A LED filament

Abstract

A light emitting diode, LED, filament (1) configured to emit LED filament light (2) and comprising an elongated carrier (3) comprising a first major surface (31) and a second major surface (32), an array of a plurality of LEDs (4) configured to emit LED light (41) and being arranged on the first major surface (31), a first encapsulant (5) at least partly enclosing the plurality of LEDs (4), at least partly covering the first major surface (31), comprising a first luminescent material (53) configured to convert at least part of the LED light (41) into first converted light (42), and comprising an outer surface (51) facing away from the array of a plurality of LEDs (4). The LED filament (1) further comprises a semi-reflective light-transmissive metallic layer (6) enclosing the outer surface (51) of the first encapsulant (5) and being configured to partially reflect ambient light (11) into reflected ambient light (12) and partially transmit the ambient light (11) into transmitted ambient light (13). The reflectivity of the semi-reflective light-transmissive metallic layer (6) is in a range from 50 % to 83 %.

Description

[0001] A LED filament

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a light emitting diode, LED, filament configured to, in an on-state, emit LED filament light. The invention further relates to a lamp and a luminaire comprising such a LED filament.

[0004] As used herein, when referring to a surface, the term “irregular” is intended to mean that the surface is not even or balanced in shape.

[0005] As used herein, when referring to a surface, the term “roughened” is intended to mean that the surface has been subjected to a treatment intended to provide the surface with a surface structure being rougher than its original or initial surface structure.

[0006] As used herein, the term “integral reflectivity” of a layer is intended to mean the average reflectivity of the total layer taking into account possible holes in the layer. In other words, the layer may have no holes and then the reflectivity may be in a first range. If the layer, however, has holes then the intrinsic reflectivity of the layer needs to be in a second range having a larger lower limit than the first range. Hence, and as used herein, the term “intrinsic reflectivity” of a layer is intended to mean the reflectivity of the material of the layer between any holes present in the layer.

[0007] BACKGROUND OF THE INVENTION

[0008] Recent market research shows customers desire hiding the yellow-orange color appearance of LED filaments. With significant marketing effort, companies are introducing white LED filaments in which a white layer is covering the light converting encapsulant of LED filaments.

[0009] A LED filament is providing LED filament light and comprises a plurality of light emitting diodes (LEDs) arranged in a linear array. Preferably, the LED filament has a length L and a width W, wherein L > 5W. The LED filament may be arranged in a straight configuration or in a non-straight configuration such as for example a curved configuration, a 2D / 3D spiral, or a helix. Preferably, the LEDs are arranged on an elongated carrier like for instance a carrier, that may be rigid (made from, e.g., a polymer, glass, quartz, metal, or sapphire) or flexible (e.g., made of a polymer or metal, e.g., a film or foil). Alternatively, or additionally, the LEDs are arranged in the carrier.

[0010] In case the elongated carrier comprises a first major surface and an opposite second major surface, the LEDs may be arranged on at least one of these surfaces. The elongated carrier may be reflective or light transmissive, such as translucent and preferably transparent.

[0011] As used herein, the terms carrier and elongated carrier may be used interchangeably, such that the elongated carrier may also simply be denoted carrier.

[0012] The LED filament may comprise an encapsulant at least partly covering at least part of the plurality of LEDs. The encapsulant may also at least partly cover at least one of the first major surface and second major surface. The encapsulant may be a polymer material which may be flexible such as for example a silicone. Further, the LEDs may be arranged for emitting LED light, e.g., of different colors or spectrums. The encapsulant may comprise a luminescent material that is configured to convert at least a part of the LED light into converted light. The luminescent material may be a phosphor such as an inorganic phosphor and / or quantum dots or rods (QDs).

[0013] The LED filament may comprise multiple sub-filaments.

[0014] US 2019 / 0128481 Al discloses a LED filament comprising one or more LED dies coated with an underlying layer of a phosphor material exhibiting a colored appearance, and an over-coated layer comprising a resinous material loaded with a scattering agent that causes the LED filament to appear white.

[0015] However, the white layer in white LED filaments decreases the efficiency dramatically due light trapping, i.e., light-absorption on the white pigment due to many reflections. In order to get a good hiding function, a thick white layer with a high concentration of white particles is needed. The reflectivity of said white particles is typically about 90 %. By having many reflections, a significant part of the LED filament light is lost.

[0016] It is thus desired to improve the optical performance (in particular reducing or even avoiding light losses, but also for example improving light quality and correlated color temperature, CCT, control and so forth) the efficiency, and / or the appearance (such as light mixing) of LED filaments.

[0017] US 2022 / 221112 discloses a LED filament arrangement which comprises an elongated, flexible LED filament having a plurality of LEDs arranged along the elongation of the filament. The arrangement further comprises a bending unit having a body in which a channel is formed. A portion of the LED filament is arranged within the channel of the bending unit, and the bending unit is at least partially curved and adapted to induce a bend in the LED filament.

[0018] SUMMARY OF THE INVENTION

[0019] It is an object of the invention to provide a LED filament which comprise an improved optical performance, an improved efficiency and / or an improved optical appearance.

[0020] It is a further object of the invention to provide a LED filament with one or more of reduced or even avoided light losses, an improved efficiency, an improved CCT control and an improved light mixing.

[0021] According to a first aspect of the invention, this and other objects are achieved by means of a light emitting diode, LED, filament configured to, in an on-state of the LED filament, emit LED filament light, the LED filament comprising an elongated carrier comprising a first major surface and a second major surface opposite to the first major surface, an array of a plurality of LEDs configured to, in the on-state, emit LED light and being arranged on the first major surface of the elongated carrier or in the carrier, a first encapsulant at least partly enclosing / covering the plurality of LEDs and at least partly covering the first major surface of the elongated carrier, the first encapsulant comprising a first luminescent material configured to, in the on-state, convert at least part of the LED light into first converted light, the first encapsulant comprising an outer surface facing away from the array of the plurality of LEDs, wherein the LED filament further comprises a semi- reflective light-transmissive metallic layer at least partly covering the outer surface of the first encapsulant, wherein the semi-reflective light-transmissive metallic layer comprises (or is) a silver layer, and wherein the integral reflectivity of the semi-reflective light-transmissive metallic layer is in a range from 50 % to 83 %.

[0022] Thereby, a LED filament with one or more of reduced or even avoided light losses, an improved light quality, an improved efficiency, an improved CCT control and an improved light mixing is provided. Such a LED filament is furthermore capable of changing the off state color appearance e.g. hiding the yellow-orange appearance.

[0023] Especially, providing such a semi-reflective light-transmissive metallic layer enables covering and hiding the (light-converting) fist (and second, cf. further below) encapsulant of the LED filament while reducing or even avoiding light losses, which in turn increases the efficiency of the LED filament. The use of a silver layer further provides a very low absorption since silver exhibits a reflectivity being very high compared to other materials. Particularly, silver has a higher reflectivity than aluminum for light with wavelengths above 500 nm, thus improving the efficiency of the LED filament for light with such wavelengths (i.e. less light is lost). The silver layer may further be a continuous layer.

[0024] Furthermore, the integral reflectivity of the semi-reflective light-transmissive metallic layer is chosen to be in a range from 50 % to 83 % to ensure on the one hand that the LED filament comprises a minimum contrast of at least 1 to have sufficient hiding power when the LED filament is in an off-state, and on the other hand to ensure that the LED filament still has sufficient good efficiency, i.e., in the on-state.

[0025] The semi-reflective light-transmissive metallic layer may fully enclose the first light-converting encapsulant. Alternatively, the semi-reflective light-transmissive metallic layer may enclose at least 90 % of the first light-converting encapsulant.

[0026] The semi-reflective light-transmissive metallic layer may cover, hide or enclose the outer surface of the first light-converting encapsulant fully.

[0027] The semi-reflective light-transmissive metallic layer may be configured to partially reflect ambient light into reflected ambient light and partially transmit the ambient light into transmitted ambient light.

[0028] The first encapsulant may be configured to convert at least a part of the transmitted ambient light into phosphor converted ambient light.

[0029] The integral reflectivity of the semi -reflective light-transmissive metallic layer may be in a range from 52 % to 70 %.

[0030] Thereby, a LED filament with one or more of a further improved light quality, an improved efficiency, an improved CCT control and an improved light mixing is provided for. Especially a reflectivity of the semi-reflective light-transmissive metallic layer within the above-mentioned intervals provides for an improved hiding power, and therefore an optimal compromise between the desire to hide the encapsulant and the desire to improve the efficiency of the LED filament.

[0031] The semi-reflective light-transmissive metallic layer may comprise a metallic pattern comprising one or more openings. The one or more openings may have a surface coverage of at most 40 %, or a surface coverage being in a range from 20 % to 40 %.

[0032] Thereby, it becomes possible to provide the semi-reflective light-transmissive metallic layer with a larger thickness, which in turn provides for a further improved efficiency and durability of the LED filament.

[0033] The reflectivity of areas between the one or more openings of the semi- reflective light-transmissive metallic layer may be in a range from 70 % to 90 %. Thereby, a further improved efficiency of the LED filament is provided for in case the semi -reflective light-transmissive metallic layer is provided with openings.

[0034] The size of the openings may be in a range from 1 to 500 micrometers.

[0035] The metallic pattern may comprise metallic dots having a diameter in a range from 1 to 500 micrometers.

[0036] The LED filament may further comprise a second encapsulant at least partly covering the second major surface of the elongated carrier, the second encapsulant comprising a second luminescent material, wherein the semi -reflective light-transmissive metallic layer further encloses the second encapsulant.

[0037] In embodiments, the first encapsulant may be a first elongated (lightconverting) encapsulant and / or the second encapsulant may be a second elongated (lightconverting) encapsulant.

[0038] The semi-reflective light-transmissive metallic layer, and especially the silver layer thereof, may be covered on one or both sides by one or more of at least one protecting layer, and at least one light-transparent non-absorbing protecting layer.

[0039] Thereby, the semi-reflective, light-transmissive metallic layer, and especially the silver layer, is protected from outside influences, especially exposure to air and corrosive components therein and thus to corrosion, and thus the LED filament becomes more durable. Furthermore, covering layer(s) may improve the reflectivity and / or may protect the semi- reflective, light-transmissive metallic layer against metal oxidation and in the case of silver protect it against reacting with sulfur compounds.

[0040] The semi-reflective light-transmissive metallic layer may be specular reflective.

[0041] Thereby, the light-converting encapsulant of the LED filament is particularly well covered and hidden, which in turn increases the efficiency of the LED filament further.

[0042] The semi-reflective light-transmissive metallic layer may be configured to at least partially scatter incident LED light in virtue of one or more of the following: the semi -reflective light-transmissive metallic layer comprises a thickness, T, varying over the surface area, A, of the semi-reflective light-transmissive metallic layer, the semi-reflective light-transmissive metallic layer is an irregular metallic layer, and the semi -reflective light-transmissive metallic layer comprises a roughened surface layer. Providing the semi-reflective light-transmissive metallic layer with such a scattering effect ensures that the coverage and hiding of the light-converting encapsulant of the LED filament is improved even further, which in turn increases the efficiency of the LED filament further. Also, the above options provide the advantageous scattering effect in particularly simple ways.

[0043] The semi-reflective light-transmissive metallic layer may comprise a carrier or a carrying layer, wherein the silver layer is arranged on the carrier or the carrying layer.

[0044] The semi-reflective light-transmissive metallic layer may comprise scattering components.

[0045] Inventors surprisingly found that when the semi-reflective, light-transmissive metallic layer is made slightly scattering, the appearance of the LED filament becomes optimal without compromising the efficiency.

[0046] The semi-reflective light-transmissive metallic layer may comprise a thickness, T, being in a range from 15 nm to 30 nm, or in a range from 17 nm to 28 nm.

[0047] Thereby, it is in a particularly simple manner ensured that the semi-reflective, light-transmissive metallic layer is indeed semi-reflective and light-transmissive. Furthermore, the inventors have found that a thickness in the range of 22 nm to 37 nm exhibits the best optical performance.

[0048] The semi-reflective light-transmissive metallic layer may be flexible. Alternatively, or additionally, the flexible semi -reflective light-transmissive metallic layer may be arranged curving and enclosing the first encapsulant.

[0049] Thereby, a LED filament is provided which may be shaped in a desired manner, such as to be spiraling or curving, without any risk of the semi -reflective light- transmissive metallic layer being damaged in the process.

[0050] One or more of the following may apply: (i) the semi-reflective light- transmissive metallic layer is arranged in physical contact with the first encapsulant, and (ii) the semi -reflective light-transmissive metallic layer is attached to the encapsulant using any one of an optical grade glue, a silicone or polydimethylsiloxane, PDMS.

[0051] Thereby, the semi-reflective, light-transmissive metallic layer, and thus the LED filament, is made especially robust. Using a glue such as an optical grade glue, a silicone or polydimethylsiloxane (PDMS) has the further advantage that such a glue may also be used as a matrix material in the elongated light-converting encapsulant.

[0052] The invention further relates to a LED filament arrangement wherein the LED filament arrangement, comprises the LED filament and a sealed envelope (9), wherein the LED filament (1) is arranged in the sealed envelope (9), and wherein the sealed envelope (9) contains an inert gas (91). The LED filament may be arranged in a sealed envelope, and the sealed envelope may contain an inert gas. The inert gas may for instance comprise Helium. The concentration of the helium may be at least 10 % or at least 10% Helium (with respect to other gasses in the sealed envelope).

[0053] Thereby, a LED filament with a higher stability and durability is obtained, especially when the semi -reflective light-transmissive metallic layer comprises silver.

[0054] The semi-reflective light-transmissive metallic layer may comprise any one or more of a thermoplastics material, poly(methyl methacrylate) (PMMA), polycarbonate (PC), polyethylene terephthalate (PET) and combinations thereof.

[0055] Thereby, the semi-reflective, light-transmissive metallic layer may be made specular reflective in a particularly simple manner.

[0056] The semi-reflective light-transmissive metallic layer may be provided or made or manufactured using physical vapor deposition, PVD, or chemical vapor deposition, CVD. Alternatively, the semi-reflective light-transmissive metallic layer may be provided or made or manufactured using a hot transfer method or a hot stamping method.

[0057] Thereby, the semi-reflective, light-transmissive metallic layer may be produced and arranged on the encapsulant in a particularly simple manner.

[0058] The semi-reflective light-transmissive metallic layer may comprise structures or surface structures, such as a gratings.

[0059] Thereby, the semi-reflective light-transmissive metallic layer may be used for showing various optical effects.

[0060] The invention further relates to a LED filament lamp comprising a LED filament according to the invention.

[0061] The LED filament lamp may further comprise a light transmissive envelope at least partly enclosing the LED filament and a base for electrically and mechanically connecting the LED filament lamp to a socket or a socket of a luminaire.

[0062] The invention still further relates to a luminaire comprising a LED filament lamp according to the invention.

[0063] The invention still further relates to a luminaire comprising a LED filament according to the invention.

[0064] It is noted that the invention relates to all possible combinations of features recited in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

[0066] Fig. 1 shows a cross-sectional view of a light emitting diode, LED, filament according to the invention and comprising a semi-reflective light-transmissive metallic layer.

[0067] Fig. 2 shows a perspective view of the LED filament according to Fig. 1.

[0068] Fig. 3 shows an enlarged view of the section III in Fig. 1 and illustrating further details of a semi-reflective light-transmissive metallic layer of the LED filament according to Fig. 1.

[0069] Fig. 4 shows a cross-sectional view of another semi -reflective light- transmissive metallic layer of a LED filament according to the invention.

[0070] Fig. 5 shows a cross-sectional view of another semi -reflective light- transmissive metallic layer of a LED filament according to the invention.

[0071] Fig. 6 shows a cross-sectional view of another LED filament according to the invention.

[0072] Fig. 7 shows a graph illustrating the reflection of, absorption in, and transmission through, respectively, of light through an exemplary semi-reflective light- transmissive metallic layer made of silver as a function of its thickness.

[0073] Fig. 8 shows a graph illustrating, on the right-hand Y-axis the contrast of, and on the left-hand Y-axis the transmission of light through, an exemplary semi-reflective light- transmissive metallic layer made of silver as a function of its thickness.

[0074] Fig. 9 shows a graph illustrating on the right-hand Y-axis the contrast of, and on the left-hand Y-axis the absorption of light in, an exemplary semi -reflective light- transmissive metallic layer made of silver as a function of its thickness.

[0075] Fig. 10 shows a graph illustrating the absorption of light in an exemplary semi -reflective light-transmissive metallic layer made of silver, comprising openings, and having a thickness of 25 nm as a function of the fraction of openings in percent.

[0076] Fig. 11 shows a graph illustrating the absorption of light in an exemplary semi -reflective light-transmissive metallic layer made of silver, comprising openings, and having a thickness of 30 nm as a function of the fraction of openings in percent.

[0077] Fig. 12 shows a schematical side view of a lamp comprising a LED filament according to the invention.

[0078] Fig. 13 shows a schematical side view of a luminaire comprising a lamp and a LED filament according to the invention. As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

[0079] DETAILED DESCRIPTION

[0080] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

[0081] Fig. 1 shows a cross-sectional view of a light emitting diode, LED, filament 1 according to the invention, and Fig. 2 shows a perspective view of the LED filament 1.

[0082] The light emitting diode, LED, filament 1 is configured to, in an on-state, emit LED filament light 2. Generally, and irrespective of the embodiment, the LED filament 1 comprises an elongated carrier 3, an array of a plurality of LEDs 4 and a light-converting encapsulant 5.

[0083] The elongated carrier 3 comprises a first major surface 31 and a second major surface 32 opposite to the first major surface 31. The elongated carrier 3 may be a substrate such as a printed circuit board (PCB) configured to provide the plurality of LEDs 4 with electrical power.

[0084] The array of a plurality of LEDs 4 is configured to, in an on-state, emit LED light 41. The array of a plurality of LEDs 4 is arranged on the first major surface 31 of the elongated carrier 3. Alternatively, or additionally, the LEDs may be arranged in the carrier. The LEDs may be any suitable type of LEDs, such as LEDs configured to emit white light, green light, red light, or blue light.

[0085] Generally, and irrespective of the embodiment, the light-converting encapsulant 5 encloses the plurality of LEDs 4 at least partly. Generally, and irrespective of the embodiment, the encapsulant 5 further covers the elongated carrier 3, and particularly the first major surface 31, at least partly. As shown in Fig. 1, the encapsulant 5 encloses the plurality of LEDs 4, and covers the elongated carrier 3, fully. The light-converting encapsulant 5 may be an elongated light-converting encapsulant 5. The light-converting encapsulant 5 is configured to convert at least a part of the LED light 41 into converted light 42. The light-converting encapsulant 5 comprises a first luminescent material 53 configured to, in the on-state, convert at least part of the LED light 41 into converted light 42. The LED filament light 2 may thus comprise LED light 41 and converted light 42 or only converted light 42. The encapsulant 5 comprises an outer surface 51 facing away from the array of a plurality of LEDs 4.

[0086] The LED filament 1 further comprises a semi-reflective light-transmissive metallic layer 6. The semi-reflective light-transmissive metallic layer 6 is arranged on the outer surface 51 of the encapsulant 5. The semi-reflective light-transmissive metallic layer 6 may be arranged on a part of the outer surface 51 of the encapsulant 5. Alternatively, the semi -reflective light-transmissive metallic layer 6 may be arranged on all of the outer surface 51 of the encapsulant 5. The semi-reflective light-transmissive metallic layer 6 is configured to partially reflect incident LED light 41 and / or incident converted light 42 in a direction away from the array of a plurality of LEDs 4 and thus in a direction out of or away from the LED filament 1. The semi-reflective light-transmissive metallic layer 6 is further configured to partially reflect ambient light 11 into reflected ambient light 12 and partially transmit ambient light 11 into transmitted ambient light 13. The first encapsulant 5 is further configured to convert at least a part of the transmitted ambient light 13 into phosphor converted ambient light 14.

[0087] The semi-reflective light-transmissive metallic layer 6 comprises a silver layer 6a. The semi-reflective light-transmissive metallic layer 6 further comprises an optional carrier 6b or a carrying layer. Where a carrier 6b or a carrier layer is provided, the silver layer 6a is arranged on the carrier 6b or the carrying layer. The semi -reflective light-transmissive metallic layer 6 comprises an integral reflectivity, R, being in a range from 25 % to 70 %, in a range from 50 % to 83 % or in a range from 52 % to 70 %. The semi -reflective light- transmissive metallic layer 6 may optionally comprise a flexible substrate, such as a polymer substrate, which may be made of, for instance, PPMA, PC, PET, or the like. The flexible semi -reflective light-transmissive metallic layer 6 is arranged curving around and covering the encapsulant 5.

[0088] Referring also to Fig. 3, the semi-reflective light-transmissive metallic layer 6 comprises a third major surface with a surface layer 61 facing away from the encapsulant 5 and a fourth major surface 62 opposite to the surface layer 61. The semi -reflective light- transmissive metallic layer 6 comprises a thickness, T. The thickness, T, is measured as the distance between and perpendicular to both of the surface layer 61 and the fourth major surface 62. The thickness, T, may be in a range from 20 nm to 70 nm. Alternatively, the thickness, T, may be in a range from 15 nm to 50 nm, 22 nm to 37 nm or 35 nm to 55 nm.

[0089] The semi-reflective light-transmissive metallic layer 6 may be provided or made or manufactured using physical vapor deposition, PVD, or chemical vapor deposition, CVD. Alternatively, the semi-reflective light-transmissive metallic layer may be provided or made or manufactured using a hot transfer method or a hot stamping method. Alternatively, or additionally, the semi-reflective light-transmissive metallic layer 6 may be provided as an optical foil. The semi-reflective light-transmissive metallic layer 6 may further comprise optional structures or surface structures 15, such as a grating (cf. Fig. 6).

[0090] The LED filament 1 is arranged in a sealed envelope 9. It is noted that the sealed envelope 9 is an optional feature. The sealed envelope 9 may contain an inert gas 91, such as for instance Helium or another noble gas. The LED filament 1 may, alternatively or additionally, comprise an optional light-transparent non-absorbing layer 8 (cf. Fig. 5). The light-transparent non-absorbing layer 8 is also an optional feature. The light-transparent nonabsorbing layer 8 encloses the semi-reflective light-transmissive metallic layer 6 at least partially.

[0091] Referring now also to Figs. 3 to 5 various optional features of the semi- reflective light-transmissive metallic layer 6 will be described.

[0092] Fig. 3 shows an enlarged view of the section III in Fig. 1 and illustrating further details of the semi-reflective light-transmissive metallic layer 6 of the LED filament 1. The semi-reflective light-transmissive metallic layer 6 is configured to partially scatter incident LED light 41. Therefore, the semi-reflective light-transmissive metallic layer 6 comprises scattering components 63. The scattering components 63 may for instance be scattering particles. The scattering components 63 may for instance be induced by creating small silver islands and / or peninsulas which can cause scattering of incident light. Such an irregular silver layer may for instance be created by using an optical foil with a rough surface layer. The semi-reflective light-transmissive metallic layer 6 is arranged in physical contact with the encapsulant 5.

[0093] Fig. 4 shows a cross-sectional view of another semi -reflective light- transmissive metallic layer 601 of a LED filament 1 according to the invention.

[0094] The semi-reflective light-transmissive metallic layer 601 is also configured to partially scatter incident LED light 41. However, the semi -reflective light-transmissive metallic layer 601 differs from the semi-reflective light-transmissive metallic layer 6 shown in Figs. 1 and 3 in that its light scattering property is obtained in at least one of the following ways. The semi-reflective light-transmissive metallic layer 601 comprises a thickness T that varies over the surface area A of the semi-reflective light-transmissive metallic layer 601. For instance, the semi-reflective light-transmissive metallic layer 601 may have a first thickness, Tl, measured at one place and a second thickness, T2, measured at another place, where T1 differs from T2. Alternatively, or additionally, the surface layer 61 of the semi -reflective light-transmissive metallic layer 601 may be roughened. Alternatively, or additionally, the semi -reflective light-transmissive metallic layer 601 may be an irregular metallic layer.

[0095] Fig. 5 shows a cross-sectional view of another semi -reflective light- transmissive metallic layer 602 of a LED filament 1 according to the invention.

[0096] The semi-reflective light-transmissive metallic layer 602 is specular reflective.

[0097] The semi-reflective light-transmissive metallic layer 602 is an optic foil. The semi -reflective light-transmissive metallic layer 602 comprises any one or more of a thermoplastics material, poly(methyl methacrylate) (PMMA), polycarbonate (PC), polyethylene terephthalate (PET)and combinations thereof. The semi-reflective light- transmissive metallic layer 6 is attached to the encapsulant 5 using an optical grade glue 7, a silicone or polydimethylsiloxane, PDMS. The semi -reflective light-transmissive metallic layer 602 further comprises one or more openings 64. The semi-reflective light-transmissive metallic layer 64 may be a metallic layer with a plurality of holes or metallic dots. The openings 64 are optional. The openings 64 may extend all the way through the semi- reflective light-transmissive metallic layer 602. The openings 64 may have a surface coverage of at most 40 %, such as in a range from 20 % to 40 %. The areas of the semi- reflective light-transmissive metallic layer 6 between the plurality of holes 64 may have a reflectivity being in a range from 70 % to 90 %.

[0098] Furthermore, the semi -reflective light-transmissive metallic layer 602 is covered on one or both sides by at least one covering layer 8. The covering layer 8 encloses the semi -reflective light-transmissive metallic layer 602 at least partially. The covering layer 8 may be any suitable type of layer. One example is a light-transparent non-absorbing layer 8.

[0099] It is noted that the various features of the reflective light-transmissive metallic layers 6, 601 and 602, respectively, described with reference to Figs. 3 to 5 may be combined in many possible ways. For instance, the reflective light-transmissive metallic layer 6 shown in Fig. 3 may also be provided with a thickness T that varies over the surface area A of the semi -reflective light-transmissive metallic layer 6 and / or a roughened surface layer 61 and / or the semi -reflective light-transmissive metallic layer 6 may be an irregular metallic layer. Likewise, the semi -reflective light-transmissive metallic layer 601 may be provided with scattering particles 63. Also, any of the semi -reflective light-transmissive metallic layers 6 and 601 may be provided as an optical foil and / or be attached to the encapsulant 5 using an optical grade glue 7, and / or comprise a thermoplastics material as described above.

[0100] Fig. 6 shows a cross-sectional view of another LED filament 100 according to the invention. The LED filament 100 differs from the LED filament 1 shown in Fig. 1 in virtue of the following.

[0101] As shown in Fig. 6, the encapsulant 5 encloses the plurality of LEDs 4. Furthermore, the encapsulant 5 covers the elongated carrier 3 at least partly. More particularly, the encapsulant 5 covers the first major surface 31 of the elongated carrier 3 at least partly. The encapsulant 5 does not cover the second major surface 32 of the elongated carrier 3. Also, the LED filament 100 does not comprise any sealed envelope 9, nor any light- transparent non-absorbing layer 8. The semi-reflective light-transmissive metallic layer 6 covers the encapsulant 5, and thus partly covers the first major surface 31 of the elongated carrier 3. The semi-reflective light-transmissive metallic layer 6 does not cover the second major surface 32 of the elongated carrier 3. The semi -reflective light-transmissive metallic layer 6 further comprises a structure or surface structure 15, such as a grating.

[0102] The LED filament 100 further comprises an optional further encapsulant 5’ at least partly covering the second major surface 32 of the elongated carrier 3. The further encapsulant 5’ may comprise a further luminescent material 53’. The semi-reflective light- transmissive metallic layer 6 may further enclose the further encapsulant 5’.

[0103] Figs. 7 to 11 shows graphs illustrating the performance of various exemplary semi -reflective light-transmissive metallic layers 6 suitable for a LED filament 1, 100 according to the invention. Generally, it is in accordance with the invention desired to provide a LED filament 1, 100 having a silver appearance while still having a high efficiency. To evaluate whether this is obtained, two parameters may be used, namely contrast and absorption. In this connection, the desired contrast for obtaining silver appearance of a filament may be defined as follows:

[0104] (ambient light reflected from the semi-reflective light-transmissive metallic layer 6) / (ambient light reflected from the elongated light converting encapsulant 5) > 1.

[0105] The desired absorption for obtaining a high efficiency may be defined as the condition that the light absorption by the semi -reflective light-transmissive metallic layer 6 must be smaller than 15 %. Figs. 7 to 9 show results for a first exemplary semi-reflective light- transmissive metallic layer 6 which is made of silver. Fig. 7 shows a graph illustrating the reflection of, absorption in, and transmission through, respectively, light through the first exemplary semi -reflective light-transmissive metallic layer 6 as a function of its thickness. Fig. 8 shows a graph illustrating, on the right-hand Y-axis the contrast of, and on the lefthand Y-axis the transmission of light through, an exemplary semi-reflective light- transmissive metallic layer 6 as a function of its thickness. Fig. 9 shows a graph illustrating on the right-hand Y-axis the contrast of, and on the left-hand Y-axis the absorption of light in, an exemplary semi-reflective light-transmissive metallic layer 6 as a function of its thickness.

[0106] As may be seen from Fig. 7, the absorption of light in the semi -reflective light- transmissive metallic layer 6 is largely independent of its thickness, at least in the size interval shown. The reflection of light from the semi -reflective light-transmissive metallic layer 6 increases exponentially with the thickness, passing 50 % at a thickness of about 20 nm and reaching 90 % at a thickness of about 25 nm. The transmission of light through the semi -reflective light-transmissive metallic layer 6 decreases exponentially with the thickness, passing 50 % at a thickness of about 20 nm and reaching 10 % at a thickness of about 25 nm.

[0107] Bearing the above conditions regarding contrast and absorption in mind, it may be seen from the graphs of Figs. 7-9 that the integral reflectivity of the semi-reflective light-transmissive metallic layer 6 is in a range from 50 % to 83 %, that the transmission through the semi-reflective light-transmissive metallic layer 6 is in a range from 84 % to 97 %, and that an appropriate thickness for a semi-reflective light-transmissive metallic layer 6 made of silver is in the range of approximately 16 to 28 nm.

[0108] Turning now to Figs. 10 and 11, Fig. 10 shows a graph illustrating the absorption of light in a second exemplary semi-reflective light-transmissive metallic layer 602 made of silver, comprising openings 64, and having a thickness of 25 nm as a function of the fraction of openings 64 in percent. Fig. 11 shows a graph illustrating the absorption of light in a third exemplary semi-reflective light-transmissive metallic layer 602 made of silver, comprising openings 64, and having a thickness of 30 nm as a function of the fraction of openings 64 in percent.

[0109] As is illustrated by the graphs in Figs. 10 and 11, the appropriate thickness of the semi -reflective light-transmissive metallic layer may be increased up 30 nm, but then the semi -reflective light-transmissive metallic layer 602 (cf. Fig. 5) needs to be provided with openings 64, particularly openings 64 with a fraction of up to 40 %, such as in the range of 20-40 %, to satisfy the aforesaid conditions. Also, when a semi-reflective light-transmissive metallic layer with a thickness of 25 nm is used, it may optionally comprise openings 64 with a fraction of up to 35 %.

[0110] Fig. 12 shows an exemplary lamp 300 comprising a LED filament 1, 100 according to any embodiment of the invention. In the embodiment shown, the LED filament 1, 100 comprises a substantially straight LED filament. The LED filament of such a lamp may in other embodiments be a LED filament with another shape, such as, but not limited to, spiral-shaped, helix-shaped, meandering, twisted, flat and combinations thereof.

[0111] The lamp 300 further comprises a driver or controller 305 configured for controlling the plurality of LEDs 4 of the LED filament 1, 100. The controller 305 is configured to power the plurality of LEDs 4 via electrical circuitry (not visible on the figures) of the LED filament 1, 100. The LED filament 1, 100 may also comprise a controller, which may or may not be separate from the controller 305. In other words, the controller 305 and the controller of the LED filament 1, 100 may be integrated into one and the same driver or controller, or they may be mutually separate units.

[0112] The lamp 300 further comprises an envelope 301 at least partially enveloping the at least one LED filament 1, 100. The lamp 300 further comprises a cap 303. As shown in Fig. 12, the controller 305 is arranged within the envelope 301. When comprising a cap 303, the controller 305 may also be arranged inside the cap 303 such that it is hidden from view. The lamp 300 further comprises threading 302 for connection to a socket, and a terminal 304 for connection to a source of electrical energy.

[0113] The envelope 301 of the lamp 300 may further and optionally be provided with a coating (not shown), such as a reflective coating, covering at least a part of the envelope 301.

[0114] Turning finally to Fig. 13, an exemplary luminaire in the form of a pendant 400 is shown. The pendant 400 comprises a LED filament 1, 100 according to any embodiment of the invention. The LED filament 1, 100 is as shown in Fig. 13 provided within a lamp 300 in the form of a light bulb. The LED filament 1, 100 as shown in Fig. 13 comprises a substantially straight LED filament.

[0115] As is also mentioned above, the light bulb further comprises a transparent envelope (cf. transparent envelope 301 of lamp 300) at least partially enveloping the at least one LED filament 1, 100. The transparent envelope may be shaped in any feasible shape, for example such as to resemble the shape of any one of a standard light bulb, a globe light bulb, a candlelight bulb, a customized light bulb and even a spiral light bulb. The transparent envelope may comprise a luminescent material. The transparent envelope may be a glass envelope.

[0116] The pendant 400 further comprises a socket 401 for connecting the lamp 300, and thereby the LED filament 1, 100, to the pendant 400. The socket 401 is adapted to cooperate with the base 303 of the lamp 300. The socket 401 may comprise a threading adapted to cooperate with the threading 302 of the lamp 300. The socket 401 may comprise a terminal adapted to cooperate with the terminal 304 of the lamp 300. The pendant 400 further comprises a reflector or screen 403.

[0117] The pendant 400 may further comprise a driver 402 configured for controlling the LED filament 1, 100. The driver 402 may or may not be the same unit as the controller 305 described above. In other words, the driver 402 and the controller 305 may be integrated into one and the same driver or controller, or they may be mutually separate units. Alternatively, or additionally, the LED filament 1, 100 may also comprise a controller, which may or may not be separate from one or both of the driver 402 and the controller 305.

[0118] As shown in Fig. 13, the driver 402 is arranged on a reflector or screen 403 of the pendant 400. The driver may also be arranged within or incorporated into the reflector or screen 403. The pendant 400 further comprises an electrical wiring 404 for connection to a source of electricity, such as a mains.

[0119] It is noted that the pendant 400 shown in Fig. 13 is only one example of a luminaire according to the invention. Any suitable type of luminaire may be envisaged, such as but not limited to, a standing luminaire, a wall hung luminaire, a chandelier, a reading luminaire, an outdoor luminaire, and a table luminaire.

[0120] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0121] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person 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 measured cannot be used to advantage.

Claims

CLAIMS:

1. A light emitting diode, LED, filament (1) configured to, in an on-state of the LED filament, emit LED filament light (2), the LED filament comprising: an elongated carrier (3) comprising a first major surface (31) and a second major surface (32) opposite to the first major surface; an array of a plurality of LEDs (4) configured to, in the on-state, emit LED light (41) and being arranged on the first major surface (31) of the elongated carrier or in the carrier; a first encapsulant (5) at least partly enclosing or covering the plurality of LEDs (4) and at least partly covering the first major surface of the elongated carrier (3), the first encapsulant comprising a first luminescent material (53) configured to, in the on-state, convert at least part of the LED light (41) into first converted light (42), the first encapsulant (5) comprising an outer surface (51) facing away from the array of the plurality of LEDs (4), wherein the LED filament (1) further comprises a semi -reflective light- transmissive metallic layer (6) at least partly covering the outer surface (51) of the first encapsulant; wherein the semi-reflective light-transmissive metallic (6) layer comprises a silver layer (6a); wherein the integral reflectivity, that is the average reflectivity of the total layer taking into account possible holes in the layer, of the semi -reflective light-transmissive metallic layer (6) is in a range from 50 % to 83 %; and wherein the semi-reflective light-transmissive metallic layer (6) comprises a metallic pattern comprising one or more openings (64), the one or more openings having a surface coverage of at most 40 % of the semi -reflective light-transmissive metallic layer (6).

2. A LED filament according to claim 1, wherein the integral reflectivity of the semi-reflective light-transmissive metallic layer (6) is in a range from 52 % to 70 %.

3. A LED filament according to claim 1 or 2, wherein at least one of the following applies:the surface coverage of the plurality of openings (64) is in a range from 20 % to 40 %; the size of the openings (64) is in a range from 1 to 500 micrometer; the metallic pattern comprises metallic dots having a diameter in a range from 1 to 500 micrometer.

4. A LED filament according to any one of the preceding claims, wherein the reflectivity of areas between the plurality of openings (64) of the semi -reflective light- transmissive metallic layer (6) is in a range from 70 % to 90 %.

5. A LED filament according to any one of the preceding claims, and further comprising a second encapsulant (5’) at least partly covering the second major surface (32) of the elongated carrier (3), the second encapsulant (5’) comprising a second luminescent material (53’), wherein the semi -reflective light-transmissive metallic layer (6) further encloses or covers the second encapsulant.

6. A LED filament according to any one of the preceding claims, wherein the silver layer (6a) is covered on one or both sides by one or more of: at least one protective layer (8), and at least one light-transparent non-absorbing protective layer (8).

7. A LED filament according to any one of the preceding claims, wherein the semi -reflective light-transmissive metallic layer (6) is specular reflective.

8. A LED filament according to any one of the above claims, wherein the semi- reflective light-transmissive metallic layer (6) is configured to at least partially scatter incident LED light (41) in virtue of one or more of the following: the semi -reflective light-transmissive metallic layer (6) comprises a thickness -(T) varying over the surface area (A) of the semi-reflective light-transmissive metallic layer, the semi -reflective light-transmissive metallic layer (6) is an irregular metallic layer, and the semi -reflective light-transmissive metallic layer (6) comprises a roughened surface layer (61).

9. A LED filament according to any one of the above claims, wherein the semi- reflective light-transmissive metallic layer (6) comprises a carrier (6b), wherein the silver layer (6a) is arranged on the carrier.

10. A LED filament according to any one of the above claims, wherein the semi- reflective light-transmissive metallic layer (6) comprises a thickness (T) being in a range from 15 nm to 30 nm, or in a range from 17 nm to 28 nm.

11. A LED filament according to any one of the above claims, wherein the semi- reflective light-transmissive metallic layer (6) is flexible, and wherein the flexible semi- reflective light-transmissive metallic layer (6) is arranged curving and enclosing the first encapsulant (5).

12. A LED filament according to any one of the above claims, wherein one or more of the following applies:(i) the semi -reflective light-transmissive metallic layer (6) is arranged in physical contact with the first encapsulant (5), and(ii) the semi -reflective light-transmissive metallic layer (6) is attached to the first encapsulant (5) using any one of an optical grade glue (7), a silicone or polydimethylsiloxane, PDMS.

13. A LED filament arrangement, comprising the LED filament according to any one of the above claims, further comprising a sealed envelope (9), wherein the LED filament (1) is arranged in the sealed envelope (9), and wherein the sealed envelope (9) contains an inert gas (91).

14. A LED filament lamp (300) or a luminaire (400) comprising the LED filament (1) according to any one of the preceding claims.

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