Abstract:
A lens and an LED using the lens to achieve homogeneous illumination include a region. The region around the optical axis of a lens is designed to be concave and form a divergent surface. The upper surface of the lens is a continuous curved surface to diverge the high-intensity light emitted by the LED in the vicinity of the optical axis through refractions. Therefore, the LED can homogeneously illuminate a larger area.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a lens and a light-emitting diode (LED) using the lens for illumination systems and, in particular, to an LED with homogeneous illumination. 
   2. Related Art 
   A traditional LED  10  is shown in  FIG. 1A . It is formed by packing an LED chip  12  in a lens  11 . The most commonly seen embodiment of the lens is illustrated in the drawing. The lens  11  is a hemisphere to converge the light emitted by the LED chip  12 . The radiation pattern (directions and energy distribution of the emitted light) has to be restricted in the vicinity of the optical axis  13 . Since the energy is more concentrated, one has to rely on the use of a light-guiding plate  14  to improve the homogeneity in the case of a surface plane light source. However, if the LED&#39;s  10  are directly distributed under the light-guiding plate, it requires a fair amount of LED&#39;s  10  because each one of them has a very small illuminating area. Therefore, one usually adopts the side-illuminating structure to achieve the required homogeneity of a surface plane light source, as shown in  FIG. 1B . However, since the LED&#39;s  10  can provide the optical energy only at the edge of the light-guiding plate  14 , the distance between the central part and the adjacent light sources becomes larger when the area of the surface plane light source is big. Therefore, the optical energy transmitted to the central part by virtue of total reflections become insufficient as the area enlarges. In other words, there is a certain limit for the area of the surface plane light source. 
   To overcome this problem, U.S. Pat. No. 6,679,621 uses a special lens shape to totally reflect the light coming out of the top surface of the LED and to deflect the light coming out of its side surfaces. All beams are emitted in parallel but orthogonal to the optical axis. Therefore, in the construction of a planar light surface, the LED&#39;s can be disposed in the middle of the light-guiding plate, instead of being limited to its edges. As the area of the surface plane light source gets larger, there will not be any problem of insufficient optical energy on the light-guiding plate due to the far distances to the light source. Nevertheless, the design relies on the total reflection and refraction of light. The overall light-emitting efficiency is lowered due to the attenuation of multiple reflections and refractions inside the light source system. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an object of the invention is to provide a lens and a homogeneously illuminating LED using the lens. The invention can provide a large-area, homogeneous light-emitting surface. It can be directly used in a surface plane light source system. It further reduces the use of LED&#39;s to save the cost. 
   To achieve the above object, the disclosed lens and the LED that uses the lens have a lens designed on an action surface and in such a way that a concave divergent surface is formed around the optical axis. This design can deflect the light emitted in the vicinity of the optical axis away from the optical axis, so that the LED can provide a homogeneous and large-area light-emitting surface. According to a first embodiment, the invention includes a substrate, an LED chip, and a lens. The lens is bonded onto the substrate by packaging and covers the LED chip. The lens has a divergent surface around the optical axis and an adjacent convergent surface. The divergent surface diverges the light around the optical axis, and the convergent surface converges light of large angles toward the optical axis. Thus, the LED&#39;s can provide more homogeneous light in a larger area. 
   In a second embodiment of the invention, the lens is designed to have a concave part at its bottom and to be directly connected to the substrate. The concave part covers the LED chip. 
   In a third embodiment of the invention, the concave part at the bottom of the lens is designed to be the top surface of a conventional LED. The lens is directly installed on the conventional LED. 
   In a fourth embodiment, the lens is in the form of a Fresnel lens. Its equivalent lens is the same as before. However, the thickness of it is reduced to have more practical applications. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1A  is a schematic view of a conventional LED and its light-emitting style. 
       FIG. 1B  is a schematic view of a conventional backlit module. 
       FIG. 2  is a schematic view of the disclosed LED according to the first embodiment of the invention. 
       FIG. 3  is a light intensity distribution diagram of the disclosed LED. 
       FIG. 4  compares the light-emitting area of the disclosed LED. 
       FIG. 5  is a schematic view of the disclosed LED according to the second embodiment of the invention. 
       FIG. 6  is a schematic view of the disclosed LED according to the third embodiment of the invention. 
       FIG. 7  is a schematic view of the disclosed LED according to the fourth embodiment of the invention. 
       FIG. 8  shows an application of the invention. 
       FIGS. 9A and 9B  to  12 A and  12 B give the experimental data in our simulations. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In observation of the fact that the conventional LED usually has stronger light intensity around its optical axis, the disclosed lens and the LED using the disclosed lens diverge light in the vicinity of the optical axis. The first embodiment of the invention is shown in  FIGS. 2 and 3 . It contains a substrate  21 , an LED chip  22 , and a lens  23 . The LED chip  22  is installed on the substrate  21  and has an optical axis  24 . The lens  23  is connected to the substrate  21  by packaging to cover the LED chip  22 . On the surface of the lens  23  and in the vicinity of the optical axis  24  is formed with a concave part as a divergent surface  25 . The divergent surface  25  is surrounded by a convergent surface  26  with the optical axis  24  as a center. The divergent surface  25  is recessed towards the optical axis  24 . The height difference from the divergent surface  25  to the bottom of the LED chip  22  increases from the optical axis outwards. The height difference from the convergent surface  26  to the bottom of the LED chip  22  decreases from the maximum outward. The upper action surface of the lens  23  is a continuous curved surface. The action surface referred herein means the surface above the lens  23  for the light emitted by the LED chip  22  to deflect. With this action surface, one is able to obtain a large-area and homogeneous beam of light. Therefore, the action surface does not necessarily mean the whole upper surface of the lens  23 . It can be only a part of the upper surface. The size of the action surface is determined by the width and height of the lens  23 , and type of the LED chip  22 . The light emitted by the LED chip  22  and going through the divergent surface  25  is deflected away from the optical axis  24 , whereas the light emitted by the LED chip  22  and going through the convergent surface  26  is deflected toward the optical axis  24 . Therefore, the light intensity within a certain range becomes more homogeneous. The light intensity distribution is shown in  FIG. 3 . As one can see, the maximum intensity region is shifted outward away from the optical axis  24 . The overall light-emitting region is larger and more homogeneous. In particular, the distance to the maximum light intensity region can be adjusted according to the distance d 1  from the highest point on the surface of the lens  23  to the optical axis  24  and the height difference d 2  from the lowest point of the divergent surface  25  to the highest point. A common design is that the lens  23  is a cylinder symmetric about the optical axis  24 . Therefore, the intersection between the divergent surface  25  and the optical axis  24  is the lowest point. The distance d 1  and the height difference d 2  can be adjusted according to the refractive index, temperature coefficient, thickness, and height of the lens, the LED chip, and the desired homogeneous light-emitting area. 
   Since the divergent surface  25  is used to diverge the light near the optical axis  24 , it is preferably to use a concave surface. However, it can be a mixture of concave and convex surfaces. The convergent surface  26  is used to converge light with large emission angles. Therefore, it is preferably to be a convex surface. As shown in  FIG. 4 , when projecting light on a screen  70  at a distance of H, the disclosed LED  20  has a larger and more homogeneous light-emitting range, in comparison with the conventional LED  10 . 
   With reference to  FIGS. 9A and 9B , one sees that the homogeneous light-emitting area is about 33 mm from the optical axis according to the first experimental data set. The LED chip has a distance of 30 mm. Therefore, we obtain a subtending angle of 47.7 degrees. Likewise, the subtending angle in  FIGS. 10A and 10B  is 53.8 degrees, that in  FIGS. 11A and 11B  is 67.4 degrees, and that in  FIGS. 12A and 12B  is 72.3 degrees. We therefore see that the subtending angle is enlarged to 45 to 75 degrees using the invention. 
   Please refer to  FIGS. 9A and 9B  again. The height difference (recession depth) d 2  from the lowest point of the divergent surface  25  to the highest point is around 0.05 mm. The total height from the highest point to the bottom of LED chip  22  is around 2.8 mm. Therefore, the height ratio of recession depth to the total height is around 0.018 (0.05/2.8). As depicted above, the subtending angle in  FIGS. 9A and 9B  is 47.7 degrees. 
   Please refer to  FIGS. 10A and 10B . The height difference (recession depth) d 2  from the lowest point of the divergent surface  25  to the highest point is around 0.16 mm. The total height from the highest point to the bottom of LED chip  22  is around 2.36 mm. Therefore, the height ratio of recession depth to the total height is around 0.068 (0.16/2.36). As depicted above, the subtending angle in  FIGS. 10A and 10B  is 53.8 degrees. 
   Please refer to  FIGS. 11A and 11B . The height difference (recession depth) d 2  from the lowest point of the divergent surface  25  to the highest point is around 0.33 mm. The total height from the highest point to the bottom of LED chip  22  is around 1.91 mm. Therefore, the height ratio of recession depth to the total height is around 0.173 (0.33/1.91). As depicted above, the subtending angle in  FIGS. 11A and 11B  is 67.4 degrees. 
   Please refer to  FIGS. 12A and 12B . The height difference (recession depth) d 2  from the lowest point of the divergent surface  25  to the highest point is around 0.45 mm. The total height from the highest point to the bottom of LED chip  22  is around 1.74 mm. Therefore, the height ratio of recession depth to the total height is around 0.259 (0.45/1.74). As depicted above, the subtending angle in  FIGS. 12A and 12B  is 72.3 degrees. 
   In the embodiments in  FIGS. 9A-12B , the height ratios are from 0.018 to 0.259. The recession depth varies from 0.05 mm to 0.45 mm when the total height varies from 2.8 mm to 1.74 mm. The subtending angles are from 47.7 to 72.3 degrees. A larger subtending angle provides a broader/larger uniform (homogeneous) flux of emitted light. To sum up, the higher the height ratio, the larger the subtending angle. In other words, in order to get a larger subtending angle, the height ratio should be designed higher. 
   On the other hand, the second embodiment of the invention is shown in  FIG. 5 . The lens  30  is designed to be a mask. Its surface also has a divergent surface  32  and a convergent surface  33 . Its bottom has a concave part  31  that is greater than the LED chip  22 . When the lens  30  is connected to the substrate  21 , the concave part  31  covers the LED chip  22 . A large-area homogeneous light-emitting area is also obtained. 
   A third embodiment of the invention is shown in  FIG. 6 . The concave part  31  is designed according to the shape of the LED  10  so that it can be directly combined with a conventional LED  10 . This design is more convenient in use. However, the light-emitting efficiency is slightly lower due to the extra deflection. 
   A fourth embodiment is shown in  FIG. 7 . The surface of the lens  40  has the form of a Fresnel lens  42 . The bottom also has a concave part  41  for accommodating an LED chip  22 . The equivalent lens of the Fresnel lens-like surface  42  is the same as the above-mentioned lens  23  (see  FIG. 2 ). This design can reduce the thickness of the LED and allow wider applications. 
   As shown in  FIG. 8 , pluralities of LED&#39;s  20  are distributed on the frame  61  of a backlit module. A diffusive plate  62  is combined from the top. This can be used in a backlit module because the LED&#39;s  20  have better homogeneity and light-emitting range. In comparison with the prior art, the required number of LED&#39;s  20  for a backlit module of the same thickness can be reduced. In this case, it is less likely to have focused light spots. 
   Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.