Abstract:
A lighting unit that includes a light source and a reflector assembly upon which the light source is mounted. The reflector assembly includes a first reflector having a first curved reflective surface extending away from the light source, and a second reflector having a second curved reflective surface extending away from the light source. The first curved reflective surface faces and opposes the second curved reflective surface. The first curved reflective surface has a curvature that is different from that of the second curved reflective surface. The first and second curved reflective surfaces both preferably terminate in convexities that reduce the size of dark band areas of illumination.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/789,726, filed Apr. 5, 2005, and entitled Improved LED Luminaire Reflector Design. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a reflector design especially ideal for Light-Emitting Diode (LED) lighting unit (luminaire) applications. More particularly, the present invention relates to a method and apparatus for efficiently redirecting light from LED applications so as to provide desirable angular distributions.  
       BACKGROUND OF THE INVENTION  
       [0003]     Light-Emitting Diodes (LEDs) have been used in many applications to replace conventional incandescent lamps, fluorescent lamps, neon tube and fiber optics light sources in order to reduce costs and to increase reliability. Due to the fact that LEDs consume less electrical energy than most conventional light sources, while exhibiting a much longer lifetime, many designs have been invented for various applications, such as traffic signal lights, channel letter modules, conventional illuminated commercial signs, street name signs, and street lights.  
         [0004]     LEDs typically have a hemispherical top and are centered on an optical axis through the center of the LED. An LED lamp typically radiates symmetrically in a Lambertian or Batwing pattern 360 degrees around the center of the optical axis. The angular intensity distribution of a Lambertian pattern peaks at the optical axis and decreases according to the cosine law of the angle from the axis. A Batwing pattern peaks off the optical axis, with a lower intensity at the optical axis. For street light applications, a lighting unit comprising an LED lamp is often installed 25 feet or higher from the street surface, such that LED light rays are redirected towards a desired location by way of a reflector apparatus. Such designs, however, often yield narrow light patterns that are focused on a limited area just below the lighting unit, if the shape of the reflector is not appropriately designed, which is not desirable for many street light applications.  
         [0005]     Many different types of reflectors have been used, including cone-shaped reflectors. In  FIG. 1 , a schematic diagram of a cone-shaped reflector  20  coupled to a light source  10  is shown. As illustrated, light source  10  emits a plurality of light rays including light rays  30 ,  32 ,  34 ,  40 ,  42 , and  44 . Of these light rays, only light rays  30  and  40  are reflected by reflector  20 . Namely, light rays  30  and  40  each reflect off of reflector  20  and are then incident upon locations  54  and  64 , respectively. The remaining light rays  32 ,  34 ,  42 , and  44 , however, are not reflected, and are thus directly incident upon locations  52 ,  54 ,  62 , and  64 , respectively.  
         [0006]     Cone-shaped reflector designs inherently cause some areas to have greater light intensities than others, which results in undesirable darker bands in the illuminated area. The areas of greater intensity, for example, result from some locations being illuminated simultaneously by both directly emitted light rays and reflected light rays, such as locations between  54  and  64 . Meanwhile, the areas of lesser intensity result from locations being illuminated by directly emitted light rays as well as rays reflected from the far side of the reflector, but not from reflections from the near side of the reflector, such as locations  52  and  62 . It should be noted that the cross-sectional area depicted by line segment  70  in  FIG. 1  represents the plurality of locations whereby higher intensity light from directly emitted light rays and reflected light rays are incident, while the cross-sectional areas depicted by line segments  50  and  60  represent the plurality of locations where directly emitted light rays but less reflected light rays are incident. The light intensity of the darker bands  50  and  60  is further compromised because these areas are offset from the peak of the Lambertian or Batwing pattern. Thus, even the directly emitted light rays in these areas have less intensity than the directly emitted light rays closer to the center of the light distribution pattern of the light source (i.e. area  70 ).  
         [0007]     In addition to the cone-shaped reflectors previously discussed, reflector tray designs have also been used for street light applications. In  FIG. 2A , a schematic diagram of a flat-surface multi-LED reflector tray is provided as an example of such a design. As illustrated, reflector tray  100  comprises LED array  110 , bottom reflector  120 , top reflector  122 , right reflector  124 , and left reflector  126 . In  FIG. 2B , a cross-sectional view of a reflector tray is provided to help illustrate the distribution created by such design. As shown, light source  110  emits a plurality of light rays including light rays  130 ,  132 ,  134 ,  140 ,  142 , and  144 . Of these light rays, only light rays  130  and  140  are reflected. Namely, light rays  130  and  140  each reflect off of reflectors  120  and  122 , respectively, and are then incident upon locations  154  and  164 , respectively. The remaining light rays  132 ,  134 ,  142 , and  144 , however, are not reflected, and are thus directly incident upon locations  152 ,  154 ,  162 , and  164 , respectively  
         [0008]     For street light applications, reflector tray designs provide more flexibility with respect to angular distribution than cone-shaped reflectors. Because most street light applications require light to be directed down towards the street, such flexibility is often desirable. In  FIG. 2B , for example, bottom reflector  120  and top reflector  122  are positioned according to their respective angles of inclination  121  and  123 , so that light from source  110  is generally directed downwards and forward in the direction toward the other side of the street.  
         [0009]     Nevertheless, similar to cone-shaped reflectors, reflector tray designs can provide for undesirable dark bands created by some portions of the illuminated area having greater light intensities than others. In  FIG. 2B , for example, because locations between  154  and  164  are illuminated simultaneously by both directly emitted light rays and reflected light rays, these locations have a greater light intensity than locations  152  and  162 , which are illuminated by directly emitted light rays and light rays reflected by the far side of the reflector. And, areas  152  and  162  are illumined by directly emitted light rays having a lesser intensity given their location within the Lambertian or Batwing distribution pattern. Here, however, it should be noted that light rays  130  and  140  are respectively incident upon reflectors  120  and  122  at angles  131  and  141 , respectively. Light rays  130  and  140  are then respectively reflected by reflectors  120  and  122  at angles of  133  and  143 , respectively (wherein, angle  131 =angle  133 , and angle  141 =angle  143 ). The angles at which light rays reflect off of a particular reflector thus dictates where, if at all, such reflected rays will be coupled with a directly emitted ray so as to create an illuminated area having a greater light intensity. In  FIG. 2B , the cross-sectional area depicted by line segment  170  represents the plurality of locations whereby the higher intensity portion of the directly emitted light rays and reflected light rays are incident, while the cross-sectional areas depicted by line segments  150  and  160  represent the plurality of locations where mainly the lower intensity of the directly emitted light rays are incident as well as some reflected light rays from just the far side of the reflector.  
         [0010]     In light of these limitations, there is currently a need for a more efficient reflector design. It is therefore desirable to develop a method and apparatus for redirecting light from LED lighting unit applications so as to provide wider and more efficient angular distributions. Moreover, it is desirable to provide an improved reflector surface design that can efficiently spread light over a wider area and minimize dark bands. Such a reflector design would represent a major improvement in lighting unit output light pattern management.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention solves the aforementioned problems by providing multiple and varying curved-surface reflectors, which substantially reduce dark band areas and distribute light to a wider area than conventional designs.  
         [0012]     A lighting unit includes a light source and a reflector assembly upon which the light source is mounted. The reflector assembly includes a first reflector having a first curved reflective surface, and a second reflector having a second curved reflective surface, wherein the first curved reflective surface has a curvature that is different from that of the second curved reflective surface.  
         [0013]     In another aspect, a lighting unit includes a light source and a reflector assembly upon which the light source is mounted. The reflector assembly includes a first reflector having a first curved reflective surface extending away from the light source, and a second reflector having a second curved reflective surface extending away from the light source. The first curved reflective surface faces and opposes the second curved reflective surface. The first curved reflective surface has a curvature that is different from that of the second curved reflective surface.  
         [0014]     Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic diagram of a conventional cone-shaped reflector.  
         [0016]      FIG. 2A  is a schematic diagram of a conventional flat-surface multi-LED reflector tray.  
         [0017]      FIG. 2B  is a schematic cross section view of a conventional flat-surface multi-LED reflector tray.  
         [0018]      FIG. 3  is a schematic cross section view of a curved-surface multi-LED reflector tray according to an embodiment of the present invention.  
         [0019]      FIG. 4A  is a plot of contour lines illustrating the varying lighting unit intensities of a flat-surface reflector tray over a particular area.  
         [0020]      FIG. 4B  is a plot of contour lines illustrating the varying lighting unit intensities of a curved-surface reflector tray over a particular area.  
         [0021]      FIG. 5  is a schematic cross section view of an apparatus with multiple rows of curved-surface multi-LED reflector trays according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The present invention is an improved reflector design for LED lighting unit applications. More particularly, the present invention is a method and apparatus for efficiently redirecting light from LED lighting unit applications so as to reduce dark bands and increase the scope of a lighting unit&#39;s 50% peak intensity contour line.  
         [0023]     The present invention provides a curved-surface reflector design, which substantially reduces dark band areas and distributes light to a wider area than conventional designs.  FIG. 3  illustrates a cross-sectional view of a lighting unit  200  having a curved-surface multi-LED reflector tray design. As illustrated, lighting unit  200  comprises a light source  210  mounted onto a reflector assembly  202 . Reflector assembly  202  includes a back plate  204 , and lower and upper curved reflectors  220 ,  222  extending therefrom (although back plate  204  can be omitted or formed integrally as part of the curved reflectors  220 ,  222 ). In a preferred embodiment, lower curved reflector  220  is positioned at an angle of inclination  221 , which is smaller than the angle of inclination  223  of upper curved reflector  222  (wherein angle of inclination  223  is approximately ninety degrees). The inner surfaces of reflectors  220 ,  222  (i.e. those facing each other) are reflective. Optional back plate  204  may or may not have a reflective surface.  
         [0024]     The reflective inner surface of upper curved reflector  222  further comprises a concavity  227  proximate to light source  210  and positioned so as to reflect more light toward location  250 . The reflective surface of upper curved reflector  222  terminates in a convexity  229 , which is rounded outwardly away from reflector  220 , so as to reflect light relative to a line  290  tangent to the curvature of convexity  229 . In  FIG. 3 , for example, light ray  240  is incident upon convexity  229  at an angle  241  relative to tangent line  290 . Light ray  240  is then reflected at an angle  243  relative to tangent line  290  and becomes incident upon location  264 . Here, it should be noted that, upon comparing  FIG. 2B  with  FIG. 3 , the curvature of convexity  229  causes light ray  240  to be reflected at an angle  243  which is smaller than reflection angle  143  of flat-surface reflector  122 . As a result of this smaller reflection angle  243 , light rays  240  and  244  are both incident upon location  264  which, together with location  262  whereupon only light ray  242  is incident, create a dark band area  260  that is smaller than the analogous dark band area  160  created by flat-surface reflector  122  in  FIG. 2B .  
         [0025]     The reflective surface of lower curved reflector  220  also preferably terminates in a convexity  225 , which similarly minimizes dark band area  250  relative to dark band area  150  in  FIG. 2B . As illustrated, convexity  225  is rounded outwards and reflects light relative to a line  280  tangent to the curvature of convexity  225 . For example, light ray  230  is incident upon convexity  225  at an angle  231  relative to tangent line  280 . Light ray  230  is then reflected at an angle  233  relative to tangent line  280  and becomes incident upon location  254 , along with light ray  234 . Here again, because reflection angle  233  is smaller than reflection angle  133  in  FIG. 2B , the dark band area  250  between location  254  and location  252  (whereupon only light ray  232  is incident) is smaller than the analogous dark band area  150  of flat surface reflector  120 . Accordingly, as a consequence of dark band areas  250  and  260  being reduced, an increase in the area of greater light intensity  270  is achieved. At the same time, the contrast between the intensity in locations  270  and  250  is relatively smaller than that between locations  170  and  150  of  FIG. 2B . Consequently, darker bands are not as visible as well.  
         [0026]     In street light applications, how far the 50% peak intensity contour line can reach on the pavement surface in terms of the mounting height (MH) define the “Type” of street light. For instance, a Type II lighting unit is one in which the 50% contour line reaches the region between 1.0 MH and 1.75 MH, while a Type III lighting unit is one in which the 50% contour line reaches between 1.75 MH and 2.75 MH, according to the Parking Lot and Area Luminaires section of the July 2004 NLPIP (National Lighting Product Information Program) Specifier Reports (Vol. 9 No. 1).  
         [0027]     The improved performance of the curved-surface reflector design of the present invention, relative to conventional flat-surface reflector designs, is quantified in the ASAP optical simulations provided in  FIGS. 4A and 4B . In  FIG. 4A , a plot of contour lines illustrating the varying lighting unit intensities of a flat-surface reflector tray over a particular area is provided, wherein two reference lines have been drawn to identify the 1.0 MH and 1.75 MH markers. In  FIG. 4B , a similar plot is provided with respect to the varying lighting unit intensities of a curved-surface reflector tray according to an embodiment of the present invention.  
         [0028]     As mentioned above, Type II lighting units are those whose 50% peak intensity contour line reaches the region between 1.0 MH and 1.75 MH. A comparison of  FIGS. 4A and 4B  shows that the 50% contour line  400  of a lighting unit using a curved-surface reflector tray covers a wider area than the 50% contour line  300  of a flat-surface reflector lighting unit. It should also be noted that contour line dimples, such as the dimples identified by arrows  302  and  304  in  FIG. 4A , denote areas in which dark bands may appear. In  FIG. 4B , however, contour line  400  extends into the area identified by arrows  402  and  404 , which indicates that no dark bands are present.  
         [0029]     The present invention addresses the need for an improved LED reflector apparatus that reduces dark bands and increases the scope of a lighting unit&#39;s 50% peak intensity contour line so that a higher value of the mount height (MH) count can be obtained for a type II or type III distribution. Moreover, by having the curvature of reflector  222  differ from that of reflector  220  (i.e. the reflective surface of reflector  222  includes both a concavity and a convexity while that of reflector  220  only includes a convexity, having a different radius of curvature, etc.), the areas of illumination can be offset from the center position of the light source. Therefore, even if the light source  210  is facing straight down onto a street, the area of illumination can be offset such that it is not centered directly below the light source.  
         [0030]     It should be appreciated that, although the present invention has been described above with reference to particular embodiments, those skilled in the art will recognize that changes and modifications may be made in the above described embodiments without departing from the scope of the invention. In  FIG. 5 , for example, a schematic cross-sectional view of an apparatus having multiple rows of curved-surface multi-LED reflector assemblies  202  and light sources  210  is provided, wherein reflector assemblies  500 ,  600 , and  700  are shown. Furthermore, while the present invention has been described with respect to upper and lower curved reflectors  220  and  222 , those skilled in the art will recognize that further enclosing a reflector assembly  202  with right and left curved reflectors having a similar design to reflectors  220 ,  222  as described above may be desirable. These and other changes and modifications are intended to be included within the scope of the present invention.