Lighting device

Abstract

A lighting device comprising an optical plate extending over a plurality of LEDs arranged on a PCB main surface of a PCB. In a mounted configuration, the PCB and optical plate are interconnected by fasteners. Each fastener extends through a through-hole in the optical plate in a plane parallel to the PCB main surface with a clearance in a direction transverse to the PCB main surface. The through-hole has a wide portion extending a depth D from a first main surface of the optical plate facing away from the PCB towards a second main surface of the optical plate facing towards the PCB, and the through-hole narrows at the depth D to a neck portion. Further, each fastener rests on a resilient washer seated in the wide portion of the through-hole.

Description

Background Technology

[0001] An LED engine is a lighting device that typically includes a printed circuit board (PCB) on which LEDs are arranged (e.g., mounted). To collimate and control the light emitted by the LEDs, the PCB with the LEDs usually has a monolithic optical plate, such as an optical plate including a lens. Typically, in known LED engines, the lens is mounted in a fixed, aligned position relative to the LED by means of fasteners to optimally control the beam shape and direction of the light emitted by the LED. The PCB material typically includes aluminum, for example, commonly used in metal-core PCBs (MCPCBs), while the optical plate material is typically polymethyl methacrylate (PMMA) or polycarbonate (PC). However, aluminum has a linear coefficient of thermal expansion of approximately 23 × 10⁻⁶ at 20°C. -6 K -1 The linear thermal expansion coefficients of PC and PMMA are approximately 70*10⁻⁶ respectively. -6 K -1 and 77*10 -6 K -1 The thermal expansion coefficients are significantly different. Specifically, during the LED engine's lighting (and therefore warming-up) period, this difference can lead to misalignment between the LED and the lens of the optical board. Misalignment typically increases with the size of the monolithic optical board. To counteract this misalignment, the optical board is fixedly mounted to the PCB. However, this has the disadvantage of imposing relatively high mechanical constraints on the PCB and / or optical board due to the thermal expansion difference between the PCB and the optical board combined with the desired alignment of the LED and lens. These high mechanical constraints involve an enhanced and relatively large risk of crack formation in the PCB and / or optical board, and thus contribute to premature failure of the LED engine. Summary of the Invention

[0002] The object of this invention is to provide a lighting device of the type described in the opening paragraph, wherein at least one disadvantage is offset. To this end, the lighting device according to the invention includes an optical plate extending above a plurality of LEDs arranged on the main surface of a PCB, wherein, in a mounted configuration, the PCB and the optical plate are interconnected by a plurality of fasteners.

[0003] Each fastener extends through a corresponding, associated through-hole in the optical board in a direction transverse to the main surface of the PCB.

[0004] The associated via has a wide portion extending a depth D from a first main surface of the optical board facing away from the PCB toward a second main surface of the optical board facing the PCB, where the via narrows into a neck.

[0005] Each fastener extends through the through-hole with a gap S in a plane P parallel to the main surface of the PCB, and rests against a corresponding, associated elastic washer located in the width of the associated through-hole.

[0006] The wide portion includes a tapered section that tapers gradually, preferably continuously, along depth D, and

[0007] The washer is a toothed washer, preferably with teeth that extend outwards.

[0008] The wide section forms a seat for the elastic washer. The fastener rests against the washer with elastic force, and the washer is elastically seated in the wide section.

[0009] The narrowing can be gradual (e.g., in one step) and / or tapered. Preferably, the tapering begins at the first main surface of the optical plate and preferably proceeds gradually and smoothly to a depth D. Thus, at depth D, the width narrows to the width of the neck. This gradual tapering or narrowing from the width to the neck forms a seat for the resilient washer. The fastener includes a head and an elongated portion, with its head resting against the washer while its elongated portion passes through a central opening in the washer and extends through the optical plate toward the PCB, where it grips the PCB, i.e., the fastener is in a fixed position relative to the PCB, and / or grips the PCB to a heatsink to which it can be mounted. The washer has a central hole that fits tightly around the elongated portion of the fastener with virtually no gap. Furthermore, the washer typically has a shape that closely matches the shape / diameter of the width, e.g., the outer diameter. Thus, when the optical plate shifts relative to the fastener—and therefore the washer also shifts—the washer elastically deforms, thereby pushing the optical plate back to its original position. Due to the difference in thermal expansion between the PCB and the optical board, the optical board may have a local offset relative to the PCB. However, due to the gaps in the fasteners within the narrow portion (also known as the neck) of the through-hole and the possibility of the gaskets shifting in the wider portion due to their elasticity, the optical board is able to locally offset from its original position relative to the PCB during the preheating of the LED engine. This local offset can vary at different locations. Once cooled, with the LED engine off, the elasticity of the gaskets allows the optical board to return to its original position relative to the PCB.

[0010] The gasket can have a circular or polygonal perimeter (e.g., triangular, square, rectangular, hexagonal, or octagonal). The lighting fixture can be any type of illuminator or lighting device—including PCBs with LEDs attached to a (monolithic) optical board—e.g., streetlights, wattage lamps, ceiling lights, vehicle headlights, etc. The optical board can be made from a variety of transparent, light-transmitting materials—e.g., glass; PET: polyethylene terephthalate (PET); polymethyl methacrylate (PMMA); polyethylene naphthalate (PEN); cyclic olefin copolymer (COC); polycarbonate (PC); high-temperature polycarbonate (HTPC); polyetherimide (PEI); polyarylate (PAR); polyphenylene sulfide (PPS); polyethersulfone (PES); polyetheretherketone (PEEK); polyimide (PI); and polyamide-imide (PAI).

[0011] The lighting device can have the following characteristics: a gap S in every direction in a plane P parallel to the main surface of the PCB. The gap can essentially only be in one direction (e.g., a gap obtained through an elongated slot), but preferably, the gap is in all directions, and therefore, the gap appears as a relatively large hole compared to the diameter of the elongated portion of the fastener extending through the hole. For this purpose, the lighting device can have the following characteristics: the fastener includes an elongated portion and a head at one end thereto, the elongated portion having a diameter Df in a direction parallel to the main surface, wherein the gap S is in the neck of the through-hole and at the elongated portion of the fastener, the gap S being in the range of 0.1*Df ≤ S ≤ 0.3*Df. Of course, the head of the fastener is fitted within the width of the through-hole, having so much room for movement that it also has at least the same gap. The elasticity of the washer allows it to deform with a gap of at least 0.3*Df without plastic deformation, but only elastic deformation.

[0012] To give just one example, the linear coefficient of thermal expansion of aluminum at 20°C is approximately 23 × 10⁻⁶. -6 K -1 The linear thermal expansion coefficients of PC and PMMA are approximately 70*10⁻⁶ respectively. -6 K -1 and 77*10 -6 K -1 The differences are obvious. Therefore, the difference in thermal expansion is approximately 50*10. -6 K -1 Therefore, for a 1 m optical board with a preheated ∆T of approximately 50°C, the mutual expansion difference between the optical board and the PCB is approximately 0.5 mm. For an M4 screw with a diameter of 4 mm, S is between 0.4 and 1.2 mm, which closely matches the calculated difference of 0.5 mm (maximum).

[0013] The lighting device may have the following characteristics: an optical plate with a local thickness T at the through-hole, wherein the wide portion is a groove extending to a depth D in the first main surface of the optical plate, where 0.25*T≤D≤0.85*T. The minimum depth of the groove is to provide a reliable seat for the washer to counteract any unintentional slippage of the washer from the seat, and to provide sufficient depth for the washer to bend elastically without protruding from the first main surface of the optical plate. The maximum depth of the groove is to ensure that the optical plate has sufficient local strength to withstand the (relatively low) mechanical constraints involved in the elastic deformation of the elastic washer. Typically, the through-hole is not located at the position of a (protruding) lens, but in a flat region located between adjacent lenses. The thickness T is then related to the thickness of the optical plate in the flat region. However, if the through-hole is located at a lens, then the thickness T is related to the thickness of the optical plate at the lens.

[0014] The lighting device has the following characteristics: the wide portion includes a tapered section that tapers gradually, preferably continuously, along its depth D. This taper preferably extends the entire depth D, allowing for the use of washers of relatively wide dimensions to fit into the gradually tapering through-hole. In this case, the wide portion can extend the entire thickness of the optical plate, i.e., D = T. Depending on the comparison between the washer size and the maximum size of the wide portion—which varies with the minimum size of the neck—the seat position of the washer varies along the depth of the wide portion. Therefore, the characteristic of the lighting device to accommodate the variation in a predetermined distance between the optical plate and the PCB can be addressed by using fasteners(s) of the same size. This predetermined distance can be determined by the shape of the lens and the desired beam pattern—such as the desired beam angle or beam width, or the desired mixing of light from adjacent LEDs. This predetermined distance can be readily obtained by using a spacer. Therefore, the lighting device can have the following characteristics: it includes a spacer of a predetermined height positioned between the optical plate and the PCB at the location of the through-hole, and corresponding, associated fasteners of a plurality of fasteners extend through the through-hole.

[0015] The lighting device may have the following characteristics: the tapered portion has an average cone angle α relative to the normal of the first main surface, where 30°≤α≤70°, preferably 40°≤α≤60°. When the tapering is straight, the cone angle is the same throughout the depth. However, the tapering can also be curved, for example according to the branching of a parabola observed in the cross-section, and then the cone angle varies with depth, i.e., the cone angle generally increases with increasing depth. This angular range facilitates relatively easy sliding of the washer along the depth of the through-hole when seated, and makes it easy to control the degree of curvature of the washer (and therefore the strength of the washer), thereby pulling the optical board and the PCB towards each other using the combination of fasteners and washers.

[0016] Lighting fixtures may have the following characteristics: washers are made of spring steel or elastic polymer / plastic. The advantage of steel springs is that they are a convenient and reliable material with a long service life for mechanically elastic parts. The advantage of polymer / plastic materials is that they are electrically insulating, thus counteracting unintentional electrical contact by the user from fasteners that come into contact with the PCB. Alternatively or additionally, fasteners may be made of plastic / polymer materials. Of course, fasteners can also be made of metal. Typically, suitable shapes for fasteners include threaded fasteners, bolts, screws, snap-clicks, and split-pen fasteners.

[0017] The lighting device has the following characteristics: the washer is a toothed washer, preferably with outwardly extending teeth. Toothed washers are easily designed to provide the desired elastic force. This can be achieved, for example, by selecting the material, material thickness, tooth length and / or width, number of teeth, and tooth spacing. Furthermore, the position and orientation of the teeth can affect the elasticity of the washer. For example, an external toothed washer is one where the teeth extend outward from the center of the central ring; or an internal toothed washer is one where the teeth extend inward toward the center of the outer ring. Either one can be chosen according to preference, but this may depend, for example, on where the elastic deformation of the washer is desired, and therefore on which part of the lighting device the elastic force is most concentrated—that is, in the case of an internal toothed washer near the center, the elastic force is most concentrated at the fastener, or in the case of an external toothed washer around the perimeter, the elastic force is most concentrated on the wall of the wide portion of the through-hole in the optical plate.

[0018] The lighting device may have the following feature: the washer is a serrated washer. In the context of this invention, a serrated washer must be understood as a washer with slotted teeth like the teeth of a washer, which are permanently bent out from a common (or shared) plane, for example, because the teeth point to the apex. Preferably, the teeth are located at an angle β to the normal of the base ring of the washer (which lies in the common or shared plane), where β has approximately the same value as the average taper angle α in the width of the through-hole, i.e., β≈α. However, it is preferable that β≈α plus a few degrees, for example, β≈α+5°. Thus, the washer is slightly wider than / appears to be wider than the tapered width of the through-hole so that the washer sits in the through-hole in a pre-tensioned manner when the teeth follow the contour of the wall of the width. Thus, it is ensured that the optical board and the PCB are permanently pushed by a permanent but relatively low force to present a mutually aligned (original) position.

[0019] The lighting device may have the following characteristics: the washer comprises n teeth, wherein a suitable number of n appears to be in the range of 3 ≤ n ≤ 24, preferably 4 ≤ n ≤ 12. The lighting device may also have the following characteristics: the resilient washer is a star-shaped washer, i.e., the star-shaped washer has outwardly extending tapered teeth, these teeth being shaped such that when the optical plate and the PCB are forced to face each other due to mutual offset in the plane of the PCB main surface and / or by mutual offset in a direction transverse to the PCB main surface, these teeth do not abut or overlap each other. Such a washer may also be described as a star-shaped washer with n points or an n-pointed star washer, the points forming the teeth of the washer. Preferably, the teeth of the star-shaped washer each have a rounded tip to enhance the smooth movement of the washer within the width of the through-hole.

[0020] The lighting fixture may have the following characteristics: the washer is annular and has a maximum diameter Dw, and the teeth have a length L, where 0.1*Dw≤L≤0.4*Dw, for example 0.15*Dw≤L≤0.3*Dw, such as 0.2*Dw. It seems that teeth with lengths within this range provide well the desired flexibility and clearance. Attached Figure Description

[0021] The invention will now be further illustrated with the aid of illustrative drawings, which are not intended to limit the scope of the invention, but rather to show its full possibilities. In the drawings:

[0022] Figure 1A-Figure 1B Perspective views of partial cross-sections of a luminaire including a first embodiment of a lighting device according to the present invention are shown respectively;

[0023] Figure 2 Components of a conception of a lighting device according to a second embodiment of the present invention are shown;

[0024] Figures 3A-3D The final stage of assembling an embodiment of the lighting device according to the invention is shown, along with details regarding elastic deformation and gaps; and

[0025] Figures 4A-4F Suitable washers of various types are shown for use in lighting devices according to the present invention. Detailed Implementation

[0026] Figure 1A-Figure 1BPerspective views and partial cross-sections of a luminaire 101 comprising a first embodiment of a lighting device 103 according to the present invention are schematically shown. In this figure, the luminaire is a street luminaire, but this could alternatively be any other type of luminaire or lighting device including a PCB 105 having LEDs 107 attached to a (monolithic) optical board 109—e.g., a wattage lamp, a ceiling light, or a vehicle headlight. In this figure, the street luminaire includes a housing 111 mounted on a pole 113. The first embodiment of the lighting device according to the present invention is housed within the housing. The lighting device includes an optical board extending above a plurality of LEDs 107 arranged on a PCB main surface 115 of the PCB. A first main surface 117 of the optical board faces the PCB main surface, and a second main surface 119 faces away from the PCB main surface.

[0027] In the installed configuration, the PCB and the optical board are interconnected by a plurality of fasteners 121 at positions aligned with LEDs on the corresponding optical axes 123 of the respective lenses 125 of the optical board. Each fastener is spaced apart in a plane P parallel to the main surface of the PCB in a direction transverse to the main surface of the PCB. Figure 1A-Figure 1B Not shown in the image, see [link / reference]. Figure 3C The fastener extends through a corresponding, associated through-hole 127 in the optical plate. The associated through-hole has a width 129 that extends a depth D from a first main surface of the optical plate facing away from the PCB toward a second main surface of the optical plate facing the PCB. In this figure, the width protrudes from the first main surface by a depth D (or has a height D), where D is approximately 0.7 * T, where T is the thickness of the optical plate. The through-hole narrows into a neck 131 at the depth D. Each fastener rests with its head 133 against a corresponding, associated resilient washer (not shown) located within the width of the associated through-hole. Each fastener extends with its elongated portion 135 through the neck 131 of the corresponding through-hole and is screwed onto the housing by at least a portion of the fastener's threaded portion 139.

[0028] Figure 2 The diagram schematically illustrates the independent components of a second embodiment of a lighting device 203 according to the present invention prior to assembly of the lighting equipment. The components of the second embodiment include two M4 bolts as fasteners 221, two resilient 10-point (10-tooth) star washers 241, an optical plate 209 made of PMMA (however, the optical plate may alternatively be made of, for example, PC), and a PCB 205 having electronic components such as LEDs 207, resistors, sensors, conductive pattern layouts, etc. In this embodiment, the optical plate includes only two through-holes 227; however, of course, the number of through-holes and fasteners in the optical plate can be more than two, and depends on the size of the (monolithic) optical plate; the larger the size, the more through-holes and fasteners.

[0029] For example, assembly can be performed as follows: the optical plate is placed in an aligned position on the PCB, wherein the LED on the PCB is located on the corresponding optical axis 223 of the associated lens 225 of the optical plate. Subsequently, washers are inserted into corresponding through-holes in the optical plate. Finally, bolts are inserted into the through-holes, wherein their elongated portions 235 extend through the openings 243 in the corresponding washers and through the corresponding through-holes, and are screwed into corresponding threaded grooves in the PCB, thereby pulling the optical plate and the PCB towards each other and securing the optical plate to the PCB, with gaps (not shown) in each direction in the plane P parallel to the main surface 215 of the PCB.

[0030] Figures 3A-3D A partial cross-sectional view of the final stage of assembling an embodiment of the lighting device 303 according to the present invention is shown, along with details regarding elastic deformation and gaps. Figure 3A The diagram shows an optical plate 309 and PCB 305 in aligned positions, with LED 307 on the optical axis 323 of lens 325. A washer 341 is located in a through-hole 327 of the optical plate, and a fastener 321 rests against the washer with its head 333. An elongated portion 335 of the fastener extends through the washer, through-hole, and PCB to a connectable heat sink (not shown) and / or housing (not shown). The through-hole in the optical plate has a wide portion 329 that tapers gradually from a first main surface 317 of the optical plate toward a second main surface 319. The depth D of the through-hole in the optical plate is the same as the thickness T of the optical plate, and tapers to an opening with a minimum diameter through-hole in the second main surface. This minimum opening in the second main surface is a neck 331.

[0031] exist Figure 3A The image shows a washer positioned as a flat washer within a through-hole, with no (permanent) force applied to the optical board and PCB. Upon tightening / screwing the fastener, the washer undergoes elastic deformation and, seated within the through-hole, takes on a gradually tapered shape. Due to this elastic deformation, a permanent force is applied to the optical board and PCB, pulling them towards each other. Figure 3B As shown. The gradually tapering hole has a wall 345, which is shaped as a branch of a parabola rotating about the axis of the parabola (in Figure 3C (As shown in more detail below). The branch with an average angle of α is positioned at an angle relative to the normal of the first principal surface, where α is approximately 40°.

[0032] Figure 3C yes Figure 3BAn enlarged, detailed cross-sectional view is shown, illustrating the assembly configuration of the optical plate 309, PCB 305, gasket 341, and fastener 321, wherein the gap S in the plane P parallel to the main surface 315 of the PCB is determined / measured. Figure 3C An LED 307, mounted on a PCB and aligned with the optical axis 323 of the associated lens 325 of the optical board, is also shown. A washer 341 has a central opening 343 that is sufficiently larger than the diameter Df of the elongated portion 335 of the fastener (e.g., 15% larger), allowing the washer to be easily fitted around the elongated portion 335 of the fastener 321. The washer is positioned within a through-hole 327 of the optical board, and the fastener rests against the washer with its head 333 and extends through the washer, through the through-hole, and securely to the PCB with its elongated portion 335. The fastener is screwed into the PCB in such a tight manner that the washer is clamped between the fastener head and the PCB, thus forcing it to remain in place relative to the PCB. On one side of the fastener, the clamping area 347 is indicated in the figure as a gray vertical post (although it is presented as a circumferential area around the fastener). The elasticity of the washer allows the optical board to move relative to the PCB (and the washer and fastener) in a direction parallel to plane P. The through-hole in the optical plate has a wide portion 329 that tapers gradually from the first main surface 317 of the optical plate toward the second main surface 319. The depth D of the through-hole in the optical plate is the same as the thickness T of the optical plate and gradually tapers to a neck 331. The fastener has a gap S in the neck of the through-hole, approximately 0.15*Df on each side in the figure. The washer has an elasticity S of at least the same size (indicated by dashed lines in the figure).

[0033] Figure 3DThis is an enlarged, detailed partial cross-sectional view of another embodiment of the lighting device, showing the assembly configuration of the optical plate 309, PCB 305, washer 341, and fastener 321, wherein a gap S in a plane P parallel to the main surface 315 of the PCB is determined / measured. The washer 341 has a central opening 343 slightly larger than its diameter Df (e.g., 1% larger), causing the washer to fit snugly (i.e., with virtually no gap) around the elongated portion 335 of the fastener 321 (which has the diameter Df). The washer sits in a through-hole 327 of the optical plate, and the fastener rests against the washer with its head 333 and extends through the washer, through the through-hole, and securely to the PCB with its elongated portion 335. The fastener is screwed into the PCB tightly enough to counteract movement of the optical plate and washer in a direction perpendicular to plane P, but not so tight as to prevent the washer and optical plate from being clamped between the head of the fastener and the PCB, thus allowing the optical plate to move in a direction parallel to plane P. The through-hole in the optical plate has a width 329 that tapers gradually from the first main surface 317 of the optical plate toward the second main surface 319. The depth D of the through-hole in the optical plate is approximately 85% of the thickness T of the optical plate, and tapers to a neck 331. Fasteners have a gap S in the neck of the through-hole, approximately 0.25 * Df on each side in the figure. Washers have the same elasticity S (indicated by dashed lines in the figure).

[0034] Figures 4A-4F Suitable washers of various types are shown for use in lighting devices according to the present invention. Figure 4A A perspective view of a flat, star-shaped resilient washer 441 is shown. The washer includes a central ring 447 with a central opening 443, wherein a plurality of teeth 449 (or tips) extend radially outward from the central ring, and wherein each tooth includes a rounded tip 451. In the figure, the plurality of teeth includes ten teeth, but the plurality can be any number from three to one hundred, such as four, six, eight, twelve, or twenty. This also applies to all washers having a plurality of teeth.

[0035] Figure 4B and Figure 4C Perspective and cross-sectional views of a serrated, star-shaped resilient washer 441 are shown, respectively. The washer includes a central ring 447 with a central opening 443, wherein a plurality of teeth 449 extend outward from the central ring and beyond the plane Q defined by the central ring. The serrated washer can be described as a washer slotted like saw teeth, which are permanently bent out of the plane formed by the central ring, for example because the teeth point to the apex. Preferably, the teeth are located at an angle β to the normal N of the base ring of the washer (which lies in the common or shared plane), β is approximately 45° in the figure, but β is preferably in the range of 30° ≤ β ≤ 60° and slightly greater than α (see α). Figure 3C).

[0036] Figure 4D , Figure 4E and Figure 4F Various types of serrated, non-star-shaped elastic washers 441 are shown. Figure 4D The washer 441 shown includes an outer ring 451 with a central opening 443, the outer ring 451 having a plurality of teeth 449 (eight shown in the figure) extending inward from the outer ring to the central opening. The teeth extend out of the plane defined by the outer ring at an angle of approximately 15°, i.e., β approximately 75° (not indicated). Figure 4E The washer 441 shown includes an inner ring 447 with a central opening 443, the inner ring 447 having a plurality of spokes 453 (six shown in the figure) extending outward from the inner ring to an outer hexagonal ring 455. The spokes extend out of the plane defined by the outer ring at an angle of approximately 12°, i.e., β approximately 78° (not indicated). Figure 4F The washer 441 shown is essentially a square, serrated, resilient washer, including a central opening 443. The washer has four indentations 457 at the corners of the square washer, wherein the edge regions 459 of the washer are curved. The curved edge regions are oriented at an angle of approximately 45° to the plane defined by the central opening.

Claims

1. A lighting device comprising an optical board extending above a plurality of LEDs disposed on a main surface of a PCB, wherein, in a mounted configuration, the PCB and the optical board are interconnected by a plurality of fasteners. Each fastener extends through a corresponding, associated through-hole in the optical board in a direction transverse to the main surface of the PCB. The via has a wide portion extending a depth D from a first main surface of the optical board facing away from the PCB toward a second main surface of the optical board facing the PCB, the via narrowing into a neck at depth D. Each fastener extends through the through-hole with a gap S in a plane P parallel to the main surface of the PCB, and rests against a corresponding, associated elastic washer located in the width of the through-hole. The wide portion includes a tapered section that tapers gradually along depth D, and The elastic washer mentioned above is a toothed washer.

2. The lighting device according to claim 1, wherein the gap S is in each direction in the plane P parallel to the main surface of the PCB.

3. The lighting device according to claim 1, wherein the optical plate has a local thickness T at the through hole, and wherein the wide portion is a groove extending to a depth D in the first main surface of the optical plate, wherein 0.25*T≤D≤0.85*T.

4. The lighting device according to any one of claims 1-3, wherein the fastener comprises an elongated portion and a head at one end thereof, the elongated portion having a diameter Df in a direction parallel to the main surface, wherein the gap S in the neck is in the range of 0.1*Df≤S≤0.3*Df.

5. The lighting device according to any one of claims 1-3, wherein the tooth is located at an angle β to the normal of the base ring of the elastic washer, wherein the width of the through hole has an average cone angle α, wherein α≤β≤α+5°.

6. The lighting device according to any one of claims 1-3, wherein the tapered portion has an average cone angle α relative to the normal of the first main surface, wherein 30°≤α≤70°.

7. The lighting device according to any one of claims 1-3, wherein the elastic washer is made of spring steel or elastic polymer / plastic.

8. The lighting device according to any one of claims 1-3, wherein the fastener is selected from the group consisting of threaded fasteners, bolts, screws, clips, and split fasteners.

9. The lighting device according to any one of claims 1-3, wherein the optical plate and the PCB are maintained at a predetermined distance by the plurality of fasteners.

10. The lighting device according to any one of claims 1-3, comprising a spacer of predetermined height positioned between an optical board and a PCB at the location of the through-hole, and a corresponding, associated fastener of the plurality of fasteners extending through the through-hole.

11. The lighting device according to any one of claims 1-3, wherein the resilient washer is a serrated washer.

12. The lighting device according to any one of claims 1-3, wherein the elastic washer comprises n teeth, where 3 ≤ n ≤ 24.

13. The lighting device according to any one of claims 1-3, wherein the resilient washer is a star-shaped washer.

14. The lighting device according to any one of claims 1-3, wherein the elastic washer is annular and has a maximum diameter Dw, and the teeth have a length L, wherein 0.1*Dw≤L≤0.4*Dw.

15. The lighting device according to any one of claims 1-3, wherein the tapered portion is continuously tapered along depth D.

16. The lighting device according to any one of claims 1-3, wherein the resilient washer has outwardly extending teeth.

17. The lighting device according to claim 6, wherein 40°≤α≤60°.

18. The lighting device according to claim 12, wherein 4 ≤ n ≤ 12.

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