Light extraction from LEDs with light pipes

[0002] This invention relates generally to light extraction from LEDs, as for example to be directed into light pipes; and more specifically concerns provision of a non-imaging optical element that receives light from an LED, or array of LEDs, and efficiently redirects the light into a light pipe, without allowing any air gaps between the LED or LEDs and the light pipe entrance.

[0003] Recently, light-emitting diodes (LEDs) have become commercially important illumination sources for high-luminance applications. LEDs are miniature semiconductor light sources installed as a thin, active layer within a block of partially transparent material with high index of refraction. For example, Aluminum Indium Gallium Phosphide has optical index 3.6, while Indium Gallium Nitride has index 2.4. The high index has two deleterious effects that act to trap light within the absorbing semiconductor, especially when the die block is in air. First, total internal reflection (TIR) reduces the active layer's nearly-Lambertian emission out the top of the die by about 1/n², where n is the optical index value. Second, this fraction of directly emitted light is further subject to Fresnel reflectance, which varies from [(n−1)/(n+1)]²at normal incidence, or 25% at n=3, to unity at the TIR angle. Thus, about 90% of the emitted light suffers at least one reflection, and must propagate through the lossy semiconductor medium to another die boundary, and most likely suffer yet another reflection.

[0004] LEDs typically employ a transparent encapsulant, often of epoxy, with refractive index about 1.5. This increases the emission fraction to one fourth, and reduces Fresnel reflectance to one ninth (ironically, swapping their values). The two most commercially significant encapsulant geometries are the bullet shaped lens and the globbed die-on-board (i.e. encapsulant covering the die mounted on a circuit board). In both cases, however, light that does escape the die must undergo further light trapping of about 50%, offsetting the full improvement possible. The losses of the bullet lens are inherent in its convenient shape, presently manufactured by the billions.

[0005] It is a major object of the invention to provide a solution and/or solutions to the above problems and difficulties. Basically, a non-imaging coupler is provided, as in the form of a figure of resolution (about the optical axis), and comprises

[0006] a) a light transmitting body defining a recessed input cavity, a transparent-droplet encapsulant for a light emitting diode, or for an array of diodes in the cavity,

[0007] b) there being a body curved side wall shaped to totally internally reflect all the light emitted from the LED and encapsulant within a predetermined distance from the diode or center of said array,

[0008] c) there being a body cylindrical transition section extending from said curved side wall, and forwardly, and

[0009] d) a planar exit face at the forward end of the body, transverse to an axis of said figure of revolution.

[0010] It is another object to provide, in association with the coupler, an endwise transparent rod that is juxtaposed to said body exit face, in order to receive light from the coupler. A bend is typically provided at the distal end of the rod when delivering light to a dental-curing site in a patient's mouth.

[0011] FIGS. 1-5 show various coupler combinations, in axial planar cross-sections; and

[0012] FIG. 6 shows an endwise transparent rod to associate with the coupler and transmit light, for example to be used in dental whitening.

[0013] The term ‘die on board’ as used herein refers to the installation of an LED chip onto a general planar substrate, e.g. a printed circuit board. An encapsulant is typically used, typically consisting of a droplet (glob) of liquid synthetic resin placed over the die, typically hardened thereafter by the application of ultraviolet light.

[0014] FIG. 1 shows light emitting diode 10, with emitting layer 11. The LED is mounted on circuit board 15, inside light transmitting epoxy glob 20, shaped predominantly by surface tension. Metal fins 16 attached to board 15 serve to convey the LED's heat to the outside air. The optically ideal glob shape is typically a full hemisphere, but this is difficult to accomplish in a production environment, so its potential for improvement is reduced. Instead, the surface tension of the liquid tends to flatten the droplet surface 20arelative to a hemisphere surface. As shown in FIG. 2, this gives rise to internal reflection of side-going light. See reflected rays 32. Active diode layer 11 emits an exemplary fan 30 of rays. Some are transmitted through the surface 20aof glob 20 and become rays 31. Others are totally internally reflected, becoming rays 32, which strike the top of circuit board 15 on which the LED is typically mounted to be absorbed or scattered. The present invention seeks to prevent these losses by ensuring efficient extraction of the diode's light emission, as for example all or substantially all such emissions.

[0015] State-of-the art high-power LEDs require highly effective means of heat removal, so they are typically installed directly on a heat exchanger. The present invention seeks to couple the light from such a high-power LED into a light pipe, specifically blue light (λ˜465 nm) for transmitting such light, as for example is used in curing of dental materials in situ. Longer wavelengths, such as red, however, are of interest in light pipes for dental whitening and for photodynamic therapy, and shorter wavelengths such as ultraviolet (UV) can be used for curing UV epoxies or exciting fluorescence via quantum dots and/or organic fluorescent materials.

[0016] The present invention enables coupling of light from a diode, or an array of high-power, globbed LEDs, into a cylindrical light pipe or fiber optic waveguide, through which the light propagates to be emitted at its distal end. Unlike most LEDs, which use ultraviolet-hardened epoxy for globs, ultraviolet-light-emitting LEDs require a different type of glob material. The present invention is suitable for ultraviolet-emitting LEDs as well, as long as such LEDs and associated structures are constructed from an ultraviolet-transparent material.

[0017] The invention contemplates provision of a non-imaging optical element that receives the light from an LED or array of LEDs and redirects it into a light pipe, excluding air gaps between the LED die and the light pipe. Furthermore, the non-imaging element operates solely by total internal reflection and thus needs no mirror-coating.

[0018] FIG. 3 shows a cross-section of non-imaging coupler 100, with recessed cavity or indentation 101 to receive an LED encapsulant glob 120, curved side wall 102, cylindrical section 103, and exit face 104. The optical axis appears at 121, walls 102 and 103 being figures of revolution about 121. Typically, the device 100 is manufactured by injection molding, with acrylic being the most common candidate light-transmitting plastic material.

[0019] FIG. 4 shows non-imaging coupler body 100 with ray fan 110, emitted from the focal point 130, which is used to determine or calculate the shape of curved side wall 102, which consists of two sections. Section 102ais shaped by choosing a local slope that totally internally reflects incident rays 110. Surface of 102ahas increasing curvature toward rim 20bof cavity 20c. Section 102bis a portion of a parabola with focus at the origin of ray fan 110, and oriented and sized to smoothly join with both spiral section 102aand cylindrical section 103. This parabolic section 102breflects rays 110 into parallel rays shown at 111 and having incidence angle 105 with body exit face 104. For acrylic material, the angle is selected as θ_M=41.5°, quite near the optically critical reflectance angle of θ_c=sin⁻¹(1/1.5)=41.81°.

[0020] FIG. 5 shows non-imaging coupler 100 with two exemplary rays 111 and 112, emitted from the radially outermost part of LED array 106. Ray 111 makes angle θ₁with the plane of array 106, and strikes side wall 102 at a point 111ahaving slope angle ρ₁, which is determined so that the incidence angle equals θ_m, slightly more than θ_c, according to

[0021] ρ₁=θ₁−θ_m+90°.  (1)

[0022] In FIG. 5, ray 112 makes angle θ₂with the plane of the array and strikes side wall 102 at a point having slope angle ρ₂, which is determined by having to reflect ray 112 onto exit face 104 at an incidence angle equal to θ_M, such that

[0023] ρ₂=(90°−θ₂−θ_M)/2.  (2)

[0024] FIG. 6 shows coupler 100 with LED array 106, with index-matching fluid 108 in cavity formed by indentation 101. Transparent rod 200, which may also be formed from a group of closely compacted plastic or glass fibers, is placed so that entry face 205 is juxtaposed to exit face 104, receiving its light output thereby. Typically the two end faces are in flat surface-to-surface engagement to exclude air gaps. Bend 210 is situated near rod end 215, for convenience of application. Use for dental material curing, or dental whitening, is indicated schematically at 150, where 150 represents dental material being cured, or teeth being whitened.