A LED filament

Abstract

A LED filament (1) configured to, in operation, emit LED filament light (22), and comprising a light-transmissive elongated carrier (2) comprising a first major surface (3) and a second major surface (4) opposite to the first major surface (3), a plurality of violet LEDs (7) configured to, in operation, emit violet light (8) and being arranged on the first major surface, a first encapsulant (9) covering the plurality of violet LEDs (7) and at least part of the first major surface (3) and comprising a blue phosphor (10) configured to convert at least a part of the violet light to blue light (11), a second encapsulant (12) fully covering the first encapsulant (9) and being an elongated encapsulant comprising a first green-yellow phosphor (13) and a first red phosphor (14), the first green-yellow phosphor being configured to convert a part of the blue light (11) to first green-yellow light (15) and the first red phosphor being configured to convert a part of the blue light (11) to first red light (16), and a third encapsulant (17) covering at least a part of the second major surface (4) and being an elongated encapsulant comprising a second green-yellow phosphor (18) and a second red phosphor (19), the second green-yellow phosphor being configured to convert a part of the blue light (11) to second green-yellow light (20) and the second red phosphor being configured to convert a part of the blue light (11) to second red light (21), the LED filament light (22) comprising the first green-yellow light (15), the first red light (16), the second green-yellow light (20), and the second red light (21).

Description

[0001] A LED FILAMENT

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a LED filament comprising a light-transmissive elongated carrier comprising a first major surface and a second major surface opposite to the first major surface.

[0004] As used herein, “violet light” is to be understood as encompassing light having a peak wavelength in a wavelength range of 380 nm - 420 nm. As used herein, “blue light” is to be understood as encompassing light having a peak wavelength in a wavelength range of 430 nm - 490 nm. As used herein, “green-yellow light” is to be understood as encompassing light having a peak wavelength in a wavelength range of 500 nm - 580 nm. As used herein, “red light” is to be understood as encompassing light having a peak wavelength in a wavelength range of 600 nm - 680 nm.

[0005] BACKGROUND OF THE INVENTION

[0006] A trend in lighting is LED filament lamps. A LED filament lamp is A LED lamp which is designed to resemble a traditional incandescent light bulb with a visible filament for aesthetic and light distribution purposes, but with the high efficiency of lightemitting diodes.

[0007] US 2019 / 0191516 Al discloses a white light emitting device or LED-filament comprising a LED operable to generate excitation light which may comprise blue and / or violet light, a first phosphor associated with the LED to generate light with a peak emission wavelength in a range 500 nm to 575 nm, and a second phosphor associated with the solid- state light emitter to generate light with a peak emission wavelength in a range 600 nm to 650 nm, wherein a percentage decrease in conversion efficiency corresponding to an increase in excitation light photon density exhibited by the second phosphor is larger than a percentage decrease in conversion efficiency corresponding to the same increase in excitation light photon density exhibited by the first phosphor.

[0008] It is desired to improve the optical performance and / or appearance of LED filaments. It is further desired to solve the spottiness problem in LED filaments which is especially present when LED filaments are used in a low brightness (low luminous flux) mode.

[0009] SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to overcome this problem, and to provide a LED filament with at least one of an improved performance and an improved appearance.

[0011] It is a further object to provide a LED filament with which the spottiness in LED filaments, which is especially present when LED filaments are used in a low brightness (low luminous flux) mode, is avoided, or at least reduced considerably.

[0012] According to a first aspect of the invention, this and other objects are achieved by means of a LED filament configured to, in operation, emit LED filament light, the LED filament comprising a light-transmissive elongated carrier comprising a first major surface and a second major surface opposite to the first major surface, a plurality of violet LEDs configured to, in operation, emit violet light, the violet light emitted by the plurality of violet LEDs having a peak wavelength in a wavelength range of 380 nm to 420 nm, the plurality of violet LEDs being arranged on the first major surface, a first encapsulant covering the plurality of violet LEDs and at least part of the first major surface, the first encapsulant comprising a blue phosphor configured to convert at least a part of the violet light to blue light, a second encapsulant fully covering the first encapsulant, the second encapsulant being an elongated encapsulant comprising a first green-yellow phosphor and a first red phosphor, the first green-yellow phosphor being configured to convert a part of the blue light to first green-yellow light and the first red phosphor being configured to convert a part of the blue light to first red light, and a third encapsulant covering at least a part of the second major surface of the light-transmissive elongated carrier, the third encapsulant being an elongated encapsulant comprising a second green-yellow phosphor and a second red phosphor, the second green-yellow phosphor being configured to convert a part of the blue light to second green-yellow light and the second red phosphor being configured to convert a part of the blue light to second red light, where the LED filament light comprises the first green-yellow light, the first red light, the second green-yellow light, and the second red light.

[0013] With such a LED filament it is obtained that the first encapsulant with the blue phosphor works essentially as a new light source. Thereby, a LED filament having an improved optical performance and / or appearance is obtained. Also, with such a LED filament not the LEDs but the phosphor become the new light source to pump the green-yellow and red phosphors. Since the LEDs are point light sources, they are visible when the LED filament is used in a low brightness mode. Therefore, and further because the first encapsulant comprising the blue phosphor is much bigger and elongated as compared to the LEDs, a LED filament is provided with which the spottiness in LED filaments, which is especially present when LED filaments are used in a low brightness (low luminous flux), mode is avoided, or at least reduced considerably.

[0014] The blue phosphor may be configured to fully convert the violet light to blue light.

[0015] The term “fully convert” is in this connection intended to mean that at least 95% or at least 97% of the incident light is converted. The obtained effect is a further improved optical performance and appearance of the LED filament. The reason is that the blue phosphor is in such a configuration the only new light source for pumping the greenyellow and red phosphors.

[0016] The violet light emitted by the plurality of violet LEDs may have a peak wavelength in a wavelength range of 400 nm to 420 nm.

[0017] The obtained effect is an improved efficiency of the LED filament. The reason is that higher wavelength violet light is used, and thus that Stokes losses are limited.

[0018] The blue phosphor may be a Phosphate phosphor configured to be effectively excited by the violet light emitted by the plurality of violet LEDs and comprising an emission peak wavelength in the wavelength range of 460-480 nm or 460-490 nm.

[0019] The obtained effect is improved human centric lighting of the LED filament. The reason is that light of this particular wavelength range is suitable for suppressing the melatonin production.

[0020] The first green-yellow phosphor and the second green-yellow phosphor may be the same, and the first red phosphor and the second red phosphor may be the same.

[0021] Thereby, a LED filament with a particularly simple structure is provided for. Alternatively, the first green-yellow phosphor and the second green-yellow phosphor may be mutually different, and / or the first red phosphor and the second red phosphor may be mutually different.

[0022] The obtained effect is an improved light quality of the light emitted by the LED filament. The reason is that different spectra are used, and thus that a more continuous (white) light spectrum is provided. One or both of the first green-yellow phosphor and the second green-yellow phosphor may comprise a luminescent material of the type A3Si6Nl 1 :Ce3+ 20 , wherein A comprises one or more of Y, La, Gd, Tb and Lu, and one or both of the first red phosphor and the second red phosphor may be a luminescent material of the type M’xM2-2xAX6 doped with tetravalent manganese, where M’ comprises an alkaline earth cation, M comprises an alkaline cation, and x is in the range of 0-1, wherein A comprises a tetravalent cation, for instance comprising one or more of silicon and titanium, 15 wherein X comprises a monovalent anion, at least comprising fluorine.

[0023] Alternatively, or additionally, one or both of the first green-yellow phosphor and the second green-yellow phosphor comprises a luminescent material may comprise a luminescent material of the type A3Si6Nl 1 :Ce3+ 20, where A comprises one or more of Y, La, Gd, Tb and Lu, such as in embodiments one or more of La and Y. In specific embodiments, alternative to or additional to a garnet luminescent material, the luminescent material may comprise a luminescent material of the type A3Si6Nl 1 :Ce3+, where A comprises one or more of Y, La, Gd, Tb and Lu, such as in embodiments one or more of La and Y.

[0024] The obtained effect is that the LED filament is provided with an optimal phosphor system. The reason is that the above proposed phosphors together with the blue phosphor show optimal light conversion.

[0025] One or both of the first green-yellow phosphor and the second green-yellow phosphor may be an Yttrium Aluminum Garnet phosphor having an absorption peak close to 450 nm and dominant emission wavelength in the wavelength range of 540-560 nm.

[0026] Such a green-yellow phosphor has the advantage of mainly converting blue light and providing a dominant emission wavelength in a 540-560nm range.

[0027] One or both of the first red phosphor and the second red phosphor may be a KSF phosphor configured to be excited by blue light with a strongest emission peak wavelength at 631 nm, and a full width at half maximum (FWHM) being smaller than 30 nm.

[0028] A KSF phosphor may be defined as a luminescent material of the type M’XM2- 2XAXe doped with tetravalent manganese, wherein M’ comprises an alkaline earth cation, M comprises an alkaline cation, and x is in the range of 0-1, wherein A comprises a tetravalent cation, for instance comprising one or more of silicon and titanium, wherein X comprises a monovalent anion, at least comprising fluorine.

[0029] Relevant alkaline cations (M) are sodium (Na), potassium (K) and rubidium (Rb). Optionally, also lithium and / or cesium may be applied. In a preferred embodiment, M comprises at least potassium. In yet another embodiment, M comprises at least rubidium. The phrase “wherein M comprises at least potassium” indicates for instance that of all M cations in a mole M’xM2-2xAX6 , a fraction comprises K+and an optionally remaining fraction comprises one or more other monovalent (alkaline) cations (see also below). In another preferred embodiment, M comprises at least potassium and rubidium. Optionally, the M’XM2- 2XAXe luminescent material has the hexagonal phase. In yet another embodiment, the M’XM2- 2xAXe luminescent material has the cubic phase. For x=0, the composition is M2AX6.

[0030] Relevant alkaline earth cations (M’) are magnesium (Mg), strontium (Sr), calcium (Ca) and barium (Ba), especially one or more of Sr and Ba.

[0031] The term “tetravalent manganese” refers to Mn4+. This is a well-known luminescent ion. In the formula as indicated above, part of the tetravalent cation A (such as Si) is being replaced by manganese. Hence, M’xM2-2xAX6 doped with tetravalent manganese may also be indicated as M’xM2-2xAi-mMnmX6. The mole percentage of manganese, i.e. the percentage it replaces the tetravalent cation A will in general be in the range of 0.1-15 %, especially 1-12 %, i.e. m is in the range of 0.001-0.15, especially in the range of 0.01-0.12.

[0032] As indicated above, X relates to a monovalent anion, but at least comprises fluorine. Other monovalent anions that may optionally be present may be selected from the group consisting of chlorine (Cl), bromine (Br), and iodine (I).

[0033] In an embodiment, M’xM2-2xAX6 comprises K^SiFe (indicated herein also as KSiF system). As indicated above, in another preferred embodiment, M’xM2-2xAX6 comprises KRbSiFe (herein also indicated as K,Rb system). As indicated above, part of silicon is replaced by manganese (i.e. the formula may also be described as K2Sii-mMnmF6 or KRbSii-mMnmF6, with m as indicated above, or as KRbSiFe:Mn and K2SiFe:Mn, respectively). As manganese replaces part of a host lattice ion and has a specific function, it is also indicated as “dopant” or “activator”. Hence, the hexafluorosilicate is doped or activated with manganese (Mn4+).

[0034] In specific embodiments, the luminescent material may comprise (K,Rb)2SiFe:Mn4+. Alternatively, or additionally, in embodiments the third luminescent material may comprise K2SiFe:Mn4+. Alternatively, or additionally, in embodiments the third luminescent material may comprise K2TiFe:Mn4+. In embodiments, the third luminescent material may comprise K2(Si,Ti)Fe:Mn4+. As can be derived from the above, “Si,Ti” may indicate one or more of Si and Ti.

[0035] Red phosphors of the KSF class have the advantage of being particularly effectively excited by blue light. The light transmissive elongated carrier may have a transmission of at least 60 % or at least 80 % in a wavelength range corresponding to that of the blue light emitted by the blue phosphor of the first encapsulant.

[0036] The obtained effect is a LED filament with an improved light distribution. The reason is that the new blue light source, that is the first encapsulant with the blue phosphor, also provides sufficient new blue light source light in an opposite direction, i.e., through said carrier.

[0037] The first encapsulant has a minimum first thickness, Tl, measured perpendicular to the substrate from a light output surface of the plurality of violet LEDs to the second encapsulant, the second encapsulant has a second thickness, T2, measured perpendicular to the substrate at a position of the plurality of violet LEDs from the first encapsulant to the light emitting surface of the second encapsulant, and Tl and T2 may be chosen such that Tl > T2 e.g. Tl > 1.5 • T2 or Tl > 2 • T2.

[0038] The obtained effect is further improved optical performance and / or appearance of the LED filament. The reason is that the first encapsulant is relatively thick, while the second encapsulant is relatively thin.

[0039] The first encapsulant has a minimum first width, Wl, measured perpendicular to the first thickness, Tl, the second encapsulant has a second width, W2, measured perpendicular to the second thickness, T2, the third encapsulant has a third width, W3, measured perpendicular to the third thickness, T3, and Wl, W2 and W3 may be chosen such that W1<W2 and / or W1<W3. Furthermore, W2 may be equal to W3.

[0040] The first encapsulant has a first cross-sectional shape, and the first cross- sectional shape may be rectangular or square, with or without rounded corners opposite to a point of contact with the elongated carrier.

[0041] The second encapsulant has a second cross-sectional shape, and the second cross-sectional shape may be C-shaped or U-shaped such as to fit around the first encapsulant with the first cross-sectional shape.

[0042] The first encapsulant and the second encapsulant may form one continuous element.

[0043] The third encapsulant has a third cross-sectional shape, and the third cross- sectional shape may be rectangular or square, with or without rounded corners opposite to a point of contact with the elongated carrier.

[0044] The first green-yellow phosphor has a first concentration, Cl, in the first encapsulant, the first red phosphor has a second concentration, C2, in the first encapsulant, the second green-yellow phosphor has a third concentration, C3, in the second encapsulant, the second red phosphor has a fourth concentration, C4, in the second encapsulant, and the first, second, third and fourth concentrations may be chosen such that (C1+C2) / (C3+C4) > 1.5 or (C1+C2) / (C3+C4) > 2.

[0045] The obtained effect is a LED filament with an improved appearance.

[0046] The reason is that by using a very low concentration in the third encapsulant its thickness can be increased. In this way the difference in the curvature of the encapsulants on both sides is reduced.

[0047] The first encapsulant has a minimum thickness, Tl, measured perpendicular to the substrate from a light output surface of the plurality of violet LEDs to the second encapsulant, the second encapsulant has a second thickness, T2, measured perpendicular to the substrate at a position of the plurality of violet LEDs from the first encapsulant to a light emitting surface of the second encapsulant, the third encapsulant has a third thickness, T3, measured perpendicular to the substrate at a position of the plurality of violet LEDs from the second major surface to a light emitting surface of the third encapsulant, wherein the first, second and third thicknesses are chosen such that 1.3 > T3 / (T1+T2) > 0.7 or 2 > T3 / (T1+T2) > 0.5.

[0048] The obtained effect is a LED filament with an improved optical performance and / or appearance. The reason is that the encapsulants on both sides of the carrier have about the same thickness.

[0049] The third encapsulant may fully cover the second major surface of the light- transmissive elongated carrier.

[0050] In this context the term “fully covered” is intended to mean covering at least 90 % or at least 95 % of the surface area available.

[0051] Thereby, a LED filament with a more uniform appearance is provided for. The elongated carrier further comprises a first minor surface and a second minor surface opposite to the first minor surface, the first minor surface and the second minor surface connecting the first major surface and the second major surface, and the first minor surface and the second minor surface may be free from the second encapsulant and the third encapsulant.

[0052] The obtained effect is improved manufacturability of the LED filament. The reason is that the first, second and third encapsulant may be provided by dispensing encapsulate material, e.g., a silicone, comprising one or more phosphors. The first encapsulant may fully cover the first major surface of the light- transmissive elongated carrier.

[0053] In this context the term “fully” is intended to mean at least 90 % or at least 95 % of the surface area available.

[0054] The obtained effect is an improved optical performance of the LED filament. The reason is that the new blue light source, that is the first encapsulant, thereby is relatively large.

[0055] The light-transmissive elongated carrier further comprises a first minor surface and a second minor surface opposite to the first minor surface, the first minor surface and the second minor surface connecting the first major surface and the second major surface, and the first minor surface and the second minor surface may be at least partially covered by at least one of the second encapsulant and the third encapsulant.

[0056] The obtained effect is a LED filament with an improved appearance. The reason is that also blue converted light which enters said carrier and exits at said minor surfaces is converted.

[0057] The layer covering the first minor surface and the second minor surface may be a continuous layer consisting of or comprising at least one of the second encapsulant and the third encapsulant.

[0058] In a further embodiment, the LED filament light comprises part of the blue light, the first green-yellow light has a first spectral light distribution having a first full width at half maximum (FWHM1) of at least 80 nm, the second green-yellow light has a second spectral light distribution having a second full width at half maximum (FWHM2) of at least 80 nm, the first red light has a third spectral light distribution having a third full width at half maximum (FWHM3) of less than 40 nm, the second red light has a fourth spectral light distribution having a fourth full width at half maximum (FWHM4) of less than 40 nm, the blue light has a fifth spectral light distribution having a fifth full width at half maximum (FWHM5) of at least 60 nm, and the LED filament light is white light having a correlated color temperature in a range from 1700K to 6500K and a color rendering index of at least 85, preferably at least 90.

[0059] The obtained effect is that the LED filament may thereby emit high quality white light. The reason is the combination of broadband green-yellow light, narrow-band red light and rather broad-band blue light.

[0060] In embodiments, the LED filament light may be white light having a correlated color temperature in a range from 1700 K to 6500 K and / or a CRI of at least 80. Preferably, correlated color temperature in a range from 1700 K to 3500 K and / or a CRI of at least 85.

[0061] The invention further relates to a LED lamp comprising at least one LED filament according to the invention. The lamp may comprise a connector and an envelope. The envelope may at least partly enclose the LED filament (or more LED filaments). The connector may be configured to electrically and mechanically connect to a socket of a luminaire. The luminaire may have a mounting part for mounting the luminaire to a ceiling or a wall.

[0062] More particularly, the invention further relates to a LED filament lamp comprising a connector configured to electrically and mechanically connect the LED filament lamp to a socket of a luminaire, and an envelope at least partly enclosing a LED filament according to the invention.

[0063] The invention still further relates to a luminaire comprising a LED lamp with at least one LED filament according to the invention.

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

[0065] BRIEF DESCRIPTION OF THE DRAWINGS

[0066] 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.

[0067] Fig. 1 shows a schematic transversal cross-sectional view of an embodiment of a LED filament according to the invention.

[0068] Fig. 2 shows a schematic longitudinal cross-sectional view of an embodiment of a LED filament according to Fig. 1.

[0069] Fig. 3 shows a graph depicting intensity as a function of wavelength for an exemplary blue phosphor of a LED filament according to the invention.

[0070] Fig. 4 shows a graph depicting intensity as a function of wavelength for an exemplary green-yellow phosphor of a LED filament according to the invention.

[0071] Fig. 5 shows a graph depicting intensity as a function of wavelength for an exemplary red phosphor of a LED filament according to the invention.

[0072] Fig. 6 shows a cross-sectional side view of a lamp comprising a LED filament according to the invention.

[0073] 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.

[0074] DETAILED DESCRIPTION

[0075] 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.

[0076] Referring first to Figs. 1 and 2, a LED filament 1 according to the invention is illustrated in a transversal cross-sectional view and a longitudinal cross-sectional view respectively.

[0077] Generally, and irrespective of the embodiment, the LED filament 1 is configured to, in operation, emit LED filament light 22 and comprises a light-transmissive elongated carrier 2, a plurality of violet LEDs 7, a first encapsulant 9, a second encapsulant 12, and a third encapsulant 17.

[0078] The light-transmissive elongated carrier 2, and thus the LED filament 1, generally comprises a length direction L, a height direction H extending perpendicular to the length direction L and a width direction W extending perpendicular to both the length direction L and the height direction H. The light-transmissive elongated carrier 2, and thus the LED filament 1, generally extends in the length direction L. The light-transmissive elongated carrier 2 comprises a first major surface 3 and a second major surface 4. The second major surface 4 is arranged opposite to the first major surface 3 as seen in the height direction H. The elongated carrier 2 further comprises a first minor surface 5 and a second minor surface 6. The second minor surface is arranged opposite to the first minor surface 5 as seen in the width direction W. The first minor surface 5 and the second minor surface 6 connect the first major surface 3 and the second major surface 4. The light-transmissive elongated carrier 2 may for instance be a printed circuit board, PCB. The light transmissive elongated carrier 2 may comprise a transmission of at least 80 % in a wavelength range corresponding to that of the blue light 11 emitted by a blue phosphor 10 comprised in the first encapsulant 9.

[0079] The plurality of violet LEDs 7 are configured to emit violet light 8. The violet light 8 may comprise a peak wavelength in a wavelength range of 400 nm to 420 nm. The plurality of violet LEDs 7 are arranged on the first major surface 3 of the elongated carrier 2. The plurality of violet LEDs 7 are generally arranged in one or more rows extending in the length direction L of the elongated carrier - cf. Fig. 2.

[0080] The first encapsulant 9 covers the plurality of violet LEDs 7. The first encapsulant 9 further covers at least part of the first major surface 3 of the elongated carrier 2. Alternatively, the first encapsulant 9 may fully cover the first major surface 3 of the light- transmissive elongated carrier 2. The first encapsulant 9 comprises a blue phosphor 10 (cf. Fig. 2). The blue phosphor 10 is configured to convert at least a part of the violet light 8 to blue light 11. The blue phosphor 10 may further be configured to fully convert the violet light 8 to blue light 11. The first encapsulant 9 has a minimum thickness T1 (cf. Fig. 1) measured perpendicular to the light-transmissive elongated carrier 2 from a light exit surface 23 of the plurality of violet LEDs 7 to the second encapsulant 12.

[0081] By way of example, the blue phosphor 10 may be a Phosphate phosphor, such as a Phosphate phosphor configured to be effectively excited by the violet light emitted by the plurality of violet LEDs and comprising an emission peak wavelength in the wavelength range of 460-480 nm. Fig. 3 shows a graph depicting intensity as a function of wavelength for such a Phosphate phosphor.

[0082] The second encapsulant 12 fully covers the first encapsulant 9. The first minor surface 5 and the second minor surface 6 of the elongated carrier 2 may be free from the second encapsulant 12 (cf. Fig. 1). Alternatively, the first minor surface 5 and the second minor surface 6 of the elongated carrier 2 may be at least partially covered by the second encapsulant 12. The second encapsulant 12 is an elongated encapsulant. The second encapsulant 12 comprises a first green-yellow phosphor 13 and a first red phosphor 14 (cf. Fig. 2). The second encapsulant 12 comprises a second thickness T2 (cf. Fig. 1) measured perpendicular to the light-transmissive elongated carrier 2 at a position of the plurality of violet LEDs 7 from the first encapsulant 9 to a light emitting surface 24 of the second encapsulant 12. The first thickness T1 of the first encapsulant 9 may be chosen to be larger than the second thickness T2 of the second encapsulant 12.

[0083] The first green-yellow phosphor 13 is configured to convert a part of the blue light 11 to first green-yellow light 15. The first green-yellow phosphor 13 may for instance comprise a luminescent material of the type A3Si6Nl 1 :Ce3+, where A comprises one or more of Y, La, Gd, Tb and Lu. The first encapsulant 9 comprises a first concentration Cl of the first green-yellow phosphor 13. By way of example, the first green-yellow phosphor 13 may be an Yttrium Aluminum Garnet phosphor having an absorption peak close to 450 nm and dominant emission wavelength in the wavelength range of 540-560 nm. Fig. 4 shows a graph depicting intensity as a function of wavelength for such an Yttrium Aluminum Garnet phosphor.

[0084] The first red phosphor 14 is configured to convert a part of the blue light 11 to first red light 16. The first red phosphor 14 may be a luminescent material of the type M’xM2-2xAX6 doped with tetravalent manganese, where M’ denotes an alkaline earth cation, M denotes an alkaline cation, and x is in the range of 0-1, where A denotes a tetravalent cation, for instance comprising one or more of silicon and titanium, and where X denotes a monovalent anion, at least comprising fluorine. The first encapsulant 9 comprises a second concentration C2 of the first red phosphor 14. By way of example, the first red phosphor 14 may be a KSF phosphor configured to be excited by blue light with a strongest emission peak wavelength at 631 nm, and a full width at half maximum (FWHM) being smaller than 30 nm. Fig. 5 shows a graph depicting intensity as a function of wavelength for such a KSF phosphor.

[0085] The third encapsulant 17 covers at least a part of the second major surface 4 of the light-transmissive elongated carrier 2. The first minor surface 5 and the second minor surface 6 of the elongated carrier 2 may be free from the third encapsulant 17 (cf. Fig. 1). Alternatively, the first minor surface 5 and the second minor surface 6 of the elongated carrier 2 may be at least partially covered by the third encapsulant 17. The third encapsulant 17 is an elongated encapsulant. The third encapsulant 17 comprises a second green-yellow phosphor 18 and a second red phosphor 19 (cf. Fig. 2). The third encapsulant 17 comprises a third thickness T3 (cf. Fig. 1) measured perpendicular to the light-transmissive elongated carrier 2 at a position of the plurality of violet LEDs 7 from the second major surface 4 to a light emitting surface 25 of the third encapsulant 17. The first thickness Tl, the second thickness T2 and the third thickness T3 may be chosen such that the relation 1.3 > T3 / (T1+T2) > 0.7 is fulfilled.

[0086] The second green-yellow phosphor 18 is configured to convert a part of the blue light 11 to second green-yellow light 20. The second green-yellow phosphor 18 may be different from the first green-yellow phosphor 13. Alternatively, the second green-yellow phosphor 18 may be the same as the first green-yellow phosphor 13. The second greenyellow phosphor 18 may for instance comprise a luminescent material of the type A3Si6Nl 1 :Ce3+, where A comprises one or more of Y, La, Gd, Tb and Lu. The second encapsulant 12 comprises a third concentration C3 of the second green-yellow phosphor 18. By way of example, the second green-yellow phosphor 18 may be an Yttrium Aluminum Garnet phosphor having an absorption peak close to 450 nm and dominant emission wavelength in the wavelength range of 540-560 nm. Fig. 4 shows a graph depicting intensity as a function of wavelength for such an Yttrium Aluminum Garnet phosphor.

[0087] The second red phosphor 19 is configured to convert a part of the blue light 11 to second red light 21. The second red phosphor 19 may be different from the first red phosphor 14. Alternatively, the second red phosphor 19 may be the same as the first red phosphor 14. The second red phosphor 19 may be a luminescent material of the type M’xM2- 2xAX6 doped with tetravalent manganese, where M’ denotes an alkaline earth cation, M denotes an alkaline cation, and x is in the range of 0-1, where A denotes a tetravalent cation, for instance comprising one or more of silicon and titanium, and where X denotes a monovalent anion, at least comprising fluorine. By way of example, the first red phosphor 14 may be a KSF phosphor configured to be excited by blue light with a strongest emission peak wavelength at 631 nm, and a full width at half maximum (FWHM) being smaller than 30 nm. Fig. 5 shows a graph depicting intensity as a function of wavelength for such a KSF phosphor. The second encapsulant 12 may comprise a fourth concentration C4 of the second red phosphor 19. The first concentration Cl, the second concentration C2, the third concentration C3 and the fourth concentration C4 may be chosen such that the relation (C1+C2) / (C3+C4) > 1.5 is fulfilled.

[0088] The LED filament light 22 comprises or is a combination of the first greenyellow light 15, the first red light 16, the second green-yellow light 20, and the second red light 21. Furthermore, the LED filament light 22 may comprise a part of the blue light 11. The first green-yellow light 15 may have a first spectral light distribution having a first full width at half maximum, FWHM1, of at least 80nm. The second green-yellow light 20 may have a second spectral light distribution having a second full width at half maximum, FWHM2, of at least 80nm. The first red light 16 may have a third spectral light distribution having a third full width at half maximum, FWHM3, of less than 40nm. The second red light 21 may have a fourth spectral light distribution having a fourth full width at half maximum, FWHM4, of less than 40nm. The blue light may have a fifth spectral light distribution having a fifth full width at half maximum, FWHM5, of at least 60nm. The LED filament light 22 may be white light having a correlated color temperature in a range from 1700K to 6500K and a color rendering index of at least 85 or at least 90.

[0089] Finally, Fig. 6 shows an exemplary lamp 40 comprising a LED filament 1 according to any embodiment of the invention. In the embodiment shown, the LED filament 1 is a substantially straight LED filament. The LED filament 1 of such a lamp may in other embodiments be a LED filament with another shape, such as, but not limited to, spiralshaped, helix-shaped, meandering, twisted, flat and combinations thereof.

[0090] The lamp 40 further comprises a driver or controller 41 configured for controlling the LEDs 3 of the LED filament 1. The controller 41 is configured to power the plurality of violet LEDs 7 via electrical circuitry (not visible on the figures) of the LED filament 1. The controller 41 may further be configured for controlling at least one of the CCT of the LED filament light 22 and the CRI of the LED filament light 22. The controller 41 may also be configured for controlling other parameters related to the violet LEDs 7 and the LED filament light 22.

[0091] The lamp 40 further comprises an envelope 42 at least partially enveloping the at least one LED filament 1. The lamp 40 further comprises a cap 43. As shown in Fig. 6, the controller 41 is arranged within the envelope 42. When comprising a cap 43, the controller 41 may also be arranged inside the cap 43 such that it is hidden from view. The lamp 40 further comprises threading 44 for connection to a socket, and a terminal 45 for connection to a source of electrical energy.

[0092] The envelope 42 of the lamp 40 may further and optionally be provided with a coating (not shown), such as a reflective coating, covering at least a part of the envelope 42.

[0093] 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.

[0094] 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 LED filament (1) configured to, in operation, emit LED filament light (22), the LED filament comprising: a light-transmissive elongated carrier (2) comprising a first major surface (3) and a second major surface (4) opposite to the first major surface, a plurality of violet LEDs (7) configured to, in operation, emit violet light (8), wherein the violet light (8) emitted by the plurality of violet LEDs has a peak wavelength in a wavelength range of 380 nm to 420 nm, the plurality of violet LEDs being arranged on the first major surface, a first encapsulant (9) covering the plurality of violet LEDs (7) and at least part of the first major surface, the first encapsulant comprising a blue phosphor (10) configured to convert at least a part of the violet light to blue light (11), a second encapsulant (12) fully covering the first encapsulant (9), the second encapsulant being an elongated encapsulant comprising a first green-yellow phosphor (13) and a first red phosphor (14), the first green-yellow phosphor being configured to convert a part of the blue light to first green-yellow light (15) and the first red phosphor being configured to convert a part of the blue light to first red light (16), and a third encapsulant (17) covering at least a part of the second major surface (4) of the light-transmissive elongated carrier, the third encapsulant being an elongated encapsulant comprising a second green-yellow phosphor (18) and a second red phosphor (19), the second green-yellow phosphor being configured to convert a part of the blue light to second green-yellow light (20) and the second red phosphor being configured to convert a part of the blue light to second red light (21), wherein the LED filament light (22) comprises the first green-yellow light (15), the first red light (16), the second green-yellow light (20), and the second red light (21).

2. A LED filament according to claim 1, wherein the blue phosphor is configured to fully convert the violet light to blue light.

3. A LED filament according to any one of the preceding claims, wherein the violet light (8) emitted by the plurality of violet LEDs has a peak wavelength in a wavelength range of 400 nm to 420 nm.

4. A LED filament according to any one of the preceding claims, wherein the blue phosphor (10) is a Phosphate phosphor configured to be effectively excited by the violet light emitted by the plurality of violet LEDs and comprising an emission peak wavelength in the wavelength range of 460-480 nm.

5. A LED filament according to any one of the preceding claims, wherein one or both of the following applies: the first green-yellow phosphor (13) and the second green-yellow phosphor (18) are mutually different, and the first red phosphor (14) and the second red phosphor (19) are mutually different.

6. A LED filament according to any one of the preceding claims, wherein: the first and / or second green-yellow phosphor comprises a luminescent material of the type A3 Si6N 11 :Ce3+, wherein A comprises one or more of Y, La, Gd, Tb and Lu; and the first and / or second red phosphor is a luminescent material of the type M’xM2-2xAX6 doped with tetravalent manganese, wherein M’ comprises an alkaline earth cation, M comprises an alkaline cation, and x is in the range of 0-1, wherein A comprises a tetravalent cation, for instance comprising one or more of silicon and titanium, and wherein X comprises a monovalent anion, at least comprising fluorine.

7. A LED filament according to any one of the preceding claims, wherein the light transmissive elongated carrier has a transmission of at least 80 % in a wavelength range corresponding to that of the blue light (11) emitted by the blue phosphor of the first encapsulant.

8. A LED filament according to any one of the preceding claims, wherein the first encapsulant (9) comprises a minimum thickness (Tl) measured perpendicular to the light-transmissive elongated carrier (2) from a light exit surface (23) of the plurality of violetLEDs (7) to the second encapsulant, wherein the second encapsulant (12) comprises a second thickness (T2) measured perpendicular to the light-transmissive elongated carrier (2) at a position of the plurality of violet LEDs from the first encapsulant to a light emitting surface (24) of the second encapsulant, and wherein T1 > T2.

9. A LED filament according to any one of the preceding claims, wherein the first green-yellow phosphor has a first concentration (Cl) in the first encapsulant (9), the first red phosphor has a second concentration (C2) in the first encapsulant (9), the second greenyellow phosphor has a third concentration (C3) in the second encapsulant (12), the second red phosphor has a fourth concentration (C4) in the second encapsulant (12), wherein (C1+C2) / (C3+C4) > 1.5.

10. A LED filament according to any one of the preceding claims, wherein the first encapsulant (9) has a minimum thickness (Tl) measured perpendicular to the light- transmissive elongated carrier (2) from a light exit surface (23) of the plurality of violet LEDs (7) to the second encapsulant, the second encapsulant (12) has a second thickness (T2) measured perpendicular to the light-transmissive elongated carrier (2) at a position of the plurality of violet LEDs (7) from the first encapsulant (9) to a light emitting surface (24) of the second encapsulant, the third encapsulant (17) has a third thickness (T3) measured perpendicular to the light-transmissive elongated carrier (2) at a position of the plurality of violet LEDs (7) from the second major surface (4) to a light emitting surface (25) of the third encapsulant, wherein 1.3 > T3 / (T1+T2) > 0.7.

11. A LED filament according to any one of the preceding claims, wherein the elongated carrier (2) further comprises a first minor surface (5) and a second minor surface (6) opposite to the first minor surface, the first minor surface and the second minor surface connecting the first major surface and the second major surface, and wherein the first minor surface (5) and the second minor surface (6) are free from the second encapsulant (12) and the third encapsulant (17).

12. A LED filament according to any one of the preceding claims 1-9, wherein the first encapsulant (9) fully covers the first major surface of the light-transmissive elongated earner.

13. A LED filament according to any one of the preceding claims 1-9 and 11, wherein the light-transmissive elongated carrier (2) further comprises a first minor surface (5) and a second minor surface (6) opposite to the first minor surface, the first minor surface and the second minor surface connecting the first major surface and the second major surface, and wherein the first minor surface (5) and the second minor surface (6) are at least partially covered by at least one of the second encapsulant (12) and the third encapsulant (17).

14. A LED filament according to any one of the preceding claims, wherein: the LED filament light (22) comprises part of the blue light; the first green-yellow light has a first spectral light distribution having a first full width at half maximum (FWHM1) of at least 80nm, the second green-yellow light has a second spectral light distribution having a second full width at half maximum (FWHM2) of at least 80nm; the first red light has a third spectral light distribution having a third full width at half maximum (FWHM3) of less than 40nm, the second red light has a fourth spectral light distribution having a fourth full width at half maximum (FWHM4) of less than 40nm, the blue light has a fifth spectral light distribution having a fifth full width at half maximum (FWHM5) of at least 60nm ; and the LED filament light (22) is white light having a correlated color temperature in a range from 1700K to 6500K and a color rendering index of at least 85, preferably at least 90.

15. A LED filament lamp comprising a connector configured to electrically and mechanically connect the LED filament lamp to a socket of a luminaire, and an envelope at least partly enclosing a LED filament (1) according to any one of the preceding claims.

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