Lighting device having improved optical efficiency

Abstract

A lighting device (10) is provided comprising a first light emitting element (11); an envelope (20) enclosing the first light emitting element (11), the envelope (20) being translucent, the envelope (20) having a wall thickness TE and a mean free path. A second light emitting element (12) arranged in connection with the envelope (20), such that the envelope (20) is arranged to act as a light guide for light emitted by the second light emitting element (12). The envelope (20) comprises a matrix material in which scattering particles are dispersed in a concentration such that the mean free path is larger than the wall thickness TE. The matrix material has a matrix material refractive index RIM and the scattering particles have a scattering particle refractive index RISP, and 0.5 ≤ |RIM - RISP| ≤ 1.5.

Description

[0001] LIGHTING DEVICE HAVING IMPROVED OPTICAL EFFICIENCY

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a lighting device having an improved optical architecture and optical efficiency.

[0004] BACKGROUND OF THE INVENTION

[0005] Home and professional environments contain a large number of lighting devices for creation of functional, ambient, atmosphere, accent or task lighting. LED (Light Emitting Diode) light bulbs are a popular lighting option due to their energy efficiency and long lifespan. However, despite their many advantages, LED lighting devices still face certain limitations in terms of efficacy and performance and there is a continuous drive to increase the efficacy (Im / W) of LED lighting devices.

[0006] The efficacy is often determined for the LED light bulb as a whole and various methods have been developed to increase different aspects of the efficacy of LED light bulbs. One of the challenges is that improvements in efficacy can impact the ability of the LED light bulb to emit light in a uniform manner, resulting in uneven lighting patterns, reduced brightness in certain areas, in color distortion, and / or in uneven color distribution.

[0007] There is thus still a need for further innovation in the field of LED lighting technology to improve the efficacy and performance of LED light bulbs.

[0008] SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to at least partly overcome one or more of the aforementioned disadvantages of the prior art, or to provide a useful alternative.

[0010] In a first aspect the invention provides a lighting device comprising a first light emitting element, an envelope enclosing the first light emitting element, the envelope being translucent, the envelope having a wall thickness TE and a mean free path. A second light emitting element is arranged in connection with the envelope, such that the envelope is arranged to act as a light guide for light emitted by the second light emitting element. The envelope comprises a matrix material in which scattering particles are dispersed in a low concentration such that the mean free path is larger than the wall thickness TE, the matrix material having a matrix material refractive index RIM and the scattering particles having a scattering particle refractive index RIsp. The absolute difference between RIM and RISP is in the range of 0.5 to 1.5.

[0011] The invention provides a lighting device having an improved optical architecture having a high optical efficiency. The first light emitting element can provide light at a very high efficiency. Due to the scattering properties of the envelope, the light emitted by the first light emitting element can exit the lighting device substantially unhindered, i.e. unscattered. At the same time, the second light emitting element, by the use of the envelope as a light guide, may emit light in a large area, i.e. the entire envelope. The addition of light emitted by the second light emitting element does not (or hardly) compromise the optical efficiency of the first light emitting element. In this way, the lighting device may deliver functional and efficient lighting having a uniform color and / or brightness distribution. Hence, the lighting device of this invention can provide a high optical efficiency with the optical efficiency of light emitted by the first light emitting element being higher than 95%, and the optical efficiency of the light emitted by the second light emitting element being higher than 85%.

[0012] The envelope may also be referred to a light guide. The envelope is made from a translucent, preferably transparent material with relatively low light scattering properties comprising a low concentration of scattering particles dispersed in the matrix material of the envelope. The light scattering particles have a high refractive index compared to the matrix material. The mean free path of the envelope may be at least equal to the wall thickness but is preferably larger or significantly larger than the wall thickness. The mean free path may for example be larger than 1.25 times, preferably larger than 2 times, such as larger than 3 times the wall thickness.

[0013] The wall thickness TE of the envelope may be between 0.2 mm to 4 mm, preferably between 0.5 mm to 2 mm, such as 1 mm.

[0014] The mean free path (also indicated as “MFP”) is a parameter known in the art which is commonly and widely used for characterizing scattering properties of light guides and optical fibers. The mean free path of the envelope needs to be understood as the average distance a photon can travel before it is scattered within the material of the light guide. The mean free path depends among others on the concentration of the scattering particles, the type of scattering particles (i.e. determining the refractive index), and size (distribution) of the scattering particles. The matrix material of the envelope has a matrix material refractive index RIM and the scattering particles have a scattering particle refractive index RIsp. The absolute difference between RIM and RISP may be in the range of 0.2 to 1.8, preferably in the range of 0.5 to 1.5, such as in the range of 0.7 to 1.3. RISP may be larger than RIM, or RISP may be lower than RIM. The refractive index of a material is an inherent material property and is an important and commonly used parameter in optics. The refractive index is used to predict the behavior of light as it enters or exits a material, being a measure of how much a material changes the direction of light as it passes through it. Materials with higher refractive indices bend light more than those with lower refractive indices.

[0015] The scattering particles may have a particle size being in a range of 0.5 to 50 pm, preferably between 1 to 10 pm, such as between 1 to 5 pm.

[0016] An envelope with the above-described features will show a low scattering effect over the wall thickness of the envelope and will therefore look substantially transparent or clear to a user of the lighting device. The user may perceive only a slight haze when looking at the envelope. However, the envelope will show a desired high scattering effect when the injected light from the second light emitting element flows through the light guide, over the outside, that is the length direction, of the envelope. When the light emitted by the second light emitting element is coupled into the light guide, i.e. via a light incoupling surface or an entrance face, the optical path length within the envelope is multiple times higher than the wall thickness. The optical path length may be more than 50 times higher, preferably more than 100 times higher, such as more than 200 times higher than the wall thickness. Hence, there is a high probability that substantially all light emitted by the second light emitting element is scattered and extracted from the envelope such that it leaks gradually out of the light guide creating a uniformly glowing surface. The fraction of scattered and extracted light is a combination of the mean free patch and the optical path length.

[0017] Therefore, in practice, measuring the mean free path of the envelope may not be required. An envelope material looking substantially clear and transparent in the thickness direction but showing substantial and sufficient light scattering in a length direction can be considered to have a mean free path larger than the wall thickness, rendering it suitable for the use in the lighting device of this invention.

[0018] The surface area of the envelope may be at least three or five times larger, e.g. 10-20 times or even more than 50 times larger than the first light emitting element, depending on the type of first light emitting element and envelope. Luminance is measured in candela per square meter and may be perceived as brightness. The area may e.g. be the area of the envelope or a projection of the light emitted by the lighting device. Any known method of measuring luminance may be used to ensure that it does not differ more than a factor of five between said portion(s) of the envelope and the average.

[0019] The lighting device may e.g. be a light bulb or a luminaire.

[0020] The envelope may be of any suitable shape, preferably a bulb. The envelope may be flat or curved (in one or two directions) or elongated. The light and / or color distribution of the light emitted by the envelope can be controlled and optimized by changing the shape of the envelope, such as the length to width ratio of the envelope.

[0021] It is noted that close to the second light emitting element, the envelope may emit light with a luminance that is significantly higher than other portions of the envelope. Accordingly, the envelope may be configured to, when acting as a light-guide for the second light emitting element, emit light such that at least 50 % of the area of the envelope not including a portion closest to the second light emitting element, such as within 1 cm, emits light with a luminance that differs less than a factor of five from the average luminance of the rest of the envelope.

[0022] The first lighting element may be arranged to emit white light, and the second light emitting element may be arranged to emit white light and / or colored light.

[0023] The first light emitting element may be used to deliver functional lighting. The light from the second light emitting element is coupled into the envelope and may give a white or colorful glow to the lighting device.

[0024] In examples in which the second light emitting device is configured to emit colored light, the colour control may be done via the envelope. The dimensions and scattering properties of the envelope may be chosen such that a desired colour mixing is obtained. The height to width ratio of the envelope may be chosen to create a good light distribution in colour and white light. Such a lighting device may be configured to show a high colour and illumination uniformity in the far field and in the near field of the lighting device.

[0025] The first light emitting element comprises an LED that emits white light. The first light emitting element may thereby e.g. be an LED filament, a lightguide LED, or a direct emitting LED. The first light emitting element may additionally emit colored light.

[0026] The second light emitting element may emit colored light. The colored light may be emitted by e.g. RGB LEDs or non-white LED or other light emitting element. The second light emitting element may alternatively or additionally emit white light. The first and second light emitting element may both emit white light. The second light emitting element may have the same or different color temperature as the first light emitting element. Thereby, a different lumen, color, and / or color temperature may be set for each of the first light emitting element and the second light emitting element.

[0027] The first light emitting element may comprise a LED filament arrangement comprising a plurality of LED filaments.

[0028] LED filaments are currently applied in many lighting devices. LED filaments can achieve high efficacies. Especially non-dimmable filament bulbs can have the highest energy label (label A, i.e., efficacy > 210 Im / W). A LED filament is an example of a light source having an elongated carrier comprising an array of a plurality of light emitting diodes, LEDs, and typically an encapsulant at least partly enclosing the plurality of LEDs. Each LED filament may have a length of 30-50 mm and a diameter of 1-3 mm.

[0029] The LED filament arrangement may comprise at least two LED filaments, preferably at least 4 LED filaments, such as at least 8 LED filaments.

[0030] The plurality of LED filaments may be evenly distributed around a central elongation axis of the LED filament arrangement. Each LED filament of the plurality of LED filaments may have a longitudinal axis and the longitudinal axis of each LED filament may be positioned at an angle a with respect to the central elongation axis. The angle a may be substantially the same for each LED filament.

[0031] A filament arrangement as described above shows a high optical efficiency and a desired intensity and omnidirectional light distribution. The LED filaments placed at an angle may support in emitting more light into a desired direction.

[0032] Each LED filament of the plurality of LED filaments may have an identical shape, size, and length. Preferably, the LED filaments may be substantially straight along their longitudinal axis and may have substantially the same length.

[0033] The LED filament arrangement may have a central elongation axis. In a mounted state of the lighting device, the central elongation axis may be aligned in a length direction of the envelope.

[0034] The plurality of LED filaments may be evenly distributed around the central elongation axis. Preferably, the plurality of LED filaments may be arranged in a circular configuration having a radius r around the central elongation axis. However, also other configurations may be suitable, such as for example a square configuration, a triangular configuration, or an elliptical configuration. The LED filaments may be positioned at an angle a with respect to the central elongation axis, which may be substantially the same for each LED filament. In examples, the angle a may be a non-zero angle. The angle a may be an angle in the range of ± 45°, preferably in the range of ± 30°, such as in the range of ± 15°.

[0035] The LED filament arrangement may comprise an alternating arrangement of a first set of LED filaments and a second set of LED filaments. The first set of LED filaments may be configured to emit light of a first color or color temperature, and the second set of LED filaments may be configured to emit light of a second color or color temperature. Preferably, the first set of LED filaments may be configured to emit warm white light and the second set of LED filaments may be configured to emit cool white light

[0036] A LED filament arrangement comprising alternating warm white and cool white LED filaments creates a lighting device enabled to emit different types of white light at different correlated color temperatures.

[0037] The first set of LED filaments may be configured to emit warm white light. The first set of LED filaments may thus be configured to emit light of a first correlated color temperature (CCT). The first CCT may be in a range of 2000-3500K.

[0038] The second set of LED filaments may be configured to emit cool white light. The second set of LED filaments may thus be configured to emit light of a second correlated color temperature (CCT). The second CCT may be in a range of 4000-6000K.

[0039] The first filaments of the first set of LED filaments and the second filaments of the second set of LED filaments may be arranged in an alternating configuration while being evenly distributed around the central elongation axis of the LED filament arrangement. A first filament may thus be located next to two second filaments, and a second filament may be located next to two first filaments. To this end. the number of LED filaments in the LED filament arrangement may be required to be an even number comprising at least four LED filaments, preferably at least eight LED filaments, such as at least 12 LED filaments. In this way a good omnidirectional light distribution of both warm white and cool white light can be obtained.

[0040] The first light emitting element may further comprise a common connecting element, a first connecting element, a second connecting element, and a spacer. The common connecting element may mechanically and electrically connect the plurality of LED filaments. The first connecting element may mechanically and electrically connect the first set of LED filaments and the second connecting element may mechanically and electrically connect the second set of LED filaments. The spacer may comprise an insulating material and the spacer may be positioned to separate the first connecting element and the second connecting element.

[0041] A first light emitting element comprising connecting elements provides an accurate and precise positioning of the filaments. The connecting elements can ensure a reproducible positioning providing a certainty in light output distribution. Furthermore, a first light emitting element is provided in which the different sets of LED filaments may be controlled independently from each other.

[0042] The shape of the connecting elements may follow the shape of the LED filament arrangement. Hence, in examples in which the LED filaments are arranged in a circular configuration, the connecting elements may be substantially ring shaped. The first light emitting element may thus comprise a common ring, a first ring, a second ring, and a spacer ring.

[0043] The connecting elements may create the desired LED filament angle a and ensure accurate radial position between the LED filaments, thus creating mechanical stability of the LED filament arrangement.

[0044] The common connecting element may electrically connect all filaments to the same polarity. This may require only a single cable or wire going from the upper part of the LED filament assembly to the bottom, hence having an improved small optical impact.

[0045] The first connecting element may electrically connect the first set of LED filaments while the second connecting element may electrically connect the second set of LED filaments. The spacer may electrically separate the first and second connecting element and may thus prevent electrical connection between the different rings.

[0046] The first light emitting element may comprise more than two sets of LED filaments. In such examples, a third connection element may connect a third set of LED filaments, a fourth connection element may connect a fourth set of LED filaments, and so forth. The first light emitting element may thus comprise N sets of LED filaments, a common connection element, N connection elements, and N-1 spacers.

[0047] The second light emitting element may be an array of LED components or a LED filament.

[0048] Thereby, the second light emitting element may direct their light emission into the envelope that acts as a light guide. By the first and second light emitting elements both being LED filaments, the electronics of the lighting device may be simplified by e.g. using a same voltage.

[0049] The array of LED components may be a circular array of RGB LEDs. In examples in which the second light emitting element is configured to emit coloured light, the second light emitting element may be a circular array of RGB LEDs. The circular array of RGB LEDs may be arranged in connection with an envelope having a circular light incoupling surface, such that the envelope is arranged to act as a light guide for light emitted by the circular array of RGB LEDs. In this way, a lighting device may be obtained that can emit coloured light which can be colour-controlled.

[0050] The array of RGB LEDs may be circular and the RGB LEDs may be evenly distributed within the circular array. Preferably the RGB LEDs may be SMD (surface mounted device) LEDs mounted on a carrier.

[0051] The envelope may comprise multiple protrusions connected to a carrier to which the second light emitting element is mounted. Each protrusion of the multiple protrusions has a height Hp, the second light emitting element may ha a height HLED and a distance d to a light incoupling surface of the envelope. The height of each protrusion is such that Hp = HLED + d, and 0.1 mm < d < 0.5 mm.

[0052] The performance of the lightguide may be related to the distance between the light incoupling surface of the envelope and the second light emitting element. The distance is preferably as close as possible, but contact might damage (i.e. scratch) the lightguide and may have negative impact on the light outcoupling and / or reliability. To guarantee the optimal distance, the envelope may comprise accurate protrusions.

[0053] The height of the protrusions may be the height of the second light emitting element (i.e. the height of the RGB LED package or the height of the LED filament) plus a distance d. The distance d may be in the range of 0.1 mm < d < 0.5 mm, preferably between 0.1 mm and 0.2 mm. The protrusions may be part of the lightguide. Each of the protrusions may be positioned in between two individual RGB LEDs of the circular array of RGB LEDs.

[0054] The lighting device may further comprise an outer envelope enclosing the envelope, the outer envelope being light transmissive.

[0055] An outer envelope may protect the envelope from damage such as scratches or fingerprints. The outer envelope shape may be chosen partly based on aesthetical arguments. The outer envelope may closely surround and follow the surface shape of the envelope. Alternatively, the outer envelope may have a different shape, by that creating a space between the envelope and the outer envelope.

[0056] The outer envelope may be light diffusing and may comprise an outer surface and an inner surface. The outer envelope may comprise one or more of a surface texture on the outer surface, a surface texture on the inner surface, and scattering particles embedded in a matrix material of the outer envelope.

[0057] Adding a light diffusing outer envelope may further improve the colour and brightness distribution of the lighting device, especially in the near field of the lighting device (i.e. at a distance of 0-10 cm of the lighting device). The outer envelope further contributes to the uniform color and / or brightness distribution.

[0058] The outer envelope may spread the light according to a Gaussian intensity distribution (FWHM=30 deg.).

[0059] The outer envelope may comprise scattering particles. The mean free path of the outer envelope may be much significantly smaller than the wall thickness of the outer envelope. This may be accomplished e.g. by using a high concentration of scattering particles having a low refractive index difference with the matrix material. The matrix material of the outer envelope may have a matrix material refractive index RIM and the scattering particles may have a scattering particle refractive index RISP. The absolute difference between RIM and RISP for the outer envelope may be in the range of 0.05 - 0.1.

[0060] Alternatively or additionally, the outer envelope may comprise a surface texture on the outer surface and / or on the inner surface. Surface texture in the context of this invention needs to be understood as a repetitive or random deviation from the normal surface, or geometrical irregularities present on the surface. Surface texture may be described by its roughness, waviness, lay and flaws. A surface texture may be applied during manufacturing of the outer envelope in various ways, including for example using a mold with the desired texture in injection molding, embossing a pattern onto the polymer surface, laser or chemical etching, or sandblasting.

[0061] The envelope and / or the outer envelope may comprise one or more of PMMA, glass, polycarbonate, polystyrene, Zeonex, silicone, or polyurethane.

[0062] These materials have been shown to be sufficiently translucent and have relatively low scattering properties, which enables light outcoupling over the entire area of the envelope. A thickness and choice of material may be affected by the choice of second light emitting element, such that ideal light scattering properties are achieved.

[0063] The refractive index of the matrix material comprised by the envelope and / or the outer envelope may be in the range of 1.3-1.7.

[0064] The second light emitting element may comprise a light emitting surface S configured to emit light at a beam angle p. The second light emitting element may have a distance d to a light incoupling surface of the envelope, and the envelope wall thickness TE may be chosen such that TE > S + 2 d tan(0).

[0065] The wall thickness TE of the envelope may be chosen to match the light emitting surface and the beam angle 0 of the second light emitting element, such that light escaping the lightguide at the light incoupling surface may be avoided.

[0066] The wall thickness may be determined by the size of the light emitting surface, the beam angle of the second light emitting element, and by the distance between the top surface of the second light emitting element and the bottom of the lightguide (i.e. the light incoupling surface). The beam angle 0 can be 0 ± x°. x° is determined by tolerances of material and manufacturing process of the lightguide. This tolerance may be taken into account when choosing a suitable wall thickness.

[0067] The envelope may comprise one or more openings.

[0068] An envelope having an open structure may have multiple benefits. Firstly, in the thermal management of the lighting device by increasing the ventilation of the first light emitting element, and secondly providing an improved look and feel of the lighting device without compromising the optical efficiency.

[0069] The envelope may comprise at least 5 openings, preferably at least 10 openings, such as at least 15 openings. In examples in which the first light emitting element is a filament arrangement comprising a plurality of LED filaments, the number of openings may be related to the number of LED filaments in the LED filament arrangement. The number of openings may be equal to, be a multiple of, or be factors / divisors of the number of LED filaments.

[0070] The openings may have any suitable or desired shape. Preferably the openings may be slits oriented in the length direction of the envelope and / or along the central elongation axis of the filament arrangement. Alternatively or additionally, the envelope may comprise holes having for example a circular, hexagonal, triangular, or elliptical shape.

[0071] In examples in which the second light emitting element comprises an array of LEDs, the openings may need to be aligned with the individual LEDs, such as being positioned in between the LEDs.

[0072] The envelope may have a first side at a light incoupling surface and an opposite second side. A first wall thickness TEI at the first side may be smaller than a second wall thickness TE2 at the second side such that the wall thickness TE of the envelope may be non-constant. The envelope may have a length L in a first direction measured from the first side to the second side, and the envelope may have a width W measured in a second direction perpendicular to the first direction. The length L may be larger than the width W of the envelope.

[0073] As discussed above, how the light emitted by the second light emitting element is subsequently emitted by the envelope is controlled by the parameters of the envelope. The envelope may need to be designed to achieve good colour mixing and to create a good light distribution all around the envelope in colour and white light. This may be determined by the scattering particles, the height to width ratio of the envelope, as well as its thickness distribution.

[0074] The wall thickness may gradually increase and / or decrease, i.e. from the first wall thickness to the second wall thickness. Additionally, the wall thickness may gradually increase and / or decrease multiple times between the first side and the second side. The wall thickness may for example additionally decrease in positions in which the envelope comprises a bend.

[0075] The lighting device may further comprise a controller that is configured to control the first light emitting element and the second light emitting element independently.

[0076] Thereby, a different lumen, color, and / or color temperature may be set for each of the first light emitting element and the second light emitting element.

[0077] In an example, the lighting device may comprise a first light emitting element, the first light emitting element being a filament arrangement configured to emit white light, an envelope enclosing the first light emitting element, the envelope being translucent, the envelope having a wall thickness TE and a mean free path. A second light emitting element may be arranged in connection with the envelope, such that the envelope is arranged to act as a light guide for light emitted by the second light emitting element. The second light emitting element may be a circular array of RGB LEDs configured to emit coloured light. An outer envelope may enclose the envelope, the outer envelope may be light transmissive and light diffusing and may comprise an outer surface and an inner surface. The outer envelope may comprise one or more of a surface texture on the outer surface, a surface texture on the inner surface, and scattering particles embedded in a matrix material of the outer envelope. The envelope may comprise a matrix material in which scattering particles are dispersed in a low concentration such that the mean free path is larger than the wall thickness TE, the matrix material having a matrix material refractive index RIM and the scattering particles having a scattering particle refractive index RIsp. The absolute difference between RIM and RISP is in the range of 0.5 to 1.5. The envelope may additionally comprise light converting particles, such as phosphor particles. The envelope and / or the outer envelope may comprise a coating. The outer envelope may be coated with a layer of phosphor with a thickness of 0.05-1.0 mm. The layer of phosphor may reduce the glare of the lighting device and may improve the homogeneity of the emitted light. The phosphor will absorb some of the light emitted by the first and second light emitting elements and re-emit the light (through electron relaxation photon emissions). The layer of phosphor may cover the entire area of the outer envelope, or at least 50 % or at least 80 % of the area of the envelope.

[0078] The layer of phosphor may have a thickness of 0.05-1.0 mm, such as 0.1 mm or 0.3 mm. The thickness of the layer of phosphor may differ in different portions of the outer envelope. The layer of phosphor may form a pattern. The layer of phosphor may comprise a pattern of dots of phosphor. The dots may be very small, such as less than 1 mm in diameter, and may be invisible to the naked eye.

[0079] The layer of phosphor may further comprise scattering particles different from phosphor. If the layer of phosphor is patterned, the scattering particles may have the same or a different pattern. The layer of phosphor may comprise grinded particles of a polymer such as PMMA or PET incorporating the phosphor. Such grinded particles may scatter light.

[0080] 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.

[0081] BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0083] Fig. 1 schematically shows a lighting device according to an example of the present invention;

[0084] Fig. 2 schematically shows aspects of a lighting device according to another example of the present invention;

[0085] Figs. 3a-3b show different views of a first light emitting element being a LED filament arrangement; Figs. 4a-4c schematically depict examples of lighting devices according to the present invention;

[0086] Fig. 5 shows aspects of a first light emitting element being a LED filament arrangement;

[0087] Fig. 6 shows aspects of an envelope comprising one or more openings;

[0088] Fig. 7 schematically depicts aspects of the envelope and the second light emitting element; and

[0089] Fig. 8 shows additional aspects of the envelope and the second light emitting element.

[0090] The schematic drawings are not necessarily to scale.

[0091] DETAILED DESCRIPTION

[0092] Fig. 1 schematically shows a lighting device 10. The lighting device 10 shown in Fig. 1 is a light bulb, however other lighting devices are possible within the scope of the appended claims. For example, the lighting device 10 may be a light strip, a luminaire and may be a HUE® light source.

[0093] The lighting device 10 comprises a first light emitting element 11. In the exemplifying embodiment of Fig. 1, the first light emitting element 11 is a light emitting diode (LED) filament, however other light emitting elements are possible within the scope of the appended claims.

[0094] The lighting device 10 further comprises a translucent envelope 20 enclosing the first light emitting element 11. The envelope 20 may hermetically seal a volume of gas or vacuum that surrounds the first light emitting element 11. The envelope 20 being translucent may mean that most of the light emitted by the first light emitting element 11 will be transmitted because even if light is reflected (because of Fresnel reflection) or scattered (by internal scattering particles), the light has a second (or third, or fourth...) chance to get through the envelope 20 at a different location. The envelope 20 being translucent may mean that at least 70 % of the light emitted by the first light emitting element 11 will be transmitted through the envelope 20 at a first pass, or preferably at least 85 % of the light. The envelope 20 may comprise a surface coating to protect the envelope from damage, such as scratches or fingerprints.

[0095] The lighting device 10 also comprises a second light emitting element 12. The second light emitting element 12 is arranged in connection with the translucent envelope 20 such that the envelope 20 acts as a light guide for the second light emitting element 12. This arrangement is schematically shown in an inset of Fig 1. Accordingly, light emitted by the second light emitting element 12 is directed into the envelope 20 (i.e. via a light incoupling surface), which acts as a light guide to guide the light within the envelope 20. Additional properties of the envelope will be described further below.

[0096] It is noted that the sharp edge of the envelope 20 shown in the inset of Fig 1 is merely schematic, and this may be smooth or may be not present at all in other examples.

[0097] The second light emitting element 12 may e.g. be an array or set of LEDs or mini-LEDs, or a LED filament. In examples in which second light emitting element 12 is a filament, the filament may be a one-sided emitting LED filament or may be a two-sided emitting LED filament (e.g. by using a translucent carrier for the filament, or by placing the filament within a reflective container or tray partly surrounding the filament and reflecting the light emitted by the second light emitting element 12 towards the envelope 20).

[0098] The first lighting element may be arranged to emit white light, and the second light emitting element may be arranged to emit white light and / or colored light, e.g. being a set of RGB LEDs. The first and second light emitting elements 11, 12 may emit the same or a different color, hue, (correlated) color temperature, brightness, or intensity. To this end, the lighting device 10 may comprise a controller that is arranged to control the first light emitting element 11 and the second light emitting element 12 independently. This may comprise sending control signal(s) from a lighting control system to the light emitting element 11, 12, e.g. wired or wirelessly over Wi-Fi or Bluetooth®.

[0099] Fig. 2 schematically shows aspects of a lighting device according to another example of the present invention. Fig. 2 shows the light emitting portion of the lighting device 10 which may be arranged in connection with a socket connector comparable to the lighting device 10 as depicted in Fig. 1.

[0100] The second light emitting element 12 may be integrated with or arranged in connection with a carrier 40. The carrier 40 may be or may be connected to the surface where a socket connector of the lighting device 10 interfaces with the envelope 20. The second light emitting element 12 may be arranged as a circle or circular array (or any other shape, depending on the lighting device) along the light incoupling surface or entrance face 21 of the envelope 20, e.g. the interface between the socket connector of the lighting device 10 and the envelope 20. The individual LED packages comprised by the circular array of LEDs may be evenly spaced and located as close as possible to the entrance face of the light guide 20. In Fig. 2 an example is depicted in which the first light emitting element 11 is a LED filament arrangement 111 comprising a plurality of LED filaments, however other light emitting elements are equally suitable for the use in this example.

[0101] The envelope 20 enclosing the LED filament arrangement I l l is made of a translucent material with relatively low scattering properties, such that light from the second light emitting element 12 is outcoupled from the envelope 20 along the area of the envelope 20. Preferably, the material properties of the envelope 20 are selected such that light from the second light emitting element 12 is outcoupled from the envelope 20 along the entire area of the envelope 20. The envelope 20 may be configured e.g. by selecting a matrix material, scattering particles, shape, and thickness that achieves this. To this end, the envelope 20 has a wall thickness TE and a mean free path. The envelope 20 comprises a matrix material in which scattering particles are dispersed in a concentration such that the mean free path is larger than the wall thickness TE. The matrix material has a matrix material refractive index RIM and the scattering particles have a scattering particle refractive index RIsp. The absolute difference between and RIM - RISP is in the range of 0.5 - 1.5.

[0102] The mean free path is a parameter known in the art which is commonly and widely used for characterizing scattering properties of light guides and optical fibers. The mean free path depends among others on the concentration of the scattering particles (i.e. particles per unit volume [mm3], the type of scattering particles (i.e. determining the refractive index), and size (distribution) of the scattering particles (i.e. the radius of the particles [mm]).

[0103] The mean free path of a material can be measured in different ways. The MFP can be calculated directly from the transmittance T of a slab of scattering material of thickness L, wherein T = exp (-L / MFP). A widely used method is for example the “cutback method”, which involves cutting a small section of the light guide and measuring the light transmission through the section. Alternatively, the mean free path can be measured by shining a light through a sample of the light guide and measuring the amount of scattering and / or the scattering pattern of the light. Alternatively, the mean free path may be calculated from the properties of the scattering particles (and therefore its refractive index), the concentration, and its particle size distribution. Both, the scattering particle type and the particle size, can be determined e.g. by a combination of chemical analysis and microscopy.

[0104] The envelope 20 may be realized by using a matrix material provided with scattering particles. In this way the envelope 20 will have volume scattering while the surface does not have any outcoupling features. To realize this, the matrix material may be a polymer like for instance polymethyl methacrylate (PMMA), glass, polycarbonate, polystyrene, Zeonex, silicone, or polyurethane, or PET in which scattering particles are dispersed. Examples of scattering particles are TiO2, BaSO4 and / or A12O3 particles. These scattering particles may have an (average e.g. D50) particle size in a range of 0.5 to 50 pm, and more preferably between 1 to 5 pm. Further the particle concentration of these scattering particles may be below 106parti cl e / mm3and may be above 104parti cl e / mm3.

[0105] In an example, the matrix material of the envelope 20 may be PMMA having RIM =1.50. The scattering particles may be TiO2 having RISP =2.41 and an average particle size of 0.93 micron at a particle concentration of 1 • 105parti cles / mm3.

[0106] The envelope 20 may have a first side at a light incoupling surface 21 and an opposite second side. The envelope 20 may have a length L in a first direction measured from the first side to the second side, and a width W measured in a second direction perpendicular to the first direction.

[0107] An envelope with these features will show a low scattering effect over the thickness of the envelope material, and a desired high scattering over the optical path length, i.e. the outside or length direction of the envelope. The effect of these features is an envelope that has a more uniform light output over the entire surface of the envelope.

[0108] Optionally, the lighting device may further comprise an outer envelope 30 enclosing the envelope 20. The outer envelope 30 may be light transmissive and may serve the purpose of protecting the envelope 20 from damage.

[0109] Additionally, the outer envelope 30 may serve an optical purpose in further optimizing the light distribution and color mixing. The diffuse outer cover 30 may be beneficial to matching the intensity profiles of the light fluxes of the first light emitting element 11 and the second light emitting element 12 in the near field of the lighting device. Such a lighting device 10 may also be suitable to be mounted in a reflector optics without having large colour differences on the wall or floor. In such examples, the outer envelope may be light diffusing, i.e. the mean free path (MFP) of the outer envelope 30 may be much smaller than the wall thickness of the outer envelope to obtain a high scattering effect. The outer envelope 30 may comprise an outer surface 31 and an inner surface 32. The light diffusing effect may be achieved by the outer envelope 30 comprising one or more of a surface texture on the outer surface 31, a surface texture on the inner surface 32, and scattering particles embedded in a matrix material of the outer envelope 30.

[0110] The outer envelope 30 may comprise a high concentration of scattering particles having a low refractive index difference with the matrix material, i.e. an absolute difference in the range of 0.05 - 0.1. The size of the scattering particles in the outer envelope 30 may be larger than the size of the scattering particles comprised by the envelope 20.

[0111] In an example, the matrix material of the outer envelope 30 may be PMMA having RIM =1.50. The scattering particles may be polycarbonate having RISP =1.586 and an average particle size of 4 micron at a particle concentration of 2.5 - 105parti cles / mm3.

[0112] Figs. 3a-3b show different views of a first light emitting element 11 being a LED filament arrangement 111. Fig. 3a shows a side view of the LED filament arrangement while Fig. 3b shows a top view of the same LED filament arrangement 111.

[0113] The LED filament arrangement 111 may comprise a plurality of N LED filaments arranged such that the LED filament arrangement 111 generates the desired intensity distribution. The plurality of LED filaments may be evenly distributed / spaced around a central elongation axis CEA of the LED filament arrangement 111. Each LED filament of the plurality of LED filaments may have a longitudinal axis positioned at an angle a with respect to the central elongation axis CEA. The angle a may be substantially the same for each LED filament. The plurality of LED filaments may be arranged in a circular configuration having a radius r around the central elongation axis CEA. The LED filament arrangement 111 may be positioned at a height h spaced apart from the carrier 40 (not shown here).

[0114] The LED filament arrangement may comprise an alternating arrangement of a first set of LED filaments 1111 and a second set of LED filaments 1112. The first set of LED filaments 1111 may be configured to emit light of a first color or color temperature, preferably warm white light. The second set of LED filaments 1112 may be configured to emit light of a second color or color temperature, preferably cool white light.

[0115] Figs. 4a-4c schematically depict different examples of lighting devices 10, in particular depicting different design options for the envelope 20, the optional outer envelope 30 and the filament arrangement 111. It has to be noted that these are merely a few examples of possible designs of the lighting device 10 and that the skilled person is able to design alternatives without departing from the scope of the appended claims.

[0116] Fig. 4a shows an example in which the envelope 20 exactly follows the shape of the outer envelope 30. In Fig. 4b, the outer envelope 30 has a different shape than the envelope 20, thereby creating an (air) gap between the envelope 20 and the outer envelope 30. In Fig. 4c a different angle is chosen for the orientation of the LED filaments and another different shape is chosen for both the envelope 20 and the outer envelope 30. The shape of the envelope 20 may be described by a Bezier function. The shape of the envelope 20 may be changed resulting in changes in the intensity distribution. The intensity profile of the envelope 20 can be matched with the intensity profile of the filament arrangement 111 using for example optimization software. Parameters of the geometry of the envelope 20 that may be varied include the length L, the width w and the wall thickness TE. The wall thickness of the light guide 20 may be mostly determined by the footprint of second light emitting element 12. The curvature of the light guide 20 may be chosen such that the light only escapes from the envelope 20 after scattering at the particles.

[0117] The shape of the outer envelope 30 may be chosen to have a desired (aesthetical) shape. The light guide 20 may have a shape largely related to its performance (i.e., generating a desired beam shape).

[0118] In the example of Fig. 4b, the envelope 20 is thicker at the top of the bulb than at the bottom of the bulb. The envelope 20 has a first side at a light incoupling surface 21 and an opposite second side. The first wall thickness TEI at the first side may be smaller than the second wall thickness TE2 at the second side such that the wall thickness TE of the envelope 20 may be non-constant. Additionally, the envelope 20 has a length L in a first direction measured from the first side to the second side, and a width W measured in a second direction perpendicular to the first direction. Preferably, the length may be larger than the width.

[0119] Fig. 5 shows aspects of a first light emitting element being a LED filament arrangement. In the example depicted, first light emitting device 11 is a LED filament arrangement 111 comprising an alternating arrangement of a first set of LED filaments 1111 and a second set of LED filaments 1112. The first light emitting element 11 as depicted in Fig. 5 further comprises a common connecting element 112, a first connecting element 113, a second connecting element 114, and a spacer 115. The common connecting element 112 mechanically and electrically connects the plurality of LED filaments. The first connecting element 113 mechanically and electrically connects the first set of LED filaments 1111 and the second connecting element 114 mechanically and electrically connects the second set of LED filaments 1112. The spacer 115 comprises an insulating material, the spacer 115 being positioned to separate the first connecting element 113 and the second connecting element 114.

[0120] Fig. 6 schematically depicts an envelope 20 comprising one or more openings 22. The depicted example comprises a plurality of elongated openings 22 or slits oriented in the length direction of the envelope 20 which also may be oriented along the central elongation axis of the filament arrangement (not shown here). In this example, the openings 22 are not designed as holes completely surrounded by the envelope 20, but rather as incisions extending to the bottom, i.e. to the light incoupling surface 21, of the envelope 20 where the envelope 20 is interfacing with a carrier or socket connector. The light incoupling surface 21 may thus be split into multiple light incoupling surfaces 21 which may be aligned with the second light emitting element 12 (not shown here).

[0121] The envelope may comprise an additional opening on top as also depicted in Fig. 6. In examples in which the lighting device 10 is a light bulb mounted in a base down position, the additional hole in the top part of lightguide may remove more heat from the first light emitting element 11.

[0122] Experiments were performed with different envelope 20 configurations (full dome without openings, slits, and slits plus hole on top) and in different orientations of the lighting device 10, showing a significant reduction in average filament temperature using a lightguide 20 with openings 22.

[0123] Average filament temperature

[0124] The one or more openings 22 may thus increase the ventilation of the first light emitting element (not shown here) and thereby reduce its operating temperature and consequently increase its efficacy. However, openings 22 may also provide an improved look and feel of the lighting device 10 without compromising the optical efficiency.

[0125] The openings may have any suitable or desired shape but are preferably larger than 1 mm, preferably larger than 1.3 mm, such as larger than 2 mm. The person skilled in the art is thus able to design alternatives without departing from the scope of the appended claims. However, a good compromise may need to be found between better cooling of the first light emitting element and a (color) uniform light output without artifacts.

[0126] Fig. 7 schematically depicts aspects of the envelope 20 and the second light emitting element 12. Fig. 7 depicts an example in which the array of LED components 121 is a circular array of (RGB) LEDs. The envelope 20 may comprise multiple protrusions 22 connected to the carrier 40 to which the second light emitting element 12 is mounted. Each protrusion of the multiple protrusions 22 may have a height Hp. The second light emitting element 12 may have a height HLED and a distance d to a light incoupling surface 21 of the envelope 20. Hpmay be equal to HLED + d. The distance d may be in the range of 0.1 mm to 0.5 mm, preferably between 0.1 mm and 0.2 mm.

[0127] The performance of the envelope 20 is related to the distance d between the envelope 20, i.e. the light incoupling surface 21, and the second light emitting element 12. The distance d should be as small as possible but large enough to avoid any physical contact of the envelope 20 and the second light emitting device 12 which might scratch or damage the lightguide 20, having negative impact on the light outcoupling and / or reliability. The protrusions 22 may be part of the envelope 20 and may be positioned in between the LEDs of the array of LEDs 121.

[0128] Fig. 8 shows additional aspects of the envelope and the second light emitting element. Depicted is a cross-section through the envelope 20 in the thickness direction, zooming in on how the light emitted by the second light emitting element 12 is coupled into the light guide 20. The envelope 20 is preferably thicker than the second light emitting element 12 such that the envelope 20 efficiently catches the emitted light when acting as a light guide. The second light emitting element 12 comprises a light emitting surface S configured to emit light at a beam angle P, wherein the second light emitting element 12 has a distance d to a light incoupling surface 21 of the envelope 20. The envelope wall thickness TE is chosen such that TE > S + 2 d tan(P).

[0129] 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.

[0130] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined.

Claims

CLAIMS:

1. A lighting device (10) comprising: a first light emitting element (11); an envelope (20) enclosing the first light emitting element (11), the envelope (20) being translucent, the envelope (20) having a wall thickness TE and a mean free path; a second light emitting element (12) arranged in connection with the envelope (20), such that the envelope (20) is arranged to act as a light guide for light emitted by the second light emitting element (12); wherein the envelope (20) comprises a matrix material in which scattering particles are dispersed in a concentration such that the mean free path is larger than the wall thickness TE, wherein the matrix material has a matrix material refractive index RIM and the scattering particles have a scattering particle refractive index RISP, wherein 0.5 < |RIM - RISP| < 1.5, and wherein the second light emitting element (12) comprises a light emitting surface S configured to emit light at a beam angle P, wherein the second light emitting element (12) has a distance d to a light incoupling surface (21) of the envelope (20), and wherein the envelope wall thickness TE is chosen such that TE> S + 2 d tan(P).

2. The lighting device (10) of claim 1, wherein the first lighting element (11) is arranged to emit white light, and the second light emitting element (12) is arranged to emit white light and / or colored light.

3. The lighting device (10) of any one of the preceding claims, wherein the first light emitting element (11) comprises a LED filament arrangement (111) comprising a plurality of LED filaments.

4. The lighting device (10) of claim 3, wherein the plurality of LED filaments is evenly distributed around a central elongation axis of the LED filament arrangement (111), wherein each LED filament of the plurality of LED filaments has a longitudinal axis andwherein the longitudinal axis of each LED filament is positioned at an angle a with respect to the central elongation axis, the angle a being substantially the same for each LED filament.

5. The lighting device (10) of claim 4, wherein the LED filament arrangement (111) comprises an alternating arrangement of a first set of LED filaments (1111) and a second set of LED filaments (1112), wherein the first set of LED filaments (1111) is configured to emit warm white light, and wherein the second set of LED filaments (1112) is configured to emit cool white light.

6. The lighting device (10) of claim 5, wherein the first light emitting element(11) further comprises a common connecting element (112), a first connecting element (113), a second connecting element (114), and a spacer (115), wherein the common connecting element (112) mechanically and electrically connects the plurality of LED filaments, wherein the first connecting element (113) mechanically and electrically connects the first set of LED filaments (1111), wherein the second connecting element (114) mechanically and electrically connects the second set of LED filaments (1112), and wherein the spacer (115) comprises an insulating material, the spacer (115) being positioned to separate the first connecting element (113) and the second connecting element (114).

7. The lighting device (10) of any one of the preceding claims, wherein the second light emitting element (12) is an array of LED components (121) or a LED filament.

8. The lighting device (10) of claim 7, wherein the array of LED components (121) is a circular array of RGB LEDs.

9. The lighting device (10) of any one of claims 7-8, wherein the envelope (20) comprises multiple protrusions (22) connected to a carrier (40) to which the second light emitting element (12) is mounted, wherein each protrusion of the multiple protrusions (22) has a height Hp, wherein the second light emitting element (12) has a height HLED and a distance d to a light incoupling surface (21) of the envelope (20), wherein Hp= HLED + d, and wherein 0.1 mm < d < 0.5 mm.

10. The lighting device (10) of any one of the preceding claims, wherein the lighting device (10) further comprises an outer envelope (30) enclosing the envelope (20), the outer envelope (30) being light transmissive.

11. The lighting device (10) of claim 10, wherein the outer envelope (30) is light diffusing, the outer envelope (30) comprising an outer surface (31) and an inner surface (32), and wherein the outer envelope (30) comprises one or more of a surface texture on the outer surface (31), a surface texture on the inner surface (32), and scattering particles embedded in a matrix material of the outer envelope (30).

12. The lighting device (10) of any one of the preceding claims, wherein the lighting device (10) is a light bulb device.

13. The lighting device (10) of any one of the preceding claims, wherein the envelope (20) comprises one or more openings (22).

14. The lighting device (10) of any one of the preceding claims, wherein the envelope (20) has a first side at a light incoupling surface (21) and an opposite second side, wherein a first wall thickness TEI at the first side is smaller than a second wall thickness TE2 at the second side such that the wall thickness TE of the envelope (20) is non-constant, wherein the envelope (20) has a length L in a first direction measured from the first side to the second side, wherein the envelope (20) has width W measured in a second direction perpendicular to the first direction, and wherein L > W.

15. The lighting device (10) of any one of the preceding claims, wherein the lighting device (10) further comprises a controller that is configured to control the first light emitting element (11) and the second light emitting element (12) independently.

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