Abstract:
The dielectric casing of a light emitting diode (LED) incorporates an integral parabolic reflector system which redirects light in a collimated pattern deflected at significant angles relative to the axis of symmetry of the LED.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application 60/699,153 filed on Jul. 14, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates in general to light emitting diodes (LEDs), and more particularly to apparatus and methods for directing the light emitted from the LED.  
       BACKGROUND OF THE INVENTION  
       [0003]     The efficiency, reliability, and compact size of LEDs makes them increasingly attractive for use in lighting devices of all kinds. However, because the semiconductor die forming the heart of an LED is essentially a point source of light, LEDs inherently produce light that radiates in all directions. Thus, a problem with LEDs is that the light emitted cannot be directed as precisely as that in other optical systems without losing the essential compactness and simplicity of the LED.  
         [0004]     A number of inventors have attempted to develop or improve systems for concentrating or diffusing this multi-directional radiation in specific patterns useful for particular applications. One approach is to mold the exterior surface of the dielectric casing which houses the semiconductor die in the form of a convex or concave lens. This method provides a means of transmitting a light emitted from the semiconductor die through the lens surface in a roughly conical beam collinear with the axis of the LED. U.S. Pat. No. 5,865,529 granted Feb. 2, 1999 to Ellis Yan discloses such a device for diffusing light in a 360° viewing plane in both horizontal and vertical axes. However, this method cannot focus the dominant portion of emitted light at an angle substantially away (i.e., &gt;45° ) from the symmetric axis of the LED and lens while simultaneously excluding radiation at shallower angles to the symmetric axis (i.e., &lt;450°).  
         [0005]     Another approach is to provide a silvered or refractive reflector mechanically separate from the LED which is aligned to intercept light radiated along the axis of the LED and reflect it in a pattern suitable for the particular application. Unlike the lens method, this approach allows for deflection of the dominate portion of the emitted light at significant angles away from the symmetric axis of the LED while excluding radiation at shallower angles. U.S. Pat. No. 5,769,532 granted June 23, 1998 to H. Sasaki, U.S. Pat. No. 6,364,506 B 1 granted Apr. 2, 2002 to M. Gallo, U.S. Pat. No. 6,447,155 B 2 granted Sep. 10, 2002 to T. Kondo and H Okada, and U.S. Pat. No. 6,846,101 B2 granted Jan. 25, 2005 to C Coushaine all disclose devices employing such a mechanically separate reflector to redirect light from an LED. However, the mechanical arrangement of the LED and separate reflector increases the complexity, space required, alignment difficulty, and cost for this assembly.  
         [0006]     A third approach is to mold the exterior surface of the dielectric casing of the LED in the form of a concave cone of faceted planes or approximating curves which, by means of total internal reflection, redirects light emitted by the LED die away from the axis of the LED. These methods allow diffusion of light at substantial angles from the axis of the LED. U.S. Pat. No. 3,774,021 granted Nov. 20, 1973 to B. Johnson and U.S. Pat. No. 6,488,392 B1 granted Dec. 3, 2002 to C Lu both disclose devices using convex planar or curved surfaces to randomly diffuse light emitted from a semiconductor die in a roughly radial direction away from the symmetric axis of the LED. However, neither method produces a uniform dispersion of the reflected light consisting of parallel rays oriented at a precise angle relative to the symmetrical axis of the LED.  
         [0007]     None of these existing approaches provide an apparatus and method of maintaining precise control over the direction of light emitted by the LED rays while at the same time retaining the primary desirable characteristics of an LED, namely, simplicity and compactness.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The invention provides an LED that synergistically retains the simplicity and compactness of an LED in a light source in which the direction of the emitted light can be precisely controlled. The invention provides the economy of an integral reflector with geometry that produces reflected rays uniformly and precisely oriented at larger angles away from the LED&#39;s axis of symmetry.  
         [0009]     The invention provides a light emitting diode (LED) assembly comprising: a dielectric casing of optically transparent material; first and second electrical leads extending within the dielectric casing; and at least one semiconductor die embedded in the dielectric casing and coupled to the first and second leads in a manner allowing for transfer of electrical energy to and illumination of the at least one semiconductor die; and, wherein the dielectric casing has a surface, at least a portion of which is parabolic, the parabolic surface portion located and dimensioned for reflecting light emitted bythe at least one semiconductor die. Preferably, the dielectric casing has a plurality of the parabolic surface portions. Preferably, the parabolic surfaces are located and dimensioned to reflect light emitted by the at least one semiconductor die into a plurality of beams. Preferably, the parabolic surface portion defines a parabolic curve rotated around the axis of the LED. Preferably, the parabolic surface portion defines a parabolic curve rotated around an axis through the LED and perpendicular to the axis of the LED. Preferably, the parabolic surface portion is located and dimensioned to reflect light emitted bythe at least one semiconductor die into a region defining a disc perpendicular to the axis of the LED. Preferably, the dielectric casing has an index of refraction greater than 1.42.  
         [0010]     The invention also provides a method of directing light in a light emitting diode (LED) assembly, the method comprising: emitting light from a semiconductor die embedded in a dielectric casing; and reflecting the emitted light from a parabolic surface portion of the dielectric casing. Preferably, the reflecting comprises reflecting the light into a region in the form of a disc perpendicular to the axis of the LED. Preferably, the reflecting comprises reflecting the emitted light from a plurality of the parabolic surface portions.  
         [0011]     In another aspect, the invention provides a light emitting diode (LED) assembly having an integral reflector comprising: a dielectric casing of optically transparent material; first and second electrical leads extending within the dielectric casing; and at least one semiconductor die embedded in the dielectric casing and coupled to the first and second leads in a  4 manner allowing for transfer of electrical energy to and illumination of the at least one semiconductor die; wherein the dielectric casing includes an integral concavity substantially opposite the at least the one semiconductor die, the concavity being shaped and dimensioned for reflecting light emitted by the at least one semiconductor die; and, wherein the concavity forms a truncated cone whose surface is described by revolving around the symmetric axis of the LED a parabolic segment defined by the equation Y 2 =2Px, P representing a constant scale factor, the parabolic segment having its focus coincident with the centroid of the at least one semiconductor die and its vertex coincident with a line representing the x-axis passing through the centroid, the angle between the x-axis and the LED axis of symmetry determining the deflection angle of the reflector and being greater than 0° and less than 180° ; and, wherein reflection at the parabolic surface occurs by means of total internal reflection according to Snell&#39;s Law as expressed by the equation sin θ c =n 1 /n 2 , θ c  representing the critical minimum angle of incidence beyond which a ray striking the parabolic surface will be totaling reflected, n 1 , representing the refractive index of air, and n 2 , representing the refractive index of the dielectric casing.  
         [0012]     In yet another aspect, the invention provides a light emitting diode (LED) assembly having multiple integral reflectors, the assembly comprising: a dielectric casing of optically transparent material; first and second electrical leads extending within the dielectric casing; and at least one semiconductor die embedded in the dielectric casing and coupled to the first and second leads in a manner allowing for transfer of electrical energy to and illumination of the at least one semiconductor die; and, wherein the dielectric casing includes a multiplicity of convex surfaces substantially opposite the at least one semiconductor die, the convex surfaces shaped and dimensioned for reflecting light emitted bythe at least one semiconductor die; and, wherein each convex surface has an associated x-axis which passes through the centroid of the at least one semiconductor die; and each convex surface forms a truncated cone as described by revolving around the associated x-axis a parabolic segment defined bythe equation Y 2 =2Px, P representing a constant scale factor, the parabolic segment having its focus coincident with the centroid of the at least one semiconductor die and its vertex coincident with the associated x-axis, the angle between the associated x-axis and the LED axis of symmetry determining the deflection angle of the reflector and being greater than  0   o ° and less than 180° ; and, wherein reflection at the parabolic surface occurs by means of total internal reflection according to Snell&#39;s Law as expressed by the equation sin θ c =n 1 /n 2 , θ c  representing the critical minimum angle of incidence beyond which a ray striking the parabolic surface will be totally reflected, n 1 , representing the refractive index of air, and n 2 representing the refractive index of the dielectric casing.  
         [0013]     The invention not only provides a compact, simple solution to the problem of directing the light from the LED, but also does so without adding to the size and simplicity of the LED. LEDs according to the invention can be distinguished from conventional LEDs only by the fact that their light is precisely directed. Numerous other advantages and features of the invention will become apparent from the following detailed description when read in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an isometric view of a preferred embodiment of the invention;  
         [0015]      FIG. 2  is a longitudinal-cross sectional view of the embodiment of  FIG. 1  through the line  2 - 2  in  FIG. 1 ;  
         [0016]      FIG. 3  is an isometric view of an alternative preferred embodiment of the invention; and  
         [0017]      FIG. 4  is a longitudinal cross-sectional view of the embodiment of  FIG. 3  through the line  4 - 4  in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  represents an isometric view of a preferred embodiment of the invention, while  FIG. 2  represents a detailed longitudinal section of the invention through the line  2 - 2  of  FIG. 1 . LED  100  comprises: semiconductor die  110  which is disposed in die cup  120 ; anode lead  131  comprising anode lead embedded end  132  and anode lead connection end  133 ; cathode lead  134  comprising cathode lead embedded end  135  and cathode lead connection end  136 ; circuit wire  137 ; and dielectric casing  140 . Dielectric casing  140  is an optically clear non-conductive material which encapsulates: semiconductor die  110 , die cup  120 , anode lead connection end  133 ; cathode lead embedded end  135 ; and circuit wire  137 . Cathode lead embedded end  135  is electrically connected to die cup  120  which, in turn, is in electrical contact with cathode pole  111  of semiconductor die  110 . Anode pole  112  of semiconductor die  110  is in electrical contact with one end of circuit wire  137 . The other end of circuit wire  137  is in electrical contact with anode lead embedded end  132 .  
         [0019]     Dielectric casing  140  is preferably bounded by: flanged bottom  141 , cylindrical side  142 , and parabolic cone  143 . Die cup  120 , cylindrical side  142 , and parabolic cone  143  are preferably aligned with their axes of symmetry co-linear with LED axis  150 . The angle between X- axis  151  and LED axis  150  determines the deflection angle of the reflecting interface  146 . In this embodiment, X-axis  151  is perpendicular to LED axis  150  resulting in a deflection angle of 90°. LED axis  150  intersects parabolic cone  143  at conical vertex  152 . LED axis  150  intersects X-axis  151  at centroid  113  of semiconductor die  110 .  
         [0020]     The surface of parabolic cone  143  is described by revolving parabolic curve  144  about LED axis  150 . Parabolic curve  144  is a segment of a two-dimensional graph derived from the parabolic equation: 
 
y 2 =2Px  (Equation 1) 
 
 as constructed relative to X-axis  151 . The focus point of parabolic curve  144  coincides with centroid  113  of semiconductor die  110 , and parabolic vertex  147  lies on X-axis  151 . The constant P in Equation 1 serves as a scale factor gauging the relative opening width of parabola  145 , and in this embodiment P is chosen to be 1. 
 
         [0021]     When an electrical current is applied to anode lead connection end  133  and cathode lead connection end  136 , semiconductor die  110  will illuminate; and because semiconductor die  110  is contained within die Cup  120 , all radiation is directed toward parabolic cone  143 . Because of its small size, semiconductor die  110  can be treated as a point source of light located at the focus of parabolic segment  144 , which describes parabolic cone  143 . Rays  161  emanating from semiconductor die  110  and striking the surface  146  of parabolic cone  143  are reflected in a direction parallel to X-axis  151  and form, in this embodiment, a light distribution pattern resembling a thin flat disc  164  of rays  161  radiating perpendicular to LED axis  150 .  
         [0022]     No minrrored coating surface is required for reflection at the surface  146  of parabolic cone  143  because it forms the interface  146  between materials of differing refractive index and, according to Snell&#39;s Law, total internal reflection will occur at interface  146  if the incident angle of ray  161  exceeds the critical angle θ 0  , given bythe following formula: 
 
sin θ c   =n   1   /n   2   (Equation 2), 
 
 where n 1  is the refractive index of air (˜1.00) and n 2  is the refractive index of dielectric casing  140 . Solving for n 2  we get: 
 
 n   2 =1/sin θ c   (Equation 3). 
 
 The smallest angle of incidence for ray  161  is 45° when striking near conical vertex  153 . Substituting, we find: 
 
 n   2 =1/sin(45)  (Equation 4), 
 
and 
 
n 2 =1.42  (Equation 5.) 
 
 Thus, total internal reflection will occur when the refractive index of dielectric casing  140  exceeds 1.42. Preferably, epoxy resin is employed for this embodiment since it has a refractive index which exceeds 1.50, though other materials with suitable refractive index can be used. 
 
         [0023]     Other embodiments of the invention may include one or more of the following features. Reconfiguration of the parabolic reflecting surface can provide two collimated beams which radiate in separate directions from LED axis  150 .  FIG. 3  represents an isometric view of such an additional embodiment  200  used, for example, as a beam splitter.  FIG. 4  is a detailed longitudinal section through line  4 - 4  of  FIG. 3 . Two separate parabolic cones  242  and  243  each are formed by revolving parabolic curve  144  180° around X-axis  252  and X-axis  253 , respectively, each of which passes through centroid  213  of semiconductor die  210 . Parabolic cones  242  and  243  are mirror images of one another and split light rays  260  and  261  emanating from semiconductor die  210  into two beams  262  and  263  oriented in two separate directions toward the positive end of each associated X-axis  252  and  253 .  
         [0024]     Alternatively, one parabolic cone or more than two separate parabolic cones, each with an independent X-axis, can be arranged opposite the semiconductor die resulting in the ability to redirect one or a multiplicity of collimated beams oriented at independent deflection angles relative to LED axis  150 .  
         [0025]     In its various configurations, the invention discloses an LED with a compact integral parabolic reflector system which allows multi-directional light radiating from the semiconductor die to be precisely collimated and directed at significant angles away from the LED&#39;s axis of symmetry in useful planar and beam-shaped patterns. Examples of devices which could beneficially employ the invention include, but are not limited to, the following: edge-lit panels for instrumentation; beam splitters for fiber optic systems; planar illumination fixtures; and compact lighting devices.  
         [0026]     There has been described a novel LED having an integral parabolic reflector. It should be understood that the specific formulations and methods described herein are exemplary and should not be construed to limit the invention, which will be described in the claims below. Further, it is evident that those skilled in the art may now make numerous uses and modifications of the specific embodiments described without departing from the inventive concepts. For example, coatings maybe applied to the reflective surface to enhance the reflection; or in some embodiments, all or a portion of the reflecting parabolic surface may be formed by a silvered coating or layer. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in and/or possessed by the compositions and methods described and by their equivalents.