A LED filament

Abstract

A LED filament (1) configured to, in operation, emit LED filament light (8, 9), and comprising a light-transmissive elongated carrier (2) comprising a first major surface (4) and a second major surface (5), an array of a plurality of N LEDs (3) configured to in operation emit LED light (10) and being arranged on the first major surface (4) of the light-transmissive elongated carrier (2), an elongated encapsulant (6) comprising a luminescent material (12), being arranged to cover the plurality of LEDs (3) and at least a part of the first major surface (4), and being configured for at least party converting the LED light (10) into converted light (11), and a partially light-transmissive layer (7; 16) arranged on the light-transmissive elongated carrier (2), being configured to one or more of reflect and absorb at least a part (14) of one or more of the LED light (10) and the converted light (11), and being configured to provide a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm.

Description

[0001] A LED FILAMENT

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to 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, a plurality of LEDs configured to, in operation, emit LED light and being arranged on the first major surface of the light- transmissive elongated carrier, and an elongated encapsulant comprising a luminescent material, the elongated encapsulant being arranged to cover the plurality of LEDs and at least a part of the first major surface, and the elongated encapsulant being configured for at least party converting the LED light into converted light. The present invention further relates to a lamp or a luminaire comprising such a LED filament.

[0004] BACKGROUND OF THE INVENTION

[0005] A trend in lighting is LED filament lamps. An LED filament lamp is an 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.

[0006] US 2022 / 0186889 Al discloses a light-emitting diode, LED, filament. The LED filament comprises an elongated substrate and a plurality of light-emitting diodes, LEDs, which are mechanically coupled to the substrate. The LED filament further comprises an at least in part light-transmissive encapsulation which encapsulate the plurality of LEDs and at least partially encapsulates the substrate, and a plurality of at least partially light- reflective particles which are arranged on an outer surface of the encapsulation.

[0007] The invention in WO 2022 / 253735 relates to a LED filament configured to provide LED filament light (A, B) and comprising an elongated carrier comprising a first elongated edge portion and a second elongated edge portion arranged at a distance from the first elongated edge portion, a first surface, a second surface arranged opposite to the first surface, the first and the second surfaces being delimited by the first and the second edge portions. The LED filament further comprises a plurality of first light-emitting diodes, LEDs, distributed along the first surface of the elongated carrier, the plurality of first LEDs being configured to emit a first LED light. The LED filament also comprises a first at least partially light-transmissive elongated layer encapsulating or covering the plurality of first LEDs and at least partially encapsulating or covering the first surface of the elongated carrier and a second elongated reflective layer arranged to asymmetrically encapsulate or cover the first at least partially light-transmissive elongated layer.

[0008] It is desired to improve the performance, functionality and / or appearance of LED filament lamps.

[0009] It is further desired to provide a LED filament with which a desired target color point may be obtained with basis in an existing color point in a simple and straight forward manner.

[0010] SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to overcome this problem, and to provide a LED filament and a LED filament lamp with an improved performance, functionality and / or appearance.

[0012] It is a further object of the invention to provide a LED filament with which a desired target color point may be obtained with basis in an existing color point in a simple and straight forward manner.

[0013] 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, an array of a plurality of N LEDs configured to, in operation, emit LED light and being arranged on the first major surface of the light-transmissive elongated carrier, an elongated encapsulant comprising a luminescent material, the elongated encapsulant being arranged to cover the plurality of LEDs and at least a part of the first major surface, and the luminescent material being configured for at least party converting the LED light into converted light, and a partially light-transmissive layer arranged on the light-transmissive elongated carrier, the partially light-transmissive layer being configured to one or more of reflect and absorb at least a part of one or more of the LED light and the converted light, and comprising a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm, e.g., a non-uniform transmission of light for blue, green and red light.

[0014] It is noted that the array of LEDs comprises a plurality of N LEDs, where N is an integer being two or more. With such a LED filament, and particularly by providing that the partially light-transmissive layer is configured to one or more of reflect and absorb at least a part of one or more of the LED light and the converted light, and most importantly comprises a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm, a LED filament is provided with which a color point of the LED filament light emitted by the LED filament may be corrected or shifted.

[0015] Thereby, a desired target color point may be obtained with basis in an existing color point in a simple and straight forward manner.

[0016] Furthermore, such a LED filament is provided with at least one of an improved performance, an improved functionality, and an improved appearance.

[0017] The partially light-transmissive layer may be arranged on one or both of the second major surface of the elongated carrier and the first major surface of the elongated carrier.

[0018] Thereby, a LED filament which is provided with an improved performance and an improved appearance is obtained. If the partially light-transmissive layer is arranged on the second major surface, it becomes easier to manufacture the LED filament because no LEDs and / or electric tracks are present on the second major surface. If the partially light- transmissive layer is arranged on the first major surface, improved light recycling can be obtained.

[0019] The partially light-transmissive layer may be configured to reflect at least a part of one or more of the LED light and the converted light, and the partially light- transmissive layer may be one or more of a reflective layer and a dichroic mirror.

[0020] Thereby, the partially light-transmissive layer is configured not only to correct a color point of the LED filament light, but also to avoid light loss during the correction of a color point. Particularly, the light losses are avoided by using a reflective partially light- transmissive layer, since such a layer is more efficient than a corresponding layer absorbing light.

[0021] The LED light emitted by the array of a plurality of N LEDs may have a (dominant) peak wavelength in a first wavelength range from 430nm to 490nm, the converted light may comprise green-yellow light having a (dominant) peak wavelength in a second wavelength range from 510nm to 580nm and red light having a (dominant) peak wavelength in a third wavelength range from 600nm to 680nm, and the partially light-transmissive layer may have a first (averaged) reflectivity, Rl, in the first wavelength range, a second (averaged) reflectivity, R2, in the second wavelength range and a third (averaged) reflectivity, R3, in the third wavelength range, where the second reflectivity, R2, is higher than the first reflectivity and / or the third reflectivity, R3, is higher than the first reflectivity R1. That is, R2 > R1 and / or R3 > R1.

[0022] The reflectivity Rl, R2 and / or R3 may be an averaged reflectivity. The reflectivity Rl, R2 and R3, respectively, is typically dependent on the angle, from which it is measured. In the present case it is to be understood that the reflectivity Rl, R2 and R3, respectively, are measured in a direction perpendicular to the reflective surface or layer in question.

[0023] Thereby, a LED filament which is provided with an improved performance, especially when employing a plurality of N LEDs emitting LED light with a peak wavelength in a first wavelength range from 430nm to 490nm, is obtained.

[0024] The peak wavelength of any one or more of the LED light emitted by the array of a plurality of N LEDs, the green-yellow light and the red light may be a dominant peak wavelength.

[0025] Thereby, a LED filament which is provided with a particularly improved performance is obtained.

[0026] The luminescent material may comprise a first phosphor and a second phosphor.

[0027] Thereby, the LED light may be converted by the luminescent material to obtain converted light with two different peak wavelengths, and the phosphors may be chosen in accordance with the two different peak wavelengths desired for a particular application.

[0028] The first phosphor and a second phosphor may be YAG and KSF, respectively.

[0029] Such phosphors are particularly well suited for converting the LED light to green-yellow light and red light, respectively.

[0030] The first reflectivity, Rl, may be below 20 %, preferably below 15 %, more preferably below 10 %, most preferably below 5 %, where R2-R1 > 30 %, and where R3-R1 > 30 %.

[0031] The above choice of the respective reflectivity Rl, R2 and R3 is reasoned as follows. The blue LED light is mainly emitted from the LEDs of the array of LEDs in a direction facing away from the light output surface of the LED. Some of the LED light will be scattered by the phosphor and / or a light scattering material to other directions including in a direction of the elongated carrier. The converted light is emitted in all directions and thus is more directed to the elongated carrier with respect to the blue LED light. Thus, a higher reflectivity R2 and R3 for the converted light as compared to the reflectivity R1 for the LED light is desired.

[0032] The second reflectivity, R2, and the third reflectivity, R3, may fulfill the relation 1.2 > R2 / R3 > 0.8.

[0033] Thereby, it is achieved that the ratio between green-yellow converted light and red converted light emitted from different sides of the LED filament is rather similar.

[0034] The LED filament light may be white light having a correlated color temperature, CCT, in a range from 1500K to 6500 K or in a range of 1500 K to 2500 K, and a CRI of at least 80.

[0035] The choice of a CCT in a range of 1500 K to 2500 K corresponds to extreme warm white light. In this case the measures suggested for the reflectivity Rl, R2 and R3, respectively, as described above are particularly advantageous due to the low content of blue light in extreme warm white light. For such a range there is much more red light than green light, and much more green light than blue light.

[0036] The LED filament light may comprise a first portion and a second portion, where the first portion of the LED filament light is light emitted from a front side of the LED filament, and where the second portion of the LED filament light is LED filament light transmitted through the partially light-transmissive layer and emitted from a back side of the LED filament, and where the first portion of the LED filament light is white light and the second portion of the LED filament light is white light.

[0037] Thereby, a LED filament is provided with which white light may be emitted from both sides or major surfaces of the filament, thereby ensuring a more uniform light output and a more uniform appearance of the LED filament, at least when it is emitting light.

[0038] The light-transmissive elongated carrier may be a flexible light-transmissive elongated carrier.

[0039] The light-transmissive elongated carrier may be made of any one of glass, sapphire, quartz, and polymer.

[0040] Thereby, a carrier, and thus a LED filament, which is provided with an improved performance is obtained.

[0041] The elongated encapsulant may be made of silicone.

[0042] Silicone is a particularly useful material for encapsulants for LED filaments, and in particular for stretchable LED filaments, as it is both flexible and stretchable and also resistant due to wear caused by the LED light emitted by the plurality of LEDs. The LED filament light may comprise a first portion and a second portion, where the first portion of the LED filament light is light emitted from a front side of the LED filament, and where the second portion of the LED filament light is LED filament light transmitted through the partially light-transmissive layer and emitted from a back side of the LED filament, and where at least one of the following applies: the luminous flux of the second portion of LED filament light is 0.2 to 0.6 times the luminous flux of the first portion of LED filament light, and the first portion of LED filament light comprises a correlated color temperature, CCT, being within 300 K of the CCT of the second portion of LED filament light.

[0043] By ensuring that the luminous flux of the second portion of LED filament light is 0.2 to 0.6 times the luminous flux of the first portion of LED filament light, it is ensured that the luminous flux of the light emitted from the front side of the LED filament is considerably stronger than that emitted from the back side of the LED filament.

[0044] By ensuring that the first portion of LED filament light comprises a correlated color temperature, CCT, being within 300 K of the CCT of the second portion of LED filament light, it is ensured that the light emitted from the front side of the LED filament and the light emitted from the back side of the LED filament comprise a similar correlated color temperature, and thus a similar appearance.

[0045] In both cases, a LED filament with an improved performance and appearance is obtained.

[0046] Each LED of the array of a plurality of N LEDs may emit light from (i) its top surface and (ii) its side surface and / or its bottom surface.

[0047] Because the plurality of N LEDs each also emit light from the side surface and / or the bottom surface, considerably more blue light is directed to the partially light- transmissive layer. Thereby a LED filament with an improved performance is obtained.

[0048] The elongated encapsulant may comprise a non-luminescent light scattering material configured to scatter LED light.

[0049] In this way, more blue light is directed to the partially light-transmissive layer. Thereby a LED filament with an improved performance is obtained.

[0050] The LED filament may further comprise a further elongated encapsulant arranged in between the elongated encapsulant and the light-transmissive elongated carrier, the further elongated encapsulant comprising a non-luminescent light scattering material configured to scatter LED light. In this way, more blue light is directed to the partially light-transmissive layer. Thereby a LED filament with an improved performance is obtained.

[0051] The LED filament may further comprise a backside encapsulant free from luminescent material, where the backside encapsulant is arranged on the second major surface of the LED filament opposite to the first major surface, and where the backside encapsulant is arranged to cover at least a major part of the partially light-transmissive layer and at least a part of the second major surface.

[0052] Thereby, a LED filament is provided with which the light outcoupling of the LED filament light is improved.

[0053] The backside encapsulant may comprise a light scattering material configured to scatter light which is transmitted through the partially light-transmissive layer.

[0054] Thereby, a LED filament is provided with which the homogeneity of the LED filament light, and in particular the second portion of the LED filament light, is improved.

[0055] The light scattering material of the backside encapsulant may have a concentration being lower than the concentration of the luminescent material in the elongated encapsulant.

[0056] The mentioned concentrations of light scattering material may for instance be defined as the percentual volume of light scattering material with respect to the total volume of the encapsulant in question.

[0057] The light scattering material may for instance be one or more of BaSCU, TiCE, and AI2O3.

[0058] Such light scattering materials have been proved to be particularly well suitable for improving the homogeneity of the LED filament light, and in particular the second portion of the LED filament light.

[0059] The invention further relates to a LED lamp or a luminaire comprising at least one LED filament according to the invention. The lamp may comprise a base and an envelope. The envelope may at least partly enclose the LED filament (or more LED filaments). The base 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.

[0060] It is noted that the invention relates to all possible combinations of features recited in the claims.

[0061] BRIEF DESCRIPTION OF THE DRAWINGS 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.

[0062] Fig. 1 shows a cross-sectional side view of a LED filament according to the invention.

[0063] Fig. 2 shows a cross-sectional side view of another LED filament according to the invention.

[0064] Fig. 3 shows a color spectrum of a LED filament according to Fig. 1.

[0065] Fig. 4 shows a close-up of the section IV of the color diagram of Fig. 3 and illustrating an exemplary shift or correction of color point obtained for a LED filament according to Fig. 1.

[0066] Fig. 5 shows a cross-sectional side view of a lamp comprising a LED filament according to the invention.

[0067] Fig. 6 shows a graph illustrating the transmission in percent of the partially light-transmissive layer of a LED filament according to the invention as a function of the wavelength of the LED filament light.

[0068] Fig. 7 shows a graph illustrating the reflection in percent of the partially light- transmissive layer of a LED filament according to the invention as a function of the wavelength of the LED filament light.

[0069] 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.

[0070] DETAILED DESCRIPTION

[0071] 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.

[0072] Fig. 1 shows a cross-sectional side view of a LED filament 1 according to an embodiment of the present invention. The LED filament 1 is configured to, in operation, emit LED filament light 8, 9. The LED filament light 8, 9 is white light. A portion 8 of the LED filament light 8, 9 is emitted from a front side 17 of the LED filament 1, and a portion 9 of the LED filament light 8, 9 is emitted from a back side 18 of the LED filament 1. Generally, and irrespective of the embodiment, the LED filament 1 comprises a light-transmissive elongated carrier 2, an array of LEDs comprising a plurality of LEDs 3, an elongated encapsulant 6 and a partially light-transmissive layer 7.

[0073] The light-transmissive elongated carrier 2 comprises a first major surface 4 and a second major surface 5. The first major surface 4 and the second major surface 5 are mutually opposite surfaces. The first major surface 4 and the second major surface 5 may be mutually parallel surfaces. The light-transmissive elongated carrier 2 may be a flexible light- transmissive elongated carrier. For instance, the light-transmissive elongated carrier 2 may be made of glass, sapphire, quartz, or polymer. The light-transmissive elongated carrier 2 may be translucent, preferably transparent. The light-transmissive elongated carrier 2 may have a light transmission of at least 60%, preferably at least 70%, more preferably at least 80%, most preferably at least 90%.

[0074] The plurality of LEDs 3 are arranged in an array. The plurality of LEDs 3 are configured to, in operation, emit LED light 10. The plurality of LEDs 3 comprises N LEDs, where N is an integer being two or more. The plurality of LEDs 3 are arranged on the first major surface 4 of the light-transmissive elongated carrier 2. Preferably, there are no LEDs arranged on the second major surface 5. Each LED of the plurality of LEDs 3 is configured to emit light from its top surface 31 (Fig. 1). Each LED of the plurality of LEDs 3 may further be configured to emit light from one or both of its side surface 32 and its bottom surface 33.

[0075] The elongated encapsulant 6 is arranged to cover the plurality of LEDs 3 and at least a part of the first major surface 4. The elongated encapsulant 6 may also be arranged to cover all of the first major surface 4. The elongated encapsulant 6 may be is made of silicone. The elongated encapsulant 6 comprises a luminescent material 12. The luminescent material 12 is configured for at least partly converting the LED light 10 into converted light 11. The luminescent material 12 may comprise a first phosphor and a second phosphor. For instance, the first phosphor may be YAG, and the second phosphor may be KSF. The elongated encapsulant 6 may be one continuous encapsulant. The elongated encapsulant 6 may further comprise a non-luminescent light scattering material configured to scatter the LED light 10.

[0076] The partially light-transmissive layer 7 is arranged on the light-transmissive elongated carrier 2. More particularly, in the embodiment shown in Fig. 1, the partially light- transmissive layer 7 is arranged on the second major surface 5 of the elongated carrier 2. The partially light-transmissive layer 7 is configured to reflect at least a part 14 of one or more of the LED light 10 and the converted light 11, or to absorb at least a part of one or more of the LED light 10 and the converted light 11, or to partially reflect and partially absorb at least a part of one or more of the LED light 10 and the converted light 11. The partially light- transmissive layer 7 is configured to correct a color point of the LED filament light 8, 9. The partially light-transmissive layer 7 is further configured to provide a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm. The partially light- transmissive layer 7 may be a reflective layer configured to correct a color point of the LED filament light 8, 9. For instance, the partially light-transmissive layer 7 may be or comprise a dichroic mirror. The coverage of the partially light-transmissive 7 layer on the light- transmissive elongated carrier 2, or in the embodiment shown in Fig. 1 particularly on the second major surface 5, may be at least 70%, preferably at least 80%, more preferably at least 90%, most preferably at least 95% such as, e.g., fully covered.

[0077] As an example of the transmissivity of the partially light-transmissive layer 7, Fig. 6 shows a graph illustrating the transmission in percent of the partially light-transmissive layer 7 of a LED filament according to the invention as a function of the wavelength of the LED filament light 8, 9.

[0078] The LED light 10 may be blue light, and the converted light 11 may be greenyellow and red light. The LED light 10 may have a peak wavelength in a first wavelength range from 430nm to 490nm, e.g., from 440nm to 465nm. The peak wavelength of the LED light 10 may be a dominant peak wavelength.

[0079] The converted light 11 may comprise green-yellow light having a peak wavelength in a second wavelength range from 510nm to 580nm, e.g., from 540nm to 570nm and red light having a peak wavelength in a third wavelength range from 600nm to 680nm, e.g., from 620nm to 640nm. The peak wavelength of the converted light 11, such as the peak wavelength of one or both of the green-yellow light and the red light, may be a dominant peak wavelength.

[0080] The partially light-transmissive layer 7 may have a first (averaged) reflectivity, Rl, in the first wavelength range, a second (averaged) reflectivity, R2, in the second wavelength range and a third (averaged) reflectivity, R3, in the third wavelength range. The second (averaged) reflectivity R2 is higher than the first (averaged) reflectivity Rl and / or the third (averaged) reflectivity R3 is higher than the first (averaged) reflectivity RL That is, R2 > Rl and R3 > RL The first (averaged) reflectivity, Rl, may be below 20 %, below 15 %, below 10 %, or even below 5 %, R2-R1 may be larger than or equal to 30 %, and R3-R1 may be larger than or equal to 30 %. Also, the second reflectivity, R2, and the third reflectivity, R3, may fulfill the relation 1.2 > R2 / R3 > 0.8. This is illustrated in Fig. 7 showing a graph illustrating the reflection in percent of the partially light-transmissive layer 7 of a LED filament according to the invention as a function of the wavelength of the LED filament light 8, 9.

[0081] Furthermore, the absorption of the partially light-transmissive layer 7 is preferably below 5%, more preferably below 3%, most preferably below 2% such as 1% or 0%.

[0082] The LED filament light 8, 9 is white light. For instance, the LED filament light 8, 9 may be light comprising a correlated color temperature, CCT, in a range from 1500 K to 6500 K, or a CCT in a range from 1500 K to 2500 K. For instance, the LED filament light 8, 9 may be light comprising a color rendering index, CRI, of at least 80. For instance, the LED filament light 8, 9 may be white light having a correlated color temperature, CCT, in a range from 2700 K to 3500 K, and a CRI of at least 80. For instance, the LED filament light 8, 9 may be white light having a correlated color temperature, CCT, in a range from 4000 K to 6500 K, and a CRI of at least 80.

[0083] The LED filament light 8, 9 comprises a first portion 8 and a second portion 9. The first portion 8 of the LED filament light 8, 9 is light emitted from a front side 17 of the LED filament 1, that is in a direction facing away from the LEDs 3. The first portion 8 of the LED filament light 8, 9 is thus light transmitted through the elongated encapsulant 6. The second portion 9 of the LED filament light 8, 9 is LED filament light transmitted through the partially light-transmissive layer 7 and emitted from a back side 18, that is opposite to the front side 17, of the LED filament 1. The luminous flux of the second portion 9 of the LED filament light 8, 9 may be 0.2 to 0.6 times the luminous flux of the first portion 8 of the LED filament light 8, 9, or 0.3 to 0.5 times the luminous flux of the first portion 8 of the LED filament light 8, 9. Alternatively, or additionally, the first portion 8 of the LED filament light 8, 9 may comprise a correlated color temperature, CCT, being within 200 K or within 300 K of the CCT of the second portion 9 of the LED filament light 8, 9.

[0084] Fig. 2 shows a cross-sectional side view of a LED filament 100 according to another embodiment of the present invention. The LED filament 100 differs from the LED filament 1 shown on Fig. 1 and described above only in virtue of the following features.

[0085] The LED filament 100 comprises a further partially light-transmissive layer 16. The further partially light-transmissive layer 16 is arranged on the elongated carrier 2. More particularly, in the embodiment shown in Fig. 2, the further partially light-transmissive layer 16 is arranged on a part of the first major surface 4 of the elongated carrier 2. Alternatively, the further partially light-transmissive layer 16 may be arranged on all of the first major surface 4 of the elongated carrier 2. The further partially light-transmissive layer 16 is configured to reflect at least a part of one or more of the LED light 10 and the converted light 11, or to absorb at least a part of one or more of the LED light 10 and the converted light 11, or to partially reflect and partially absorb at least a part of one or more of the LED light 10 and the converted light 11. The further partially light-transmissive layer 16 is further configured to provide a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm. The further partially light-transmissive layer 16 may be a reflective layer configured to correct a color point of the LED filament light 8, 9. For instance, the further partially light-transmissive layer 16 may be or comprise a dichroic mirror. The coverage of the partially light-transmissive 16 layer on the light-transmissive elongated carrier 2, or in the embodiment shown in Fig. 2 particularly on the first major surface 4, may be at least 70%, preferably at least 80%, more preferably at least 90%, most preferably at least 95% such as, e.g., fully covered.

[0086] The first portion 8 of the LED filament light 8, 9, which is light emitted from a front side 17 of the LED filament 100, and which is light transmitted through the elongated encapsulant 6, thus in this embodiment comprises light transmitted through or reflected by the further partially light-transmissive layer 16.

[0087] The LED filament 100 also comprises a backside encapsulant 13. The backside encapsulant 13 is arranged to cover the light transmissive layer 7 and at least a part, particularly at least a major part, of the second major surface 5. The backside encapsulant 13 is arranged on or adjacent to at least a part of the second major surface 5 of the light- transmissive elongated carrier 2. The backside encapsulant 13 is free from luminescent material. The backside encapsulant 13 may comprise a light scattering material 15. More particularly, the backside encapsulant 13 may comprise a concentration (for instance measured in terms of v / v%) of the light scattering material 15 being lower than the concentration of the luminescent material present in the elongated encapsulant 6. The light scattering material 15 may for instance be BaSCU, TiCL, AI2O3 or a combination thereof. The light scattering material 15 is configured to scatter light which is transmitted through the partially light-transmissive layer 7.

[0088] The second portion 9 of the LED filament light 8, 9, which is light emitted from a back side 18 of the LED filament 100, thus in this embodiment comprises light transmitted through the backside encapsulant 13. The LED filament 100 also comprises a further elongated encapsulant 19. The further elongated encapsulant 19 is arranged in between the elongated encapsulant 6 and the light-transmissive elongated carrier 2. The further elongated encapsulant 19 comprises a non- luminescent light scattering material configured to scatter LED light 10.

[0089] In yet another embodiment the further partially light-transmissive layer 16 may be substituted for a reflective layer such as to avoid loss of light from regions of the first major surface 4 not covered by the elongated encapsulant 6.

[0090] Reference is now made to Figs. 3 and 4 to illustrate the effect of a shift or correction of color point which may be obtained for a LED filament 1, 100 according to the invention. Fig. 3 shows a color spectrum of a LED filament 1, 100 according to the invention. Fig. 4 shows a close-up of the section IV of the color diagram of Fig. 3.

[0091] Fig. 3 illustrates a black body locus, BBL, on which it is desired that the color points, CP, of the LED filament light 8, 9 emitted by the LED filament 1, 100 is located. As is illustrated in Fig. 4, it may however, be the case that a color point CPI of the LED filament light 8, 9 emitted by the LED filament 1, 100 is located well off the desired black body locus BBL. Also illustrated on Fig. 4 is another color point CP2, which is located on the BBL, or at least sufficiently close to the BBL to fulfill the needed and desired requirements. Simulations have shown that with a LED filament 1, 100 according to the invention the color point CPI may be corrected or shifted in position to attain the position illustrated by the color point CP2 due to the LED filament 1, 100 according to the invention comprising the partially light- transmissive layer 7 configured to provide a non-uniform transmission of light, and particularly a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm. Thus, with a LED filament 1, 100 according to the invention a color point of the LED filament light 8, 9, such as the color point CPI, may be corrected or shifted to obtain a desired target color point.

[0092] Finally, Fig. 5 shows an exemplary lamp 20 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.

[0093] The lamp 20 further comprises a driver or controller 25 configured for controlling the LEDs 3 of the LED filament 1. The controller 25 is configured to power the plurality of LEDs 3 via electrical circuitry (not visible on the figures) of the LED filament 1. The controller 25 may further be configured for controlling at least one of the CCT of the LED filament light 8, 9 and the CRI of the LED filament light 8, 9. The controller 25 may also be configured for controlling other parameters related to the LEDs 3 and the LED filament light 8, 9.

[0094] The lamp 20 further comprises an envelope 21 at least partially enveloping the at least one LED filament 1. The lamp 20 further comprises a cap 22. As shown in Fig. 5, the controller 25 is arranged within the envelope 21. When comprising a cap 22, the controller 25 may also be arranged inside the cap 22 such that it is hidden from view. The lamp 20 further comprises threading 23 for connection to a socket, and a terminal 24 for connection to a source of electrical energy.

[0095] The envelope 21 of the lamp 20 may further and optionally be provided with a coating (not shown), such as a reflective coating, covering at least a part of the envelope 21.

[0096] 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.

[0097] 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 white LED filament light(8, 9), the LED filament comprising: a light-transmissive elongated carrier (2) comprising a first major surface (4) and a second major surface (5), opposite to said first major surface; an array of a plurality of N LEDs (3) configured to, in operation, emit LED light (10) and being arranged on the first major surface (4) of the light-transmissive elongated carrier (2); an elongated encapsulant (6) comprising a luminescent material (12), the elongated encapsulant being arranged to cover the plurality of LEDs (3) and at least a part of the first major surface (4), and the luminescent material being configured for at least party converting the LED light (10) into converted light (11); and a partially light-transmissive layer (7; 16) arranged on the light-transmissive elongated carrier (2), wherein the partially light-transmissive layer (7; 16): is configured to one or more of reflect and absorb at least a part (14) of one or more of the LED light (10) and the converted light (11), and has a non-uniform transmission of light in a wavelength range of from 400 nm to 800 nm, wherein the LED light emitted by the array of a plurality of N LEDs (3) has a peak wavelength in a first wavelength range from 430nm to 490nm, wherein the converted light comprises green-yellow light having a peak wavelength in a second wavelength range from 510nm to 580nm and red light having a peak wavelength in a third wavelength range from 600nm to 680nm; and wherein the partially light-transmissive layer (7; 16) has a first reflectivity (Rl) in the first wavelength range, a second reflectivity (R2) in the second wavelength range and a third reflectivity (R3) in the third wavelength range, and wherein the second reflectivity (R2) is higher than the first reflectivity (Rl), and wherein the third reflectivity (R3) is higher than the first reflectivity (Rl).

2. A LED filament according to claim 1, wherein the partially light-transmissive layer (7; 16) is arranged on one or both of the second major surface (5) of the elongated carrier (2) and the first major surface (4) of the elongated carrier (2).

3. A LED filament according to claim 1 or 2, wherein the partially light- transmissive layer (7; 16) is configured to reflect at least a part (14) of one or more of the LED light (10) and the converted light (11), and wherein the partially light-transmissive layer (7; 16) is one or more of a reflective layer and a dichroic mirror, and wherein the partially light-transmissive layer (7; 16) is configured to correct a color point of the LED filament light (8, 9).

4. A LED filament according to any of the above claims, wherein the first reflectivity (Rl) is below 20 %, wherein R2-R1 > 30%, and wherein R3-R1 > 30%.

5. A LED filament according to claim 4, wherein 1.2 > R2 / R3 > 0.8.

6. A LED filament according to any one of the above claims, wherein the LED filament light (8, 9) comprises a first portion (8) and a second portion (9), wherein the first portion (8) of the LED filament light is light emitted from a front side (17) of the LED filament (1), and wherein the second portion (9) of the LED filament light is LED filament light transmitted through the partially light-transmissive layer (7) and emitted from a back side (18) of the LED filament (1), and wherein the first portion (8) of the LED filament light is white light and the second portion (9) of the LED filament light is white light.

7. A LED filament according to any one of the above claims, wherein the LED filament light (8, 9) comprises a first portion (8) and a second portion (9), wherein the first portion (8) of the LED filament light is light emitted from a front side (17) of the LED filament (1), and wherein the second portion (9) of the LED filament light is LED filament light transmitted through the partially light-transmissive layer (7) and emitted from a back side (18) of the LED filament (1), and wherein one or both of the following applies: the luminous flux of the second portion (9) of LED filament light is 0.2 to 0.6 times the luminous flux of the first portion (8) of LED filament light, andthe first portion (8) of LED filament light comprises a correlated color temperature, CCT, being within 300 K of the CCT of the second portion (9) of LED filament light.

8. A LED filament according to any one of the above claims, wherein each LED of the array of a plurality of N LEDs emit light from (i) its top surface (31) and (ii) its side surface (32) and / or its bottom surface (33).

9. A LED filament according to any one of the above claims, wherein the elongated encapsulant (6) comprising a non-luminescent light scattering material configured to scatter the LED light.

10. A LED filament according to any one of the preceding claims, further comprising a further elongated encapsulant (19) arranged in between the elongated encapsulant (6) and the light-transmissive elongated carrier, the further elongated encapsulant (19) comprising a non-luminescent light scattering material configured to scatter LED light.

11. A LED filament according to any one of the above claims, and further comprising a backside encapsulant (13) free from luminescent material, wherein the backside encapsulant (13) is arranged on the second major surface (5) of the LED filament opposite to the first major surface (4), and wherein the backside encapsulant (13) is arranged to cover at least a major part of the partially light-transmissive layer (7) and at least a part of the second major surface (5).

12. A LED filament according to claim 11, wherein the backside encapsulant (13) comprises a light scattering material configured to scatter light which is transmitted through the partially light-transmissive layer (7).

13. A LED filament according to claim 12, wherein the light scattering material in the backside encapsulant (13) has a concentration lower than the concentration of the luminescent material in the elongated encapsulant (6).

14. A LED lamp or a luminaire (20) comprising at least one LED filament (1) according to any one of the above claims.

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