nm, or a direct emission green wavelength excitation emitter having a range 495 nm-575 nm. White light may be produced by exciting a yellow phosphor with light from blue LEDs. The LEDs are typically mounted on a substrate  2  which provides electrical connections, thermal dissipation, and mechanical support. Placement and quantity of LEDs  1   a  may vary from the placement shown in  FIG. 1 . Typical dimensions in millimeters of the light engine are shown in  FIG. 1 . For instance, the light engine of  FIG. 1  is shown having an optical axis coincident with the central LED, and having an optical center at (x,y,z) coordinates of (0, 0, 5.47) mm, in which the Z-axis is measured from the surface of the substrate  2 . The diameter of the central reflective cavity is 6.00 mm as indicated in  FIG. 1 . The diameter of each reflective cavity is similar, but it will be understood by those skilled in the art that the diameter of each reflective cavity may vary by ±0.5 mm or more, and each cavity may have a similar but different diameter. 
       FIG. 3  shows a perspective view of one embodiment of the present invention, having the following features designed to enhance the collimation and mixing of light, with each of these features discussed in greater detail below: light engine  1  having a plurality of LEDs  1   a  (not all LEDs  1   a  are labeled); folded path faceted primary reflector  3 ; support spars  4 ; faceted secondary reflector  5 ; support struts  6 . The light engine  1  has a plurality of LEDs and is preferably the light engine shown in  FIG. 1 . 
       FIG. 4  shows a cut-away side view of one embodiment of the present invention, additionally showing the substrate  2  and mounting fasteners  7  which may be screws, bolts, or the like in any combination. 
     Structure of the Optical Assembly 
     Referring to  FIGS. 3 and 4 , the optical assembly includes a substrate  2 , on which a plurality of LEDs  1   a  are mounted, forming a light engine  1  (not labeled in  FIG. 4 ) such as the light engine  1  shown in  FIG. 1 . Each LED  1   a  is situated within a cup-like cavity  1   b  having reflective interior walls. The tops of the cup-like cavities  1   b  are co-planar in a plane parallel to substrate  2 , and also co-planar with the bottom of a secondary reflector  5 . In a preferred embodiment, the bottom of secondary reflector  5  has an opening which is seen more clearly in  FIG. 3 . 
     Referring to both  FIGS. 3 and 4 , secondary reflector  5  is an upwardly concave structure having a reflective inner surface. In one embodiment, secondary reflector  5  has two openings. The first opening of secondary reflector  5  is an entrance aperture that forms an opening which surrounds the light engine  1 , through which light enters the secondary reflector  5 . The second opening is at the top of secondary reflector  5  and is an exit aperture from which light emerges. Adjacent to the exit aperture at one or more points, and on the exterior side of secondary reflector  5 , is a mounting area  9 , shown in  FIG. 3  as a flat lip encircling the exit aperture of secondary reflector  5 . Mounting area  9  need not completely encircle the exit aperture of secondary reflector  5 , and may be more than one mounting area  9  unconnected to each other and located at different points around the perimeter of the exit aperture. 
     Under the secondary reflector  5  are two or more support struts  6 , which stabilize and provide physical support to secondary reflector  5 . The preferred configuration is three support struts  6  approximately equally-spaced, as shown in  FIG. 3 . The lower end of support strut  6  is attached to substrate  2 . The upper end of support strut  6  includes a strut head  6   a , which engages with mounting area  9 , thereby stabilizing and providing physical support to secondary reflector  5 . Strut head  6   a  may optionally include a locking portion which has limited flexibility, in which at least part of the locking portion may be physically snapped over at least a portion of the top of the mounting area  9 , exerting a compression force between the locking portion of strut head  6   a , mounting area  9 , and the remainder of strut head  6   a,  thereby further stabilizing and providing physical support to secondary reflector  5 . 
     Those skilled in the art will recognize that other means may be used to position, support, and align the secondary reflector  5  with respect to the primary reflector  3 , e.g., a truss; or support ribs embedded in secondary reflector  5 ; or if secondary reflector  5  provides adequate stiffness then no additional support may be required. 
     In one embodiment, a first end  4   a  of one or more support spars  4  is attached to the mounting area  9 , preferably at a location of mounting area  9  that is supported by a support strut  6 . The means of attaching support spar  4  to the mounting area  9  may include bonding with an adhesive, or by having a portion of support spar  4  located between mounting area  9  and the locking portion of strut head  6   a , thereby causing the first end  4   a  of support spar  4  to be physically held in place by the compression force exerted by the locking portion of strut head  6   a.    
     In other embodiments, the first end  4   a  of the one or more support spars  4  may be attached to one or more struts  6 , or directly to the substrate  2 . 
     The second end of support spar  4  is attached to mounting ring  10 . The means for attaching support spar  4  to mounting ring  10  may include adhesive, a physical snap connection similar to that which may be used to attach the locking portion of strut head  6   a  to the secondary reflector  5 , or any combination of such methods. The lower surface of mounting ring  10  is attached to the upper surface of the folded path primary reflector  3 . 
     The folded path primary reflector  3  is a structure having a reflective surface facing the light engine  1 , and having a cross-section at least partially within the beam width produced by the light engine  1 . Support spar  4  acts to hold the folded path primary reflector  3  in the required position within the beam width of light engine  1 , and with the required degree of stability. Although one or two support spars  4  may be adequate to hold the folded path primary reflector  3  if the support spars  4  have adequate stiffness, three support spars  4  are preferred in order to provide a more stable support. 
     Preferred embodiments of the optical assembly are compact and low profile but may exhibit reduced efficiency due to light blockage by the support spars  4  and some uncaptured light from light engine  1  that does not strike both the folded path primary reflector  3  and secondary reflector  5 . 
     Operation of the Optical Assembly 
       FIG. 5  illustrates the operation of the optical assembly by presenting a raytrace of representative light rays  8  traveling through an embodiment of the present invention. Light emitted by the LEDs  1   a  strikes the folded path primary reflector  3 , and is reflected by its reflective surface. The reflected light rays then strike the secondary reflector  5  and are reflected, thereby forming a light beam having the desired level of intensity and collimation. The operation of the optical assembly is presented below in greater detail. 
     Referring to  FIG. 3 , light is generated by the LEDs  1   a  of the light engine  1 . Each LED is located within a cup-like cavity  1   b . The interior walls of cavities  1   b  are reflective, and act to restrict the light produced by the light engine  1  to within a beam angle, e.g., approximately 60°, oriented upward. 
     Light emitted from the LEDs  1   a  superimposes to produce a beam of light having a desired level of uniformity. In one embodiment, generally acceptable uniformity includes an illuminance distribution which deviates by less than 20% within 5° of the optical axis of the light engine system. The field width of the intensity dispersion is 100°.  FIG. 6  shows the typical luminous intensity distribution emitted by an exemplary Titan™ light engine at a typical far field distance of 1 meter or approximately 6 times the distance of the maximum diameter of the collimation system. 
     The primary reflector  3  is located within the beam angle of light from the light engine  1 . The primary reflector  3  has a reflective surface facing the light engine  1  which may include facets to improve the light mixing. The facets include a simple tessellation (i.e., a repeating pattern) of the spline from a continuously varying function to that of a discrete function. The facets are flat. Faceting may also be included on the reflective surface of the secondary reflector  5 . Table 1 presents five embodiments of facet design. The design of facet level 0 provides a relatively small number of larger facets, progressing to facet level 4 which provides a relatively large number of smaller facets. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Reflector Facets 
               
             
          
           
               
                 Facet Level 
                 Primary Reflector 
                 Secondary Reflector 
                   
               
               
                 0-4 
                 Facet # 
                 Facet # 
                 Total Facet # 
               
               
                   
               
             
          
           
               
                 0 
                 11 
                 26 
                 37 
               
               
                 1 
                 72 
                 54 
                 126 
               
               
                 2 
                 264 
                 104 
                 368 
               
               
                 3 
                 2115 
                 720 
                 2835 
               
               
                 4 
                 5500 
                 1650 
                 7150 
               
               
                   
               
             
          
         
       
     
     The preferred embodiment of facet design among the levels of Table 1 is facet level 3, having 2,835 facets, providing a preferred combination of simple facets producing a 10° beam with acceptable uniformity. Persons skilled in the art will recognize that the number of facets of each facet level may be varied 5-10% from the exact values given in Table 1 without producing an unacceptable change in beam width or uniformity from that of the nearest facet level. Generally, the higher the number of facets the lower the intensity dispersion and uniformity. 
     Optical devices and features for controlled color mixing developed by the applicant, including faceting, are known and described in commonly-assigned U.S. patent application Ser. No. 11/737,101, the entire content of which is incorporated by reference herein in its entirety. 
     The primary reflector  3 , aside from the faceting, is rotationally symmetric, having an approximate shape similar to a cone having a narrow end pointed toward the light engine  1 . More specifically, the primary reflector  3  has a cross-sectional profile in the X-Z plane described as a free-form bezier spline.  FIG. 21  shows an approximate polynomial fit of the bezier splines which form an embodiment of the cross-sectional profiles of both the primary reflector  3  and the secondary reflector  5 . 
     Light emitted by the light engine  1  at an angle of approximately 45° to 90° with respect to the surface of substrate  2  will reflect from the primary reflector  3  toward the secondary reflector  5 . Light emitted by the light engine  1  at an angle of approximately 0° to 30° will strike the secondary reflector  5  directly and be reflected to the side, forming side light. Light emitted by the light engine  1  at an angle of approximately 30° to 45° is uncaptured spill light. 
     Both side light and spill light are undesirable because they lessen the amount of light in the main beam produced by the optical assembly. In order to lessen the amount of spill light, the angle of emissions from light engine  1  that produces spill light can be reduced by constraining the optic assembly into as low a profile as possible. 
     The secondary reflector  5  is generally of an upwardly concave shape with a reflective inner surface facing the primary reflector  3 . The secondary reflector  5  has a cross-sectional profile in the X-Z plane which is more precisely described as a free-form bezier spline. The secondary reflector  5  receives light reflected by the primary reflector  3 , and reflects the light upward with the desired amount of collimation by performing a cosine correction by which collimation of the light is improved. The secondary reflector  5  may include facets on its inner surface, thereby improving the uniformity of the light beam reflected from the secondary reflector  5  with minimal degradation to intensity dispersion. The facets are produced by converting a circle into a polygon by dividing the 360 degrees of the circle into “N” segments of approximately equal size, where N is the number of sides of the polygon. The facets are simple square facets having a flat surface shape. Support spars  4  block a small portion of the light.  FIG. 20  shows a typical calculated raytrace diagram of light emitted from an embodiment of the present invention, having a beam angle of 10°. 
     Embodiments of the present invention provide a more compact assembly compared to the prior art.  FIG. 7  shows a comparison of the cross-section in the X-Y plane of the prior art  11  and an embodiment  12  of the present invention.  FIG. 8  shows a similar comparison of cross-sections in the X-Z plane. 
       FIG. 9  shows a wireframe view of an embodiment of the present invention, without facets.  FIGS. 10-14  show wireframe views of additional embodiments of the present invention, showing increasing numbers of facets on the primary reflector  3  and the secondary reflector  5 .  FIG. 15  shows a comparison of the resulting intensity distributions for the embodiments shown in  FIGS. 9-14 , in which the Y-axis is the normalized relative intensity and the X-axis is degrees off the main axis of the beam of light.  FIG. 15  depicts the impact of facet size on intensity dispersion. Coarse facets which roughly discretize the smoothly revolved bezier spline architecture of the optical reflectors widen the intensity dispersion dramatically, whereas smaller facets disrupt the dispersion of the light less. 
       FIG. 16  shows a comparison of relative intensities of the improved light collimation available with embodiments of the present invention, compared to 20° narrow optics known in the prior art. The improved collimation allows the light to be projected further. 
       FIG. 17  shows a comparison of the illuminance of specific 20° narrow optics known in the prior art with that of embodiments of the present invention, at distances of 1, 2, 5 and 10 meters. An embodiment of the present invention converts the 60° primary beam of light exiting a 7-cavity LED light engine array into a 10° beam of light. The 10° intensity dispersion throws more illuminance (units of Lux) over a greater distance than either a 20° optic, or the 60° beam from the stock light engine. At a 10 meter distance, an embodiment of the present invention throws 68 Lux when the primary light engine produces 850 source lumens. 
       FIG. 18  shows a typical illuminance chart at a distance of 2 meters.  FIG. 19  shows a photograph of this illuminance at a 2 meter distance. 
     The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     This application may disclose several numerical range limitations. Persons skilled in the art would recognize that the numerical ranges disclosed inherently support any range within the disclosed numerical ranges even though a precise range limitation is not stated verbatim in the specification because this invention can be practiced throughout the disclosed numerical ranges. The entire disclosure of the patents and publications referred in this application are hereby incorporated herein by reference.