WO2000019547A1

WO2000019547A1 - Biconic reflector for collecting radiation from both top and side surfaces of led die

Info

Publication number: WO2000019547A1
Application number: PCT/US1999/019182
Authority: WO
Inventors: James S. Sherwin
Original assignee: Maxim Integrated Products, Inc.
Priority date: 1998-09-25
Filing date: 1999-08-23
Publication date: 2000-04-06
Also published as: TW428332B

Abstract

An apparatus for directing radiation from a light-emitting diode is described. In the apparatus, a biconic reflector which utilizes two conic sections is used to direct light from both the side surfaces and a top surface of the light-emitting diode.

Description

BICONIC REFLECTOR FOR COLLECTING RADIATION FROM BOTH TOP AND SIDE SURFACES OF LED DIE

Field of the Invention

The present invention applies to the field of optics. More specifically, the invention is directed toward reflector devices to direct radiant energy from a light-emitting diode.

Background Of The Invention

Light-emitting diodes (LEDs) are widely used in a number of devices including displays and communication devices. In many of these devices, it is useful to maximize the captured output light intensity of the LED while minimizing the power consumption of the LED. One method of improving the output of a LED for each unit of power used by the LED is to use a simple conic reflector, as shown in Figure 1.

Figure 1 illustrates a typical LED 100, which has a top surface 104 and side surfaces 108. In the embodiment illustrated in Figure 1, the LED top surface 104 is almost perpendicular to the approximately vertical side surfaces 108 of LED 100. LED 100 is mounted on a base 112 which is typically a conductive mounting material .

A standard LED 100 emits light from both a top surface 104 and four side surfaces 108. Figure 1 includes examples of light rays 124, 128 emitted from side surfaces 108 of LED 100 and light rays 132 emitted from top surface 104 of LED 100. Three intensity plot circles 136, 140, 144 are illustrated in Figure 1. Each intensity plot circle plots the intensity of light output in various directions from corresponding output points 149, 150, 148. An output point is where an intensity plot circle is tangent to a surface of LED 100. The intensity of light in a given direction from an output point can be determined by multiplying an intensity ray which starts from the output point and ends at the perimeter of the intensity plot circle by a proportionality constant. For example, at output point 148 where intensity plot circle 144 is tangent to LED 100, the maximum intensity of light is output in a direction perpendicular to the top surface 104 of LED 100, where an intensity ray 151 is at a maximum. The minimum intensity of light is output in a direction parallel to the plane of top surface 104.

In many applications such as fiber optic or non- guided optical communications, it is desirable to direct the light output of LED 100 along an optic axis 152. Conic reflector 156 reflects light emitted from side surface 108 of LED 100 towards optic axis 152. However, off-axis light emitted from top surface 104, such as light traveling in the direction of intensity ray 153 , is not captured by conic reflector 156 and is lost.

Figure 2A and Figure 2B illustrate a side view and top view of conic reflector 156 including LED 100 with integral bond pad 204. In Figure 2A, a bond pad 204 couples a wire 208 to the top surface 104 of the LED 100. In a typical LED system, a current flows through wire 208, the bond pad 204, and LED 100 to base 112. In one embodiment, base 112 is made of a conductive mounting material. An electric current passing through LED 100 excites quantum states in LED 100 causing the emission of light. Light emitted from side surface 108 of LED 100 reflects off conic reflector 156, as well as base 112 , preferably in a direction approximately parallel to optic axis 152 of Figure 1. The output of light illustrated in Figure 1 is idealized and does not take into account opaque bond pad 204 which blocks light output from center point 148 of Figure 1.

Figure 2B illustrates a top view of LED 100 and conic reflector 156 of Figure 1 and Figure 2A. Bond wire 208 couples to bond pad 204 on top surface 104 of LED 100 and transfers current to LED 100. The opaque bond pad 204 prevents light from being output where coupling occurs. In the embodiment of Figure 2, bond pad 204 is located on LED 100 at the center of top surface 104. Thus, light output occurs in areas of the top surface 104 not covered by bond pad 204 and along side surface 108 of LED 100.

In Figure 2B, ring 250 indicates the junction between base 112 and conic reflector 156. The inner surface 254 of conic reflector 156 is highly reflective and angled to reflect light output from side surfaces 108 of LED 100 approximately along optic axis 152 of Figure 1. The outer circle outlines larger opening 258 of the truncated conic section which forms the conic reflector 156.

Present current conic reflectors capture little of the off-axis light emitted by top surface 104. Thus, a method and apparatus for recovering off-axis light emitted by the top surface of an LED is needed. Recovering lost light improves the usable light output of LED 100 per unit of power consumed. SUMMARY OF THE INVENTION

An improved reflector system for directing light output by a LED is described. The apparatus includes a reflector formed by joining two conic sections. Each conic section of the two or more conic sections is approximately defined by a corresponding conic equation. A first conic section primarily directs light output from side surfaces of the LED in a desired direction. A second conic section primarily directs light output by a top surface of the LED in the desired direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a conceptual drawing of the output of a LED mounted in a prior art conic reflector.

Figure 2A and Figure 2B illustrate a cross- sectional side and a top view of the structure of a LED mounted in a prior art conic reflector.

Figure 3 is a cross-sectional view illustrating the output of an LED mounted in a biconic reflector system as described in one embodiment of the current invention.

Figure 4A and Figure 4B illustrate cross-sectional and top views of a biconic reflector system as described in one embodiment of the current invention.

Figure 4C illustrates a top view of the LED structure of Figure 4A in one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an apparatus for directing the light output from a light-emitting diode (LED) . The invention utilizes a second conic section coupled to a first conic section, the second conic section primarily redirects light emanating from a top surface of a LED. In the following description, various details will be given, such as the tilt of the conic sections, the geometric shape of the LED, and the dimensions of the conic sections. These details, as well as others, are given to facilitate understanding of the invention and are not intended to limit the invention.

Figure 3 illustrates a cross-sectional view of a light apparatus 300, including a LED light source 304 and a biconic reflector which includes a first conic section 308 and a second conic section 312. A side surface of LED 304 emits light rays 316, 320, 324 while a point 336 on the top surface of LED 304 emits light rays 328, 332. LED 304 is mounted on a base 340.

An ideal conic section represented by a mathematical equation extends forever. In practice, the first conic section 308 and the second conic section 312 are truncated to form truncated conic sections . One end typically a smaller opening of truncated first conic section 308 couples to base 340 at joint 344 and an opposite end or larger opening of truncated first conic section 308 couples to a smaller opening of truncated second conic section 312 at joint 348. The joints 344, 348 may be abrupt or gradual, depending on the design of the system and fabrication procedures . Intensity plot circles 352, 356 plot as a function of angle the intensity of light output from corresponding points on the LED. For example, intensity plot circle 356 represents the intensity of light output from point 336 in each direction. At point 336, LED 304 outputs the maximum intensity of light in a direction perpendicular to the top surface of LED 304. An optic axis 360 is also perpendicular to the top surface of LED 304. In the illustrated embodiment, the biconic reflector is axially symmetric around optic axis 360.

In the biconic reflector, first conic section 308 is optimized to receive light from side surfaces of LED 304 and second conic section 312 is optimized to receive light from a top surface of LED 304. Thus, in one embodiment, over half of the light incident on the first conic section 308 from LED 304 originates from the side surface of LED 304 and over half of the light incident on the second conic section 308 from LED 304 originates from the top surface of LED 304.

Light intensity is proportional to the number of photons per unit area of a surface. The area of shaded area 357 divided by the total area of intensity plot circle 352 is equal to the number of photons that reach first conical section 308 from point 364 divided by the total number of photons output from point 364. Likewise, the area of shaded area 358 compared to the total area of intensity plot circle 352 represents the percentage of photons from point 364 reaching second conic section 312. The significantly larger area of shaded area 357 relative to shaded area 358 illustrates that more photons emitted from point 364 (and side surfaces in general) will be directed toward the first conic section 308 than the second conic section 312. The volume (rotational) of shaded area 359 divided by the total volume of intensity plot sphere 356 represents the percentage of photons from point 336 reflected by second (rotational) conic section 312. In the illustrated embodiment, the shaded area 359 represents approximately 12% of the area of intensity plot circle 356. Thus, the second conic section 312 may increase intensity directed along a predetermined path from point 336 by up to approximately 12%. The actual quantity of light from each LED surface incident on each conic section can be determined using computer programs which integrate the light output from all points on a surface of the LED 304.

A biconic reflector is generated using two truncated conic sections such as first conic section 308 and second conic section 312. A surface of each conic section may be mathematically described by rotating a surface described by a conic equation. Examples of such conic equations include equations describing a circle, parabola, hyperbola, or ellipse. The plot of a conic section with a radius r and a curvature C equal to y_τ may be described by the following equations:

y² - 2rx + px² = 0

2

Ipy⁴ _| 1 • 3p²y⁶ ₍ 1 • 3 • 5p³y⁸ x = - + + 2r 2²2! r 2³3! r⁵ 2⁴4i r⁷

1 • 3 * 5 . 7p⁴y¹⁰ , 2⁵5! r⁹

Ellipse p > 1 conic constant Kappa ^*= p - 1 Circle p = 1 conic eccentricity e = -Jl - p

Ellipse 1 > p > 0

Parabola p = 0

Hyperbola p < 0

These equations are well-known to one of ordinary skill in the art. The definition and a further description of conic sections is provided in Warran J. Smith, Modern Optical Engineering - The Design of Optical Systems . ©1990 by McGraw-Hill, Inc., pages 438 through 446 and hereby incorporated by reference.

Maximum focusing of light can be achieved using conic sections in which an inner surface is described by a conic equation. However, to simplify fabrication, it is sometimes preferable to approximate the conic sections using flat or planar surfaces which has been "rotated" into curved surfaces in a cone shape. Thus, in one embodiment of the invention, a first flat surface rotated into a cone shape approximates the conic equation used to describe the first conic section 308 and a second flat surface rotated into a second curved surface approximates the conic equation used to describe the second conic section 312.

In the illustrated embodiment of Figure 3, assuming a trapezoidal LED 304 with almost vertical side surfaces, first tilt angle θ 364 between first conic section 308 and a base plane containing base 340 may be approximately 45 degrees. A second tilt angle Φ 368 between second conic section 312 and the same base plane containing base 340 is larger, possibly on the order of 60 degrees. In one embodiment, each conical section is curved rather than straight and the curvature is calculated by means of computer ray tracing.

In one embodiment of the invention, the inner surface of conic sections 308, 312 are highly reflective to redirect light along optic axis 360. The biconic reflector directs light in a cone around optic axis 360 to a lens system 372. In one embodiment of the invention, typically a system for communicating communications signals between a personal computer and associated peripherals such as keyboards and computer input devices, the lens system 372 guides the light from a peripheral to a detector in the computer.

Figures 4A and 4B illustrate cross section side and top views of one embodiment of the present invention. In Figure 4A, a wire 404 carries electric power or current to a bond pad 408 integral with a top surface of LED 304. LED 304 is mounted to base 340. The base 340 preferably includes a conductive mounting material, such that current flows from bond wire 404 to pad 408, through LED 304 and to the conductive mounting material of base 340. Opaque bond pad 408 prevents light from exiting the center top surface of LED 304. Thus, in the embodiment of Figure 4A LED 304 outputs light from the top surface of LED 304 in a "squared" ring around bond pad 408.

First conic section 308 is coupled to base 340 and second conic section 312. In the illustrated embodiment, joint 348 coupling first conic section 308 to second conic section 312 is an abrupt joint. The two conic sections 308, 312 form a cavity or well around LED 304. Increasing the depth of the well improves control of light from LED 304, by capturing and redirecting a larger portion of photons emitted from top surface 306 of LED 304. However, increasing the depth of the well makes fabrication more difficult. In particular, a deep well complicates fabrication of the electrical connection which couples bond wire 404 to pad 408 on LED 304.

The structure illustrated in Figure 4A is typically quite small. In optical communications systems, the light from LED 304 is directed into lens 372. In one embodiment, the LED 304 measures .014 inches on each side and has a height of approximately .008 inches. Base 340 measures approximately 0.024 inches in diameter. The truncated outer rim of second conic section 312 has a diameter of approximately 0.060 inches. The well depth from base 340 to the top rim of second conic section 312 is typically .018 inches.

Figure 4B illustrates a top view of the LED structure, including the biconic reflector of Figure 4A. In Figure 4B, the opaque bond pad 408 is typically metal and transfers electric current to the LED. Junction 344 between base 340 and first conic section 308 forms a circle with a diameter of approximately 0.024 inches. Joint 348 between first conic section 308 and second conic section 312 forms a second ring which typically has a diameter of .04 inches. The larger opening of second conic section 312 forms an outer ring 440 which, in one embodiment, has a 0.06 inch diameter.

Figure 4C illustrates a top view of the LED structure of Figure 4A with a modification of the biconic reflector illustrated in Figure 4B. In the embodiment of the invention illustrated in Figure 4C, the inner surfaces of conic sections 308, 312 of Figure 4B have been segmented to form surfaces which are less curved. In Figure 4C, the inner surface of first conic surface 308 has been modified to form a set of four approximately planar surfaces 450. Second conic surface 312 of Figure 4B has been modified to form four approximately planar surfaces 454, 458, 462, 466. Although the surfaces 454, 458, 462, 466 are described as approximately planar, it is understood that each approximately planar surface may also be represented by a slightly curved surface.

Each approximately planar surface is joined at joints 470, 472, 474, 476 as illustrated. The four sided surface formed by approximately planar surfaces 454, 458. 462, 466 directs light from the four side surfaces and the top surface of the approximately square LED 304.

Although this invention has been shown in relation to a particular embodiment, it should not be considered to be so limited. Rather, the invention is limited only by the scope of the appended claims .

Claims

CLAIMSI claim:

1. A reflector system to direct light from a light-emitting diode (LED) , the reflector system comprising: a first conical section having a smaller opening and a larger opening, an inner surface of the first conical section approximately described by a first conical equation; and a second conical section having an opening coupled to the larger opening of said first conical section, an inner surface of the second conical section approximately described by a second conical equation.

2. The reflector system of claim 1 wherein the inner surface of the first conical section is approximated by a first flat cone surface.

3. The reflector system of claim 1 wherein the second inner surface of the second conical section is approximated by a second flat cone surface.

4. The reflector system of claim 1 wherein a tilt angle of the first conic section is between 35 and 55 degrees .

5. The reflector system of claim 1 wherein a tilt angle of the second conic section is between 45 and 80 degrees .

6. The reflector system of claim 1 wherein the reflector is less than 0.10 inches in diameter and said base section is less than 0.05 inches in diameter.

7. An apparatus for emitting light comprising: a light-emitting diode (LED) including a top surface and a side surface; a first conic section having an inner surface defined by a first conic equation, the first conic surface positioned to primarily direct light emitted from the side surface of said LED; and a second conic section having an inner surface defined by a second conic equation, the second conic surface positioned to primarily direct light emitted from the top surface of said LED.

8. The apparatus of claim 7 further comprising: a base section supporting the LED, said base section approximately parallel to the top surface of the LED, the base section reflecting light from the side section of the LED.

9. The apparatus of claim 7 wherein the first conic section is approximated by a first curved surface in a first cone shape.

10. The apparatus of claim 7 wherein the second conic section is approximated by a second curved surface in a second cone shape.

11. The apparatus of claim 7 wherein the LED includes an opaque metal bond pad mounted to the top surface to provide electric current to the LED.

12. The apparatus of claim 7 further comprising: a lens positioned over the top surface of the

LED to further collimate light reflected from the first conic section and the second conic section.

13. The apparatus of claim 7 wherein at least 52% of the light reflected by the first conic section originates from the side surface of the LED.

14. The apparatus of claim 7 wherein at least 52% of the light reflected by the second conic section originates from the top surface of the LED.

15. The apparatus of claim 7 wherein the diameter of the second conic section is less than 0.1 inches.

16. The apparatus of claim 7 wherein a joint between the first conic section and the second conic section is abrupt .

17. The apparatus of claim 7 wherein a joint between the first conic section and the second conic section is non-abrupt.

18. A system for optical communications comprising: a light-emitting diode (LED) including a top surface and a side surface; an opaque metal contact coupled to the LED to transfer electrical power to the LED; a first conic section reflector defined by a first conic equation to primarily reflect light from the side surface of the LED; a second conic section reflector defined by a second conic equation to primarily reflect light from the top surface of the LED; a lens system to focus the light reflected from the first conic section and the second conic section; and a fiber optic cable to transfer the light output by the lens to a remote location.

19. An apparatus for emitting light comprising: a light-emitting diode ("LED") including a top surface and a side surface; a first section having a smaller opening and a larger opening, an inner surface of the first section to primarily direct light emitted from the side surface of the LED; and a second section having an inner surface defined by a plurality of approximately planar surfaces, the second section positioned to primarily direct light emitted from the top surface of the LED.

20. The apparatus of 19 wherein the top surface of the LED forms an approximate square and the second section includes at least four approximately planar surfaces .

21. The apparatus of claim 19 wherein the smaller opening of the first section forms an approximate circle and a larger opening of the first section forms an approximate square .

22. The apparatus of claim 19 wherein the smaller opening of the first section forms an approximate rectangle .

23. The apparatus of claim 19 wherein the planar surface is approximately trapezoidal in shape.