Abstract:
An illumination-redistribution lens comprising a thick aspheric lens collecting a high proportion of the luminous output of a compact LED with a quasi-hemispheric pattern. After receiving the highly nonuniform illuminance from the nearby LED, the lower surface refractively deflects these rays into a less diverging angular pattern that results in uniform illuminance on the upper surface of the lens, which itself is shaped so its distribution of slope angles refractively deflects the uniform illuminance distribution into an exiting beam that will produce uniform illuminance on a distant target, such as a table below a ceiling-mounted unit. When square-cut sections of such lenses are laterally arrayed to form a downlight, a uniform rectangular spot will be produced on the target.

Description:
This application is based on provisional application Ser. No. 60/627,287 filed Nov. 12, 2004. 

   BACKGROUND OF THE INVENTION 
   Illumination lenses act to collect light from a source and gather it into a useful beam to cast upon a target. Frequently, uniform illumination is desired, but most often not attained. Sometimes this is because the target, such as a room&#39;s floor, has widely varying distance and slant to the luminaire, so that uniform intensity becomes nonuniform illumination. In most cases, though, the luminaire itself has widely varying distance from its light source to its different light-redirecting sections. Thus both mirrors and lenses typically have nonuniform illumination across their exit apertures. The present invention addresses this problem with a general method of making a lens achieve uniform exit-aperture irradiance as well as uniform target irradiance. This capability is particularly important for the next generation of high-brightness light-emitting diodes (LEDs). Their comparatively high cost per lumen puts a premium on luminaire uniformity and efficiency, for which the present invention provides a significant boost. 
   A particular illumination field for such a device is downlights, ubiquitously seen in ceilings as comprising an incandescent spotlight bulb in a recessed can. These 50-100 W lamps must typically be replaced at least once per year. An LED downlight would be of great benefit, due to the much lower power consumption and ten times longer life of LEDs. Also, the low voltage of LEDs would enable LED downlights to be installed without the extremely high price of licensed electricians who must be used to install the otherwise low-priced conventional downlight cans. 
   Conventional LED optics, however, provide neither uniform output nor the high directionality required for a downlight to produce, for example, a small spot of light on a table. A standard target, moreover, is not round but a three-foot square that is nine feet distant from the lamp. Currently only expensive and bulky projection lamps can produce a square light beam, and quite inefficiently at that, by using large lenses to form an image of a square aperture placed in the output beam of a collimating lens or mirror that uses an incandescent lamp. 
   The present invention remedies the current lack of suitable optics for LED downlights, and in particular provides an LED downlight with a square spot. 
   SUMMARY OF THE INVENTION 
   The present invention relates generally to illumination lenses, especially for LEDs, that produce uniform output-irradiance from highly nonuniform inputs. In particular, the present invention discloses several LED downlights providing marked improvements over the conventional cans. In general form the present invention comprises a thick aspheric lens lying close to a quasi-hemispheric light source, typically an LED but possibly a combination of incandescent lamp and reflector. The lower surface of the lens acts primarily to redistribute the flux it receives by refracting it in such a way that it has a uniform distribution by the time it has intercepted the front surface, which typically has the form of a thick aspheric section. This top surface then refracts the uniform distribution into a suitable beam. 
   One preferred embodiment of this new lens produces a very tightly collimated beam with uniform irradiance. This is advantageous for LED downlights because different diffuser-covers can go across this beam to produce a variety of circular or elliptical beam patterns, albeit with fuzzy boundaries. 
   Another preferred embodiment produces a square diverging beam that uniformly illuminates a square target, a valuable trait for LED downlights, particularly because of the sharp boundary of the square. 
   These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which; 

   
     DRAWING DESCRIPTION 
       FIG. 1  is an intensity graph showing the distinction between Lambertian and sub-Lambertian emitters; 
       FIG. 2  shows a receiving aperture and a nearby compact light-emitter; 
       FIG. 3  shows the highly nonuniform distribution of irradiance across the aperture, and compares its cumulative distribution with that of uniform irradiance; 
       FIG. 4  shows how finite source size causes a lateral spread of received rays refracted through a flat aperture; 
       FIG. 5  shows an irradiance-redistribution lens; 
       FIG. 6  shows how this lens uniformly illuminates a standard target; 
       FIG. 7  shows the square distribution possible with this lens; 
       FIG. 8 . shows the squared-off lens making a square illuminance distribution; 
       FIG. 9  shows an array of such squared-off lenses; 
       FIG. 10  shows a square downlight based on such an array; 
       FIG. 11  shows a hexagonal array of lenses; 
       FIG. 11A  shows the entire downlight; 
       FIG. 12  shows downlights with different holographic diffusers, powered by low-voltage wiring; 
       FIG. 13  shows the mathematical generation of a lens profile; 
       FIG. 14  shows a wide-angle irradiance-redistribution lens; 
       FIG. 14A  shows the beam of same; 
       FIG. 15A  shows an extremely wide-angle irradiance-redistribution lens; 
       FIG. 15B  shows the beam of same; 
       FIG. 16  shows a collimating irradiance-redistribution lens; and 
       FIG. 17  shows a luminaire utilizing semicircular lens to form a semicircular output beam. 
   

   Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
   DETAILED DESCRIPTION 
   A Lambertian light source presents constant luminance at all angles, so that off-axis foreshortening of its flat output aperture gives a cosine dependence of intensity. Actual LED chips can differ from this ideal pattern.  FIG. 1 , as an example, shows intensity graph  10  with horizontal angular scale  11  and vertical scale  12  showing relative intensity. Curve  13  is the measured output of the Flash-LED made by the Lumileds Corporation. For comparison, curve  14  graphs the cosine dependence of a perfectly Lambertian emitter, showing that curve  13  may be called ‘sub-Lambertian’, in spite of horizontal branch  13   h . This latter feature is due to lateral leakage out of the top of the Flash package. 
   An important feature of an intensity distribution is the cumulative flux distribution, defined as the normalized angular integral of the intensity I(θ):
 
 J (ψ)= 0 ┌ ψ   I (θ)sin θ dθ/   0 ┌ 90   I (θ)sin θ dθ 
 
Curve  15  graphs this function, showing that 90% of the flux lies within 63° of the axis, a solid angle equal to half that of a hemisphere&#39;s 2π steradians. A Lambertian emitter, however, has the cumulative function sin 2  (θ, equal to 79% at 63°.
 
   Both the ideal Lambertian emitter and the Flash LED emit into a hemisphere of solid angle, with far more than half their output going into half the hemisphere. Such emitters can be called ‘quasi-hemispheric’. For their luminosity to be usefully collected, quasi-hemispheric emitters require wide-angle illumination optics.  FIG. 2  is a perspective view of such a situation, showing Flash LED  21  illuminating nearby flat aperture  22 , which subtends a 63° half angle. Quasi-hemispheric emission  23  can be seen to be vertically stronger, because it has the intensity distribution of curve  13  of  FIG. 1 , so that 90% of its rays are intercepted by aperture  22 . Arrowed rays  24  represent the 10% of the emission missing the lens because they fall in the lower half-hemisphere of solid angle. Because the outer portions of aperture  22  are much further away than the center, with the less intense off-axis rays hitting it at a slant, the illumination distribution across  22  is far more nonuniform than emission  23  itself. 
   For aperture  22 ,  FIG. 3  shows normalized illuminance graph  30  with horizontal scale  31 , in radial millimeters from aperture center, and vertical scale  32  in percent. Curve  33  graphs the highly nonuniform relative illuminance on the aperture. Broken-line curve  34  shows the relative encircled flux for radius r along scale  31 . Parabolic curve  35  is the cumulative distribution for uniform illuminance. Arrows  36  run horizontally from curve  34  to curve  35  to show how the nonuniform illuminance of curve  33  must be redistributed radially outward to produce uniform illuminance. That is, the rays intercepting the aperture at radius r must be shifted outwards to radius R. Carrying out the redistribution function R[r] is an objective of the present invention, and this mapping of cumulative distributions constitutes its method of producing uniform illuminance from initial nonuniformity. 
     FIG. 4  shows LED package  41  sending ray-triplets  42  to points  43  on refracting flat surface  44  of a transparent dielectric. Note the relatively small size (a few square millimeters) of the emitter to the relatively large size (hundreds of mm 2 ) of the aperture. Such a small emitter is termed a ‘compact’ light source, and is said to be ‘nearby’ from such a wide-angle aperture. This source-compactness means that above aperture  44  source  41  subtends small angle  45 . This means that deflecting a ray triplet from r to R will not greatly spread out the flux received at point r.  FIG. 4  shows the different ray-triplets  42  stopping at different heights, deliberately suggestive of an upper surface to go with surface  44 . In fact, such a configuration, of redistributive lower surface and beam-forming convex upper surface, encompasses all preferred embodiments of the present invention. The ratio shown in  FIG. 4  of the width of emitter  41  to that of receiving surface  44  is the cause of the narrow spread  45  of the luminosity through any point  43 . If emitter  41  were made even larger, a size would eventually be reached that where the R[r] function did not deliver uniform illuminance. Said ratio in  FIG. 4  is less than ⅕. 
     FIG. 5  shows a preferred embodiment in profile  50  of a 35 mm-diameter circularly symmetric illumination-redistribution lens formed of polycarbonate and comprising concave lower surface  50 L, upper exit-surface  50 U, and optically inactive flange  50 F. Flash LED  51  emits ray-fan  52 , extending to 63° off-axis and thereby representing 90% of the luminosity of source  51 . Rays  52  are received along concave lower surface  50 L, each at a radius r. A diagram quite similar to  FIG. 3  would describe the encircled flux at each radius r and give it the corresponding top-surface radius R. Both r and R are shown in  FIG. 5 . 
   Furthermore in  FIG. 5 , the ray going from radius r on lower surface  50 L to radius R on upper surface  50 U is refractively deflected therefrom toward radius RT on target surface  53 , herein shown for clarity in close proximity to lens profile  50 , but customarily more distant. Radius RT on target  53  follows a parabolic profile of encircled flux, a property of uniform illuminance thereupon. 
     FIG. 6  shows the practical situation for which the present invention is especially useful, diagonal  63  of a 3-ft square at distance h=9′ from lens  60  forming beam  62  from LED source  61 . Distance RT is in correspondence with that of  FIG. 5 , as a part of uniform illumination of target  63 . This is a common commercial lighting specification, but a square pattern does not seem to be commercially available today except via bulky and expensive imaging-spotlights, a market deficit the present invention ably remedies. 
     FIG. 7  shows a square illumination pattern in the form of flux pattern  70 , a perspective view of a flat-topped square distribution over x and y coordinate scales in mm. 
     FIG. 8  shows the preferred embodiment producing the pattern of  FIG. 7 . Illumination lens  80  comprises upper surface  80 U, vertical planar side walls  80 S, one of which is removed, and lower surface  80 L. Just below lens  80  is nearby compact light source  81 . 
     FIG. 9  shows an array of such squared-off lenses. Array  90  is positioned above sources  91 , which are mounted on circuit board  92 . 
   Illustrating the use of such a lens array,  FIG. 10  shows downlight  100 , comprising lens array  101 , case  102 , and rectangular convective heat-sink fins  103 , oversized for longer operating life of the LEDs they cool (not shown). 
     FIG. 11  shows how the lens of  FIG. 5  can be hexagonally packed into a circular configuration. Downlight  110  comprises an array of 17 lenses  111 . It is positioned over round circuit board  112  upon which are mounted LEDs  113 . Lower lens surfaces  111 L are visible, also showing hexagonal-cell boundary-line  111 H. Although this configuration will produce a round spot instead of a square, more of the luminosity of its LEDs will end up in its beam. 
     FIG. 11A  shows downlight  110  from another angle, as well as case  114  and translucent cover  115 . Case  114  is cut away to reveal lenses  111 , circuit board  112 , and LEDs  113 . Several lower lens surfaces  111 L are visible, as well as hexagonal-cell boundary-lines  111 H. 
   It is possible to utilize a holographic diffuser on the translucent cover, to alter the beam pattern for particular situations. In this case the lenses would make a narrow beam and different diffusers could be selected to get different wide beams.  FIG. 12  shows the illumination function of the present invention. Alcove  120  comprises floor  121 , wall  122 , and false-ceiling  123 . Display table  124  is illuminated by downlights  125 ,  126 , &amp;  127 , respectively casting small elliptical beam  125 B, large elliptical beam  126 B, and tight circular beam  127 B. Each of these beams is produced by a different version of the holographic diffuser on cover  115  of  11  of  FIG. 11A . Low-voltage wiring system  127  is fed by low-voltage power supply  128 . 
   It can be seen that differing distances and target sizes will call for lenses with somewhat differing profiles than that of  FIGS. 5 &amp; 6 . Such profiles are circularly symmetric and generated by a differential equation relating the bottom-surface coordinates to the slope angle of the bottom surface, via the bottom-surface deflection angle required by value-matching the cumulative distribution C[r] of bottom illuminance with the parabolic distribution of uniform illuminance, D[R]=(R/R MAX ) 2 . That is, the deflection function R[r] comes from inverting the redistribution equation C{r}=D(R[r]), as shown by the horizontal arrows of  FIG. 3 . 
   Given this function R[r}, the lens profile can be calculated by the method of  FIG. 13 , which is a close-up view of the edge of irradiance-redistribution lens  130 , in the vicinity of flange  130 F, showing lower surface  130 L and upper surface  130 U. The mathematical generation of lower surface  130 L begins with its outer edge, at defined point (r,z). Flange  130 F has defined thickness t, so that the corresponding initial point of upper surface  130 U is (R,z+t). Vectors are expressed in terms of horizontal unit vector i and vertical unit vector k, so that the input ray vector of ray  131  is A=i sin α+k cos α, where α is the inclination of ray  131  from vertical vector k. The lower-surface inclination angle is ρ 1 , so that the corresponding normal vector is N 1 =i cos ρ 1 +k sin ρ i . Similarly on upper surface  130 U having inclination angle ρ 2 , the normal vector is N 2 =i cos ρ 2 +k sin ρ 2 . 
   From initial points (r 1 ,z 1 ) and (R 1 ,Z 1 ), with Z 1 =z 1 +t, comes the interior angle γ=tan −1  t/(R−r), and thence the ray-vector C=i sin γ+k cos γ. The exit-angle β is derived from the known target distance ZT and the known radius RT on the target, which is given by the requirements of uniform illumination thereupon: β=tan −1  (ZT−Z 1 )/(RT−R 1 ). Thence ray-vector B is given by B=i sin β+k cos β. The normal vector N 1  is that which refracts A into C, and is given by
 
 N   1 =( C−A )/∥( C−A )∥
 
where ∥x∥ is the scalar magnitude of vector x. Similarly on upper surface  130 U, normal vector N 2  is that which refracts C into B:
 
 N   2 =( B−C )/∥( B−C )∥
 
   Given vector N 1 , the next point on lower surface  130 L is (r 2 ,z 2 ) on ray  132 , at small distance ds from the first point (r 1 ,z 1 ). These next coordinates are given by
 
 r   2   =r   1   −ds  cos ρ 1  z 2   =z   1   +ds  cos ρ 1  
 
From lower-surface coordinate r 2  comes upper-surface coordinate R 2 =R[r 2 ], from which can be calculated the other upper-surface coordinate: Z 2 =Z 1 +tan ρ 2 (R 2 −R 1 ). By using small intervals ds, smooth surfaces  130 L and  130 U can thus be mathematically iterated from the periphery to the center, with different resultant shapes dependent upon the size and distance of the illumination target, such as target  63  of  FIG. 6 . The narrower the target, the thicker the lens, but uniform illumination is always the result.
 
     FIG. 14  shows the profile of wide-angle irradiance-redistribution lens  140 , illuminated by LED  141  with rays  142 .  FIG. 14A  also shows target  143  uniformly illuminated by rays  142 . 
     FIG. 15A  shows the profile of extremely wide-angle irradiance-redistribution lens  150 , illuminated by LED source  151 , with rays  152  that illuminate the arched interior surface  150   i , which advantageously can extend to the same level as source  151 , more fully intercepting its light than previous, narrower-angle lenses. External rays  153  are shaped to uniformly illuminate a 160° wide planar target. Central rays  153   c  are diverging because the target center is so close, while side rays  153   s  are nearly collimated for illuminating the distance flanks of the target. 
     FIG. 15B  is a side view of planar target  155  being uniformly illuminated by wide-angle rays  153  from lens  150 , including central rays  153   c  and side rays  153   s  from  FIG. 15A . 
     FIG. 16  shows the profile of collimating irradiance-redistribution lens  160 , illuminated by LED  161  with rays  162  that uniformly illuminate target  163 . 
     FIG. 17  is a perspective view of semicircular luminaire  170 , comprising semilens  170 , vertical planar mirror  171 , and LED light source  172  adjacent thereto. This preferred embodiment produces a semicircular beam suitable for gaming tables of that shape. 
   The preceding description of the presently contemplated best mode of practicing the optical transformer described herein is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
   It will be appreciated by those skilled in the art, in view of these teachings, that alternative embodiments may be implemented without deviating from the spirit or scope of the invention. This invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.