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Title:
LIGHT EMITTING DEVICE WITH SELF-ALIGNING PREFORMED LENS
Document Type and Number:
WIPO Patent Application WO/2015/110927
Kind Code:
A1
Abstract:
A lens structure is pre-formed with features that facilitate accurate alignment of a light emitting chip within the lens structure. To ease manufacturing, the features include tapered walls that allow for easy insertion of the light emitting chip into the lens structure, the taper serving to accurately align the light emitting chip when the chip is fully inserted. The taper may include linearly sloped or curved walls, including complex shapes. An adhesive may be used to secure the light emitting chip to the lens structure. The light emitting chips may be picked-and-placed into an array of lens structures, or picked-and-placed onto a substrate that may be overlaid by the array of lens structures.

Inventors:
DAESCHNER, Walter (Building 5, AE Eindhoven, NL-5656, NL)
FOUKSMAN, Mikhail (Building 5, AE Eindhoven, NL-5656, NL)
MALA, Mohiuddin (Building 5, AE Eindhoven, NL-5656, NL)
HAQUE, Ashim Shatil (Building 5, AE Eindhoven, NL-5656, NL)
Application Number:
IB2015/050050
Publication Date:
July 30, 2015
Filing Date:
January 04, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
KONINKLIJKE PHILIPS N.V. (High Tech Campus 5, AE Eindhoven, NL-5656, NL)
International Classes:
F21V5/04; F21V17/04; G02B3/00; G02B19/00
Domestic Patent References:
WO2013084155A12013-06-13
Foreign References:
US20130039050A12013-02-14
US20110110097A12011-05-12
JP2011113755A2011-06-09
US20030168670A12003-09-11
US20060078246A12006-04-13
US7452737B22008-11-18
US7214116B22007-05-08
Attorney, Agent or Firm:
VAN EEUWIJK, Alexander Henricus Walterus et al. (High Tech Campus Building 5, AE Eindhoven, NL-5656, NL)
Download PDF:
Claims:
CLAIMS:

1. A lens structure that includes:

a cavity having an opening that receives a light emitting device, the cavity having a taper such that a cross-section area within the cavity is smaller than an area of the opening, and

an optical element that provides a particular light output pattern when illuminated by the light emitting device within the cavity.

2. The lens structure of claim 1 , wherein the taper includes one or more walls that are linearly sloped.

3. The lens structure of claim 2, wherein the taper is such that the light emitting device is arranged at a fixed location within the lens structure.

4. The lens structure of claim 1, wherein a depth of the cavity is such that electrical contacts of the light emitting device protrudes from the cavity when the light emitting device is fully inserted into the cavity.

5. The lens structure of claim 1, wherein the taper includes a compound form.

6. The lens structure of claim 1, wherein the taper includes one or more curved walls.

7. The lens structure of claim 1, including one or more channels that allow displacement of materials when the light emitting device is inserted in the cavity.

8. The lens structure of claim 1, including one or more protrusions within the cavity that serve to reduce the cross-section area.

9. The lens structure of claim 1, including the light emitting device and a reflective material between an edge of the light emitting device and a wall of the cavity.

10. A sheet comprising:

a plurality of lens structures, each lens structure having one or more tapered cavities for receiving a light emitting element and an optical element that provide a particular light output pattern when the light emitting element emits light,

wherein each of the tapered cavities has an opening for receiving the light emitting element, a surface to which the light emitting element is optically coupled, and at least one cross section area that is less than a cross section area of the opening.

11. The sheet of claim 10, wherein the sheet comprises a silicone material.

12. The sheet of claim 11, wherein the sheet includes a plurality of light emitting element situated in each cavity, and the silicone material is in a partially cured state.

13. The sheet of claim 10, wherein the sheet comprises glass.

14. A method for creating light emitting products, comprising:

providing a sheet that includes a plurality of lens structures, each lens structure having one or more tapered cavities for receiving a light emitting element and an optical element that provide a particular light output pattern when the light emitting element emits light, and each of the tapered cavities has an opening for receiving the light emitting element, a surface to which the light emitting element is optically coupled, and at least one cross section area that is less than a cross section area of the opening;

inserting a light emitting element into each of the tapered cavities; and singulating the lens structures with corresponding light emitting elements to form the light emitting product.

15. The method of claim 14, wherein the sheet comprises silicone that is in a partially cured state when the light emitting elements are inserted into the cavities, and the method includes curing the silicone after the light emitting elements are inserted into the cavities.

Description:
LIGHT EMITTING DEVICE WITH SELF-ALIGNING PREFORMED LENS

FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to a light emitting device that is formed by placing a self supporting light emitting element into a preformed lens having a cavity with sloped walls that facilitate insertion of the light emitting element into the lens and facilitate adhering the light emitting element to the lens.

BACKGROUND OF THE INVENTION

Conventional light emitting devices include a light emitting element, such as a light emitting diode chip (LED chip) mounted on a substrate and encased in a protective enclosure that may serve as an optical lens. The substrate provides the structural support required to facilitate handling of the light emitting device during subsequent processes, such as the mounting of the light emitting device on a printed circuit board. The protective enclosure may include a wavelength conversion material that converts at least a part of the light emitted from the light emitting chip to light of a different wavelength. The wavelength conversion material may alternatively be provided as a discrete element between the light emitting chip and the enclosure/lens.

Common techniques for providing a light emitting device as described above include attaching the light emitting element to a wire frame substrate that serves to allow external power connections to the light emitting element, then encapsulating the light emitting element and the portion of the wire frame to which it is attached with a silicone mold. The wire frame may be part of a carrier that includes multiple frames for mounting light emitting elements, such that the encapsulation can be performed for all of the light emitting elements as a single molding process. In another embodiment, the light emitting element is mounted on a ceramic substrate that includes conductors to which the light emitting element is attached. USP 7,452,737, "MOLDED LENS OVER LED DIE", issued 18 November 2008 to Grigony Basin, Robert Scott West, and Paul S. Martin, discloses a ceramic substrate that accommodates multiple light emitting elements, and a mold that forms a lens element over each of the light emitting elements. The ceramic substrate may subsequently be sliced/diced to provide 'singulated' light emitting devices that include external connections to the light emitting element on the ceramic substrate.

In another embodiment, the substrate includes cup-like cavities within which the light emitting elements are attached to conductors for coupling the light emitting element to a power source. The light emitting elements are encapsulated by filling the cavities with a low viscosity silicone and curing the silicone. The cavity may be shaped to provide a particular optical effect, and/or a mold may be used to form a desired lens structure above the cup. USP 7,214,116, "LIGHT -EMITTING DIODE AND METHOD FOR ITS PRODUCTION", issued 8 May 2007 to Akira Takekuma, discloses placing a preformed lens atop the silicone within the cup. After curing the silicone, the substrate is diced to provide the singulated light emitting devices.

Each of the above processes requires singulating the light emitting dies, mounting each die on the substrate, encapsulating the dies on the substrate, then slicing/dicing the substrate to singulate the completed light emitting devices. In addition to the additional manufacturing cost and effort associated with the double-handling involved with the intermediate step of mounting the light emitting dies on a substrate, this double-handling process also challenges applications wherein the light emitting die is required to have a particular location with respect to the optics of the enclosing structure. In many applications, if the light emitting element is Off-center' relative to the optics of the enclosing structure, the formed light emitting device may be discarded as 'failed' in the manufacturing process, or may pass the manufacturing test and result in a defective product when it is incorporated into the product. For example, in a camera-flash application, if the camera/cell-phone/tablet/etc. produces pictures with non-uniform illumination, the purchaser of the camera/cell-phone/tablet/etc. will likely demand a replacement. Although fairly simple techniques are available to properly align the substrate with the tool that provides the molded lens structure, such as creating alignment features in each of the substrate and the tool, achieving a correspondingly proper alignment of the light emitting element on the substrate is a more challenging and costly task, requiring, for example, a high-precision 'pick-and-place' machine to place each light emitting element at a highly-precise location on the substrate.

In order to avoid the aforementioned double-handling of the light emitting element, technologies have evolved to provide light emitting dies that are self-supporting, and can be handled directly. WO 2013/084155, "FORMING THICK METAL LAYERS ON A SEMICONDUCTOR LIGHT EMITTING DEVICE", published 13 June 2013 for Schiaffion, Akram, Basin, Munkhol, Lei, and Nickel, and incorporated by reference herein, discloses light emitting elements that have thick metal layers that provide the structural support required for routine handling of the elements, eliminating the need for a supporting substrate. Because the self-supporting chip can be handled without further packaging, it is commonly termed a "Chip Scale Package" (CSP).

However, even though these self-supporting chips do not require a structural substrate, the conventional encapsulation processes still require that these chips be placed on some form of substrate, to allow multiple chips to be encapsulated at the same time, with the accompanying difficulty in assuring alignment of the light emitting chip and the attached lens structure.

SUMMARY OF THE INVENTION

It would be advantageous to provide a method and system that facilitates accurate alignment of light emitting chips and their associated lens structures. It would also be advantageous if this method and system are suitable for mass production processes.

To better address one or more of these concerns, in an embodiment of this invention, a lens structure is pre-formed with features that facilitate accurate alignment of a light emitting chip within the lens structure. To ease manufacturing, the features include tapered walls that allow for easy insertion of the light emitting chip into the lens structure, the taper serving to accurately align the light emitting chip when the chip is fully inserted. The taper may include linearly sloped or curved walls, including complex shapes. An adhesive may be used to secure the light emitting chip to the lens structure. The light emitting chips may be 'picked and placed' into an array of lens structures, or 'picked and placed' onto a substrate that may be overlay ed by the array of lens structures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIGs. 1 A-IB illustrate an example profile and bottom view of a lens structure with sloped walls and channels that facilitate the exit of air bubbles and adhesives.

FIG. 2 illustrates an example profile view of a lens structure with stepped and sloped walls.

FIGs. 3 A and 3B illustrate example sheets of lens structures with tapered cavities.

FIGs. 4A-4D illustrate example profiles of complex tapered cavities.

FIG. 5 illustrates an example bottom view of a lens structure with a conic-section cavity. FIGs. 6A and 6B illustrate alternative optical elements.

Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. DETAILED DESCRIPTION

In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention.

However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

FIGs. 1 A-1B illustrate an example profile and bottom view of a lens 100 that includes a cavity 150 for receiving a light emitting device (LED) 110, and an optical element 140 that provides a desired light output pattern when light is emitted from the LED 110. In this example, the optical element 140 is a hemispherical dome that provides a substantially uniform light output pattern across its field of view.

The lens 100 may comprise silicone, a silicone epoxy hybrid, glass, or any transparent optical material with an appropriate refractive index. The LED 110 may be a self-supporting device, such as a chip-scale-package (CSP), or a thin film die mounted on a ceramic substrate (die on ceramic, DOC), with contacts 120 on the surface opposite the light emitting surface 130. Other LED structures may also be used.

As illustrated, to ease assembly, the cavity 150 is tapered, and includes sloped walls 160. The bottom surface 170 of the cavity 150 is dimensioned so as to situate the light emitting device 110 at a fixed location within the cavity 150 within a given precision, based on the requirements of the intended application. In this example, the bottom surface 170 has substantially the same dimensions as the light emitting device, although it may be slightly larger, depending upon the tolerances of the light emitting device. The required precision of the location of the light emitting device 110 with respect to the lens structure 100 may dictate the allowable over-sizing, if any, of the bottom surface 170. An adhesive having a refractive index that is equal to the refractive index of the LED 110 or the lens 100, or a value between the refractive indexes of the LED 110 and lens 100 may be dispensed into the cavity 150 before the LED 110 is inserted into the cavity. Depending upon the particular assembly technique, the adhesive may also, or alternatively, be dispensed upon the LED 110 prior to insertion into the cavity 150.

As illustrated in FIGs. 1A and IB, channels 180 may be provided to enable air and excess adhesive to escape during the assembly process. These channels 180 are illustrated as cylindrical borings in FIGs. 1 A and IB, although other shapes may be used; for example, if the cavity is formed by a molding process, the channels may have the same slope as the sloped walls 160.

The channels 180 are illustrated at each corner of the cavity 150, although other locations, and fewer or more channels may be provided. In one alternative channels located at the sides of the LED 100 and away from the corners may be used to avoid rotational alignment errors. The size, shape, and location of the channels may be altered depending upon multiple factors including, for example, the viscosity of the adhesive, and the overall size of the LED 110.

In another embodiment, the LED 110 is inserted into the cavity without an adhesive between the light emitting surface 130 and the bottom surface 170 of the cavity 150. A thin film of index-matched liquid may be used to provide an efficient optical coupling between the LED 110 and the bottom surface 170. After insertion, an adhesive may be administered in the space between the LED 110 and the sloped walls 160. This post-insertion application of the adhesive may eliminate or minimize the need for the channels 180.

To ease subsequent mounting of the lens 100 with LED 110 on a subsequent substrate, such as a printed circuit board, the depth of the cavity 150 may be determined such that the contacts 120 extend slightly above ('proud of) the underside 101 of the lens 100 when the light emitting device is fully situated within the cavity. A depth that is about 50-500um less than the total height of the LED 110, including contacts 120, generally provides a sufficient prideO of the contacts beyond the underside 101 of the lens 100, although other depths may be used, depending upon the tolerance requirements of the application. For example, if the LED 110 is a self-supporting chip-scale package, with fine tolerances, a nominal proud as small as 5um may be used.

By shaping the taper such that the opening of the cavity 150 is larger than the dimensions of the LED 110, insertion of the LED 110 into the cavity 150 is simplified. By shaping the taper such that the cross-section of the cavity 150 narrows in a direction toward the bottom surface 170, variance in the location of the LED 110 within the lens 100 is substantially controlled, providing for a self-alignment of the LED 110 as it is inserted into the lens 100. This taper also provides this self-alignment independent of the means used to insert the LED 110 into the cavity 150. Even a manual insertion of the LED 110 into the cavity 150 will provide the same accuracy and precision as an automated insertion using a highly accurate and precise pick-and-place machine. In like manner, a pick-and-place machine of minimal accuracy and precision may be used while still maintaining the same high level accuracy and precision.

As illustrated in FIG. 2, the profile of the cavity 250 of lens 200 may be adjusted to conform to the shape of the light emitting device 210. In this example, the light emitting device 210 includes a wavelength conversion layer 230, such as a phosphor- embedded silicone that is molded upon the light emitting device 210. A recess 265 at the entry to the cavity 250 is shaped to accommodate the lip 235 formed by this example wavelength conversion layer 230.

Below the recess 265, the cavity 250 includes sloped walls 260 to facilitate insertion of the light emitting device 210, and a bottom surface 270 that serves to locate the light emitting device within the lens 200 within a given precision, as detailed above with regard to surface 170 of lens 100. FIGs. 3A and 3B illustrate example sheets 300, 300' of lenses 100, 100' with cavities 150. Although only a few lenses 100, 100' are illustrated, one of skill in the art will recognize that the sheets 300, 300' may include hundreds of lenses 100, 100'. For ease of illustration, the venting channels 180 of each cavity 150 of FIGs. 1A-1B are not illustrated, but may be present.

In the example of FIG. 3 A, sheet 300 includes sixteen lenses 100, each with a single cavity 150. This sheet may comprise, for example silicone, a silicone epoxy hybrid, glass, or any other transparent optical material that can be formed with defined cavities.

In an example manufacturing process, a pick and place machine may be used to insert each LED 110 (not illustrated) into each cavity 150. The pick and place machine may be configured to place each LED 110 at the center of each cavity 150, but with sufficient compliance during the insertion to enable the LED 110 to be guided by the walls of the cavity 150 into the desired location. Alternatively, the pick and place machine may place each LED 110 partially into each cavity 150, and a subsequent process, such as a plate press may be used to complete the insertion of the LEDs 110 into the cavities 150.

In an alternative process, the LEDs are arranged on a temporary substrate, such as a conventional "dicing tape", at appropriate locations, and the sheet 300 is mated with these LEDs on the substrate, by either overlaying the sheet 300 upon the LEDs, or overlaying the dicing tape with attached LEDs over the sheet 300.

In an example embodiment, the sheet 300 is a partially cured silicone that is cured after the LED 110 is inserted into each cavity 150. The subsequent curing may serve to adhere each LED 110 to each lens 100, thereby avoiding the need to include an adhesive bond.

In an alternative embodiment, the sheet 300 is fully formed, and an adhesive may be applied to each cavity 150, or to each LED 110, to secure each LED 110 to each lens 100. In some embodiments, the adhesive is applied after the LEDs 110 are inserted into the cavities 150, adhering the edges of the LEDs 110 to the walls of the cavities 150. In other embodiments, detailed below, the sheet 300 may comprise a material with some resilience, and the insertion of the LED 110 into the cavity 150 may provide a sufficient friction force to maintain the LED 110 at the appropriate location within the lens 100.

A material that facilitates optical coupling between the light emitting surfaces of the LEDs 110 and the lenses 100 of the sheet 300 may be applied to either the cavities 150 or the LEDs 110.

In like manner, a material that serves to reflect light that strikes the edges of the LED 110 may be applied to the edges of the LED 110, for example, by filling the gap between the LED 110 and the sloped walls of the cavity 150 with such material.

Upon completion of the insertion and adhering of the LEDs 110 in the cavities 150 of the lenses 100, the sheet 300 may be sliced/diced along the cutting lines 320-370 to provide singulated LED with lens assemblies. In some embodiments multiple LED with lenses may be provided as a single assembly, for example, by only slicing along lines 330 and 360, providing four assemblies, each assembling including four LEDs with individual lenses.

One of skill in the art will recognize that the example one-to-one relationship between LEDs and lenses of the previous figures is merely one of many configurations. For example, FIG. 3B illustrates an embodiment wherein multiple LEDs are intended to be inserted into multiple cavities 150 of each lens 100'. In such an embodiment, the cavities 150 of each lens 100' may be more closely situated than the cavities 150 of each lens 100 of FIG. 3 A.

In some embodiments, one or more of the cavities 150 may be configured to accommodate multiple LED dies, which may be arranged on a single substrate. In other embodiments, the cavities 150 within each lens 100' may be of different sizes, to accommodate a mix of different LED types within the lens 100', such as a combination of different color LEDs. As in the example of FIG. 3 A, the LEDs 110 (not illustrated) may be inserted into each cavity manually, or via a pick-and-place process. Or, the LEDs 110 may be arranged on a temporary substrate at locations corresponding to cavities 150 on the sheet 300', and subsequently mating the sheet 300' and the substrate containing the LEDs 110. Similarly, the LEDs 110 may be adhered to the lenses 100' using any of the above described techniques, or any other viable and reliable technique.

Upon completion of the insertion and adhering of the LEDs 110 into the cavities 150 of each lens 100', the lenses 100' may be singulated by slicing/dicing the sheet 300' along the cutting lines 380, 390.

One of skill in the art will recognize, in view of this disclosure, that this invention is not limited to the example use of cavities 150 with linearly sloped walls 160.

FIGs. 4A-4D illustrate alternative cavity profiles. As in FIGs. 3A-3B, the venting channels 180 of FIG. 1 are not illustrated in these figures, for ease of illustration, but may be included in each example embodiment.

FIG. 4A illustrates a profile comprising wall segments 410, 420 having different slopes. The upper wall segment 410 has a relatively shallow slope to provide a wide opening for inserting the LED (not illustrated), while the wall segment 420 has a relatively steep slope, and may be orthogonal to the surface 470, to provide a larger surface area for constricting the edges of the LED to maintain the proper location of the LED within the cavity.

Depending upon the material in which the cavity is formed, the closeness of the fit between the size of the LED and the size of the surface 470, the slope of the lower wall segment 420, and the size of the venting channels 180 (not illustrated), this embodiment may require substantial force to insert each LED into each cavity. FIGs. 4B- 4D illustrate alternative profiles that may require less insertion force. In FIG. 4B, the upper wall segment 430 is sloped to provide an opening that is larger than the size of the intended LED, and the lower wall segment 420 is sloped in an opposite direction to create protrusions 435 that serve to constrict the edges of the LED to maintain the proper location of the LED within the cavity. However, as compared to FIG. 4A, the edges of the LED will only contact these protrusions 435, and not the entire surface of the lower wall segment 440. The reversed slope of the wall segment 440 provides a lower surface 470 that is wider than the LED that is containing between the protrusions 435, providing some room for the displaced air or adhesive, reducing or eliminating the reliance on the venting channels 180.

In FIG. 4C, a curved wall segment 450 is used to gradually reduce the cross section area in the direction of the surface 470 in a non-linear fashion, so that the lower portion of the wall segment 450 may be more constraining of the LED compared to the linearly sloped walls 160 of FIG. 1, but less constraining compared to the linear wall segment 420 of FIG. 4A, particularly if the segment 420 is orthogonal to the surface 470. The continuous curvature of the wall segment 450 may also ease the insertion of the LED, compared to the abrupt edges at the transition between wall segment 410 and 420 of FIG. 4A.

FIG. 4D illustrates a combination of curved 460 and linear 490 wall segments, as well as the addition of features 480 that may secure the LED while introducing minimal insertion resistance. The features 480 may be a continuous ridge within the cavity, or a plurality of individual bead-like protrusions from the wall segment 490. If individual protrusions are used, the insertion resistance is reduced, and the space between the protrusions allows for the displaced air and adhesive to escape, potentially avoiding the need for the venting channels 180 of FIG. 1.

One of skill in the art will recognize, in light of this disclosure, that any of a variety of other profiles may be used to fix the location of the LED within the lens within a given tolerance, while also allowing for practical insertion forces. One of skill in the art may also recognize that the shape of the cavity, or the shape of the surface of the cavity, need not match the shape of the LED. Depending upon the processes and materials used to create the lens, creating a rectangular cavity, such as illustrated in FIG. IB, may not be economically viable. If, for example, the lens is a rigid material, boring or grinding a circular cavity may be substantially less expensive than creating a rectangular cavity.

FIG. 5 illustrates an example lens 500 that includes a conical cavity 550 with a sloped wall 560 that forms a circular bottom surface 570. The diameter of the surface 570 is such that it circumscribes the LED 110, providing contact points 590 on the wall of the cavity that center the LED 110 at the center of the surface 570. The semicircular gaps 575 around LED 110 allows for the displaced air and adhesive to escape, potentially avoiding the need for the venting channels 180 of FIG. 1

As contrast to the rectangular surface 170 of FIG. IB, the conic cavity 550 and circular surface 570 may allow the LED 110 to rotate during the insertion process, but if the optical properties of the lens 500 are symmetric about the center axis, the rotation of the LED 110 about this center axis will have no effect on the accuracy and precision of locating the LED 110 at that center axis. If the lens 500 is a partially cured silicone, the compliance of the partially cured silicone may enable the LED 100 to "dig in" to the silicone at the corners 590, thereby controlling or limiting the rotation.

It is significant to note that all of the above example profile views could also be profile views of half-sections of conic cavities, although the profiles of FIGs. 4B and 4D would more likely be formed by a molding process, rather than a boring or grinding process, and achieving a rectangular cavity via a molding process is relatively straightforward.

One of skill in the art will also recognize that the optical element of the lens is not limited to the hemispherical dome 140 of FIGs. 1A-1B. FIGs. 6A and 6B illustrate an example side emitting optical element 600, and an example collimating optical element 650, respectively. Other optical elements may be used to achieve desired light output patterns. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, it is possible to operate the invention in an embodiment wherein additional elements may be included within the cavity. For example, a wavelength conversion material may be inserted into the cavity before the light emitting device is inserted. Alternatively, or additionally, the lens may include a wavelength conversion material, or the light emitting device may include a wavelength conversion material. In some embodiments, the wavelength conversion material may serve as an adhesive layer between the light emitting device and the lens.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.