LED filament comprising encapsulants

Abstract

A light emitting diode, LED, filament (100) configured to provide LED filament light (110) is provided. The LED filament comprises an elongated carrier (120) and LEDs (130) arranged on the elongated carrier, wherein the LEDs are configured to emit LED light (150). The LED filament further comprises an elongated luminescent encapsulant (200) at least partially covering the elongated carrier and at least partially enclosing the LEDs. The LED filament further comprises an elongated white encapsulant (300) comprising a polymer matrix (310) and particles (320) dispersed in the polymer matrix, wherein the polymer matrix has a refractive index, npm, and the particles have a refractive index, npa, wherein |npa – npm| is in a range of 0.03 – 0.15, wherein the polymer matrix has a reflectivity, R, in a range of 12 – 30 %, and wherein the particles have a particle diameter, D, in a range of 1 – 5 μm.

Description

[0001] LED filament comprising encapsulants

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to a light emitting diode, LED, filament. More specifically, the present invention relates to a LED filament comprising a coating.

[0004] BACKGROUND OF THE INVENTION

[0005] The use of light emitting diodes, LEDs, for illumination purposes continues to attract attention. Compared to incandescent lamps, fluorescent lamps, neon tube lamps, etc., LEDs provide numerous advantages such as a longer operational life, a reduced power consumption, and an increased efficiency related to the ratio between light energy and heat energy.

[0006] Due to the advantageous aspects of the use of LEDs, the interest has rapidly increased to replace conventional light sources with LEDs in many lighting arrangements. It will be appreciated that this replacement, also called retrofitting, is appreciated and desired by users who wish to have the look of an incandescent bulb. The light source replacement (retrofitting) is often performed by removing the conventional light source(s) from the luminaire (e.g. a lamp holder) of the lighting arrangement and attaching the LEDs, LED arrangement(s) or LED device(s) into the luminaire. One of these concepts is based on LED filaments which are placed in a bulb. The visible LED filament(s) may provide a light distribution which is effective and decorative at the same time, whilst taking advantage of LED technology.

[0007] It is desired to even further improve the performance, functionality and / or appearance of LED filaments and / or LED filament lamps. In particular, it is desirable to hide the phosphor color of LED filaments, and prior art discloses LED filaments with a coating in order to try to achieve this. However, these suggested coatings have a significantly negative impact on the LED filament efficiency.

[0008] Based on the above, there is a wish to provide an efficient hiding effect of the LED filament phosphor color whilst still providing an efficient LED filament lighting during operation, and an improved performance and / or functionality of the light emission and / or light distribution from the LED filaments. Another purpose is to augment the appearance and / or the decorative aspect of the LED filaments and / or the LED filament lamps, both during operation (on-state) as well as when switched off (off-state).

[0009] US 2019 / 128481 discloses an overcoated LED filament. The LED filament comprises 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.

[0010] US 2022 / 349530 discloses a LED filament lamp which provides LED filament lamp light. The at least one LED filament provides LED filament light and comprises an array of a plurality of LEDs and an encapsulant at least partially enclosing the plurality of LEDs. The encapsulant comprises a light scattering material. In the direction from a first end to a second end of the LED filament, at least one of (i) the thickness (TL) of the encapsulant, and (ii) the concentration (CL) of the light scattering material in the encapsulant, along the LED filament length (L) increases over at least two adjacent first LEDs and decreases over at least two adjacent second LEDs different from the at least two adjacent first LEDs, and wherein the thickness (TL) and / or concentration (CL) firstly increases and secondly decreases at least along a portion of the LED filament length (L).

[0011] SUMMARY OF THE INVENTION

[0012] It is of interest to explore the possibility of combining one or more of the numerous advantages of LED filaments comprising LEDs, whilst improving the LED filaments’ performance and / or functionality, via the properties of the light emission and / or distribution from the LED filaments, and improving the appearance and / or the decorative aspect of the LED filaments and / or the LED filament lamps during on-state and off-state.

[0013] This and other objects are achieved by providing a LED filament having the features in the independent claim. Preferred embodiments are defined in the dependent claims.

[0014] According to the present invention, there is provided a light emitting diode, LED, filament configured to provide, in an on-state, LED filament light. The LED filament comprises an elongated carrier, a plurality of light emitting diodes, LEDs, arranged on a first major surface of the elongated carrier, wherein the plurality of LEDs is configured to emit LED light. The LED filament further comprises an elongated luminescent encapsulant at least partially covering the first major surface of the elongated carrier and at least partially enclosing the plurality of LEDs, wherein the elongated luminescent encapsulant comprises a luminescent material configured to at least partly convert the emitted LED light into converted light. The LED filament further comprises an elongated white encapsulant at least partially enclosing the elongated luminescent encapsulant, wherein the elongated white encapsulant comprises a light-transmissive polymer matrix, and particles dispersed in the polymer matrix. The polymer matrix has a refractive index, npm, and the particles have a refractive index, npa, wherein |npa- npm| is in a range of 0.03 - 0.21, preferably in a range from 0.03 - 0.15, more preferably in a range from 0.03 - 0.14 or most preferably in a range from 0.03 - 0.13, the polymer matrix has a reflectivity, R, in a range of 12 - 30 %, and the particles have a particle diameter, D, in a range of 1 - 5 pm.

[0015] Thus, the present invention is based on the idea of providing a LED filament comprising an elongated white encapsulant which at least partially covers (hides) the elongated luminescent encapsulant whilst providing a relatively low back reflection for a high efficiency. More specifically, by the features of the light-transmissive polymer matrix and the particles dispersed therein of the elongated white encapsulant, the LED filament achieves a significant hiding effect of the phosphor color of the elongated luminescent encapsulant whilst providing a high efficiency of the LED filament during operation. Hence, the LED filament provides an improved performance and / or functionality, as well as an improved appearance and / or decorative aspect thereof.

[0016] The present invention is advantageous in that the difference between the refractive index, npa, of the particles and the refractive index, npm, of the polymer matrix, |npa- npm|, is in the (very low) range of 0.03 - 0.15, thereby achieving much forward scattering of the converted light and optionally part of the LED light. Consequently, the relatively large level of forward scattering leads to a significantly improved efficiency compared to a highly reflective coating as has been implemented in the prior art. The prior art suggests a highly reflective white coating in order to try to hide the phosphor color of a LED filament, but suffers greatly by its low efficiency because of high back reflection. In sharp contrast, the present invention provides an elongated white encapsulant with a polymer matrix and particles therein, providing much forward scattering of the converted light and optionally part of the LED light, and consequently, a very high efficiency of the LED filament during operation.

[0017] The present invention is further advantageous in that the polymer matrix of the elongated white encapsulant has a reflectivity, R, in a range of 12 - 30 %, thereby achieving an efficient hiding effect of the phosphor color of the LED filament whilst providing a high efficiency of the emission of LED filament light. The thickness of the elongated white encapsulant and / or the concentration of particles in the polymer matrix thereof may be tuned conveniently to achieve the mentioned reflectivity, R.

[0018] The present invention is further advantageous by the diameter, D, of the particles dispersed in the polymer matrix of the elongated white encapsulant being in the range of 1 - 5 pm. As the particle diameter, D, is much larger than the wavelength of visible light, the LED filament does not show any wavelength dependent scattering.

[0019] It will be appreciated that the elongated luminescent encapsulant of the LED filament may have a yellow-orange color which, by the construction and / or features of the LED filament, may be hidden by the elongated white encapsulant. This efficiently and conveniently augments the appearance and decorative aspect of the LED filament.

[0020] The present invention is further advantageous in that one or more properties of the elongated white encapsulant may be easily and conveniently customized for desired properties of the light distribution from, and / or aesthetic appearance of, the LED filament. More specifically, one or more properties of the elongated white encapsulant may be set or tuned for optimizing the hiding of the yellow-orange color of the elongated luminescent encapsulant to provide an aesthetically attractive LED filament look and to provide a desired LED filament light distribution.

[0021] The present invention is further advantageous in that the elongated white encapsulant has a white color in an off-state of the LED filament, rendering the LED filament attractive for an observer.

[0022] The present invention is further advantageous in that the elongated white encapsulant may improve the heat management of the LED filament during operation.

[0023] The present invention is further advantageous in that the numerous advantages of using LED technology may be combined with the attractiveness and the appealing properties of the LED filament arrangement as disclosed.

[0024] The present invention is further advantageous in that the LED filament of the present invention comprises relatively few components. The low number of components is advantageous in that the LED filament is relatively inexpensive to fabricate. Moreover, the low number of components of the LED filament implies an easier recycling, especially compared to devices or arrangements comprising a relatively high number of components which impede an easy disassembling and / or recycling operation.

[0025] There is provided a light emitting diode, LED, filament configured to provide, in an on-state, LED filament light. Preferably, the LED filament has a length, LF, and a width, WF, wherein LF > 5WF. 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. The LED filament may extend along a length axis, LX. The LED filament comprises an elongated carrier and a plurality of light emitting diodes, LEDs, arranged on a first major surface of the elongated carrier, wherein the plurality of LEDs is configured to emit LED light. The elongated carrier may, for instance, be a substrate, 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). The elongated carrier may be reflective or light transmissive, such as translucent and preferably transparent. The plurality of LEDs may be arranged on the elongated carrier in a linear array. By the term “array”, it is here meant a linear arrangement or chain of LEDs, or the like. The LED filament further comprises an elongated luminescent encapsulant at least partially covering the first major surface of the elongated carrier and at least partially enclosing the plurality of LEDs, wherein the elongated luminescent encapsulant comprises a luminescent material configured to at least partly convert the emitted LED light into converted light. By the term “encapsulant”, it is here meant an elongated material, element, arrangement, or the like, which in the present context is configured or arranged to at least partially cover, surround, encapsulate and / or enclose the elongated carrier (and in the case of the elongated luminescent encapsulant, also the plurality of LEDs). The elongated luminescent encapsulant may be a polymer material which may be flexible such as for example a silicone. The luminescent material of the elongated luminescent encapsulant may be a phosphor such as an inorganic phosphor and / or quantum dots or rods. The LED filament further comprises an elongated white encapsulant at least partially enclosing the elongated luminescent encapsulant. Hence, the elongated white encapsulant at least partially covers, surrounds, encapsulates and / or encloses the elongated luminescent encapsulant. The elongated white encapsulant comprises a light-transmissive polymer matrix and particles dispersed in the polymer matrix. The particles may, for example, be defined as light-directing particles, as the particles may be arranged to direct impingent light. The polymer matrix has a refractive index, npm, and the particles have a refractive index, npa, wherein (a difference) An = |npa- npm| is in a range of 0.03 - 0.15, preferably 0.1 - 0.15. Furthermore, the polymer matrix has a reflectivity, R, in a range of 12 - 30 %, preferably at least 15 % and / or at most 28 %. Furthermore, the particles have a particle diameter, D, in a range of 1 - 5 pm, preferably in a range of 2 - 5 pm.

[0026] According to an embodiment of the present invention, the LED filament light may comprise the converted light and part of the LED light, wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1700 - 6500 K and e.g. a color rendering index, CRI, of least 80, and wherein the elongated white encapsulant has, in an off-state of the LED filament, a white appearance. Hence, the elongated white encapsulant renders the LED filament white (for an observer of the LED filament), when the LED filament is in an off-state (i.e. switched off). The obtained effect is a visually attractive off-state white appearance of the LED filament, while the LED filament further provides an improved lighting performance. Preferably, the LED filament light may be white light having a correlated color temperature, CCT, in a range of 1700 - 3500 K. The reason is that this type of white light is desired for LED filaments and in order to obtain this type of white light the elongated luminescent encapsulant has an orange color which has an especially undesired appearance (with respect to more yellowish elongated luminescent encapsulant which provide higher correlated color temperatures).

[0027] According to an embodiment of the present invention, the concentration of particles may be in a range of 5- 104- 4.9- 106particles / mm3. The present embodiment is advantageous in that the mentioned concentration of particles dispersed in the polymer matrix of the elongated white encapsulant achieves a high level of forward scattering, and thereby enhances the efficiency of the LED filament, whilst attaining an efficient hiding effect of the phosphor color of the elongated luminescent encapsulant of the LED filament. In other words, the present embodiment provides an optimization of a mean free path for best hiding effect. It should be noted that the concentration of particles may be in a relatively low (sub) interval of the range of 5- 104- 4.9- 106particles / mm3in case of relatively large particles, whereas the concentration of particles may be in a relatively high (sub) interval of the mentioned range in case of relatively small particles.

[0028] According to an embodiment of the present invention, the concentration of the particles may be in a range of 0.03 - 5 v / v%. The present embodiment is advantageous in that the mentioned volume percentage or concentration of particles dispersed in the polymer matrix of the elongated white encapsulant achieves a high level of forward scattering, and thereby enhancing the efficiency of the LED filament, whilst attaining an efficient hiding effect of the phosphor color of the elongated luminescent encapsulant of the LED filament.

[0029] According to an example of the present invention, the LED filament may comprise a first concentration of particles in a range of 1 - 4.9- 106particles / mm3with a particle diameter, D, in a range of 1 - 3 pm, such as 2 - 3 pm, and a second concentration of particles < 6.0 - 104particles / mm3with a particle diameter, D, in a range of 3 - 5 pm, such as According to an example of the present invention, the reflectivity, R, of the polymer matrix may be determined based on a thickness, T, of the elongated white encapsulant, and a concentration of particles dispersed in the polymer matrix.

[0030] According to an embodiment of the present invention, a thickness, T, of the elongated white encapsulant may be in a range of 0.25 - 1.5 mm, preferably 0.5 - 1 mm. The present embodiment is advantageous in that this relatively thin elongated white encapsulant provides a slim LED filament look and a white appearance of the LED filament, which is aesthetically appealing, whilst the LED filament at the same time achieves a high efficiency and a significant hiding effect of the elongated luminescent encapsulant.

[0031] According to an embodiment of the present invention, the polymer matrix may comprise a crosslinked silicone selected from the group consisting of poly dimethylsiloxane, PDMS, polymethylphenylsiloxane, PMPS, and poly diphenylsiloxane, PDPS. The present embodiment is advantageous in that these mentioned materials are optimal concerning the ability to withstand high (levels of) flux of the emitted LED light. Hence, the selection of this (these) material(s) in the polymer matrix increases the service life of the LED filament and / or promotes the operation of the LED filament, thereby even further increasing the reliability of the LED filament.

[0032] According to an embodiment of the present invention, the particles are transparent and may comprise transparent beads, e.g. transparent organic beads. The present embodiment is advantageous in that the transparent beads provide higher efficiency because of less back scattering.

[0033] According to an example of the present invention, the light-transmissive polymer matrix may be transparent. The present example is advantageous in that the transparent polymer matrix provides high light transmission.

[0034] According to an embodiment of the present invention, the particles may comprise cross-linked silicone beads. The present embodiment is advantageous in that the silicone beads augment the robustness and the flexibility of the polymer matrix of the elongated white encapsulant to an even further extent, thereby increasing the LED filament’s reliability during operation. For example, the cross-linked silicone beads may comprise crosslinked polydemethyl siloxane (PDMS), crosslinked poly dephenyl siloxane (PDPS) and / or crosslinked polymethylphenyl siloxane (PMPS).

[0035] According to an embodiment of the present invention, the elongated carrier may be light-transmissive and the elongated luminescent encapsulant may at least partially cover a second major surface, opposite the first major surface, of the elongated carrier. The present embodiment is advantageous in that a substantially omnidirectional light distribution from the LED filament may be obtained, whereby this optimal light distribution even further contributes to the optical performance and / or aesthetical attractiveness of the LED filament.

[0036] According to an embodiment of the present invention, the elongated white encapsulant may comprise a first elongated white encapsulant portion and a second elongated white encapsulant portion, wherein the elongated luminescent encapsulant may comprise a first elongated luminescent encapsulant portion which at least partly covers the first major surface and a second elongated luminescent encapsulant portion which at least partly covers the second major surface, the first elongated white encapsulant portion enclosing the first elongated luminescent encapsulant portion and the second elongated white encapsulant portion enclosing the second elongated luminescent encapsulant portion. The present embodiment is advantageous in that the first elongated white encapsulant portion efficiently covers or encloses the first elongated luminescent encapsulant portion and that the second elongated white encapsulant portion efficiently covers or encloses the second elongated luminescent encapsulant portion, thereby conveniently hiding the phosphor color of the elongated luminescent encapsulant. The present embodiment is advantageous in that the first and second elongated white encapsulant portions may be optimized (e.g. differently) with respect to the (e.g. different properties of the) first and second elongated luminescent encapsulant portions.

[0037] According to an embodiment of the present invention, at least one of a concentration, CM2, of the luminescent material in the second elongated luminescent encapsulant portion is lower than a concentration, CMI, of the luminescent material in the first elongated luminescent encapsulant portion, and a thickness, TE2, of the second elongated luminescent encapsulant portion is smaller than a thickness, TEI, of the first elongated luminescent encapsulant portion, may be fulfilled. Hence, the concentration, CM2, of the luminescent material in the second elongated luminescent encapsulant portion may be lower than the concentration, CMI, of the luminescent material in the first elongated luminescent encapsulant portion and / or the thickness, TE2, of the second elongated luminescent encapsulant portion may be smaller than the thickness, TEI, of the first elongated luminescent encapsulant portion, i.e. CM2 < CMI and / or TE2 < TEI. The present embodiment is advantageous by its customization of the LED filament concerning luminescent material concentration and / or thickness of the (first / second) elongated luminescent encapsulant portions dependently on orientation of the LED filament and / or luminous intensity of the LED filament light. For example, in case of a higher level of luminous intensity of the emitted LED light from the first major surface of the LED filament compared to the second major surface of the LED filament during operation, the relatively high luminescent material concentration and / or thickness associated with the first elongated luminescent encapsulant portion compared to the second elongated luminescent encapsulant portion achieves an efficient omnidirectional hiding effect of the LED filament phosphor color whilst still providing an efficient omnidirectional LED filament lighting during operation.

[0038] According to an embodiment of the present invention, at least one of a concentration, CP2, of the particles in the second elongated white encapsulant portion is lower than a concentration, CPI, of the particles in the first elongated white encapsulant portion, and a thickness, TWE2, of the second elongated white encapsulant portion is smaller than a thickness, T EI, of the first elongated white encapsulant portion, may be fulfilled. Hence, the concentration, CP2, of the particles in the second elongated white encapsulant portion may be lower than a concentration, CPI, of the particles in the first elongated white encapsulant portion and / or the thickness, TWE2, of the second elongated white encapsulant portion may be smaller than the thickness, TWEI, of the first elongated white encapsulant portion, i.e. CP2 < CPI and / or TWE2 < TWEI. The present embodiment is advantageous by its customization of the LED filament concerning particle concentration and / or thickness of the (first / second) elongated white encapsulant portions dependently on orientation of the LED filament and / or luminous intensity of the LED filament light. For example, in case of a higher level of luminous intensity of the emitted LED light from the first major surface of the LED filament compared to the second major surface of the LED filament during operation, the relatively high particle concentration and / or thickness associated with the first elongated white encapsulant portion compared to the second elongated white encapsulant portion achieves an efficient omnidirectional hiding effect of the LED filament phosphor color whilst still providing an efficient omnidirectional LED filament lighting during operation.

[0039] According to an embodiment of the present invention, the elongated white encapsulant may fulfil one of enclosing the elongated luminescent encapsulant to at least 90%, and fully enclosing the elongated luminescent encapsulant. Hence, the elongated white encapsulant may enclose the elongated luminescent encapsulant to at least 90% or fully enclose the elongated luminescent encapsulant. The present embodiment is advantageous in that the elongated white encapsulant hereby fully, or almost fully, encloses or covers the elongated luminescent encapsulant, thereby minimizing the exposure of the (phosphor color of) the elongated luminescent encapsulant of the LED filament, leading to an even more appealing LED filament. According to an embodiment of the present invention, there is provided a LED filament arrangement comprising at least one LED filament according to any one of the preceding embodiments. The LED filament arrangement may further comprise a controller coupled to the at least one LED filament, wherein the controller is configured to control the LED light emitted from the plurality of LEDs. By “controller”, it is here meant any device, unit, or the like, which is able to control the LED light either by wire or via wireless technology. In case the LED filament arrangement comprises a plurality of LED filaments, the controller may control each LED filament individually. The present embodiment is advantageous in that the controller may conveniently and efficiently control the emitted LED light, thereby even further ameliorating the light emission from the LED filament(s) and improving the appearance and / or the decorative aspect of the LED filament(s) and / or LED filament arrangement.

[0040] According to an embodiment of the present invention, there is provided a LED filament lamp, comprising one of at least one LED filament according to any one of the preceding embodiments, and the LED filament arrangement according to the preceding embodiment. The LED filament lamp further comprises a light-transmissive envelope (or cover) at least partly enclosing the at least one LED filament, and a connector for electrically and mechanically connecting the LED filament lamp to a socket of a luminaire. Hence, the LED filament lamp comprises either one or more LED filaments or a LED filament arrangement, which in turn comprises one or more LED filaments. The present embodiment is advantageous in that the LED filament lamp, comprising the LED filament(s), combines the aspects of a desired light emission and aesthetical appearance provided via the LED filament arrangement and / or via the feature(s) of the lamp.

[0041] Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art will realize that different features of the present invention can be combined to create embodiments other than those described in the following.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] 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.

[0044] Fig. 1 shows a LED filament lamp according to the prior art, Figs. 2a and 2b schematically show a LED filament arrangement according to an exemplifying embodiment of the present invention,

[0045] Fig. 3 schematically shows a LED filament arrangement comprising at least one LED filament according to an exemplifying embodiment of the present invention,

[0046] Fig. 4 schematically shows a portion of the LED filament arrangement according to an exemplifying embodiment of the present invention,

[0047] Fig. 5 schematically shows a simulation diagram of a LED filament according to an exemplifying embodiment of the present invention, and

[0048] Fig. 6 shows a LED filament lamp comprising a LED filament according to an exemplifying embodiment of the present invention.

[0049] DETAILED DESCRIPTION

[0050] Fig. 1 shows a LED filament lamp 10 according to the prior art, comprising a plurality of LED filaments 20. LED filament lamps 10 of this kind are highly appreciated as they are very decorative, as well as providing numerous advantages compared to incandescent lamps such as a longer operational life, a reduced power consumption, and an increased efficiency related to the ratio between light energy and heat energy. However, it is of interest to improve the properties of the light distribution emitted from the LED filaments 20, and to even further augment the decorative appearance and / or aspect of the LED filaments 20 and / or the LED filament lamps 10.

[0051] Fig. 2a schematically shows a LED filament 100 according to an exemplifying embodiment of the present invention. The LED filament 100 is configured to provide (emit) LED filament light 110 in an on-state of the LED filament 100. The LED filament 100 extends along a length axis, LX, and comprises an elongated carrier 120. An array of a plurality of LEDs 130 is arranged on a first major surface 145 of the elongated carrier 120. It should be noted that the LED filament 100 may comprise substantially any number of array(s) of the plurality of LEDs 130. The plurality of LEDs 130 preferably comprises more than 5 LEDs, more preferably more than 8 LEDs, and even more preferred more than 10 LEDs. The plurality of LEDs 130 may be direct emitting LEDs which provide a color. The plurality of LEDs 130 is configured to emit LED light. The LED filament 100 further comprises an elongated luminescent encapsulant 200 at least partially covering the first major surface 145 of the elongated carrier 120 and at least partially enclosing the plurality of LEDs 130. The elongated luminescent encapsulant 200 comprises a luminescent material configured to at least partly convert the emitted LED light into first converted light. The luminescent material of the elongated luminescent encapsulant 200 may be a light-scattering material, e.g. a polymer matrix comprising BaSO4, AI2O3 and / or TiCh particles. The luminescent material of the elongated luminescent encapsulant 200 may be a phosphor such as an inorganic phosphor (e.g. YAG, LuAG, ECAS, KSiF, etc.) and / or quantum dots or rods. The phosphor may further be e.g. a (blue) green / yellow and / or red phosphor. Although not shown, a concentration of the luminescent material in the elongated luminescent encapsulant 200 may vary over the length of the LED filament 100. The LED filament 100 further comprises an elongated white encapsulant 300 at least partially enclosing the elongated luminescent encapsulant 200, wherein the elongated white encapsulant 300 comprises a light- transmissive polymer matrix and particles dispersed in the polymer matrix.

[0052] Fig. 2b schematically shows a cross-section of a LED filament 100, corresponding to the LED filament 100 of Fig. 2a, and according to an exemplifying embodiment of the present invention. It is furthermore referred to Fig. 2a and the associated text for features and / or functions of the LED filament 100. In Fig. 2b, an array of a plurality of LEDs 130 is arranged on an elongated carrier 120, and is arranged to emit LED light 150. The LED filament 100 further comprises an elongated luminescent encapsulant 200 at least partially covering the first major surface of the elongated carrier 120 and at least partially enclosing the plurality of LEDs 130. The elongated luminescent encapsulant 200 comprises a luminescent material configured to at least partly convert the emitted LED light 150 into converted light 210. The LED filament light 110 may comprise the converted light 210 and part of the LED light 150, wherein the LED filament light 110 may be white light having a correlated color temperature, CCT, in a range of 1700 - 6500 K and a color rendering index, CRI, of least 80. The light-transmissive polymer matrix 310 of the elongated white encapsulant 300, wherein the elongated white encapsulant 300 encloses the elongated luminescent encapsulant 200, has a refractive index, npm. The particles 320 dispersed in the polymer matrix 310 have a refractive index, npa, wherein |npa- npm| is in a range of 0.03 - 0.15, preferably 0.1 - 0.15. Furthermore, the polymer matrix 310 of the elongated white encapsulant 300 has a reflectivity, R, in a range of 12 - 30 %, preferably at least 15 % and / or at most 28 %. The polymer matrix 310 may comprises a crosslinked silicone such as poly dimethylsiloxane, PDMS, polymethylphenylsiloxane, PMPS, and poly diphenylsiloxane, PDPS. A thickness, T, of the elongated white encapsulant 300 may be in a range of 0.5 - 1 mm. The particles 320 of the polymer matrix 310 have a particle diameter, D, in a range of 1 - 5 pm, preferably in a range of 2 - 5 pm. The concentration of the particles 320 in the polymer matrix 310 may be in a range of 5- 104- 4.9- 106particles / mm3. Alternatively, or in addition to this, the concentration of the particles 320 in the polymer matrix 310, expressed in percent of volume of the polymer matrix 310, may be in a range of 0.03 - 5 v / v%. The particles 320 are transparent and may comprise (or constitute) transparent beads, e.g. transparent organic beads, and / or comprise cross-linked silicone beads.

[0053] Fig. 3 schematically shows a cross-section of a LED filament 100, corresponding to the LED filament 100 of Fig. 2a, and according to an exemplifying embodiment of the present invention. It is furthermore referred to Fig. 2a and the associated text for features and / or functions of the LED filament 100. In Fig. 3, the elongated carrier 120 is light-transmissive. Hence, the LED light 150 from the plurality of LEDs 130 may be emitted omnidirectionally (albeit indicated only in Fig. 3 in an upward and downward direction by the arrows). The elongated luminescent encapsulant 200 may at least partially cover a second major surface 400, opposite the first major surface 145, of the elongated carrier 120. The elongated white encapsulant 300 may furthermore comprise a first elongated white encapsulant portion 410 and a second elongated white encapsulant portion 420. The elongated luminescent encapsulant 200 may comprise a first elongated luminescent encapsulant portion 430 which at least partly covers the first major surface 145 and a second elongated luminescent encapsulant portion 440 which at least partly covers the second major surface 400. The first elongated white encapsulant portion 410 encloses the first elongated luminescent encapsulant portion 430 and the second elongated white encapsulant portion 420 encloses the second elongated luminescent encapsulant portion 440.

[0054] According to the example of the LED filament 100 of Fig. 3, the concentration, CM2, of the luminescent material in the second elongated luminescent encapsulant portion 440 may be lower than the concentration, CMI, of the luminescent material in the first elongated luminescent encapsulant portion 430, i.e. CM2 < CMI. Furthermore, the thickness, TE2, of the second elongated luminescent encapsulant portion 440 may be smaller than the thickness, TEI, of the first elongated luminescent encapsulant portion 430, i.e. TE2 < TEI. The luminescent material concentration(s), CMI, CM2, and / or thickness(es) of the (first and / or second) elongated luminescent encapsulant portions 430, 440 may depend on orientation of the LED filament 100 and / or luminous intensity of the LED filament light 110. Furthermore, the concentration, CP2, of the particles 320 in the second elongated white encapsulant portion 420 may be lower than a concentration, CPI, of the particles 320 in the first elongated white encapsulant portion 410, i.e. CP2 < CPI. Furthermore, the thickness, TWE2, of the second elongated white encapsulant portion 420 may be smaller than the thickness, TWEI, of the first elongated white encapsulant portion 410, i.e. TWE2 < TWEI. The particle concentration(s), CPI, CP2, and / or thickness(es), TWEI, TWE2, of the (first / second) elongated white encapsulant portions 410, 420 may depend on orientation of the LED filament 100 and / or luminous intensity of the LED filament light 110. The elongated white encapsulant 300 either encloses the elongated luminescent encapsulant 200 to at least 90%, or fully encloses the elongated luminescent encapsulant 200.

[0055] Fig. 4 schematically shows a portion of a LED filament 100 according to an exemplifying embodiment of the present invention. More specifically, Fig. 4 schematically indicates the (light-influencing) properties of the elongated white encapsulant 300 of the LED filament 100. A multiple scattering effect is schematically described in Fig. 4. LED light 150, which enters the elongated white encapsulant 300, is subjected to multiple scattering within the elongated white encapsulant 300, before exiting the elongated white encapsulant 300 (here at a scattering angle, 0). As the difference |npa- npm| between the refractive index, npm, of the elongated white encapsulant 300 and the refractive index, npa, of the particles 320 is very low, namely in a range of 0.03 - 0.15, and preferably 0.1 - 0.15, the forward scattering is high. Furthermore, as the particle diameter, D, is much larger than the wavelength, Z. of visible light, the LED filament 100 does not show any wavelength dependent scattering. Furthermore, the thickness, T, of the elongated white encapsulant 300 and / or the concentration of particles in the polymer matrix 310 may be tuned conveniently to achieve the desired reflectivity, R, of the polymer matrix 310.

[0056] Fig. 5 schematically shows a simulation diagram of relative hiding power, RHP (i.e. hiding effect of the LED filament phosphor color) for different particle sizes and number of particles (concentration) in the polymer matrix of the elongated white encapsulant of a LED filament according to an exemplifying embodiment of the present invention. The leftmost y-axis of the diagram shows relative hiding power, RHP, and the rightmost y-axis of the diagram shows the number of particles of the elongated white encapsulant in order to attain 12% reflectivity (4 leftmost bar pairs) and 30% reflectivity (4 rightmost bar pairs). In accordance with the properties of the LED filament 100 as described in Fig. 2b and the associated text, the polymer matrix 310 of the elongated white encapsulant 300 has a refractive index, npm, and the particles 320 have a refractive index, npa, wherein |npa- npm| is 0.1 or 0.2 according to the simulation diagram, i.e. in the range of 0.03 - 0.15, preferably 0.1 - 0.15. The particles 320 of the polymer matrix 310 have a particle diameter, D, of either 2 pm or 5 pm, i.e. in the (preferred) range of 2 - 5 pm. The concentration of the particles 320 in the polymer matrix 310 is in the range 5.85- 104- 4.9- 106particles / mm3, i.e. in the preferred range of 5- 104- 4.9- 106particles / mm3. Furthermore, the concentration of the particles 320 in the polymer matrix 310, expressed in percent of volume of the polymer matrix 310, is 0.4 - 3.9 v / v%, i.e. in the preferred range of 0.03 - 5 v / v%. A particularly suitable concentration sub-range may be 0.1-0.3 v / v%, such as 0.2 v / v%, in case of |npa- npm| = 0.1-0.3, such as 0.2, D = 1-3 pm, such as D=2 pm, and / or R=10-14%, such as R=12%. Another particularly suitable concentration sub-range may be 3.7-4.3 v / v%, such as 4 v / v%, in case of |npa- npm| = 0.05-0.15, such as 0.1, D = 4-5 pm, such as D=5 pm, and / or R=25- 35%, such as R=30%. From the simulation diagram in Fig. 5, it is shown that for particles having a diameter of 2 pm, the particle concentration is relatively large, whereas the (needed) particle concentration for larger particles having a diameter of 2 pm is relatively low.

[0057] Fig. 6 schematically shows a LED filament lamp 600 according to an embodiment of the present invention. The LED filament lamp 600, which may constitute substantially any kind of lamp or luminaire, comprises one or more LED filaments 100 according to any one of the previously described embodiments. The LED filament lamp 600 further comprises a light-transmissive envelope 610, which is exemplified as being bulbshaped. The light-transmissive envelope 610 at least partially encloses the LED filament(s) 100. The LED filament lamp 600 further comprises a connector 620 for electrically and mechanically connecting the LED filament lamp 600 to a socket of a luminaire. The LED filament lamp 600 may further comprise a controller 510 (schematically indicated by a dashed rectangle) configured to control the luminous flux of the LED filament light. In case of a plurality of LED filaments 100, the controller 510 may control each LED filament individually.

[0058] 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. For example, one or more of the LED filament 100, the elongated luminescent encapsulant 200, the elongated white encapsulant 300, etc., may have different shapes, dimensions and / or sizes than those depicted / described.

Claims

CLAIMS:

1. A light emitting diode, LED, filament (100) configured to provide, in an on- state, LED filament light (110), wherein the LED filament comprises an elongated carrier (120), a plurality of light emitting diodes, LEDs (130) arranged on a first major surface (145) of the elongated carrier, wherein the plurality of LEDs is configured to emit LED light (150), an elongated luminescent encapsulant (200) at least partially covering the first major surface of the elongated carrier and at least partially enclosing the plurality of LEDs, wherein the elongated luminescent encapsulant comprises a luminescent material configured to at least partly convert the emitted LED light into converted light (210), and an elongated white encapsulant (300) at least partially enclosing the elongated luminescent encapsulant, wherein the elongated white encapsulant comprises a light-transmissive polymer matrix (310), and particles (320) dispersed in the polymer matrix, wherein the polymer matrix has a refractive index, npm, and the particles have a refractive index, npa, wherein |npa- npm| is in a range of 0.03 - 0.15, wherein the polymer matrix has a reflectivity, R, in a range of 12 - 30 %, wherein the particles have a particle diameter, D, in a range of 1 - 5 pm, and wherein the particles (320) are transparent and wherein the polymer matrix (310) is transparent2. The LED filament according to claim 1, wherein the LED filament light comprises the converted light and part of the LED light, wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1700 - 6500 K and a color rendering index, CRI, of least 80, and wherein the elongated white encapsulant has, in an off-state of the LED filament, a white appearance.

3. The LED filament according to claim 1 or 2, wherein the concentration of particles is in a range of 5- 104- 4.9- 106particles / mm3.

4. The LED filament according to any one of the preceding claims, wherein the concentration of particles is in a range of 0.03 - 5 v / v%.

5. The LED filament according to any one of the preceding claims, wherein a thickness, T, of the elongated white encapsulant is in a range of 0.5 - 1 mm.

6. The LED filament according to any one of the preceding claims, wherein the polymer matrix comprises a crosslinked silicone selected from the group consisting of poly dimethylsiloxane, PDMS, polymethylphenylsiloxane, PMPS, and poly diphenylsiloxane, PDPS.

7. The LED filament according to any one of the preceding claims, wherein the particlesare polymer particles8. The LED filament according to claim 7, wherein the polymer particles comprise cross-linked silicone.

9. The LED filament according to any one of the preceding claims, wherein the elongated carrier is light-transmissive and the elongated luminescent encapsulant at least partially covers a second major surface (400), opposite the first major surface, of the elongated carrier.

10. The LED filament according to claim 9, wherein the elongated white encapsulant comprises a first elongated white encapsulant portion (410) and a second elongated white encapsulant portion (420), wherein the elongated luminescent encapsulant comprises a first elongated luminescent encapsulant portion (430) which at least partly covers the first major surface and a second elongated luminescent encapsulant portion (440) which at least partly covers the second major surface, the first elongated white encapsulant portion enclosing the first elongated luminescent encapsulant portion and the second elongated white encapsulant portion enclosing the second elongated luminescent encapsulant portion.

11. The LED filament according to claim 10, wherein at least one ofa concentration, CM2, of the luminescent material in the second elongated luminescent encapsulant portion is lower than a concentration, CMI, of the luminescent material in the first elongated luminescent encapsulant portion, and a thickness, TE2, of the second elongated luminescent encapsulant portion is smaller than a thickness, TEI, of the first elongated luminescent encapsulant portion, is fulfilled.

12. The LED filament according to claim 10 or 11, wherein at least one of a concentration, CP2, of the particles in the second elongated white encapsulant portion is lower than a concentration, CPI, of the particles in the first elongated white encapsulant portion, and a thickness, TWE2, of the second elongated white encapsulant portion is smaller than a thickness, TWEI, of the first elongated white encapsulant portion.

13. The LED filament according to any one of the preceding claims, wherein the elongated white encapsulant fulfils one of enclosing the elongated luminescent encapsulant to at least 90%, and fully enclosing the elongated luminescent encapsulant.

14. A LED filament arrangement (500), comprising at least one LED filament according to any one of the preceding claims, and a controller (510) coupled to the at least one LED filament, wherein the controller is configured to control the LED light emitted from the plurality of LEDs.

15. A LED filament lamp (600), comprising one of at least one LED filament according to any one of claims 1-13, a LED filament arrangement according to claim 14, a light-transmissive envelope (610) at least partly enclosing the at least one LED filament, and a connector (620) for electrically and mechanically connecting the LED filament lamp to a socket of a luminaire.

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