A LED filament

Abstract

A light emitting diode, LED, filament (1) configured to, in an on-state, emit LED filament light (2) and comprising an elongated carrier (3) comprising a first major surface (31) and a second major (32) surface opposite to the first major surface (31), an array of a plurality of LEDs (4) configured to, in an on-state, emit LED light (41) and being arranged on the first major surface (31), an elongated light-converting encapsulant (5) at least partly enclosing the plurality of LEDs (4), at least partly covering the first major surface (31) of the elongated carrier, and comprising a luminescent material adapted to convert at least part of the LED light (41) into converted light, the LED filament light (2) comprising the converted light and optionally a portion of the LED light (41), and a semi-reflective mirror (6). The semi-reflective mirror (6) encloses the elongated light-converting encapsulant (5) at least partially and comprises (i) a plurality of layers (61-67), wherein the plurality of layers comprises m layers, where m is an integer being at least seven, wherein neighboring layers of the plurality of layers (61-67) comprise different refractive indices (n1-nm), or (ii) a cholesteric liquid crystal layer.

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, the term “blue light” or “blue part of the visible wavelength spectrum” is intended to refer to light with a peak wavelength falling within the wavelength interval of 420 nm to 490 nm.

[0005] As used herein, the term “green light” or “green part of the visible wavelength spectrum” is intended to refer to light with a peak wavelength falling within the wavelength interval of 490 nm to 590 nm.

[0006] As used herein, the term “red light” or “red part of the visible wavelength spectrum” is intended to refer to light with a peak wavelength falling within the wavelength interval of 590 to 750 nm.

[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).

[0010] In case the elongated carrier comprises a first major surface and an opposite second major surface, the LEDs are arranged on at least one of these surfaces. Alternatively, or additionally, the LEDs are arranged in the carrier. 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 the 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 (such as 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 11841115 discloses an LED filament lighting device comprising, at least one LED filament, each including a plurality of LEDs mounted on an elongated substrate, at least one light scattering element, each including a light scattering shell surrounding a transparent volume, each light scattering element being arranged to encompass at least one LED of at least one LED filament, and configured to scatter light emitted from the encompassed LED(s), thereby forming a first LED set including at least one LED encompassed by said at least one scattering element, and a second LED set including a plurality of LEDs not encompassed by said at least one scattering element, wherein a light distribution from said first LED set combines with a light distribution from said second LED set to provide an improved total light distribution from said LED filament lighting device.

[0018] WO 2020 / 208009 relates to a LED filament. According to an embodiment, the LED filament comprises an elongated substrate and a plurality of light-emitting diodes, LEDs, which are mechanically coupled to the substrate. According to an embodiment, the LED filament further comprises an at least in part light-transmissive encapsulation which encapsulate the plurality of LEDs and at least partially encapsulates the substrate, and a plurality of at least partially light-reflective particles which are arranged on an outer surface of the encapsulation.

[0019] SUMMARY OF THE INVENTION

[0020] 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.

[0021] It is a further object of the invention to provide a LED filament with one or more of a reduced light loss, an improved light quality, an improved efficiency, an improved CCT control and an improved light mixing.

[0022] 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, 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 an on-state, emit LED light and being arranged on the first major surface of the elongated carrier or in the carrier, an elongated light-converting encapsulant at least partly enclosing the plurality of LEDs and at least partly covering the first major surface of the elongated carrier, the elongated light-converting encapsulant comprising a luminescent material adapted to convert at least part of the LED light into converted light, the LED filament light comprising the converted light and optionally a portion of the LED light, and a semi -reflective mirror, wherein the semi-reflective mirror at least partly covers the elongated light-converting encapsulant, wherein the semi-reflective mirror comprises:

[0023] (i) a plurality of layers, wherein the plurality of layers comprises m layers, where m is an integer being at least seven, wherein neighboring layers of the plurality of layers comprises different refractive indices (m-nm), or

[0024] (ii) a cholesteric liquid crystal layer. By adding either one of a plurality of layers and a cholesteric liquid crystal layer as described above, a LED filament which comprises an improved optical performance, an improved efficiency and / or an improved optical appearance is provided for. Especially, a LED filament with low light loss, and further with one or more of an improved light quality, an improved efficiency, an improved CCT control and an improved light mixing is provided for.

[0025] The LED filament may be white light having a correlated color temperature in a range from 1700K to 6500K and / or a color rendering index of at least 80 or at least 85.

[0026] The semi-reflective mirror may cover, hide or enclose the elongated lightconverting encapsulant fully.

[0027] The integer m may be at least 9, at least 12, or at least 15.

[0028] Adding further layers provides for a LED filament which comprises a further improved optical performance, a further improved efficiency and / or a further improved optical appearance.

[0029] The plurality of layers may be polymer layers. The polymer layers may comprise (i) one or more PET layers, (ii) one or more PC layers, (iii) one or more PMMA layers.

[0030] The plurality of layers may be inorganic layers. The inorganic layers may comprise magnesium fluoride, silicon dioxide, tantalum pentoxide, zinc sulfite, titanium dioxide.

[0031] The elongated light-converting encapsulant may comprise or consist of a plurality of light-converting encapsulant parts. The plurality of light-converting encapsulant parts may have the same properties or two or more different properties. The elongated lightconverting encapsulant may comprise a light converting encapsulant pattern.

[0032] Thereby, various patterns of LED filament light becomes possible, thus improving the light quality and the optical appearance even further.

[0033] The semi-reflective mirror may be a semi -reflective specularly reflecting mirror.

[0034] Employing a semi-reflective specularly reflecting mirror provides for a LED filament which comprises a further improved optical performance.

[0035] The cholesteric liquid crystal layer may be a polymer cholesteric liquid crystal layer, or a crosslinked polymer cholesteric liquid crystal layer. Thereby, a LED filament which comprises a further improved optical performance, a further improved efficiency and / or a further improved optical appearance is provided for.

[0036] The semi-reflective mirror may be configured to cause the LED filament to appear silver or silvery when the LED filament is in an off-state. Alternatively, or additionally, the semi-reflective mirror may be configured to cause the elongated lightconverting encapsulant to appear silver or silvery when the LED filament is in an off-state.

[0037] Thereby, an improved silver appearance of the LED filament when in the off- state is provided for. The reason is that the yellow-orange appearance of the elongated lightconverting encapsulant is hidden.

[0038] The semi-reflective mirror may be flexible. In this way, the semi-reflective mirror may be arranged around the LED filament with a minimal risk of inflicting damage.

[0039] The semi-reflective mirror may be arranged in a curved configuration around the LED filament.

[0040] Thereby, an improved silver appearance is provided for due to at least one of the flexibility and the curved arrangement of the semi-reflective mirror. Also, a LED filament allowing for obtaining various shapes is obtained.

[0041] The semi-reflective mirror may comprise a cross-section having a shape being polygonal, polygonal with at least five faces, round, circular, oval, or any combination thereof.

[0042] Thereby, a further improved silver appearance of the LED filament when in the off-state is provided for due to the semi-reflective mirror, and thus the LED filament, comprising faces at many angles.

[0043] At least one of the following may apply: (i) each layer of the plurality of layers has a thickness in a range from 100 nm to 400 nm; (ii) the neighboring layers have a difference in refractive index (An) of at least 0.08; (iii) the plurality of layers are polymer layers; (iv) the plurality of layers are arranged as function of an increase in layer thickness; and (v) the semi-reflective layer comprises a gradient of layers, for instance in terms of one or more of refractive index and thickness.

[0044] A gap, such as an air gap, may be provided between the elongated lightconverting encapsulant and the semi -reflective mirror.

[0045] Thereby, a further improved silver appearance of the LED filament when in the off-state is provided. The integer m may be at least 9, at least 12, or at least 15. The more layers to more continuous the light spectrum is. For example, the reflectivity (and transmission) of the semi -reflective layer may vary at most 20% or at most 10%.

[0046] The air gap comprises a thickness, Tg, the elongated light-converting encapsulant comprises a thickness, Te, and Tg may be smaller than or equal to Te. Alternatively, or additionally, Tg may be smaller than or equal to 1 mm.

[0047] Thereby, a further improved silver appearance of the LED filament when in the off-state is provided for.

[0048] The elongated light-converting encapsulant may at least partly cover the second major surface of the elongated carrier.

[0049] Thereby, a further improved efficiency of the LED filament is provided for.

[0050] The elongated carrier may be light transmissive.

[0051] Thereby, a further improved optical performance of the LED filament is provided for.

[0052] The semi-reflective mirror may fully enclose the elongated light-converting encapsulant. Alternatively, the semi-reflective mirror may enclose at least 90 % of the elongated light-converting encapsulant.

[0053] Thereby, a particularly improved silver appearance of the LED filament when in the off-state is provided for.

[0054] The semi-reflective mirror may comprise a reflectivity, R, being in a range from 61 % to 87 % in the visible wavelength spectrum.

[0055] Particularly, the semi-reflective mirror may comprise a reflectivity, R, being in a range from 61 % to 87 % in the blue, green, and red part of the visible wavelength spectrum.

[0056] Thereby, a further improved optical performance of the LED filament is provided for.

[0057] The semi-reflective specularly reflecting mirror may comprises a reflectivity, R, exhibiting multiple peaks in each of the blue, green, and red part of the visible wavelength spectrum.

[0058] Thereby, a particularly improved silver appearance of the LED filament when in the off-state and a further improved optical performance of the LED filament is provided for.

[0059] The semi-reflective mirror may comprise a transmission, T, being in a range from 11 % to 34 % in the visible wavelength range. Particularly, the semi-reflective mirror may comprise a transmission, T, being in a range from 11 % to 34 % in the blue, green, and red part of the visible wavelength range.

[0060] The inventors have found that a minimum transmission, T, of 11 % is required for minimizing absorbed LED light such that a LED filament with a relatively high efficiency is obtained, and that a maximum transmission, T, of 34 % is required for obtaining sufficient contrast thus sufficient hiding power. Thereby, a further improved optical performance of the LED filament is provided for.

[0061] The semi-reflective mirror may comprise an absorption, A, being less than 5 % for the LED light, or less than 3 %, or less than 2 %.

[0062] Thereby, very low light losses is ensured without affecting the appearance and optical performance of the LED filament adversely.

[0063] The semi-reflective mirror may be a foil.

[0064] Thereby, a semi-reflective mirror may be added to the LED filament in a particularly simple and cost efficient manner.

[0065] The LED filament may comprise a linear shape, and the semi -reflective mirror may comprise a (semi) cylindrical shape.

[0066] Thereby, a further improved silver appearance of the LED filament when in the off-state and a further improved optical performance of the LED filament is provided for.

[0067] At least three layers or at least five layers or at least five of the plurality of layers may have a different thickness.

[0068] Thereby, an improvement in the optical performance of the semi-reflective mirror is provided for.

[0069] The LED filament comprises a length, L, and the semi -reflective mirror may be specularly reflecting and e.g. extending at least along the complete length, L, of the elongated light-converting encapsulant.

[0070] Thereby, a further improved silver appearance of the LED filament when in the off-state over the whole length of the LED filament is provided for.

[0071] The semi-reflective mirror may comprise openings having a size in the range of 0.1 mm to 0.3 mm.

[0072] Providing such openings has been shown to improve the contrast of the LED filament. Table 1 below indicates some values for the obtained contrast for a semi-reflective mirror with a given reflectivity and a given percentage of holes. TABLE 1

[0073] The invention further relates to a LED filament lamp comprising a LED filament according to the invention.

[0074] 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.

[0075] The invention still further relates to a luminaire comprising a LED filament lamp according to the invention.

[0076] The invention still further relates to a luminaire comprising a LED filament according to the invention.

[0077] It is noted that the invention relates to all possible combinations of features recited in the claims.

[0078] BRIEF DESCRIPTION OF THE DRAWINGS

[0079] 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.

[0080] Fig. 1 shows a cross-sectional view of a light emitting diode, LED, filament according to the invention and comprising a semi-reflective mirror.

[0081] Fig. 2 shows a perspective view of the LED filament according to claim 1.

[0082] Fig. 3 shows an enlarged view of the section III in Fig. 1 and illustrating further details of a semi-reflective mirror of the LED filament according to Fig. 1.

[0083] Fig. 4 shows a cross-sectional view of another LED filament according to the invention.

[0084] Fig. 5 shows a cross-sectional view of another LED filament according to the invention.

[0085] Fig. 6 shows a graph illustrating the percentage of LED filament light absorbed (left hand Y-axis) in a semi-reflective mirror of a LED filament according to the invention and the contrast (right hand Y-axis), respectively, as a function of the transmission through the semi-reflective mirror, where the semi-reflective mirror is a wide-band multistack layer reflector with an absorption of 5 %.

[0086] Fig. 7 shows a graph illustrating the percentage of LED filament light absorbed (left hand Y-axis) in a semi-reflective mirror of a LED filament according to the invention and the contrast (right hand Y-axis), respectively, as a function of the transmission through the semi-reflective mirror, where the semi-reflective mirror is a wide-band multistack layer reflector with an absorption of 2 %.

[0087] Fig. 8 shows a schematical side view of a lamp comprising a LED filament according to the invention.

[0088] Fig. 9 shows a schematical side view of a luminaire comprising a lamp and a LED filament according to the invention.

[0089] 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.

[0090] DETAILED DESCRIPTION

[0091] 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.

[0092] 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.

[0093] The 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 an elongated light-converting encapsulant 5.

[0094] 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 light transmissive or transparent. 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.

[0095] As shown in Fig. 1, 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 also be arranged in the carrier, that is the carrier material comprises the LEDs. The LEDs may be any suitable type of LEDs, such as LEDs configured to emit white light, green light, red light, or blue light.

[0096] Generally, and irrespective of the embodiment, the elongated light-converting encapsulant 5 encloses the plurality of LEDs 4 at least partly. Generally, and irrespective of the embodiment, the elongated light-converting encapsulant 5 further covers the elongated carrier 3, and particularly the first major surface 31, at least partly. As shown in Fig. 1, the elongated light-converting encapsulant 5 encloses the plurality of LEDs 4, and covers the elongated carrier 3, fully. Thus, as shown in Fig. 1, the elongated light-converting encapsulant 5 further covers the second major surface 32, fully. The elongated lightconverting encapsulant 5 is configured to convert at least a part of the LED light 41 into converted light 42. The elongated light-converting encapsulant 5 comprises a luminescent material 52 adapted to convert at least a part of the LED light 41 into converted light 42 (cf. Fig. 1). The LED filament light 2 may thus comprise LED light 41 and converted light 42 or only converted light 42. The elongated light-converting encapsulant 5 comprises an outer surface 51 facing away from the array of a plurality of LEDs 4. The elongated lightconverting encapsulant 5 comprises a thickness, Te. The elongated light-converting encapsulant 5 may be configured to cause the LED filament 1 to appear silver or gray when the LED filament 1 in the off-state. The elongated light-converting encapsulant 5 may consist of a plurality of light-converting encapsulant parts. The plurality of light-converting encapsulant parts may have the same properties or two or more different properties. The elongated light-converting encapsulant 5 may comprise an elongated light-converting encapsulant pattern.

[0097] The LED filament 1 further comprises a semi-reflective mirror 6. The semi- reflective mirror 6 may be a semi-reflective specularly reflecting mirror. The semi-reflective mirror 6 is shown in further detail in the close-up view shown in Fig. 3. The semi-reflective mirror 6 generally, and irrespective of the embodiment, comprises a plurality of alternating layers 61-67. The semi-reflective mirror 6 comprises m layers, where m is an integer being seven or more. Alternatively, m may be at least 9, at least 12, or at least 15. The more layers the more continuous the light spectrum is. Thus, the semi -reflective mirror 6 may comprise at least seven alternating layers 61-67. Neighboring layers of the plurality of layers 61-67 comprise different refractive indices ni-nm. Thus, referring to the example shown in Fig. 1, layers 61 and 62, layers 62 and 63, layers 63 and 64, layers 64 and 65, layers 65 and 66 and layers 66 and 67, respectively, of the plurality of layers 61-67 comprise different refractive indices ni-nm. Also, alternating layers of the plurality of layers 61-67 may have a relatively high and low refractive index.

[0098] At least three layers of the plurality of layers 61-67 may have a different thickness. The layers of the plurality of layers 61-67 may have a thickness in a range from 200 nm to 400 nm. The thickness of the layers of the plurality of layers 61-67 may increase as a function of the total thickness of the plurality of layers 61-67. For instance, referring to Fig. 3, layers 62 may be thicker than layer 61, layer 63 may be thicker than layer 62, layer 64 may be thicker than layer 63, layer 65 may be thicker than layer 64, layer 66 may be thicker than layer 65, and layer 67 may be thicker than layer 66. The semi-reflective mirror 6 may have a gradient through the plurality of layers 61-67, e.g., a gradient in layer thickness.

[0099] The semi-reflective mirror 6 encloses the elongated light-converting encapsulant 5 at least partially and optionally fully as shown in Fig. 1. The semi-reflective mirror 6 may be specularly reflective. The semi-reflective mirror 6 is configured to cause the LED filament 1 to appear silver colored when the LED filament 1 is in an off-state. The semi -reflective mirror 6 may further be flexible. The semi-reflective mirror 6 is hiding the elongated light-converting encapsulant 5 at least partially and optionally fully. The semi- reflective mirror 6 may be a foil. The semi-reflective mirror 6 may be configured to cause the LED filament 1 to appear silver or gray when the LED filament 1 in the off-state.

[0100] The semi-reflective mirror 6 is as shown in Fig. 1 arranged in a curved configuration around the LED filament 1. The semi-reflective mirror 6 may also be arranged in other configurations, such as an angled configuration, around the LED filament 1. As shown in Figs. 1 and 2, the semi-reflective mirror 6 comprises a cross-section having an oval shape. Other feasible cross-sectional shapes include round and circular and any combination of the afore-mentioned cross-sectional shapes.

[0101] As shown in Fig. 1 an air gap 7 is provided between the elongated lightconverting encapsulant 5 and the semi-reflective mirror 6. Such an air gap 7 is, however, optional as is illustrated for instance in Fig. 3, in which no such air gap 7 is shown. Referring again to Fig. 1, where provided the air gap 7 comprises a thickness, Tg. The thickness Tg may be chosen such that Tg < Te, and / or such that Tg < 1 mm.

[0102] The semi-reflective mirror 6 comprises a transmissivity T, a reflectivity R, and an absorption A. The reflectivity, R, may be chosen to be in a range from 35 % to 55 % in the blue, green, and red part of the visible wavelength spectrum. Alternatively, the reflectivity, R, may be chosen to exhibit multiple peaks in each color part of the visible wavelength spectrum, or to exhibit multiple peaks in each of the blue, green, and red part of the visible wavelength spectrum. Alternatively, the reflectivity, R, may be chosen to be in a range from 61 % to 87 %, or to be in a range from 64 % to 87 %, or to be in a range from 61 % to 68 %, in the visible wavelength spectrum. The transmissivity T may be chosen to be in a range from 45 % to 65 % in the blue, green, and red part of the visible wavelength spectrum, or in a range from 11 % to 34 %, or in a range from 27 % to 34 %, in the visible wavelength spectrum. The absorption, A, may be chosen to be less than 15 % for the LED light 41, or less than 5 %, or less than 2 %.

[0103] As is apparent especially from Fig. 2, the LED filament 1 comprises a linear shape, and the semi-reflective mirror 6 comprises a cylindrical shape. Furthermore, the LED filament 1 comprises a length L. The semi-reflective mirror 6 is continuous. The semi- reflective mirror 6 extends along the complete length L of the LED filament 1.

[0104] Fig. 4 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.

[0105] As shown in Fig. 4, the elongated light-converting encapsulant 5 encloses the plurality of LEDs 4. Furthermore, the elongated light-converting encapsulant 5 covers the elongated carrier 3 at least partly. More particularly, the elongated light-converting encapsulant 5 covers the first major surface 31 of the elongated carrier 3 at least partly. The elongated light-converting encapsulant 5 does not cover the second major surface 32 of the elongated carrier 3.

[0106] The semi-reflective layer 6 comprises a cholesteric liquid crystal layer. The cholesteric liquid crystal layer may be a polymer cholesteric liquid crystal layer, or a crosslinked polymer cholesteric liquid crystal layer. The cholesteric liquid crystal layer may be supported by a support layer of two support layers each at a different side of the cholesteric liquid crystal layer.

[0107] Fig. 5 shows a cross-sectional view of another LED filament 101 according to the invention. The LED filament 101 differs from the LED filaments 1 and 100 shown in Figs. 1, 2 and 4 in virtue of the following.

[0108] The semi-reflective mirror 6 comprises a cross-section having a shape being polygonal, such as polygonal with at least five faces. As shown in Fig. 5, the semi-reflective mirror 6 comprises a cross-section having a shape being polygonal with six faces. The corners of such a polygonal shape may or may not be rounded. It is also feasible that the semi -reflective mirror 6 may comprise a cross-section having a shape being a combination of polygonal, polygonal with at least five faces, round, circular, oval, or any combination thereof.

[0109] Turning now to Fig. 6 and 7, results of a simulation performed on a semi- reflective mirror 6 of a LED filament 1 according to the invention are shown.

[0110] Fig. 6 shows a graph illustrating the percentage of LED filament light 2 absorbed (left hand Y-axis) in a semi-reflective mirror 6 of a LED filament 1 and the contrast (right hand Y-axis), respectively, as a function of the transmission through the semi-reflective mirror 6. In this case, the semi-reflective mirror 6 simulated upon was a wide-band multilayered reflector with an absorption of 5 %.

[0111] Fig. 7 shows a graph illustrating the percentage of LED filament light 2 absorbed (left hand Y-axis) in a semi-reflective mirror 6 of a LED filament 1 and the contrast (right hand Y-axis), respectively, as a function of the transmission through the semi-reflective mirror 6. In this case, the semi-reflective mirror 6 simulated upon was a wide-band multilayered reflector with an absorption of 2 %.

[0112] In both cases neighboring layers 61-67 of the semi-reflective mirror 6 were provided with different refractive indices nl-n7, and a minimum of seven layers 61-67 were provided.

[0113] In summary, the simulations illustrated by the graphs in Figs. 6 and 7 show that:

[0114] The absorption A of the semi-reflective mirror 6 was below 5%.

[0115] The absorption of LED light of the semi-reflective mirror 6 was less than 15%. The contrast of the semi-reflective mirror 6 was more than two.

[0116] The transmission T through the of the semi-reflective mirror 6 was between 11 % and 34 % for the semi-reflective mirror 6 with a 2 % absorption (Fig. 7), and between 27 % and 34 % for of the semi-reflective mirror 6 with a 5 % absorption (Fig. 6).

[0117] The reflection of the of the semi-reflective mirror 6 was between 64 % and 87 % for of the semi-reflective mirror 6 with a 2 % absorption (Fig. 7), and between 61 % and 68 % for of the semi-reflective mirror 6 with a 5 % absorption (Fig. 6).

[0118] Fig. 8 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. 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.

[0119] 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. 8, 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.

[0120] 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.

[0121] Turning finally to Fig. 9, 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. 9 provided within a lamp 300 in the form of a light bulb. The LED filament 1, 100 as shown in Fig. 9 comprises a substantially straight LED filament.

[0122] 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.

[0123] 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. 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.

[0124] As shown in Fig. 9, 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.

[0125] It is noted that the pendant 400 shown in Fig. 9 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.

[0126] 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.

[0127] 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, emit LED filament light (2), the LED filament comprising: an elongated carrier (3) comprising a first major surface (31) and a second major (32) surface opposite to the first major surface, an array of a plurality of LEDs (4) configured to, in an on-state, emit LED light (41) and being arranged on the first major surface (31) of the elongated carrier or in the carrier, an elongated light-converting encapsulant (5) at least partly enclosing the plurality of LEDs and at least partly covering the first major surface of the elongated carrier, the elongated light-converting encapsulant (5) comprising a luminescent material adapted to convert at least part of the LED light (41) into converted light, the LED filament light (2) comprising the converted light and optionally a portion of the LED light, and a semi -reflective mirror (6), wherein the semi-reflective mirror (6) at least partly covers the elongated light-converting encapsulant (5), wherein the semi-reflective mirror (6) comprises (i) a plurality of layers (61- 67), wherein the plurality of layers comprises m layers, where m is an integer being at least seven, wherein neighboring layers (61 and 62; 62 and 63; 63 and 64; 64 and 65; 65 and 66; 66 and 67) of the plurality of layers (61-67) having different refractive indices (m-nm), or (ii) a cholesteric liquid crystal layer, and wherein one or more of the following applies: the semi-reflective mirror (6) is configured to cause the LED filament (1) to appear silver or silvery when the LED filament is in an off-state, and the semi -reflective mirror (6) is configured to cause the elongated lightconverting encapsulant to appear silver or silvery when the LED filament is in an off-state.

2. A LED filament according to claim 1, wherein one or more of the following applies:the semi-reflective mirror (6) is flexible, and wherein the semi-reflective mirror (6) is arranged in a curved configuration around the LED filament (1), and the semi-reflective mirror (6) comprises a cross-section having a shape being polygonal, polygonal with at least five faces, round, circular, oval, or any combination thereof.

3. A LED filament according to any one of the above claims, wherein at least two of the following applies:(i) each layer of the plurality of layers has a thickness in a range from 100 nm to 400 nm;(ii) the neighboring layers have a difference in refractive index (An) of at least 0.08;(iii) the plurality of layers are polymer layers;(iv) the plurality of layers are arranged as function of an increase in layer thickness; and(v) the semi -reflective layer comprises a gradient of layers in terms of one or more of refractive index and thickness.

4. A LED filament according to any one of the above claims, wherein an air gap (7) is provided between the elongated light-converting encapsulant (5) and the semi- reflective mirror (6).

5. A LED filament according to claim 4, wherein the air gap (7) comprises a thickness, Tg, wherein the elongated light-converting encapsulant (5) comprises a thickness, Te, and wherein Tg < Te, or wherein Tg < 1 mm.

6. A LED filament according to any one of the above claims, wherein the elongated light-converting encapsulant (5) at least partly covers the second major surface of the elongated carrier, wherein the elongated carrier is light transmissive, and wherein the semi-reflective mirror (6) is fully enclosing the elongated light-converting encapsulant (5) or is enclosing at least 90 % of the elongated light-converting encapsulant (5).

7. A LED filament according to any one of the above claims, wherein the semi- reflective mirror (6) comprises a reflectivity, R, being in a range from 61 % to 87 % in the visible wavelength spectrum, and / or whereinthe semi -reflective mirror (6) comprises a reflectivity, R, exhibiting multiple peaks in each of the blue, green and red part of the visible wavelength spectrum.

8. A LED filament according to any one of the above claims, wherein the semi- reflective mirror (6) comprises a transmission, T, being in a range from 11 % to 34 % in the visible wavelength spectrum.

9. A LED filament according to any one of the above claims, wherein the semi- reflective mirror (6) comprises an absorption, A, being less than 5 % for the LED light (41), or less than 3 %, or less than 2 %.

10. A LED filament according to any one of the above claims, wherein the semi- reflective mirror (6) is a foil.

11. A LED filament according to any one of the above claims, wherein the LED filament (1) comprises a linear shape, and wherein the semi-reflective mirror (6) comprises a cylindrical shape.

12. A LED filament according to any one of the above claims, wherein at least three layers of the plurality of layers have a different thickness.

13. A LED filament according to any one of the above claims, wherein the LED filament (1) comprises a length (L) and wherein the semi -reflective mirror (6) is specularly reflecting and extends at least along the complete length (L) of the elongated light-converting encapsulant.

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

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