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Title:
TIR OPTICS WITH OPTIMIZED INCOUPLING STRUCTURE
Document Type and Number:
WIPO Patent Application WO/2012/042436
Kind Code:
A1
Abstract:
A re-directing structure for re-directing at least a portion of light emitted by a LED is disclosed. The re-directing structure includes a body having an inner surface and an outer surface. The inner surface is adapted to form a chamber at least partially encompassing the LED and to refract at least a first portion of light beams emitted by the LED to be incident on the outer surface. The outer surface is adapted to reflect the first portion of the light beams incident thereon by total internal reflection. The inner surface includes a peak. Such re-directing structure is advantageous in comparison with the prior art implementations because uncontrollable light leakage from the re-directing structure is significantly reduced while the overall thickness of the structure is minimized.

Inventors:
VAN DER SIJDE, Arjen Gerben (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
PFEFFER, Nicola Bettina (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
HEEMSTRA, Tewe Hiepke (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
Application Number:
IB2011/054131
Publication Date:
April 05, 2012
Filing Date:
September 21, 2011
Export Citation:
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Assignee:
KONINKLIJKE PHILIPS ELECTRONICS N.V. (Groenewoudseweg 1, BA Eindhoven, NL-5621, NL)
VAN DER SIJDE, Arjen Gerben (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
PFEFFER, Nicola Bettina (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
HEEMSTRA, Tewe Hiepke (High Tech Campus 44, AE Eindhoven, NL-5656, NL)
International Classes:
G02B17/08; F21K99/00; F21V7/00; G02B6/00
Foreign References:
US20060067640A12006-03-30
US7334933B12008-02-26
FR2841966A12004-01-09
EP0945673A11999-09-29
Attorney, Agent or Firm:
VAN EEUWIJK, Alexander et al. (Philips Ip&s - NL, High Tech Campus 44, AE Eindhoven, NL-5656, NL)
Download PDF:
Claims:
CLAIMS:

1. A re-directing structure for re-directing at least a portion of light emitted by a light emitting device (LED), the re-directing structure comprising:

an inner surface; and

an outer surface,

wherein:

the inner surface is adapted to form a chamber, where the chamber is adapted to at least partially encompass the LED;

the inner surface is adapted to refract at least a first portion of light beams emitted by the LED to be incident on the outer surface,

the outer surface is adapted to reflect the first portion of the light beams incident thereon by total internal reflection, and

the inner surface comprises a peak.

2. The re-directing structure according to claim 1, wherein the re-directing structure is rotationally symmetric with an axis of symmetry of the re-directing structure adapted to coincide with an axis of symmetry of a beam pattern of the LED.

3. The re-directing structure according to claim 1, wherein the inner surface is rotationally symmetric with an axis of symmetry of the inner surface adapted to coincide with an axis of symmetry of a beam pattern of the LED.

4. The re-directing structure according to claims 2 or 3, wherein in a cross- section of the re-directing structure, the cross-section including the axis of symmetry of the beam pattern of the LED, the peak is at a point along the axis of symmetry of the beam pattern of the LED.

5. The re-directing structure according to any one of the preceding claims, further comprising one or more structures adapted to re-direct a second portion of the light beams emitted by the LED out of the re-directing structure.

6. The re-directing structure according to claim 5, wherein the one or more structures comprise one or more concentric rings on the inner surface and/or the outer surface and/or extending between the inner surface and the outer surface, wherein the re-directing structure is rotationally symmetric and the center of the concentric rings is adapted to coincide with an axis of symmetry of the re-directing structure.

7. A lamp comprising a light emitting device (LED) and the re-directing structure according to any one of the preceding claims.

8. The lamp according to claim 7, wherein the LED comprises a light emitting die and a dome disposed over the light emitting die and configured to seal the die, and wherein the re-directing structure is disposed over the dome. 9. The lamp according to claim 7, wherein the LED comprises a light emitting die and wherein the re-directing structure is disposed immediately over the die.

10. The lamp according to any one claims 7-9, wherein the LED comprises a high- power LED capable of emitting light with luminous flux of 65 or more lumens.

Description:
TIR optics with optimized incoupling structure

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to the field of illumination systems, and, more specifically, to LED-based lamps for providing large area illumination.

BACKGROUND OF THE INVENTION

As the efficacy (measured in lumen per Watt) and the luminous flux

(measured in lumen) of light emitting devices such as e.g. light emitting diodes (LEDs) increases and prices go down, it is expected that LED illumination and LED based lamps soon will be serious alternatives to and at a competitive level with until now predominant tube luminescent (TL) based lamps for providing large-area illumination. Such lamps can e.g. be found in office buildings with form factors of 15 centimeter- to 2.4 meter-long tubes, having diameters 5/8 inch, 1 inch, 1.25 inch or even larger, providing uniform luminance along the length of the tube.

Current LED lamps attempting to replace TL lamps require re-directing structures such as e.g. light guides to realize guiding and spreading of light emitted by the LED over large distances and areas. In order to be able to use a light guide, light coupling techniques are required for coupling the light emitted by the LED into the light guide.

One technique includes using a side-emitting LED so that the emission direction of the LED coincides with the propagation direction within the light guide, making the coupling of the light emitted by the LED into the light guide relatively easy. However, for easier assembly and easier thermal management, top-emitting LEDs are preferred.

Since replacing a side-emitting LED with a top-emitting LED would result in the emission direction of the LED being perpendicular to the propagation direction within the light guide, further efforts are required to align these two directions to couple the light emitted by the LED into the light guide.

One solution includes soldering top-emitting LEDs on perpendicularly mounted Printed Circuit Board (PCB) strips to orient the emission direction of the LED to be parallel to the propagation direction of the light guide. Another solution is disclosed in EP 0 945 673 Al and is illustrated in Fig. 1 showing a light guide 2 having a through hole 5 of an inverted triangular shape above a top- emitting LED 3 disposed on a substrate 4. As shown in Fig. 1, the light guide 2 has a dome- shaped inner surface 7 surrounding the LED 3, a top surface 10, and a pair of opposite reflecting surfaces 8 radially extending from the inner surface 7 to form a radial reflection surface, where sides 5 A of the through hole 5 are used to re-direct, by total internal reflection (TIR), the light emitted by the LED in a longitudinal direction of the light guide.

A common problem with both of the above-described solutions for using the top-emitting LED is that the resulting structure is relatively thick, which is undesirable for applications requiring thin building height or applications where the incoupling structure is part of the light guide and where, therefore, the thickness should be limited for

manufacturability reasons. An additional problem of the first solution can be the added assembly complexity (LEDs mounted perpendicular to light guide backplane) requiring additional heat sinking and/or additional incoupling structure and bevel length around the LEDs. One additional problem of the second solution is that no attention seems to be given to the extended size of LED's emitting surface. Another problem of the second solution is that complexity is added by the shown symmetry where additional side mirrors are required to couple light into the light guide.

What is needed in the art is a solution that can improve on at least some of the problems described above.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a re-directing structure adapted for use with a top-emitting LED while minimizing the overall thickness of the structure and uncontrollable leakage of light from the re-directing structure.

According to one aspect of the invention, a re-directing structure for redirecting at least a portion of light emitted by a LED is disclosed. The re-directing structure includes a body having an inner surface and an outer surface. The inner surface is adapted to form a chamber, where the chamber is adapted to at least partially encompass the LED. The inner surface is further adapted to refract at least a first portion of light beams emitted by the LED to be incident on the outer surface. The outer surface is adapted to reflect the first portion of the light beams incident thereon by total internal reflection. The inner surface includes a peak. The peak may be considered to be formed by two portions of the inner surface and would preferably be adapted to be above the center of the LED. In various embodiments, in a cross-sectional view of the re-directing structure, the cross-section being taken so as to include an axis of symmetry of the beam pattern of the LED, the two portions could comprise e.g. two linear portions of the inner surface joining at an angle (i.e., the two linear portions form an angle less than 180 degrees), two convex curves, two concave curves, or any combination of each of the two portions being a linear portion, a convex curve, or a concave curve, where, optionally, the peak may further be rounded. In some embodiments, the peak may be considered to be a "head" of the inner surface of the re-directing structure while the portions of the inner surface next to the peak could be considered to be the

"shoulders." However, in other embodiments, the peak may be considered to be a "head" of the inner surface without having any "shoulders" in the inner surface of the re-directing structure.

As used herein, the inventive concept of an inner surface of a re-directing structure comprising a peak is intended to cover all embodiments where the inner surface is adapted to form a chamber, around a LED, which chamber has aspherical, generally convex shape. The term "generally convex" is used to describe a shape that, while it may include some concave or linear portions, is, in general, convex, such as e.g. the inner surfaces shown in Fig. 12. In line with these definitions, the concept of the inner surface comprising a peak does not cover the inner surface forming a dome-shaped chamber (because dome is spherical, not aspherical) and does not cover the inner surface forming a rectangular chamber with the LED adapted to be disposed along a side of the rectangle (because the rectangle is not "generally convex").

Such a re-directing structure could be used to re-direct the light emitted by the LED from substantially vertical direction to substantially horizontal (or downward horizontal) direction. Such a structure could also be used as an incoupling structure adapted to couple light emitted by the LED into a light guide, in which case the re-directing structure could be an integral part of the light guide.

The invention is based on the recognition that, when thickness of the redirecting structure is reduced to the point that the size of the LED's emitting surface is comparable with the thickness of the re-directing structure, the LED may no longer be considered to be a point light source emitting light from a single point at the center of the emitting surface. Instead, particular attention must be given to the extended size of the LED's emitting surface when determining the shape of the re-directing structure that would allow all light beams emitted by the LED and incident on the outer surface to be reflected by TIR. When such attention is not given, light beams emitted from near the edges of the LED's emitting surface could enter the re-directing structure at such an angle that a TIR condition at the outer surface of the re-directing structure is not satisfied and these light beams will escape, or leak out from, the re-directing structure. Therefore, care should be taken to ensure that light beams emitted from near the edges of the LED's emitting surface are incident on the outer surface of the re-directing structure in a manner that the TIR condition would still be satisfied. Without increasing the overall thickness of the re-directing structure, such care includes adapting the shape of the inner surface with respect to the maximum thickness of the re-directing structure and the shape of the outer surface of the re-directing structure so that all of the light beams emitted by the LED and incident on the inner surface, even the light beams emitted from near the edges of the LED's emitting surface, could be refracted by the inner surface at such angles that the angles of incidence of the refracted light beams on the outer surface of the LED would satisfy the TIR condition. Only then can substantially all of the light beams emitted by the LED be contained in the re-directing structure.

A dome-shaped inner surface used in the prior art implementations and illustrated in Fig. 1 is not optimal in terms of the minimized light leakage because some of the light beams emitted from near the edges of the LED may leak out from the outer surface of the re-directing structure, as shown in Figs. 9B and 9D and explained in greater detail below. By contrast, an inner surface according to the embodiments of the present invention including a peak above the LED is more optimal because only for such not-dome-shaped surfaces a shape could be determined, with respect to the shape of the outer surface and the maximum thickness of the structure, so that all of the light beams emitted by the LED would be refracted by the inner surface at such angles that the angles of incidence of the refracted light beams on the outer surface of the re-direction structure would satisfy the TIR condition. The resulting structure is advantageous in comparison with the structure illustrated in Fig. 1 because uncontrollable light leakage from the re-directing structure is significantly reduced while the overall thickness of the structure is minimized since there is no need to include holes in the re-directing structure that re-direct the light, as was done in the structure of Fig. 1 and since there is no need to increase the thickness of the re-directing structure to the point that the LED can be considered to be a point source emitter.

In an embodiment, the re-directing structure may be rotationally symmetric with an axis of symmetry of the re-directing structure adapted to coincide with an axis of symmetry of a beam pattern of the LED, where, as used herein, the term "beam pattern" of a LED refers to the intensity distribution of the LED which gives the flux per solid angle in all direction of space. Further, as used herein, the term "rotationally symmetric structure" refers to a structure having symmetry with respect to rotations to certain or all angles around an axis of symmetry of the structure. In one embodiment, the re-directing structure may be rotationally symmetric with respect to rotations of all angles around the axis of symmetry of the structure, thus, having a circular top profile. In another embodiment, the re-directing structure may be rotationally symmetric with respect to rotations of only particular angles around the axis of symmetry of the structure, thus, having a non-circular but still symmetric top profile. For example, a re-directing structure which has rotational symmetry with respect to rotations of 90 degrees may have a square top profile.

In some embodiments, not the entire re-directing structure but only the inner surface of the re-directing structure may be made rotationally symmetric with an axis of symmetry adapted to coincide with an axis of symmetry of a beam pattern of the LED (i.e., the chamber adapted to encompass the LED may be made rotationally symmetric) and vice versa.

In an embodiment, in a cross-sectional view of the re-directing structure, the cross-section being taken so as to include the axis of symmetry of the beam pattern of the

LED, the peak is at a point along the axis of symmetry of the beam pattern of the LED. Thus, the inner surface would include a single peak above the LED, along the axis of symmetry of the beam pattern of the LED.

In an embodiment, the re-directing structure may further include one or more structures configured to redirect a second portion of the light beams emitted by the LED out of the re-directing structure. This embodiment provides that some of the light emitted by the LED may "leak out" from the re-directing structure. In one embodiment, such "leaked out" light could comprise 5-10% of the total light emitted by the LED, however, this percentage may be further adjusted in order to e.g. obtain sufficient uniform lit appearance of a lamp comprising the re-directing structure surrounding a LED. In this manner, controlled leakage of light from the re-directing structure may be enabled.

Positions and shapes where the light leaks out (e.g., closer or further away from the center of the re-directing structure etc.) and the amount and direction of the leaked light rays may also be optimized in terms of e.g. uniform source luminance. Thus, in one embodiment, the one or more structures for redirecting the second portion of the light beams emitted by the LED out of the re-directing structure could comprise one or more concentric rings on the inner surface and/or the outer surface and/or extending between the inner surface and the outer surface, where the re-directing structure is rotationally symmetric and the center of the concentric rings is adapted to coincide with an axis of symmetry of the re-directing structure. Such rings could allow easier modification of the optics. Therefore, the symmetry of the rings is preferably linked to the symmetry of the optics (i.e., the re-directing structure) and/or the lamp incorporating such optics rather than to the symmetry of the LED. Of course, in other embodiments, the rings could be made symmetric with respect to the axis of symmetry of the beam pattern of the LED.

In other embodiments, for a purely refractive re-directing structure, controlled leakage of light can be achieved by adding carefully calculated corrugations (e.g.

micro structures like micro-lenses, 'holographic diffuser'-like structures or other

microstructures with a defined angular distribution) to selected positions of the outer surface of the re-directing structure. If the outer surface of the re-directing structure is a mirror, similar structures could be added but, in addition to that, the mirror can be locally made more or less transparent to get the desired kind of leakage.

Alternatively or additionally, if the re-directing structure is transparent, it may also contain scattering elements in the bulk or on the outer surface to induce light leakage.

According to another aspect of the invention, a lamp is disclosed. The lamp includes a LED and a re-directing structure according to one or more of the embodiments described herein.

In an embodiment, the LED may include a light emitting die and a dome disposed over the light emitting die and configured to seal the die, where the re-directing structure is disposed over the dome. This embodiment specifies that the re-directing structure may be included as "secondary optics," where, as used herein, this term refers to optical elements not included within a LED package. By contrast, the term "primary optics" typically refers to elements included within a LED package such as a light emitting die, a dome disposed over the die and configured to protect the die, a high reflecting surface onto which the die is mounted, etc.

In an embodiment, the LED may include a light emitting die and the redirecting structure may be disposed immediately over the die. In this context, the phrase "the re-directing structure may be disposed immediately over the die" does not imply that some or all of the re-directing structure is in direct contact with the die, but, rather describes that the re-directing structure may be disposed over the die in place of the protective dome typically included in an LED package. In such an embodiment there can and preferably will be an air gap between the die and the re-directing structure (alternatively, the gap may be filled with substance other than air). In contrast to the previous embodiment, this embodiment provides that the re-directing structure may be included in the LED package, or, in other words that the re-directing structure may be included as "primary optics" in a LED package.

In yet other embodiments, the LED may include a number of light emitting dies having either the dome disposed over them or directly the re-directing structure.

In an embodiment, the LED may include a high-power LED capable of emitting light with luminous flux of 65 or more lumen. Including high-power LEDs in a lamp allows using less LEDs to obtain the desired luminance output from the lamp, thereby decreasing the cost and the complexity of the lamp.

Hereinafter, an embodiment of the invention will be described in further detail. It should be appreciated, however, that this embodiment may not be construed as limiting the scope of protection for the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In all figures, the dimensions as sketched are for illustration only and do not reflect the true dimensions or ratios. All figures are schematic and not to scale. In particular the thicknesses are exaggerated in relation to the other dimensions. In addition, details such as LED chip, wires, substrate, housing, etc. have sometimes been omitted from the drawings for clarity.

Fig. 1 illustrates a light guide according to prior art;

Fig. 2A illustrates a three-dimensional view of a LED arrangement, according to an embodiment of the present invention;

Fig. 2B illustrates a cross-sectional side view of the LED arrangement of Fig. 2 A, according to one embodiment of the present invention;

Fig. 2C illustrates a cross-sectional side view for a light guide incoupling application of the LED arrangement of Fig. 2A, according to another embodiment of the present invention;

Fig. 3 provides a schematic illustration of a LED arrangement used in the description of Figs. 4-8;

Fig. 4 provides a computation result for a cross-sectional shape of the outer surface of a re-directing structure satisfying the TIR condition when the LED is considered to be a point source emitter;

Fig. 5 provides a computation result for a cross-sectional shape of the outer surface of a re-directing structure satisfying the TIR condition when the LED is considered to be an immersed extended source emitter; Fig. 6 illustrates dependence of the optics size on the size of the emitting surface of the LED for the above-described re-direction structures satisfying the TIR condition;

Fig. 7 provides a comparison of the computation results for a cross-sectional shape of the outer surface of a re-directing structure satisfying the TIR condition when the LED is considered to be a point source emitter, or an immersed extended source or an extended source encompassed by a dome-shaped inner surface of the re-directing structure;

Fig. 8 provides a comparison of the computation results for the different cross- sections of the minimum thickness TIR re-directing structures with different incoupling chambers;

Figs. 9A-9D illustrate differences with respect to light leakage between a redirecting structure forming a dome chamber around a LED and a re-directing structure according to an embodiment of the present invention forming a chamber having a sharp peak above the LED;

Fig. 10 illustrates a cross-sectional side view in the plane 250 of a LED arrangement with controlled light leakage, according to one embodiment of the present invention;

Fig. 11 illustrates a cross-sectional side view in the plane 250 of an LED arrangement with controlled light leakage, according to another embodiment of the present invention; and

Fig. 12 illustrates inner surfaces comprising peaks of different shapes, according to various embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention.

Fig. 2A illustrates a three-dimensional view of a LED arrangement 200, according to an embodiment of the present invention. In the following, the term "LED arrangement" is used to describe an optical arrangement including at least a LED and a corresponding re-directing structure. As shown in Fig. 2A, the LED arrangement 200 includes a LED 210 and a redirecting structure 230 partially encompassing the LED 210. In operation, the LED 210 is a top-emitting LED emitting light which is incident on the re-directing structure 230.

In the embodiment illustrated in Fig. 2A, the LED 210 includes a LED die 212 covered with an optional dome 214. The LED die 212 is configured to emit light in response to a drive signal. Material used as the LED die 212 primarily determines characteristics of the LED 210, such as e.g. color, brightness, and/or intensity of light. Possible materials that could be used for the LED die 212 include inorganic semiconductors, such as e.g. GaN, InGaN, GaAs, AlGaAs, or organic semiconductors, such as e.g. small-molecule

semiconductors based on Alq 3 or polymer semiconductors based e.g. on the derivatives of poly(p-phenylene vinylene) and polyfluorene.

The dome 214 is usually configured to e.g. protect the LED 212 from the environmental factors and/or determine characteristics of the LED such as e.g. maximizing the total luminous flux of the LED while still fitting within a certain envelope or matching the intensity profile of the LED for the largest number of applications. In various

embodiments, the dome 214 may actually have a different shape than the half-dome illustrated in Fig. 2, suitable for particular requirements for applications of the LED 210.

The LED 210 may be a high-power LED, capable of providing light with luminous output of 65 or more lumen. High-power LED dies capable of emitting such fluxes usually have sizes larger than lxl mm 2 .

The re-directing structure 230 comprises a body having an outer surface 292 and an inner surface 291, the inner surface 291 forming a chamber that encompasses the LED 210. As illustrated in Fig. 2A, the outer surface 292 may have a shape that resembles a portion of a sandglass. Because of such shape, the LED arrangement 200 may also be referred to herein as the "sandglass arrangement 200." The re-directing structure 230 is a lens designed to change the path of light emitted by the LED die 212. In a preferred embodiment, the re-directing structure 230 may be fabricated from a material that does not absorb light emitted by the LED die 212 and is designed to re-direct and/or guide the light emitted by the LED die 212 based on the TIR principle. For example, the re-directing structure 230 may be fabricated from transparent PMMA, polycarbonate, glass, or comparable materials. In other embodiments, the re-directing structure 230 could also be implemented with mirror surfaces (e.g. metallic or dielectric), either hollow or filled with transparent material.

As shown in Fig. 2A, a plane 250 is a plane of symmetry for the LED arrangement 200. Fig. 2B illustrates a cross-sectional side view in the plane 250 of the LED arrangement 200 shown in Fig. 2A. In Fig. 2B, elements with the same reference numbers as in Fig. 2A illustrate the same elements as in the Fig. 2A.

As shown in Fig. 2B with a chamber 235, in the illustrated embodiment, the inner surface 291 of the re-directing structure 230 encompasses the LED 210 without being in physical contact with the LED 210. Thus, the chamber 235 is the space enclosed between the dome 214 (or the die 212, if the dome 214 is absent) of the LED 210 and the re-directing structure 230. As also shown in Fig. 2B, the inner surface 291 has a sharp peak 238 above the LED 210. Angle 239 illustrates an opening angle of the peak 238, described in greater detail below.

In various embodiments, the chamber 235 may be filled with air or with a material having suitable optical properties. In the latter case, this may require a different shape of the redirecting structure 230.

In still other embodiments, the re-directing structure 230 can be part of a light guide, where the re-directing structure would serves to couple all of the light emitted by the LED into a longer piece of transparent material guiding the light further away from the LED position, as schematically shown on Fig. 2C.

In operation, the LED die 212 emits light in a substantially vertical direction (i.e., upwards) which is intended to be re-directed by the re-directing structure 230 in a substantially horizontal and/or horizontal downwards direction. As used herein, the term "horizontal downwards" is used to describe the direction towards the place on which the LED die is disposed. The re-direction may be illustrated by tracing one of the light beams emitted by the LED die 212, a ray 251, shown in Fig. 2B.

As shown, the ray 251 includes three segments: 251a, 251b, and 251c. The segment 25 la illustrates a segment of the ray 251 emitted by the LED die 212 before it is refracted at the boundary between the material filling the chamber 235 and the inner surface 291. Note that, while this is not illustrated in Fig. 2B in order not to clutter the drawing, the segment 25 la may also be refracted at the boundary between the dome 214 and the material filling the chamber 235, depending on the refractive indices of the respective materials at the boundary. The segment 251b illustrates a segment of the ray 251 refracted at the boundary between the material filling the chamber 235 and the inner surface 291 and being incident on the outer surface 292. Finally, the segment 25 lc illustrates that the ray 251 is being reflected at the outer surface 292 by TIR, thus re-directing the ray 251 from propagating in a substantially vertical direction to propagate in a substantially horizontal direction. Shaping the inner surface 291 to include the peak 238 allows operation of the LED arrangement 200 where all of the light beams emitted by the LED die 212 that are incident on the outer surface 292 after having been refracted at the inner surface 291 may be reflected at the outer surface 292 by TIR. The advantages of having an inner surface of a re- directing structure including a sharp peak above a LED and how such a shape of the inner surface may be arrived at will now be explained with reference to Figs. 3-8.

Fig. 3 provides a schematic illustration of a LED arrangement 300 used in the description of Figs. 4-8. As shown, the LED arrangement includes a LED 310 and a redirecting structure 330 having an inner surface 391 and an outer surface 392. In an embodiment, the LED 310 and the re-directing structure 330 could be analogous to the LED 210 and the re-directing structure 230 described above and, therefore, their detailed descriptions are not repeated here.

While the inner surface 391 is shown in Fig. 3 as a dome, this is not relevant for the purpose of explaining the results shown in Figs. 4-8 and, in fact, as explained below the shape of the inner surface 391 may be different. Similarly, while the outer surface 392 is shown to have a sandglass shape, this shape may also be different, depending on e.g. the desired radiation pattern. The inner surface 391 is merely shown to illustrate that "inner surface" is the surface of the re-directing structure 330 that partially encompasses the LED 310, while the outer surface 392 is shown to illustrate that "outer surface" is the TIR surface adapted to reflect light beams incident thereon by TIR. An angle between dashed lines 332 and 333 is referred to herein as an "embossing angle" of the re-directing

structure 330.

In the discussions of Figs. 4-8, the LED 310 is either considered to be a "point source" emitter (i.e. all of the light beams emitted by the LED's die are considered to originate from a single point), which is illustrated in Fig. 3 with the LED 310 being a circle, or an "extended source" emitter (i.e., the light beams emitted by the LED's die are considered to originate not from a single point, but from different points of the emitting surface of the die), which is illustrated in Fig. 3 with the LED 310 being a dashed rectangle.

Coordinate system used in the explanations of Fig. 4, 5, 7 and 8 is shown with the x-axis and the y-axis, where the point (0,0) of the coordinate system is considered to be at the center of the emitting surface of the LED 310 (be it a point source or an extended source emitter), the x-axis is used to indicate the position, in arbitrary units of length, and the y-axis is used to indicate thickness "t" of the re-directing structure 330, also measured in arbitrary units of length. As used herein, the term "thickness" of the re-directing structure refers to the distance from the x-axis (typically from the substrate on which the LED 310 and the redirecting structure 330 are disposed) to the outer surface 392, measured in the direction of the y-axis. Thus, thickness t max illustrated in Fig. 3 shows the point where the thickness of the redirection structure 330 is at its maximum (thickness = max.thickness) and thickness to illustrated in Fig. 3 shows the thickness of the re-directing structure 330 at its minimum, referred to herein as a "starting thickness" of a re-directing structure. For the sandglass- shaped re-directing structure illustrated in Fig. 3, the starting thickness happens to be measured along the axis of symmetry of the beam pattern of the LED 310 (i.e., above the center of the LED 310). The starting thickness may be given by e.g. the height of the LED chamber (i.e, the height of the dome 214 encompassing the LED die 212) and the minimum material thickness of the material of the re-directing structure 330 which has the necessary mechanical characteristics or is given by manufacturing reasons.

Note that, as used herein, the word "maximum" in the term "maximum thickness" refers to the thickness of the re-directing structure being at its maximum and that the "maximum thickness" is actually the minimum value of the thickness of the re-directing structure that would allow all light beams incident on the outer surface 392 to be reflected with TIR.

Using Snell's law and light propagation laws, a set of appropriate trigonometric equations can be established describing the ray paths of light emitted at different angles from different spots on the emitting surface of the LED travelling to the boundaries formed by the inner and outer surfaces of the re-directing structure. Including information regarding refractive indices of the materials on different sides of the boundaries formed by the inner and outer surfaces and information regarding the shape of the inner surface in a manner known in the art, a shape of the outer surface 392 may be determined that would allow all of the light beams incident on the outer surface 392 to be reflected via TIR.

In the following discussions, the re-directing structure 330 is assumed to be made of a material having a refractive index n and to be surrounded by air (i.e., surrounded by a material having a refractive index equal to 1).

To calculate the outer surface just fulfilling the TIR conditions, starting point will be above the center of the LED 310. When the refractive index of the re-directing structure 330 is equal to 1.49 (e.g. the re-directing structure 330 is made from PMMA) and the LED 310 is considered to be a point source emitter, the embossing angle of the hourglass arrangement could be calculated to be equal to 95.7 degrees. The next coordinate points (x,y) of the outer surface can be calculated from

In equation (1), t 0 represents the starting thickness of the re-directing structure 330, BTIR IS the minimum incident angle to fulfill TIR condition and P RAY is the emission angle from the LED 310. For a point source located at (0,0), is directly linked to the coordinates of the outer surface and the equation of the outer surface becomes:

Fig. 4 provides a computation result of the cross-section (x, y(x)) of such a shape of the outer surface 392 satisfying the TIR condition when the LED 310 is considered to be a point source emitter. In the simulation of Fig. 4, the re-directing structure 330 is such that the inner surface 391 forms a dome chamber encompassing the LED 310 or such that the LED 310 is immersed within the re-directing structure 330. As used herein, the term

"immersed" is used to describe an embodiment where the inner surface 391 is in direct contact with the LED 310. As becomes clear from the inspection of the simulation result of Fig. 4, in this case, the outer surface 392 could as well be chosen to be essentially a parabola (with a minimum height to satisfy TIR condition) if the resulting light beam pattern after redirection is meant to be essentially collimated.

The thinnest re-directing structure 330 above back plane based on the TIR condition (see equation (2) above), with starting thickness to above the center of the

LED 310, will have as maximum thickness of the re-directing structure 330: ma . zaness = (3) where k =

However, the LED 310 cannot always be considered to be a point source emitter. This consideration becomes especially important when the size of the emitting surface of the LED 310 is comparable with the starting thickness of the re-directing structure 330. For a LED 310 with a given size of the emitting surface (source size), at a height h above the center of the LED 310 the incident angles (on the outer surface 392) from the emitting surface will be distributed between - , which, at a height

equal to the size of the emitting surface, will lead to emitting angles being distributed from - 26 degrees to + 26 degrees above the center of the LED 310. This means that the embossing angle of the re-directing structure 330 would decrease to 55 degrees (for the re-directing structure 330 made from PMMA) to satisfy the TIR condition. As can already be made clear from a simple geometrical inspection of the relationship between the embossing angle and the maximum thickness of the re-directing structure 330 illustrated in Fig. 3, the smaller the embossing angle, the greater the maximum thickness of the re-directing structure 330. Thus, a decreased embossing angle leads to a significantly increased thickness of the re-directing structure 330 for the TIR condition to still be satisfied. Equation (2) would be changed here to include the extended source emission position to

This case is illustrated in Fig. 5, providing a computation result for a cross- sectional shape of the outer surface 392 satisfying the TIR condition when the LED 310 is considered to be an immersed extended source emitter (the dashed line in Fig. 5). For comparison, the simulation result of Fig. 4 (i.e., the LED 310 is considered to be a point source emitter) is also included in Fig. 5 (the solid line in Fig. 5).

When the LED 310 is considered to be immersed within the re-directing structure 330, the solution for the maximum thickness of the re-directing structure 330 can still be written analytically, showing the dependence on source size and optics' size: max. t icknsss

sin ^—arcian^ ; |j t "* " 45 (5)

Fig. 5 illustrates that when only the outer surface 392 of the re-directing structure 330 is optimized to make sure that all light beams incident on the outer surface 392 are reflected by TIR, thickness of the re-directing structure 330 has to increase when the size of the emitting surface of the LED 310 is such that the LED 310 may no longer be considered to be a point source emitter. Just how big should the size of the emitting surface of the LED 310 be when it may no longer be considered to be a point source emitter is illustrated in Fig. 6.

Fig. 6 illustrates dependence of the optics size on the size of the emitting surface of the LED 310, where the term "optics' thickness" refers to the maximum thickness of the re-directing structure 330. The x-axis in Fig. 6 is used to indicate the ratio between the starting thickness of the re-directing structure 330 (i.e., to) and the size of the emitting surface of the LED 310, while the y-axis is used to indicate the ratio between the maximum thickness of the re-directing structure 330 when the LED 310 is considered to be an extended source emitter and the maximum thickness of the re-directing structure 330 when the LED 310 is considered to be a point source emitter (i.e., the ratio between values calculated according to equation (5) and equation (3) provided above).

Point A in Fig. 6 illustrates that when the size of the emitting surface is approximately twice as large as the starting thickness of the re-directing structure 330 (i.e., the ratio depicted along the x-axis of Fig. 6 is equal to 0,5), the maximum thickness of the re- directing structure increases approximately 3 times compared with the maximum thickness necessary to satisfy TIR condition when the LED 310 can be considered to be a point source emitter. This means that when the ratio to/source_size is equal to 0,5, the re-directing structure 330 having a maximum thickness less than the value calculated according to equation (5), provided above, will not be able to satisfy TIR condition for all of the incident angles and some of the light will be leaked out.

Fig. 6 further illustrates that when the starting thickness of the re-directing structure 330 is increased with respect to the size of the emitting surface of the LED 310, the ratio between the optics size for the extended source and a point source emitter decreases. Thus, as shown in Fig. 6 with a point B, when the starting thickness of the re-directing structure 330 is 2,5 times bigger than the size of the emitting surface of the LED 310, the maximum thickness of the re-directing structure only needs to be increased approximately 1.25 times to satisfy the TIR condition in comparison with maximum thickness when the LED 310 could be considered a point source.

Point C in Fig. 6 corresponds to the scenarios depicted in Fig. 5 where the ratio between the starting optics' thickness and the source size is equal to 1,5 because in Fig. 5, to is shown to be equal to 1,5 units of length while the half of the extended source is shown to be equal to 0,5 units of length. Fig. 5 illustrates only a half of the extended source since, as described above, point (0,0) coincides with the center of an LED. As illustrated in Fig. 6 with point C, for such a ratio between the starting optics' thickness and the source size, the ratio between optics' thickness calculated for the extended source and for the point source is a little bit less than 1,5. This is also clear from comparing, in Fig. 5, the maximum thickness of the dashed line and the maximum thickness of the solid line.

Based on Fig. 6, it can be concluded that when the re-directing structure 330 is made thick enough, any LED 310 may be considered to be a point source and shape of the outer surface 392 may be determined so that TIR condition would be satisfied for all light beams incident thereon. However, as described in the background section, it may be desirable to make the re-directing structure 330 as thin as possible. To that end, employing a dome- shaped inner surface 391 (i.e., the inner surface 391 shaped so as to form a dome around the LED 310) allows decreasing the overall thickness of the re-directing structure 330, in comparison with the immersed LED described in Fig. 5, while still satisfying the TIR condition. This is illustrated in Fig. 7, providing a simulation result for a cross-sectional shape of the outer surface 392 satisfying the TIR condition when the LED 310 is considered to be an extended source emitter with the inner surface 391 forming a dome chamber encompassing the LED 310 (the dotted-dashed line in Fig. 7). For comparison, the simulation results of Fig. 4 (i.e., the LED 310 is considered to be a point source emitter) and Fig. 5 (i.e., the LED 310 is considered to be an extended source emitter) are also included in Fig. 7 (the solid line and the dashed line, respectively, in Fig. 7).

When the LED 310 is considered to be encompassed by a dome-shaped inner surface 391, the solution for the maximum thickness of the re-directing structure 330 can no longer be written analytically. Instead, the equation for the outer surface 392 that would satisfy TIR condition for all incident beams, given the dome shape of the inner surface 391 needs to be solved numerically, which would read:

y(x) = tan I tf„ — a retan■ : ) ) *

(6)

In equation (6), (x c , y c ) are the coordinates of the ray's intersection with the dome interface before being incident on the outer surface in (x, y).

As is clear from Fig. 7, employing a dome-shaped inner surface 391 allows decreasing the overall thickness of the re-directing structure 330 compared to the necessary minimum thickness for the immersed extended source scenario (dashed line in Fig. 7) but the overall thickness is still significantly greater than that for the point sources scenario (solid line in Fig. 7). Only by shaping the inner surface 391 beyond a dome shape, that is, by creating a peak above the LED 310, can the light beams be sufficiently refracted at the inner surface boundary to be reflected by TIR at the outer surface 392, without uncontrolled leakage and with minimum increase in the thickness of the re-directing structure 330. When the inner surface 391 is shaped as shown in Fig. 2B (such a surface may be referred to as 'peak-shaped' surface), then the numerical simulation similar to those shown in Fig. 7 would yield a shape of the outer surface 392 closest to the shape of the outer surface 392 for the point source scenario, i.e., with the minimum overall thickness of the re-directing

structure 330.

Similar to the dome-shaped inner surface, the solution for the maximum thickness of the re-directing structure 330 for the peak-shaped inner surface 391 cannot be written analytically, but may be obtained as a numerical solution.

To calculate the shape of the inner chamber, the inner surface's slope should be oriented so as to "rotate" the rays emitted from the edge of the source's surface into the direction of rays coming from the center (as the outer TIR surface was originally designed on that point). In other words, the inner surface slope should be oriented so that the rays emitted from the edge of the source's emitting surface would be refracted by the inner surface at such an angle that it would appear as if they were emitted from the center of the source's emitting surface. As used herein, the term "rotation" in the context of "rotation" of the rays emitted from the edge of the emitting surface is used to describe the change in the direction of such rays after they are refracted at the boundary with the inner surface due to the change in the shape of the inner surface.

The corresponding equation then becomes

siiif aretan j — >·.).)

(?)

In equation (7), o r is the angle that the slope of the inner interface at coordinates (xiimer,yinner) forms with the X-aXIS, Cttarget IS the target angle (i.e., angle as defined by rays emitted from the center position of the source) of the source's edge ray in the medium of refractive index n. The shape of the inner surface can then be integrated as In equation (8), yo is the starting height of the inner chamber above the center of the LED for the two-sided type redirecting structures as described here (i.e., the distance, measured in the direction of the y-axis, from the x axis to the peak of the inner chamber, such as the peak 238). Note that, depending on the total source size/ optics thickness ratio, when approaching the edge of the source, the slope increases such that the chamber will be reduced to zero thickness. To avoid this, the slope could be chosen smaller than necessary and the outer surface would need to be modified, leading to a slightly thicker solution of the optics. The thickness increase is, however, much less than with a dome-shaped inner surface, as the starting slope of the outer surface, above the center of the source, has been reduced significantly.

This is illustrated in Fig. 8, providing simulation result of the cross-sectional shape of the outer surface 392 satisfying the TIR condition when the LED 310 is considered to be an extended source emitter with the inner surface 391 forming a peak shaped chamber encompassing the LED 310 (the dotted-dashed line in Fig. 8, solution to the equations (7) and (8)). For comparison, the simulation results of Fig. 4 (i.e., the LED 310 is considered to be a point source emitter) and Fig. 5 (i.e., the LED 310 is considered to be an extended source emitter) are also included in Fig. 8 (the solid line and the dashed line, respectively, in Fig. 8). This numerical simulation yields thus a shape of the outer surface 392 closest to the shape of the outer surface 392 for the point source scenario, i.e., with the minimum overall thickness of the re-directing structure 330.

Persons skilled in the art will recognize that, depending on e.g. the final radiation pattern and mechanical constraints, the re-directing structure can be described by shapes different than the shapes illustrated in the exemplary embodiments here. However, irrespective of the particular shape of the re-directing structure, reducing the otherwise steep embossing angle in the center above the LED is the most important consideration for the final thickness of the re-directing structure, when the LED is considered to be an extended source. The peak-shaped inner surface contributes efficiently to this reduction by "rotating" the incoming rays from the edge of the source into a direction that more easily can fulfill the TIR condition at the outer surface of the re-directing structure. The peak-shaped inner surface should have an average opening angle (such as e.g. the opening angle 239 illustrated in Fig. 2B) so that, in a cross-sectional view like the one shown in Fig. 2B, a straight line drawn between the peak 238 and the edge of the LED die 212 would not cross the inner surface 291. For a starting height yo of the inner surface this condition reads:

opening angle = 2 c r (0, y 0 ) = 2 atan(source size/2/y 0 ) Further, to allow the "rotation" of the beam emitting from the edge of the source, the angle of the inner surface will be in the center for an inner surface starting height equal to the source size and for refractive index of n=1.49:

2 c r (0, source size) = 74 degrees (instead of 180 degrees as given for a dome shaped solution).

In practice, when a re-directing structure is manufactured by e.g. injection moulding, it could happen that the peak in the inner surface instead of being a sharp peak with an opening angle designed as described above is actually rounded on the very top (as shown in some exemplary structures illustrated in Fig. 12). In such a case the opening angle could be, in reality, equal to 180 degrees (because the peak itself forms a dome at the center of the inner surface of the re-directing structure). However, persons skilled in the art would still recognize that how to apply the teachings disclosed in the present invention regarding introducing an appropriate peak to the inner surface of a re-directing structure even when the peak is, in practice, rounded.

A second part of the inner surface can rather be given by mechanical constraint and minimum Fresnel reflection for rays already emitted from the LEDs in the final target direction. With these starting points in mind, optical raytracing software may be employed to find the overall compromise.

Figs. 9A-9D illustrate how, given the same maximum thickness and the same starting thickness of the re-directing structure 330 and the same shape of the outer surface 392, light beams emitted from near the edge of the emitting surface of the LED 310 could be leaked out when a dome-shaped inner surface 391 is employed (see Figs 9B and 9D). In contrast, when a peak-shaped inner surface 391 is employed, the same light beams would be reflected at the outer surface 392 by TIR (see Figs. 9A and 9C). Thus, shaping the inner surface 391 with respect to the outer surface 392 allows achieving that all of the light beams refracted at the inner surface 391 and incident on the outer surface 392 are reflected at the outer surface 392 by TIR.

Returning back to the LED arrangement illustrated in Figs. 2A and 2B, for applications that require that some of the light is leaked out of the LED arrangement 200, additional structures may be introduced to the re-directing structure 230 that allow controlled light leakage in pre-determined locations with respect to the LED 210, according to one embodiment of the present invention. To that end, Fig. 10 illustrates a cross-sectional side view in the plane 250 of an LED arrangement 1000, which is similar to the LED

arrangement 200. The LED arrangement 1000 includes a LED 1010 and a re-directing structure 1030, similar to the LED 210 and the re-directing structure 230, respectively, described above. Therefore, in the interests of brevity, a description of the LED 1010 and the re-directing structure 1030 is not repeated here. The main difference between the LED arrangement 200 depicted in Figs. 2A-2B and the LED arrangement 1000 is that the latter also includes rings 1003 on an outer surface 1092 of the re-directing structure 1030. The rings 1003 are adapted to provide controlled leakage of a portion of light beams emitted by the LED 1010. Alternatively or additionally, controlled leakage of light from the re-directing structure may also be achieved by introducing different corrugations in the outer surface of the re-directing structure, as is schematically shown in Fig. 11.

The embodiments described above have all illustrated the LED including a light emitting die covered with a dome. The LED arrangements 200 and 1000 may also be implemented without including the domes 214 and 1014. Instead, the re-directing structure itself may be used to perform the functions of the dome over the light emitting die. In such embodiments, the re-directing structure may be referred to as a "primary" optical component.

Further, LED arrangements described above could also be made rotationally symmetric with respect to all angles and/or, while the illustrated embodiments are

particularly advantageous for use with the top-emitting LEDs, the teachings of the present invention could also be applied for use with side-emitting LEDs.

In addition, while the embodiments described herein all include a sharp peak in the inner surface of the re-directing structure, embodiments with the inner surface comprising a peak of a different shape are also within the scope of the present invention. Fig. 12 illustrates portions of re-directing structures with inner surfaces comprising peaks of different shapes, according to various embodiments of the present invention.

While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. Therefore, the scope of the present invention is determined by the claims that follow.