LED filament comprising a LED filament arrangement

Abstract

There is provided a light emitting diode, LED, filament (100) comprising a LED filament arrangement (115). The LED filament arrangement comprises an elongated carrier (120), a plurality of light emitting diodes, LEDs (130), and an elongated encapsulant (160) at least partially enclosing the plurality of LEDs. The LED filament further comprises a semi-reflective specular-reflecting mirror (200) at least partially enclosing the at least one LED filament, wherein the semi-reflective specular-reflecting mirror, having a transmissivity, T, is configured to partly transmit the LED filament arrangement light into transmitted LED filament arrangement light (220), and wherein the semi-reflective specular-reflecting mirror, having a reflectivity, R, is configured to partly reflect the LED filament arrangement light into reflected LED filament arrangement light (210), wherein the semi-reflective specular-reflecting mirror has a high transmissivity, T, for green and red light, and a high reflectivity, R, for blue light.

Description

[0001] LED filament comprising a led filament arrangement

[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 LED filament arrangement.

[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 filament(s) whilst providing a sustainable LED filament. More specifically, it is desirable to improve the performance and / or functionality of the light emission and / or light distribution from the LED filament(s), whilst the LED filament(s) is (are) environmentally sustainable. Another purpose is to augment the appearance and / or the decorative aspect of the LED filament(s).

[0008] In EP 2524165 a lighting device is disclosed of which the efficiency and color temperature be improved by using wavelength shifting material, such as a phosphor, to absorb less desired wavelengths and transmit more desired wavelengths. A reflective filter (e.g., dichroic or dielectric mirror material) may pass desired wavelengths while returning or reflecting less desired wavelengths away from an optical exit back toward wavelength shifting material which may either be disposed in the optical path or on the periphery of the light source.

[0009] SUMMARY OF THE INVENTION

[0010] 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. It is desirable to ensure the (environmental) sustainability of the LED filament(s) and improve the appearance and / or the decorative aspect thereof.

[0011] 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.

[0012] According to the present invention, there is provided a light emitting diode, LED, filament configured to provide, during operation, LED filament light. The LED filament comprises a LED filament arrangement configured to provide, during operation, LED filament arrangement light. The LED filament arrangement 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, wherein the LED light has a peak emission wavelength in a blue wavelength range of 430 - 470 nm. The LED filament further comprises an elongated encapsulant at least partially covering the first major surface of the elongated carrier and at least partially covering the plurality of LEDs, wherein the elongated encapsulant comprises luminescent material configured to at least partly convert the emitted LED light into primary converted light. The LED filament arrangement light comprises a portion of the LED light constituting non-converted light, and the primary converted light. The LED filament further comprises a semi-reflective specular- reflecting mirror at least partially enclosing the LED filament arrangement. The semi- reflective specular-reflecting mirror, having a transmissivity, T, is configured to partly transmit the LED filament arrangement light into transmitted LED filament arrangement light, and wherein the semi-reflective specular-reflecting mirror, having a reflectivity, R, is configured to partly reflect the LED filament arrangement light into reflected LED filament arrangement light. The semi-reflective specular-reflecting mirror has a transmissivity, T, of at least 70 % in a green wavelength range of 500 - 540 nm for transmitting green light, and in a red wavelength range of 600 - 690 nm for transmitting red light, and a reflectivity, R, of at least 80 % in the blue wavelength range of 430 - 470 nm for reflecting blue light. The nonconverted light is reflected back to the elongated encapsulant and whereby at least a portion of the non-converted light is converted into secondary converted light. The LED filament light may be white light having a correlated color temperature, CCT, in a range of 1500K - 3500 K, preferably 1500K - 2700K.

[0013] Thus, the present invention is based on the idea of providing a LED filament comprising a LED filament arrangement at least partially enclosed by a semi-reflective specular- reflecting mirror, whereby a desired light distribution and a decorative light effect may be achieved by the environmentally sustainable LED filament. More specifically, the LED filament comprising a high-reflective blue semi-reflective specular-reflecting mirror reduces phosphor loading, leading to a more sustainable LED filament approach as less rare earth materials are needed.

[0014] The present invention is further advantageous in that the proposed LED filament has a blue appearance for an observer, indicating an efficient light source.

[0015] The present invention is further advantageous in its versatility and customization concerning encapsulant and / or mirror properties of the LED filament. For low color temperatures, which may be desired for LED filaments, the reflectivity in the blue wavelength range is preferably very high (>80%) and is preferably very low (<30%) in the green-red wavelength range.

[0016] The present invention is further advantageous in that the properties of the elongated encapsulant may be easily and conveniently customized for desired properties of the light distribution from the LED filament, as well as the aesthetic appearance of the LED filament. More specifically, properties of the elongated encapsulant such as e.g. transmissivity, reflectivity, thickness, porosity, etc. may be set or tuned for different purposes.

[0017] 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 as disclosed.

[0018] 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 arrangement 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.

[0019] There is provided a light emitting diode, LED, filament configured to provide, during operation (i.e. in an on-state) LED filament light. The LED filament light is white light having a correlated color temperature, CCT, in a range of 1500 to 2700 K, and preferably a color rendering index, CRI, of at least 80, or at least 85. The LED filament may have a length, LF, and a width, WF, wherein LF > 5WF or even LF > 10WF. 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 has a lightemitting surface that extends along the length, and preferably at least partly around the axis of elongation. The light-emitting surface is preferably arranged to homogenously emit light and / or to emit light omnidirectionally.

[0020] The LED filament arrangement of 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 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 first major surface of 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.

[0021] The LED filament arrangement further comprises an elongated 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 encapsulant comprises luminescent material configured to at least partly convert the emitted LED light into primary 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 first major surface of the elongated carrier and the plurality of LEDs. The encapsulant may be a polymer material which may be flexible such as for example a silicone. The luminescent material of the elongated primary encapsulant may be a phosphor such as an inorganic phosphor and / or quantum dots or rods. The LED filament arrangement light comprises a portion of the LED light constituting nonconverted light, and the primary converted light. The LED filament further comprises a semi-reflective specular-reflecting mirror at least partially enclosing the LED filament arrangement. The semi-reflective specular-reflecting mirror, having a transmissivity, T, is configured to partly transmit the LED filament arrangement light into transmitted LED filament arrangement light, and wherein the semi-reflective specular-reflecting mirror, having a reflectivity, R, is configured to partly reflect the LED filament arrangement light into reflected LED filament arrangement light. Hence, the semi-reflective specular-reflecting mirror, via its transmissivity, T, and reflectivity, R, transmits and reflects to LED filament arrangement light into transmitted and reflected LED filament arrangement light. The semi-reflective specular-reflecting mirror has a transmissivity, T, of at least 70 % in a green wavelength range of 500 - 540 nm for transmitting green light, and in a red wavelength range of 600 - 690 nm for transmitting red light, and a reflectivity, R, of at least 80 % in the blue wavelength range of 430 - 470 nm for reflecting blue light. Hence, the semi-reflective specular-reflecting mirror has a relatively high transmissivity, T, for transmitting green light and red light (analogously, a relatively low reflectivity, R, for reflecting green light and red light) and a relatively high reflectivity, R, for reflecting blue light, i.e. a dominant reflectivity, R, for blue light. Preferably, a majority of the non-converted light may be reflected back to the elongated encapsulant. Furthermore, a majority of the non-converted light may preferably be converted into secondary converted light.

[0022] According to an embodiment of the present invention, one of the elongated carrier is light-transmissive, and the elongated carrier is light-transmissive and the elongated encapsulant at least partially covers a second major surface, opposite the first major surface, of the elongated carrier, is fulfilled. Hence, the elongated carrier may be light-transmissive. Alternatively, the elongated carrier is light-transmissive and the elongated encapsulant at least partially covers a second major surface, opposite the first major surface, of the elongated carrier. It should be noted that part of the light which is transmitted through the light-transmissive carrier may be converted by the luminescent material. The present embodiment is advantageous in that the light transmissive elongated carrier may achieve an (almost) omnidirectional light distribution of the LED filament. The present embodiment is further advantageous in that the elongated encapsulant may influence the light transmitted via the second major surface, due to the light transmissive property of the elongated carrier, thereby even further enhancing the aesthetical appearance of the LED filament arrangement during operation. According to an embodiment of the present invention, the luminescent material of the elongated encapsulant comprises at least one of a green-yellow phosphor, e.g. of the type AsBsO Ce, wherein A comprises one or more of Y, La, Gd, Tb and Lu, and wherein B comprises one or more of Al, Ga, In and Sc, and an orange-red phosphor, e.g. (of the type) comprising at least one of one or more nitrides and one or more oxynitrides. Hence, the luminescent material of the elongated encapsulant comprises a green-yellow phosphor and / or an orange-red phosphor having the respective features described. Examples of the green-yellow phosphor may be YAG phosphor and / or LuAG phosphor. The present embodiment is advantageous in that the LED filament, via the green-yellow phosphor and / or orange-red phosphor of the elongated encapsulant, provides a more pleasing, homogenous and / or efficient light distribution and high quality of light.

[0023] According to an embodiment of the present invention, the elongated encapsulant may be configured to transmit at least 40 % (or at least 60%) of the emitted LED light. Hence, the elongated encapsulant may have a transmissivity of at least 40 % (or at least 60%) of the emitted LED light. For example, the green-yellow phosphor and the orange-red phosphor may have an absorption A < 60 % (or A < 40 %). The present embodiment is advantageous in that the relatively large transmissivity of the LED light of the LED filament provides a further decrease in phosphor loading. Inventors have found that the higher the transmission of the emitted LED light through the elongated encapsulant, the lower the phosphor loading. However, the higher the transmission of the emitted LED light through the elongated encapsulant, the higher the reflectivity of the semi-reflective specular-reflecting mirror in the blue wavelength range of 430 - 470 nm needs to be. For example, the reflectivity, R, may be at least 85 % or at least 90 % in the blue wavelength range of 430 - 470 nm for reflecting blue light.

[0024] According to an embodiment of the present invention, the transmissivity, T, may be at least 85 % in the green wavelength range of 500 - 540 nm for transmitting green light (and optionally in the yellow-orange wavelength range of 541 - 599 nm for transmitting yellow-orange light), and in the red wavelength range of 600 - 690 nm for transmitting red light. Hence, the transmissivity, T, of the semi-reflective specular-reflecting mirror to partly transmit the LED filament arrangement light into transmitted LED filament arrangement light may be at least 85 % in the green (and yellow-orange) and red wavelength ranges. The present embodiment is advantageous in that the relatively large transmissivity of the LED filament light in the green (and optionally yellow-orange) and red wavelength ranges significantly contributes to the possibility of the LED filament to provide LED filament light with high efficiency. The reason is that (converted) light in this spectral is not blocked / reflected by the semi-reflective specular-reflecting mirror which would lead in light trapping re-absorption of converted light which lowers the efficiency.

[0025] According to an embodiment of the present invention, the semi-reflective specular-reflecting mirror may fully enclose the LED filament arrangement. The semi- reflective specular-reflecting mirror may hereby fully enclose the LED filament arrangement both in the circumferential direction and in the longitudinal direction. The present embodiment is advantageous in that the effect of the fully enclosing semi-reflective specular- reflecting mirror even further augments the desired light distribution and decorative light effect whilst providing an environmentally sustainable LED filament.

[0026] According to an example of the present invention, the semi-reflective specular-reflecting mirror may cover at least 90% or at least 95% such as 100% of the elongated encapsulant.

[0027] According to an embodiment of the present invention, the LED filament light may comprise at most 6% blue light in the blue wavelength range of 430 - 470 nm. Preferably, the LED filament light may comprise at most 3% blue light in the blue wavelength range of 430 - 470 nm.

[0028] According to an embodiment of the present invention, the semi-reflective specular-reflecting mirror may have, in an off-state of the LED filament, a blue appearance. Hence, the semi-reflective specular-reflecting mirror may appear blue for an observer. The present embodiment is advantageous in that the blue appearance may indicate an efficient light source. The present embodiment is further advantageous in that the semi-reflective specular-reflecting mirror is rendered attractive for an observer.

[0029] According to an embodiment of the present invention, the elongated encapsulant may comprise a light-scattering material configured to scatter at least part of the emitted LED light. Hence, the light-scattering material of the elongated encapsulant may be configured or arranged to at least partially diffusely transmit and diffusely reflect the LED filament light. By “diffusely transmit”, it is here meant that the light scattering material is configured to transmit the LED light whilst providing diffusion of the LED light upon transmission. By “diffusely reflect”, it is here meant that the light scattering material is configured to reflect the LED light whilst providing diffusion of the LED light upon reflection. The present embodiment is advantageous in that the scattering property of the elongated encapsulant may achieve an omnidirectional, or at least almost omnidirectional, light distribution. The present embodiment is further advantageous in that the elongated encapsulant’s scattering property may even further enhance the aesthetical effect and / or optical performance of the LED filament light.

[0030] According to an embodiment of the present invention, the semi-reflective specular-reflecting mirror may be at least one of a dichroic mirror, and flexible and arranged in a curved configuration around the LED filament arrangement. Hence, the semi-reflective specular-reflecting mirror may be a dichroic mirror and / or be flexible and be arranged in a curved configuration around the LED filament arrangement. By “dichroic mirror”, it is here meant a mirror which may selectively reflect light of a (small / limited) range of colors / wavelengths while transmitting other colors / wavelengths. The flexible property of the mirror and / or the mirror’s curved configuration around the LED filament arrangement even further increases the adaptability and customization of the LED filament.

[0031] According to an embodiment of the present invention, the LED filament arrangement may have one of a spiral shape and a helix shape, and wherein the semi- reflective specular-reflecting mirror has a corresponding one of a spiral shape and a helix shape, and a linear shape, and wherein the semi-reflective specular-reflecting mirror has a cylindrical shape. Hence, the LED filament arrangement may have a spiral shape or a helix shape, and the semi-reflective specular-reflecting mirror may have a corresponding spiral shape or helix shape. Alternatively, the LED filament arrangement may have a linear shape, and the semi-reflective specular-reflecting mirror may have a cylindrical shape. The present embodiment is advantageous in that the different shapes of the LED filament arrangement even further augments the decorative light effect and desired light distribution.

[0032] According to an embodiment of the present invention, the semi-reflective specular-reflecting mirror is at least one of continuous, and extends along a complete length, L’, of the elongated encapsulant, and in contact with the elongated encapsulant or is arranged at a distance < 3 mm to the elongated encapsulant. Hence, the semi-reflective specular- reflecting mirror is continuous and extends along a complete length, L’, of the elongated encapsulant and / or in contact with the elongated encapsulant or is arranged at a distance < 3 mm to the elongated encapsulant. Alternatively, the semi-reflective specular-reflecting mirror may extend along a complete length, L, of the LED filament arrangement. The present embodiment is advantageous in that the complete LED filament arrangement may be hidden by the semi-reflective specular reflecting mirror, and the light effect provided by the semi- reflective specular-reflecting mirror may be provided all throughout the LED filament. The present embodiment is further advantageous in that the semi-reflective specular-reflecting mirror, which may be in (physical / mechanical) contact with the encapsulant, provides a slim LED filament. Alternatively, the semi-reflective specular-reflecting mirror may be arranged at a distance (e.g. < 7 mm or < 3 mm) to the encapsulant, for example using an air gap or a transparent encapsulant e.g. of silicone. The transparent encapsulant may for example be a silicone glue fixing the semi-reflective specular-reflecting mirror to (the outside of) the encapsulant of the LED filament arrangement.

[0033] According to an embodiment of the present invention, the reflectivity, R, fulfills one of at least 90 % in the blue wavelength range of 430 - 470 nm for reflecting blue light, and at least 95 % in the blue wavelength range of 430 - 470 nm for reflecting blue light, and wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1500K - 2400K. Hence, the reflectivity, R, of the semi-reflective specular-reflecting mirror is at least 90 % in the blue wavelength range of 430 - 470 nm for reflecting blue light and / or the reflectivity, R, of the semi-reflective specular-reflecting mirror is at least 90 % in the blue wavelength range of 430 - 470 nm for reflecting blue light and wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1500K - 2400K. The very high reflectivity, R, for blue light and / or the warm white light provides an even more desired light distribution, aesthetically appealing appearance whilst ensuring a (more) sustainable LED filament.

[0034] According to an example of the present invention, the LED filament arrangement may comprise a circuitry, e.g. arranged on or in the elongated carrier, electrically connected to the plurality of LEDs.

[0035] According to an example of the present invention, the plurality of LEDs may comprise at least 20 LEDs or at least 30 LEDs.

[0036] According to an example of the present invention, the semi-reflective specular- reflecting mirror may have a thickness less than 1mm or less than 0.5 mm. The obtained effect is a slim LED filament.

[0037] According to an example of the present invention, the LED filament may have a thickness, T, a width, W, and a length, L, wherein L> 10-T and / or L > 10-W.

[0038] According to an example of the present invention, the LED filament may have a thickness T, a width, W, and a length, L, wherein, the width, W, and / or the thickness, T, may be at most 6 mm and the length, L, is at least 30 mm.

[0039] According to an embodiment of the present invention, there is provided a LED filament device comprising at least one LED filament according to any one of the preceding embodiments, and a transparent tube enclosing the at least one LED filament. The present embodiment is further advantageous in that the transparent tube provides an ingress protection of the LED filament.

[0040] According to an embodiment of the present invention, there is provided a LED filament lamp (and / or LED filament luminaire). The LED filament lamp comprises one of the LED filament according to any one of the preceding embodiments, and the LED filament device according to the previous embodiment. The LED filament lamp may further comprise a light-transmissive envelope at least partly enclosing the at least one LED filament, and / or a connector for electrically and mechanically connecting the LED filament lamp to a socket of a luminaire. 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 and / or via the feature(s) of the lamp.

[0041] According to an example of the present invention, the LED filament device or the LED filament lamp may further comprise a controller configured to control the luminous flux of the LED filament light. The controller may control each LED filament individually. By “controller”, it is here meant any device, unit, or the like, which is able to control the luminous flux either by wire or via wireless technology. The present example is advantageous in that the controller may conveniently and efficiently control the LED filament(s), 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).

[0042] 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.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] 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.

[0045] Fig. 1 shows a LED filament lamp according to the prior art,

[0046] Fig. 2 schematically shows a LED filament according to an exemplifying embodiment of the present invention,

[0047] Figs. 3a and 3b schematically show diagrams of transmissivity, T, and reflectivity, R, of a semi-reflective specular-reflecting mirror of a LED filament according to an exemplifying embodiment of the present invention, Figs. 4a-c schematically show diagrams as a result of simulations of the present invention, and

[0048] Figs. 5a-d are views of various LED filament lamps according to exemplifying embodiments 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 sustainability aspect of the LED filaments 20 and / or the LED filament lamps 10.

[0051] Fig. 2 schematically shows a cross-section of a LED filament 100 according to an exemplifying embodiment of the present invention. The LED filament 100 is configured to provide, during operation, LED filament light 105. Preferably, the LED filament 100 has a length, L, and a width W, wherein L > 5W. The width, W, may for example be in a range from 0.5 to 5 mm. The LED filament 100 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 100 comprises a LED filament arrangement 115 configured to provide, during operation, LED filament arrangement light 118. The LED filament arrangement 115 comprises an elongated carrier 120 and a plurality of light emitting diodes, LEDs 130 arranged on a first major surface 145 of the elongated carrier 120. The elongated carrier 120 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 120 may be reflective or light transmissive, such as translucent and preferably transparent. In Fig. 2, the elongated carrier 120 is exemplified as being light- transmissive and at least partially covering a second major surface 300, opposite the first major surface 145, of the elongated carrier 120.

[0052] It should be noted that the LED filament arrangement 115 may comprise substantially any number of LEDs 130, e.g. arranged in one or more arrays. 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 150, wherein the LED light 150 has a peak emission wavelength in a blue wavelength range of 430 - 470 nm. The LED filament arrangement 115 further comprises an elongated encapsulant 160 at least partially covering the first major surface 145 of the elongated carrier 120 and at least partially covering the plurality of LEDs 130. The elongated encapsulant 160 comprises a luminescent material. The luminescent material of the elongated encapsulant 160 may be a green-yellow phosphor of the type AsBsOn Ce, wherein A comprises one or more of Y, La, Gd, Tb and Lu, and wherein B comprises one or more of Al, Ga, In and Sc. Alternatively, or in combination with the green-yellow phosphor, the luminescent material of the elongated encapsulant 160 may be an orange-red phosphor of the type (comprising) nitrides and / or oxynitrides. Although not shown, a concentration of the luminescent material in the elongated primary encapsulant 160 may vary over the length of the LED filament arrangement 115. The luminescent material of the elongated encapsulant 160 is configured to at least partly convert the emitted LED light 150 into primary converted light 170, whereby the LED filament arrangement light 118 comprises a portion of the LED light constituting non-converted light 180, and the converted light 170. The elongated encapsulant 160 may furthermore comprise a light-scattering material configured to scatter at least part of the emitted LED light 150.

[0053] The LED filament 100 further comprises a semi-reflective specular-reflecting mirror 200 at least partially enclosing the LED filament arrangement 115. As exemplified in Fig. 2, the semi-reflective specular-reflecting mirror 200 fully encloses the LED filament arrangement 115 both in the circumferential direction as well as in the longitudinal direction thereof. To this end, the semi-reflective specular-reflecting mirror 200 may be continuous and arranged (e.g. in a curved configuration) around the complete outer profile of the LED filament arrangement 115, and also extend along the complete length, L’, of the elongated encapsulant 160 (or alternatively, along the complete length, L, of the LED filament arrangement 115). Furthermore, in case the LED filament arrangement 115 has a linear (straight) shape, the semi- reflective specular-reflecting mirror 200 may have a corresponding cylindrical shape, e.g. a right circular or elliptical cylinder, as in Fig. 2. Alternatively, in case the LED filament arrangement 115 has a spiral or helix shape, the semi-reflective specular-reflecting mirror 200 may have a corresponding spiral or helix shape, resulting in that the LED filament 100 (overall) has a spiral or helix shape or configuration. Furthermore, the semi-reflective specular-reflecting mirror 200 may be in direct contact with the encapsulant 160 by using e.g. a glue. Alternatively, the semi-reflective specular-reflecting mirror 200 may be arranged at a distance D to the encapsulant 160, for example using an air gap or a transparent (further) encapsulant 165 e.g. of silicone. The air gap may be realized by using a holder (not shown). The holder may be attached e.g. to part of the electrodes at the ends of the LED filament 100. The transparent encapsulant 165 may for example be a silicone glue fixing the semi- reflective specular-reflecting mirror 200 to the encapsulant 160. The distance D is preferably < 3 mm. The thickness of the semi-reflective specular-reflecting mirror 200 may be in the range of 0.05-2 mm, typically 0.2 mm, whereby LED filament 100 may maintain a typical, slim appearance.

[0054] The semi-reflective specular-reflecting mirror 200 has a transmissivity, T, and is configured to partly transmit the LED filament arrangement light 118 into transmitted LED filament arrangement light 220, as schematically indicated. The semi-reflective specular- reflecting mirror 200 further has a reflectivity, R, and is configured to partly reflect the LED filament arrangement light 118 into reflected LED filament arrangement light 210, as schematically indicated. The transmissivity, T, of the semi-reflective specular-reflecting mirror 200 is at least 70 % in a green wavelength range of 500 - 540 nm for transmitting green light. Alternatively, or additionally, the transmissivity, T, of the semi-reflective specular-reflecting mirror 200 is at least 70 % in a red wavelength range of 600 - 690 nm for transmitting red light. Furthermore, the reflectivity, R, of the semi -reflective specular- reflecting mirror 200 is at least 80 % in the blue wavelength range of 430 - 470 nm for reflecting blue light. The non-converted light 180 is reflected back to the elongated encapsulant 160 and at least a portion of the non-converted light 180 is converted into secondary converted light 170b. The LED filament light 105 is white light having a correlated color temperature, CCT, in a range of 1500K - 2700K.

[0055] Fig. 3a schematically shows a diagram of transmissivity, T (in %) as a function of wavelength (in nm) of a semi-reflective specular-reflecting mirror of a LED filament according to an exemplifying embodiment of the present invention. The transmissivity, T, of the semi-reflective specular-reflecting mirror 200 is at least 70 % in a green wavelength range of 500 - 540 nm for transmitting green light. Alternatively, or additionally, the transmissivity, T, of the semi-reflective specular-reflecting mirror 200 is at least 70 % in a red wavelength range of 600 - 690 nm for transmitting red light.

[0056] Fig. 3b schematically shows a diagram of reflectivity, R (in %, y-axis) as a function of wavelength (in nm, x-axis), of a semi-reflective specular-reflecting mirror of a LED filament according to an exemplifying embodiment of the present invention. The reflectivity, R, of the semi-reflective specular-reflecting mirror 200 is at least 80 % in the blue wavelength range of 430 - 470 nm for reflecting blue light.

[0057] Figs. 4a and 4b schematically show diagrams of power (W) / nanometer (y- axis) as a function of wavelength (nm, x-axis) as a result of simulations of the present invention. In Fig. 4a, there is a peak of the power in a blue wavelength range of 450 - 470 nm, whereas the power for higher wavelengths, including a green wavelength range of 500 - 540 nm and a red wavelength range of 600 - 690 nm, is relatively low. In Fig. 4b, with a correlated color temperature, CCT, of 2629 K, a color rendering index, CRI, of 93, and a color rendering index value, R9, of 77, there is a peak of the power in a blue wavelength range of 450 - 470 nm, a shoulder / plateau in the green wavelength range of 500 - 540 nm and a peak of the power in the red wavelength range of 600 - 690 nm.

[0058] Fig. 4c schematically shows a diagram of transmitted blue intensity, TBI, (%, y-axis), as a function of blue reflectivity intensity, BRI, (%, x-axis) of the semi-reflective specular-reflecting mirror for phosphor absorption of 80%, 60%, and 40%. The minimum reflectivity percentage in the blue wavelength range of 430 - 470 nm may be at least 80 % for a desired blue off-state appearance of the LED filament, and preferably at least 90 % in order to (even further) reduce the phosphor load. For low correlated color temperatures, CCT < 2400K, it is preferred that the minimum reflectivity percentage in the blue wavelength range of 430 - 470 nm is at least 95 %. The transmitted blue intensity increases with decreasing phosphor absorption, wherein the absorption is preferably at most 60%, even more preferred at most 40 % for reduction of the phosphor load with a factor 2. The semi- reflective specular-reflecting mirror preferably has a transmissivity of at least 70 %, preferably at least 80 %, and even more preferred at least 90 %, in the green wavelength range of 500 - 540 nm the red wavelength range of 600 - 690 nm to keep the efficiency high.

[0059] Figs. 5a-d are views of various LED filament lamps or luminaires 900a-d, each comprising at least one LED filament 100. Fig. 5a is a perspective view, and Figs. 5b-d are side views. Each LED filament lamp 900a-d may further comprise a light transmissive envelope 910 inside which the at least one LED filament 100 is arranged, and a cap (comprising a base) 920 for electrically and mechanically connecting the LED filament lamp 900a-d to a socket of a luminaire (not shown). The light transmissive envelope 910 may for example be made of glass. The light transmissive envelope 910 may have various shapes, including bulb, candle, globe, etc. The cap or base 920 may for example be E14, E27, B22, etc. The LED filament lamp 900a-d may for example be a (retrofit) light bulb. In Fig. 5a, the LED filament lamp 900a comprises multiple LED filaments 100, namely four LED filaments 100. Each LED filament 100 here has a linear or straight configuration. The LED filaments 100 may be substantially vertically arranged and / or may be substantially parallel to each other. The LED filaments 100 may have semi-reflective specular-reflecting mirrors with the same or different (reflective and transmissive) properties.

[0060] In Figs. 5b and 5c, each LED filament lamp 900b, 900c comprises only one LED filament 100. The LED filament 100 here has a spiral or helix configuration. The LED filament 100 may be substantially vertically arranged, as seen in the view of Figs. 5b and 5c.

[0061] The semi-reflective specular-reflecting mirror of the LED filament 100 in Fig. 5b may for example be a dichroic mirror configured to dominantly reflect blue light. In this way, the LED filament 100 in Fig. 5b may look (more) blue when OFF, due to blue color in ambient light being reflected by the dichroic mirror.

[0062] The semi-reflective specular-reflecting mirror of the LED filament 100 in Fig. 5c has a color-variable reflection over its length, L”, varying from a lower region 40a (closer / proximal to cap 920) and gradually transitioning to a top region 40d (further from / distal of cap 920).

[0063] Fig. 5d schematically shows a LED filament lamp 900d according to an embodiment of the present invention. The LED filament lamp 900d, 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 900d further comprises a light-transmissive envelope 910, which is exemplified as being bulbshaped. The light-transmissive envelope 910 at least partially encloses the LED filament(s) 100. The LED filament lamp 900d further comprises a connector 920 for electrically and mechanically connecting the LED filament lamp 900d to a socket of a luminaire. The LED filament lamp 900d may further comprise a controller 930 configured to control the luminous flux of the LED filament light. The controller 930 is schematically indicated by a dashed rectangle in the connector 920, but it should be noted that the controller 930 may be arranged at substantially any position or place. The controller 930 may, for example, be able to control each LED filament 100 individually.

[0064] 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 encapsulant 160, the semi-reflective specular- reflecting mirror 200, 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, during operation, LED filament light (105), comprising a LED filament arrangement (115) configured to provide, during operation, LED filament arrangement light (118), wherein the LED filament arrangement 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), wherein the LED light has a peak emission wavelength in a blue wavelength range of 430 - 470 nm, an elongated encapsulant (160) at least partially covering the first major surface of the elongated carrier and at least partially covering the plurality of LEDs, wherein the elongated encapsulant comprises luminescent material configured to at least partly convert the emitted LED light into primary converted light (170), whereby the LED filament arrangement light comprises a portion of the LED light constituting non-converted light (180), and the primary converted light, a semi-reflective specular-reflecting mirror (200) at least partially enclosing the LED filament arrangement, wherein the semi-reflective specular-reflecting mirror, having a transmissivity, T, is configured to partly transmit the LED filament arrangement light into transmitted LED filament arrangement light (220), and wherein the semi-reflective specular- reflecting mirror, having a reflectivity, R, is configured to partly reflect the LED filament arrangement light into reflected LED filament arrangement light (210), wherein the semi-reflective specular-reflecting mirror has a transmissivity, T, of at least 70 % in a green wavelength range of 500 - 540 nm for transmitting green light, and in a red wavelength range of 600 - 690 nm for transmitting red light, and a reflectivity, R, of at least 80 % in the blue wavelength range of 430 - 470 nm for reflecting blue light, whereby the non-converted light is reflected back to theelongated encapsulant and whereby at least a portion of the non-converted light is converted into secondary converted light (170b), wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1500K - 2700K, and wherein the semi-reflective specular-reflecting mirror fully encloses the LED filament arrangement.

2. The LED filament according to claim 1, wherein one of the elongated carrier is light-transmissive, and the elongated carrier is light-transmissive and the elongated encapsulant at least partially covers a second major surface (300), opposite the first major surface, of the elongated carrier, is fulfilled.

3. The LED filament according to claim 1 or 2, wherein the luminescent material of the elongated encapsulant comprises at least one of a green-yellow phosphor of the type AsBsOn Ce, wherein A comprises one or more of Y, La, Gd, Tb and Lu, and wherein B comprises one or more of Al, Ga, In and Sc, and an orange-red phosphor comprising at least one of at least one nitride, and at least one oxynitride.

4. The LED filament according to any one of the preceding claims, wherein the elongated encapsulant is configured to transmit at least 40 % of the emitted LED light.

5. The LED filament according to any one of the preceding claims, wherein the transmissivity, T, is at least 85 % in the green wavelength range of 500 - 540 nm for transmitting green light, and in the red wavelength range of 600 - 690 nm for transmitting red light.

6. The LED filament according to any one of the preceding claims, wherein the LED filament light comprises at most 6% blue light in the blue wavelength range of 430 - 470 nm.

7. The LED filament according to any one of the preceding claims, wherein the semi-reflective specular-reflecting mirror has, in an off-state of the LED filament, a blue appearance.

8. The LED filament according to any one of the preceding claims, wherein the elongated encapsulant comprises a light-scattering material configured to scatter at least part of the emitted LED light.

9. The LED filament according to any one of the preceding claims, wherein the semi-reflective specular-reflecting mirror is at least one of a dichroic mirror, and flexible and arranged in a curved configuration around the LED filament arrangement.

10. The LED filament according to any one of the preceding claims, wherein the LED filament arrangement has one of one of a spiral shape and a helix shape, and wherein the semi-reflective specular-reflecting mirror has a corresponding one of a spiral shape and a helix shape, and a linear shape, and wherein the semi-reflective specular-reflecting mirror has a cylindrical shape.

11. The LED filament according to any one of the preceding claims, wherein the semi-reflective specular-reflecting mirror is at least one of continuous, and extends along a complete length, L', of the elongated encapsulant, and in contact with the elongated encapsulant or is arranged at a distance < 3 mm to the elongated encapsulant.

12. The LED filament according to any one of the preceding claims, wherein wherein the reflectivity, R, fulfills one of at least 90 % in the blue wavelength range of 430 - 470 nm for reflecting blue light, andat least 95 % in the blue wavelength range of 430 - 470 nm for reflecting blue light, and wherein the LED filament light is white light having a correlated color temperature, CCT, in a range of 1500K - 2400K.

13. A LED filament device, comprising at least one LED filament according to any one of the preceding claims, and a transparent tube enclosing the at least one LED filament.

14. A LED filament lamp (900a-d), comprising one of the LED filament according to any one of claims 1-12, and the LED filament device according to claim 13, a light-transmissive envelope (910) at least partly enclosing the at least one LED filament, and a connector (920) for electrically and mechanically connecting the LED filament lamp to a socket of a luminaire.

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