EP4671601A1

EP4671601A1 - LED FILAMENT LIGHTING DEVICE

Info

Publication number: EP4671601A1
Application number: EP24184303.6A
Authority: EP
Inventors: Erik Paul Boonekamp
Original assignee: Signify Holding BV
Current assignee: Signify Holding BV
Priority date: 2024-06-25
Filing date: 2024-06-25
Publication date: 2025-12-31
Also published as: WO2026002640A1

Abstract

A lighting device (10) configured to, in operation, provide lighting device light. The lighting device comprising a housing (20) and a light source (50). The housing comprises a cavity surface (23) defining an inner cavity (24) of the housing. The inner cavity has a first window (21) in a first direction (D1) and a second window (22) in a second direction (D2), opposite to the first direction. The first window has a first area (A1), and the second window has a second area (A2). The light source comprises a plurality of light emitting diode, LED, filaments (52) configured to, in operation, provide LED filament light within the inner cavity. The LED filaments are electrically connected between first and second tracks. The LED filament light leaves the housing through either the first window or the second window. A luminaire comprising a lighting device is also disclosed.

Description

FIELD OF THE INVENTION

The present invention generally relates to lighting devices. More specifically, the present invention is related to LED filament lighting devices intended to be suspended, e.g. from a ceiling.

BACKGROUND OF THE INVENTION

The use of light emitting diodes (LED) 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.
There is a continuous drive to improve the performance of LED based systems. A relatively new LED type is the LED filament which is frequently and exclusively used in LED lamps, typically referred to as "bulbs". A LED filament is a long rod-shaped emitter with a typical length of 30-50 mm and a diameter of 1-3 mm. The LED filament emits light over the whole surface area, comparable to a fluorescent tube. A LED filament is typically composed of a linear array of blue LED dies on a lead frame supported by a ceramic bar. The linear array is covered with a layer of blue light converting material, referred to as "phosphor". This specific "packageless" construction results in LED filaments with a high efficacy of typically 230 Im/W. Up to now, LED filaments are exclusively applied in light bulbs.
Hence, it is an object of the present invention to expand the use of LED filaments into other applications, taking advantage of the attractive properties of LED filaments.

SUMMARY OF THE INVENTION

It is of interest to utilize the advantageous properties of LED filaments with respect energy efficiency, light distribution purposes and/or aesthetics within different types of lighting devices. For instance, LED filaments can be applied in other lighting applications such as office lighting, creating several advantages compared to the more conventional small SMD LEDs based systems.
This and other objects are achieved by providing a LED filament lighting device and a luminaire having the features in the independent claims. Preferred embodiments are defined in the dependent claims.
Hence, according to a first aspect of the present invention, there is provided a lighting device configured to, in operation, provide lighting device light.
The lighting device comprises a housing and a light source, wherein the light source is provided in the housing.
The housing comprises an inner cavity bounded by a cavity surface, wherein the cavity surface is at least partly reflective.
The inner cavity of the housing has a first window in a first direction and a second window in a second direction, opposite to the first direction. The first window has a first area, and the second window has a second area.
The light source comprises a plurality of light emitting diode (LED) filaments configured to, in operation, provide LED filament light within the inner cavity. The LED filaments are electrically connected between a first track and a second track.
The LED filament light leaves the housing through at least one of the first window and the second window.
The lighting device is intended to be a so-called suspended luminaire. However, it may also be utilized in other configurations where illumination in two opposite directions is requested. These suspended luminaires require in general both a downlight with low glare and an uplight to illuminate the ceiling. Since LED filaments are omnidirectional sources, there is a natural fit with suspended luminaire applications. In other words, there is a particular advantage of using LED filaments in suspended luminaires. The light flux from the LED filaments is split into two fractions. One fraction is used to give a downward directed beam, preferably with low glare, to illuminate for example a table or a floor. The other fraction provides an upward directed beam to illuminate the ceiling. The ratio between the flux up and the flux down can be defined in the optic properties of the housing and windows.
The present invention is advantageous in that the LED filaments are natural omnidirectional light sources, emitting light in all directions. By providing windows in the housing in opposite directions, light distributed in opposite directions can easily be achieved. By the reflective property of the inner cavity surface, a majority of the emitted light can be directed to exit the lighting device through the windows.
The lighting device may have the first and second tracks provided outside the inner cavity. The electrical contacts of the LED filaments to the first and second tracks are thereby arranged through the cavity surface.
By providing the tracks outside the inner cavity, the arrangements of the tracks do not contribute to any absorption of light within the inner cavity. A high light efficiency within the inner cavity is thereby achieved. Wires, screws, connectors etc. do not interact directly with the light.
Furthermore the LED filaments may be provided in such a way that they, or at least one or a few of them, extend an entire distance between two points of the cavity surface.
In other words, one or more LED filaments may have a light emitting surface that bridges a distance in the inner cavity from a first point on a first area of the cavity surface to a second point on a second area of the cavity surface. The second area may be on opposite sides of the inner cavity 24, but they may also be on the same side of the inner cavity 24. This further contributes to removing parts that do not directly contribute to emission of light from the inner cavity, thereby increasing the light distribution efficiency further.
The housing may comprise a first compartment and a second compartment provided on opposite sides of the inner cavity. The first compartment may accommodate the first track, and the second compartment may accommodate the second track. The tracks and other parts associated therewith can thereby be hidden from being seen of touched. This increases the electrical safety and improves the aesthetic impression. By having compartments on opposite sides of the inner cavity also facilitates the use of straight LED filaments. Alternatively, if only one compartment is provided, non-straight LED filaments can be used.
The use of LED filaments in a lighting device enables controlling the properties of the emitted light in different ways. Geometrical relations and control options may thereby be further used for obtaining requested properties.
In the lighting device, the plurality of LED filaments may comprise one or more first LED filaments configured to, in operation, provide LED filament light of a first correlated color temperature. The plurality of LED filaments may further comprise one or more second LED filaments configured to, in operation, provide LED filament light of a second correlated color temperature, wherein the second correlated color temperature is different from the first correlated color temperature. The lighting device light can thereby be a mix of light from these different LED filaments, achieving an overall correlated color temperature between the first correlated color temperature and the second correlated color temperature.
As used herein, the term "warm white light" is intended to refer to light with color temperature falling within the color temperature interval of 2000 K to 3500 K.
As used herein, the term "cool white light" is intended to refer to light with color temperature falling within the color temperature interval of 4500 K to 6500 K.
The first correlated color temperature may be a cool white temperature and the second correlated color temperature may be a warm white temperature. The first correlated color temperature may in the range of 4500 K to 6500 K and the second correlated color temperature may in the range of 2000 K to 3500 K. A large range of possible overall correlated color temperatures is thereby available.
When the lighting device has first and second LED filaments as described above, it may be arranged such that the first LED filaments and the second LED filaments can be operated independently from each other. In other words, an operation of the first LED filaments may be allowed independently of an operation of the second LED filaments and likewise an operation of the second LED filaments may be allowed independently of an operation of the first LED filaments. This enables a high flexibility in modifying the properties of the lighting device light.
The first LED filaments may be interleaved with the second LED filaments in a direction along a main extension direction of the inner cavity. Preferably, the first LED filaments and the second LED filaments may be arranged alternatingly. Since the LED filaments typically are of a relatively limited length, the inner cavity has typically a shape involving a main extension direction, across which the LED filaments are provided. By interleaving the different types of LED filaments along that main extension direction, preferably in an alternating configuration, a good light mixing can be achieved.
As used herein, the term "main extension" is connected to a shape of a cavity having a high aspect ratio, giving an extension in one "main" direction that is much larger than extensions in transversal directions. One example of such a cavity is a parallelepipedal cavity with one main axis being much longer, typically more than 5 times longer, than the others. The main extension is in such an example along this main axis. In an example of an annular cavity, the extension along the tangential direction, i.e. the circumference, is typically much longer than the radial or axial extensions. In such an example, the main extension direction is along the tangential direction.
The first window may be covered by a micro lens optic plate. The provision of the micro lens optic plate enables provision of a low glare light through the first window. In applications of a suspended lighting device, such low glare illumination is highly appreciated for the downwards directed light, and the lighting device may therefore advantageously be turned with the first window facing downwards.
The present lighting device provides light in two opposite directions. A size of the first area and a size of the second area determine the amount of the LED filament light leaving the housing through the first window and the amount of the LED filament light leaving the housing through the second window.
A flux ratio between light exiting the lighting device through the first window and light exiting the lighting device through the second window may be within the range of 1 to 10, and preferably within the range of 2 to 5.
The asymmetric provision of light may be useful in many applications. For the application in a suspended lighting device, it is usually considered pleasant to have a stronger light pointing downwards towards the items intended to be illuminated and just having a weaker light illuminating the ceiling.
If the partition of the light intensity is requested to be controllable on site, at least a part of the cavity surface may be configured to allow mechanical adjustment of at least one of the first area and the second area. In this way, the flux ratio can be adapted according to the requested conditions.
The housing may further comprise a translucent plate covering the second window. Preferably, the translucent plate is a transparent plate, a plate comprising scattering particles, a plate having surface texture on at least one side, a plate having lenslets or prism structures, a single lens, and/or a plate comprising light guides. The properties of the light exiting the second window may thereby be modified in very many different ways.
The shape of the lighting device may be of various kinds. However, practical solutions are typically involving highly symmetric shapes of the inner cavity, and typically the inner cavity has a shape presenting an elongates shape in a main extension direction. This direction may e.g. be linear or circular. The inner cavity may e.g. be a linearly extending cavity, or an annular cavity. Also inner cavities being circular cavities may be useful.
According to a second aspect of the present invention, there is provided a luminaire comprising a lighting device according to the first aspect.
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.

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.

Fig. 1 shows an exploded elevational view of a lighting device.
Fig. 2 is a cross-sectional view of a lighting device.
Figs. 3A to 3E schematically illustrate different LED filaments in light sources.
Figs. 4A and 4B schematically illustrate a lighting device with a flexible window.
Figs. 5A to 5C are different views of a lighting device with an annular inner cavity.
Figs. 6A and 6B are different views of a lighting device with a circular inner cavity.
Fig. 7 is a schematic illustration of a luminaire.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present technology is based on the insight that since LED filaments are omnidirectional sources, they are promising light emitters for achieving controllable light in mor than one direction. In many applications, where a main direction of light is utilized, the omnidirectional emission may be a disadvantage, however, when a plurality of light directions is requested, the use of LED filaments instead gives a synergetic effect.
Figure 1a schematically shows an exploded elevational view of a lighting device 10. The lighting device 10 is configured to, in operation, provide lighting device light.
The lighting device 10 is a linear lighting device that extends in a main extension direction E.
The lighting device 10 comprises a housing 20, and the housing 20 comprises an inner cavity 24 that is bounded by a cavity surface 23. The cavity surface 23 is, at least to a part, a reflective surface. The inner cavity 24 has a first window 21 in a first direction D1 and a second window 22 in a second direction D2, opposite to the first direction D1. Both the first direction D1 and the second direction D2 are perpendicular to the main extension direction E.
The lighting device 10 is intended to provide light exiting upwards and light exiting downwards. The lighting device 10 is intended to be used in a suspended luminaire.
The first window 21 has a first area A1, and the second window 22 has a second area A2.
In the lighting device 10 of Figure 1, the first window 21 is covered by a translucent layer, in this case a micro lens optic plate 40. Such a micro lens optic plate 40 is arranged to provide light with a low glare, which often is requested in suspended lighting devices. Light intended to be illuminating, for example, a desk or a floor is often more attractive when having passed a micro lens optic plate 40. However, other translucent layers may be used, or may be omitted altogether, depending on the actual application.
In the lighting device 10 of Figure 1, a translucent plate 30 covers the second window 22. Such a plate is optional, but it may be preferred in order to prevent contamination of the inner cavity 24. The translucent plate 30 can be of many different types and may easily be selected in dependence to any particular demands of the intended application. If the lighting device 10 is intended for a suspended luminaire, the light intended to illuminate the ceiling can thereby be modified in any requested way. The translucent plate 30 can have a wide range of optical properties. The translucent plate 30 can be transparent, such as a plate made from any one of PC, PMMA, and glass. The translucent plate 30 may contain light scattering particles. The translucent plate 30 can have a surface texture on one or both sides. The translucent plate 30 may also have lenslets (for example, it may be a lenticular plate), or prism structures on one or more of its surfaces. It is also possible that the translucent plate 30 contains a large lens, then typically directly in front of the window 22 to shape the upward beam to the ceiling. The translucent plate 30 may also contain a light guide to guide light from the cavity close to the filaments to the upper exit. In other words, often requested translucent plates 30 are a transparent plate, a plate comprising scattering particles, a plate having surface texture on at least one side, a plate having lenslets or prism structures, a single lens, or a plate comprising light guides. One or more of these functional plates may also be combined.
The lighting device 10 of Figure 1 is a linear lighting device that extends in a main extension direction E. A light source 50 comprises a plurality of LED filaments 52. The LED filaments 52 are configured to, in operation, provide LED filament light within the inner cavity 24. This is performed according to, as such, well known procedures. The LED filament light reaches one or more of the first window 21 and the second window 22, either directly or via one or more reflections in the cavity surface 23. In other words, the LED filament light leaves the housing 20 through one or more of the first window 21 and the second window 22.
The size of the first area A1 and the size of the second area A2 determine, together with the optical properties and shape of the cavity surface 23, the amount of the LED filament light leaving the housing 20 through the first window 21 and the amount of the LED filament light leaving the housing 20 through the second window 22, respectively.
A reduction of the first area A1 will typically lead to a reduced amount of light exiting the housing 20 through the first window 21. The exact relation between a change in the first area A1 and a change in the amount of light exiting the first window 21 depends further on the shape and optical properties of the cavity surface 23 as well as on the size of the second area A2.
Likewise, a reduction of the second area A2 will typically lead to a reduced amount of light exiting the housing 20 through the second window 22. Also here, the exact relation between a change in the second area A2 and a change in the amount of light exiting the second window 22 depends further on the shape and optical properties of the cavity surface 23 as well as on the size of the first area A1.
In many applications, in particular in suspended luminaires, it is typically requested that the light towards the floor is more intense than the light towards the ceiling.
A flux ratio between light exiting the lighting device 10 through the first window 21 and light exiting the lighting device 10 through the second window 22 may be within the range of 1 to 10. In other words, the ratio of flux up and flux down can be adjusted by the area of the upper window related to the area of the lower window.
Figure 2 schematically shows a cross section of a lighting device 10. The lighting device 10 comprises an array of LED filaments 52, of which one is visible in the figure.
Electrical contacts 54, for example contact strips of the LED filaments 52, stick through the wall, i.e. the cavity surface 23, of the inner cavity 24 into side compartments 25.
To provide the required reflectivity, the cavity surface 23 may comprise a white, diffusely-reflective material.
In the side compartments 25, the electrical contacts 54 are connected to tracks 26, one for each side of the LED filament 52, providing necessary electrical connections. In other words, the LED filaments 52 are electrically connected between first and second tracks 26. Furthermore, the first and second tracks 26 are provided outside the inner cavity 24, wherein electrical contacts 54 of the LED filament 52 are arranged through the cavity surface 23. One advantage of such an arrangement is that the optical cavity 24 remains free from light absorbing parts, such as wires, screws, connectors etc. Therefore, this concept has a very high optical efficiency.
In typical arrangements, in particular where straight LED filaments 52 are used, the LED filaments 52 are provided across the inner cavity 24. In other words, the housing 20 may comprise compartments 25 - a first compartment and a second compartment - provided on opposite sides of the inner cavity 24. The first compartment accommodates the first track, and the second compartment accommodates the second track.
Each LED filament 25 has a light emitting surface, which surface may extend around the LED filament so that the LED filament is essentially an omnidirectional light source. Preferably, at least one, but more preferably all LED filaments 25 have a light emitting surface that extends an entire distance between two points of the cavity surface 23.
In other words, one or more LED filaments 25 may have a light emitting surface that bridges a distance in the inner cavity 24 from a first point on a first area of the cavity surface 23 to a second point on a second area of the cavity surface 23. The first area and the second area of the cavity surface 23 may be located on opposite sides of the inner cavity 24, but they may also be located on the same side of the inner cavity 24.
This has the advantage that all parts necessary to operate the LED filament 52 that are present within the optical cavity 24 are the light emitting parts themselves. Light absorbing parts within the optical cavity 24 are thus reduced to a minimum.
The LED filaments 52 provided in the inner cavity 24 may be of a same type, giving a same spectral distribution and a same correlated color temperature. However, this reduces the flexibility in adapting the emitted light.
Therefore, as for instance illustrated in Figure 3A, the plurality of LED filaments of the light source 50 may comprise one or more first LED filaments 52A and one or more second LED filaments 52B. The first LED filaments 52A are configured to, in operation, provide LED filament light of a first correlated color temperature. The second LED filaments are configured to, in operation, provide LED filament light of a second correlated color temperature, different from the first correlated color temperature.
Further sets of LED filaments may also be provided, such as third LED filaments, fourth LED filaments, etc., giving light of a respective correlated color temperature.
The first correlated color temperature may be a cool white temperature, in which case the second correlated color temperature may be a warm white temperature.
The first correlated color temperature may be in the range of 5500 K to 6500 K, in which case the second correlated color temperature may be in the range of 2000 K to 3000 K.
In Figure 3A, the LED filaments 52A, 52B are provided in an array having a "rope ladder" arrangement. The LED filaments 52A, 52B are then connected between tracks 26 on each side of the inner cavity 24, outside the cavity surfaces 23. The first LED filaments 52A are interleaved with the second LED filaments 52B in a direction along a main extension direction E of the inner cavity 24. In this configuration, the first LED filaments 52A and the second LED filaments 52B are arranged alternatingly. By providing the two different kinds of LED filaments close together, a mixing of their individual lights is facilitated. In this configuration, the lighting device light can have a correlated color temperature between the correlated color temperature of the first LED filaments and the correlated color temperature of the second LED filaments.
Figure 3B again shows a light source 50. Here, one side of the first LED filaments 52A are connected to one track 26A, while one side of the second LED filaments 52B are connected to another track 26B. Thereby, the different sets of LED filaments can be controlled separately. In other words, an operation of the first LED filaments 52A is allowed independently of an operation of the second LED filaments 52B and an operation of the second LED filaments 52B is allowed independently of an operation of the first LED filaments 52A. Both filament types can thus be driven independently and therefore the correlated color temperature of the lighting device light can be adjusted between the correlated color temperatures of the first and second, respectively, LED filaments 52A, 52B.
If, for instance, the warm white LED filaments have a correlated color temperature of 2200 K and the cool white LED filaments have a correlated color temperature of 6500 K, the lighting device light can have correlated color tempartures in a range from 2200 K to 6500 K by different operations of the different kinds of LED filaments.
The LED filaments of the different types do not necessarily have to be parallel.
Figure 3E illustrates a configuration of a light source 50 having a zig-zag pattern of LED filaments 52A, 52B. In this configuration, the number of penetrations of the cavity surfaces 23 is reduced.
In Figure 3D, curved LED filaments 52A, 52B are used. This opens for providing the electrical contacts through only one side of the inner cavity 24. In such a configuration, the LED filaments 52A, 52B can have a light emitting surface that bridges a distance in the inner cavity 24 from a first point on a first area of the cavity surface 23 to a second point on a second area of the cavity surface 23, wherein the first and second areas are on the same side of the inner cavity 24.
In Figure 3E, parallel LED filaments 52A, 52B, inclined compared to the main extension direction E are provided. The first LED filaments 52A are interleaved with the second LED filaments 52B in a direction along a main extension direction E of the inner cavity 24. However, there are twice as many first LED filaments 52A compared to second LED filaments 52B, which means that some first LED filaments 52A are provided next to each other.
The person skilled in the art will appreciate that there are a countless number of variations to this theme.
As was mentioned above, the ratio of the lighting device light exiting through the different windows depends on the size of the windows.
Figure 4A illustrates schematically a cross-sectional view of a lighting device 10 having a second window 22 with an area A2. In Figure 4B, a part of the cavity surfaces 23 has been tilted, resulting in that the area A2 has decreased. This will also lead to a decrease in the amount of lighting device light exiting through the second window 22. At least a part of the cavity surface 23 may be configured to allow mechanical adjustment of at least one of the first area A1 and the second area A2.
Different further configurations of a lighting device 10 are also feasible. For most of them, there is a strive to form the inner cavity into a shape capable to support the filaments and simultaneously hide most of the contact wires and other items, such as screws, connectors, etc., in the adjacent compartments.
Figures 5A to 5C illustrate schematical drawings of a lighitng device having an annular inner cavity 24.
An outer ring 27 serves as one side of the annular inner cavity 24, while a central disc 28 forms an opposite side of the annular inner cavity 24. The LED filaments 52 are provided across the annular inner cavity 24 between the outer ring 27 and the central disc 28.
The LED filaments 52 may be provided in a radial direction, as illustrated. However, LED filaments provided with a non-zero angle to the radial direction are also feasible as well as many other configurations.
The area ratio of the upper window 22 having the area A2 and the lower window 21 having the area A1 is one of the factors determining the up/down flux ratio. An advantage of this configuration may be that the beam shape of the up and downward directed beams can be perfectly rotationally symmetric.
Figures 6A and 6B illustrate another configuration of a lighting device 10. In this case, the inner cavity 24 has not any particular extended direction. Instead, the inner cavity 24 has a circular shape. The LED filaments may then be connected between different parts of the surrounding outer ring 27, whereby all tracks may be comprised in one and the same compartment 25. A slight disadvantage of this design is that it is difficult to achieve a uniform distribution of the lighting device light.
In other words, the inner cavity 24 may be a linearly extending cavity, a circular cavity, or an annular cavity.
Figure 7 is an elevational view of a luminaire 1 having a lighting device 10 that is configured according to the description hereinabove.
The luminaire 1 comprises suspension means 2 for suspending the luminaire 1 from a surface, such as from a ceiling. Electrical connections 3 provides for the electrical power to the lighting device 10.
The present technology presents the application of LED filaments in a new generation of high efficiency, preferably suspended, LED modules and is relevant for both office and retail applications. The omnidirectional nature of LED filaments has a "natural fit" with the required performance of suspended lighting modules, since it gives both upwards and downwards directed lighting. The so-called "rope ladder" configuration of the filaments is one efficient embodiment, hiding all light absorbing parts related to the filaments. This enables designs very efficient optical cavities. In combination with the high efficacy of the LED filaments, e.g. compared to conventional SMD LED types, unprecedented luminaire efficacies can be achieved.
The person skilled in the art realizes that the present invention is by no means limited to the specific configurations described above, and that many modifications and variations are possible within the scope of the appended claims.

Claims

A lighting device (10) comprising a light source (50) provided in a housing (20),
wherein the housing (20) comprises an inner cavity (24) bounded by a cavity surface (23) that is at least partly reflective, the inner cavity (24) having a first window (21) in a first direction, and a second window (22) in a second direction opposite to the first direction, the first window (21) having a first area, and the second window (22) having a second area,

wherein the light source (50) comprises a plurality of light emitting diode, LED, filaments (52; 52A, 52B) for providing LED filament light within the inner cavity (24),

wherein the LED filaments (52; 52A, 52B) are electrically connected between a first track (26, 26A, 26B) and a second track (26, 26A, 26B), and

wherein the LED filament light leaves the housing (20) through at least one of the first window (21) and the second window (22).
The lighting device (10) according to claim 1, wherein each of the first track (26, 26A, 26B) and the second track (26, 26A, 26B) is provided outside the inner cavity (24), and wherein electrical contacts (54) of the LED filaments (52; 52A, 52B) to the first track (26, 26A, 26B) and to the second track (26, 26A, 26B) are arranged through the cavity surface (23).
The lighting device (10) according to claim 2, wherein the housing (20) comprises a first compartment (25) and a second compartment (25) provided on opposite sides of the inner cavity (24), wherein the first compartment (25) accommodates the first track (26, 26A, 26B), and wherein the second compartment (25) accommodates the second track (26, 26A, 26B).
The lighting device (10) according to claim 2 or 3, wherein one or more LED filaments (52; 52A, 52B) has a light emitting surface that bridges a distance in the inner cavity (24) from a first point on a first area of the cavity surface (23) to a second point on a second area of the cavity surface (23).
The lighting device (10) according to any one of the preceding claims, wherein the plurality of LED filaments (52; 52A, 52B) comprises one or more first LED filaments (52A) for providing LED filament light of a first correlated color temperature, and one or more second LED filaments (52B) for providing LED filament light of a second correlated color temperature, the second correlated color temperature being different from the first correlated color temperature.
The lighting device (10) according to claim 5, wherein the first correlated color temperature is a cool white temperature, wherein and the second correlated color temperature is a warm white temperature.
The lighting device (10) according to claim 5 or 6, wherein the first correlated color temperature is in the range of 5500 K to 6500 K, and wherein the second correlated color temperature is in the range of 2000 K to 3000 K.
The lighting device (10) according to any one of claims 5 to 7, wherein the first LED filaments (52A) are interleaved with the second LED filaments (52B) in a direction along a main extension direction of the inner cavity (24).
The lighting device (10) according to claim 8, wherein the first LED filaments (52A) and the second LED filaments (52B) are arranged alternatingly.
The lighting device (10) according to any one of claims 4 to 9, wherein the first LED filaments (52A) and the second LED filaments (52B) can be operated independently from each other.
The lighting device (10) according to any one of the preceding claims, wherein the first window (21) is covered by a micro lens optic plate (40).
The lighting device (10) according to any one of the preceding claims, wherein a size of the first area and a size of the second area determine the amount of LED filament light leaving the housing (20) through the first window (21) and the amount of the LED filament light leaving the housing (20) through the second window (22), and wherein a flux ratio between light exiting the lighting device (10) through the first window (21) and light exiting the lighting device (10) through the second window (22) is within the range of 1 to 10.
The lighting device (10) according to any one of the preceding claims, wherein the housing (20) further comprises a translucent plate (30) covering the second window (22), and wherein the translucent plate (30) is at least one of a transparent plate, a plate comprising scattering particles, a plate having surface texture on at least one side, a plate having lenslets or prism structures, a single lens, and a plate comprising light guides.
The lighting device (10) according to any one of the preceding claims, wherein the inner cavity (24) is a linearly extending cavity, a circular cavity, or an annular cavity.
A luminaire (1) comprising a lighting device (10) according to any one of claims 1 to 14.