WO2025003063A1

WO2025003063A1 - Lighting device

Info

Publication number: WO2025003063A1
Application number: PCT/EP2024/067658
Authority: WO
Inventors: Ties Van Bommel
Original assignee: Signify Holding BV
Current assignee: Signify Holding BV
Priority date: 2023-06-27
Filing date: 2024-06-24
Publication date: 2025-01-02
Anticipated expiration: 2025-12-27

Abstract

In an aspect, there is provided a lighting device (10) comprising: a light mixing chamber (12) comprising a reflective inner surface (12a) and a diffusive light exit window (20); a plurality of first LEDs (22) arranged in the light mixing chamber (12) on a bottom portion (14) thereof, wherein the plurality of first LEDs is configured to emit first light,; and at least one second LED (24) arranged in the light mixing chamber (12), wherein the at least one second LED is configured to emit second light; wherein the lighting device (10) is configured to emit a mixture of light through the diffusive light exit window (20), wherein the mixture of light comprises the first light emitted from the plurality of first LEDs (22) and the second light emitted from the at least one second LED (24); wherein at least a majority of the first light emitted from the plurality of first LEDs (22) defines directly propagating first light propagating directly to the diffusive light exit window (20), and the second light emitted from the at least one second LED (24) is propagating directly and/or indirectly to the diffusive light exit window (20); and wherein the at least one second LED (24) is arranged such that an average optical path length (L2) for the second light, between the at least one second LED (24) and the diffusive light exit window (20), exceeds an average optical path length (L1) for the directly propagating first light, between the first LEDs (22) and the diffusive light exit window (20).

Description

LIGHTING DEVICE

FIELD OF THE INVENTION

The present invention generally relates to a lighting device.

BACKGROUND OF THE INVENTION

A light mixing chamber is a chamber or cavity, for instance box-shaped, with a reflective inner surface and a diffusive light exit window. Light mixing chambers are used in lighting applications for mixing light emitted by one or more light sources, typically light emitting diodes (LEDs), and thus provide a diffuse and even light output distribution.

In many LED-based lighting applications it is desirable to have a light mixing chamber which has a low profile (i.e. being shallow), provides a homogeneous light distribution at its light exit window (e.g. without any appreciable LED spottiness), yet has a high output efficiency. However, while a high output efficiency may be provided using a diffusive light exit window with low reflectivity, this tends to reduce the homogeneity of the light distribution (e.g. an increased appearance of LED spottiness). Simultaneously addressing the above stated aims is hence not trivial.

US 2010/290208 discloses a lighting device comprising a first group of solid state light emitters, an element containing luminescent material and a second group of solid state light emitters spaced from the element. In some embodiments, (1) at least 50 percent of light emitted by one of the first group does not mix with light emitted by the second group before the light emitted by the second group has entered the element, (2) at least 90 percent of exiting light emitted by the second group travels farther within the lighting device than 90 percent of exiting light emitted by the first group, (3) an average distance traveled by exiting light emitted by the second group is farther than an average distance traveled by exiting light emitted by the second group, and/or (4) light emitted by the first group directly exiting the lighting device exits the lighting device without being incident upon the element.

WO 2008/135927 discloses a solid-state lighting device includes a plurality of light-emitting elements configured for generating light that are thermally coupled to a heat spreading chassis configured for coupling to one or more heat sinks. The lighting device further includes a mixing chamber which is optically coupled to the plurality of light-emitting elements and configured to mix the light emitted by the plurality of light-emitting elements. A control system is operatively coupled to the plurality of light-emitting elements, and configured to control operation of the plurality of light-emitting elements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lighting device comprising a light mixing chamber which may have a low profile, yet still enabling a homogeneous light distribution at its light exit window and a high output efficiency. These and other objects may be achieved by a lighting device in accordance with the independent claim. Embodiments of the present invention are defined in the dependent claims.

Hence, according to an aspect of the present invention, there is provided a lighting device comprising: a light mixing chamber comprising a reflective inner surface and a diffusive light exit window; a plurality of first LEDs arranged in the light mixing chamber on a bottom portion thereof, wherein the plurality of first LEDs is configured to emit first light, and wherein each LED of the first LEDs comprises a first die having a first surface area, SAI, wherein the first surface area SAI has a first largest spatial extent, SEI, that is less than or equal to 100 micrometers; and at least one second LED arranged in the light mixing chamber, wherein the at least one second LED is configured to emit second light, and wherein each LED of the at least one second LED comprises a second die having a second surface area, SA2, wherein the second surface area SA2 has a second largest spatial extent, SE2, that is greater than or equal to 300 micrometers; wherein a ratio, R, defined by SA2/SA1 is equal to or larger than 10; wherein the lighting device is configured to emit a mixture of light through the diffusive light exit window, wherein the mixture of light comprises the first light emitted from the plurality of first LEDs and the second light emitted from the at least one second LED; wherein at least a majority of the first light emitted from the plurality of first LEDs defines directly propagating first light propagating directly to the diffusive light exit window, and the second light emitted from the at least one second LED is propagating directly and/or indirectly to the diffusive light exit window; and wherein the at least one second LED is arranged such that an optical path length for the second light (e.g. an average optical path length for the second light), between the at least one second LED and the diffusive light exit window, exceeds an optical path length for the directly propagating first light (e.g. an average optical path length for the directly propagating first light), between the first LEDs and the diffusive light exit window.

This aspect of the present invention is based, at least partly, on the insight that combining a plurality of first LEDs with smaller die dimensions (i.e. SAI and SEI) with one or more second LEDs of a greater die dimension (i.e. SA2 and SE2) enables a light mixing chamber with a low profile, yet providing a high output efficiency and a homogeneous light distribution. In some embodiments, the aspect ratio of a width and a height of the light mixing chamber may for example at least 8.

Note that where reference is made to surface area, this relates to the epitaxial or epitaxy (in short ‘epi’) surface area of the die which may only be on the top surface of the die.

The second LEDs may be referred to as “normal” LEDs as their size is typical for conventional LEDs. The first LEDs may be referred to as “microLEDs” or “nanoLEDs” as these are LEDs with dies of microscopic or sub-microscopic dimensions SAI, SEI. “Normal LEDs”, “microLEDs” and “nanoLEDs” are as such known in the field of LEDs.

The first LEDs are thus very small (e.g. compared to the second LEDs) and may thus appear as very tiny dots of light. Providing a plurality of first LEDs on the (reflective) bottom portion of the light mixing chamber hence enables a substantially continuous light emitting surface, which when viewed through the diffusive light exit window may be absent from any appreciable LED spottiness. This effect may be provided even when the diffusive light exit window is close to the bottom surface (thus enabling a light mixing chamber with low profile), and at a low reflectivity of the diffusive light exit window (thus enabling a high output efficiency of the light mixing chamber). The reflectivity of the diffusive light exit window may for instance be 28% or less, such as in a range from 15 to 25 %. In addition to their small size and even light distribution, microLEDs may provide benefits in the form of a greater dimming precision compared to normal LEDs.

A drawback with the small dimension first LEDs (e.g. nano-/microLEDs) is that they tend to be associated with higher material and manufacturing costs. Providing a lighting device for general lighting applications (e.g. device lighting, ambient room lighting, workspace lighting, downlighting luminaires etc.) using exclusively the first LEDs thus tends to be cost-prohibitive. Therefore, according to the present invention, the plurality of first LEDs are supplemented by at least one second LED (e.g. normal LED). However, such LEDs tend to be more prone to produce spottiness, especially if the reflectivity of the diffusive light exit window is low and/or a distance between diffusive light exit window and the LED is small.

To mitigate this tendency, each of the at least one second LEDs is according to the present invention arranged in the light mixing chamber such that the (average) optical path length for the second light from the second LED(s) is greater than the (average) optical path length for the directly propagating first light from the first LEDs. This enables a greater mixing of the second light before being emitted from the light mixing chamber through the diffusive light exit window, such that spottiness of the second LEDs is suppressed.

Reference is herein made to a “bottom portion”, a “sidewall structure” and a “top portion” of the light mixing chamber, wherein it is to be understood that they refer to respective portions of the reflective inner surface of the light mixing chamber. Hence, “bottom portion” refers to a reflective bottom portion of the reflective inner surface / the light mixing chamber, “sidewall structure” refers to a reflective sidewall structure of the reflective inner surface / the light mixing chamber, and “top portion” refers to a reflective top portion of the reflective inner surface / the light mixing chamber. In embodiments comprising a bottom portion, a sidewall structure and a top portion, the light mixing chamber is thus formed between the bottom portion, the sidewall structure, the top portion, and the diffusive light exit window.

At least a majority of the first light emitted by the first LEDs defines directly propagating first light propagating directly to the diffusive light exit window. Preferably, the directly propagating first light may constitute at least 70% of the first light, more preferably at least 80% of the first light, most preferably at least 90% of the first light. The term “at least a majority” may hereby be understood to refer to at least a majority of luminous flux (Im) of the first light emitted by the first LEDs.

As may be appreciated, light emitted from a first or second LED and exiting the light mixing chamber through the diffusive light exit window may propagate along a plurality of different optical paths between the LED and the diffusive light exit window. The term “average optical path length for the first / second light” hence refers to the average length of the plural optical paths propagated by the directly propagating first / second light, respectively. That is, according to an embodiment, the directly propagating first light emitted from the first LEDs propagates along a first plurality of optical paths extending directly between the first LEDs and the diffusive light exit window, and the second light emitted from the at least one second LED propagates along a second plurality of optical paths extending directly and/or indirectly between the at least one second LED and the diffusive light exit window, wherein the average optical path length for the second light is the average length of the second plurality of optical paths, and wherein the average optical path length for the directly propagating first light is the average length of the first plurality of optical paths.

In embodiments, the ratio, R, defined by SA2/SA1 may preferably be at least 15, more preferably at least 20, most preferably at least 25.

In some embodiments, the die of each of the first LEDs may have preferably a largest spatial extent (SEI) that is less than or equal to 80 micrometers, more preferably at most 70 micrometers, most preferably at most 50 micrometers, such as for example 50 micrometers or 40 micrometers.

In some embodiments, the die of each of the second LEDs may have preferably a largest spatial extent (SE2) that is greater than or equal to 400 micrometers, more preferably at least 500 micrometers, most preferably at least 600 micrometers such as for example 800 micrometers or 1000 micrometers.

In some embodiments, a majority of the light in the mixture of light may be formed by the second light.

In some embodiments, the average optical path length for the second light, L2, is at least 1.5 times greater than the average optical path length for the first light, LI. The additional optical path length for the second light enables a sufficient further mixing of the second light. The average optical path length for the second light, L2, may be preferably at least 1.7 times greater than the average optical path length for the first light, more preferably L2>1.9-L1, most preferably L2>2 L1.

In some embodiments, the at least one second LED is arranged in the light mixing chamber such that at least a majority of the second light (i.e. a majority of the second plurality of optical paths of the second light) emitted from the at least one second LED propagates to the diffusive light exit window via at least one reflective inner surface portion of the light mixing chamber. At least a majority of the second light emitted by the second LED(s) (e.g. normal LED(s)) may hence be indirectly emitted from the light mixing chamber, after being reflected one or more times by the reflective inner surface of the light mixing chamber. An indirect light configuration of the second LED(s) facilitates providing the light mixing chamber with a low profile. Preferably, the indirectly propagating portion of the second light may constitute (e.g. in terms of luminous flux) at least 80% of the second light, more preferably at least 90% of the second light, most preferably (substantially) all of the second light emitted by the second LED(s).

In some embodiments, a respective first die of each LED of the plurality of first LEDs face the diffusive light exit window and a respective second die of each LED of the at least one second LED is arranged at an angle 0 different from zero with respect to the first dies. The first LEDs may hence be arranged to face the diffusive light exit window, wherein the respective optical axis of the first LEDs may point directly towards the diffusive light exit window. Meanwhile, the second LEDs (and thus also the respective optical axis thereof) may angled with respect to the first LEDs to facilitate an indirect light configuration. For example, the angle 0 may be such that the one or more second dies may face one or more of the first dies. Also, the angle 0 may be such that the one or more second dies may be arranged perpendicular to the first dies. The angle 0 may for example be approximately 90 degrees and/or 180 degrees.

In some embodiments, the light mixing chamber comprises a top portion surrounding the diffusive light exit window and facing the bottom portion, and a circumferential sidewall structure extending between the bottom portion and the top portion, and wherein the at least one second LED is arranged on the sidewall structure or on the top portion. The light mixing chamber may thus be formed between the bottom portion, the sidewall structure, the top portion, and the diffusive light exit window. Either of these configurations allow the (respective) optical axis of the second LED(s) to be oriented towards a reflective inner surface portion of the light mixing chamber, and hence not towards the diffusive light exit window. In the top-portion mounted configuration, the second LED(s) may face the bottom surface and thus emit light towards the bottom surface (e.g. corresponding to the aforementioned angle 0 being approximately 180 degrees). In the sidewall-mounted configuration, the second LED(s) may face a reflective inner surface portion of the sidewall structure front located at an opposite side of the light mixing chamber (e.g. corresponding to the aforementioned angle 0 being approximately 90 degrees). Additionally, the top-portion may reduce a fraction of the second light directly propagating directly from the second LED(s) to the diffusive light exit window.

In some embodiments, the diffusive light exit window is substantially planar and/or substantially parallel to the bottom portion. The diffusive light exit window may hence be formed with a simple shape. This may for instance be suitable for implementing the light mixing chamber as a substantially flat hollow box. The aspect ratio of a width and a height of the light mixing chamber may for example be at least 8.

According to an alternative embodiment, the at least one second LED is arranged on the bottom portion of the light mixing chamber and wherein a shape of the diffusive light exit window is such that a separation between each second LED of the at least one second LED and the diffusive light exit window exceeds a separation between each first LED and the diffusive light exit window. Arranging both the first and second LEDs on the bottom portion may facilitate wiring and contribute to a rational manufacturing. Additionally, shaping the diffusive light exit window may provide further flexibility in controlling the mixing properties of the first and second light.

In some embodiments, the plurality of first LEDs is arranged (only) on a central portion of the bottom portion and the at least one second LED is arranged (only) on a peripheral portion of the bottom portion, and wherein the shape of the diffusive light exit window is such that a height of the diffusive light exit window above the bottom portion is greater at the peripheral portion than the central portion. This enables a simple shape of the diffusive light exit window, e.g. sloping towards the bottom portion along a direction towards a center axis of the light mixing chamber.

The light mixing chamber may comprise a circumferential sidewall structure extending between the bottom portion and the diffusive light exit window. The light mixing chamber may thus be formed between the bottom portion, the sidewall structure, and the diffusive light exit window. The sidewall structure (being reflective) may thus contribute to the mixing of the first and second light. Locating the second LED(s) closest to the sidewall structure may increase the proportion of light emitted by the second LED(s) and reaching the diffusive light exit window as indirect (i.e. reflected) light.

In some embodiments, the at least one second LED is instead arranged on (only) a central portion of the bottom portion and the plurality of first LEDs is arranged on (only) a peripheral portion of the bottom portion, and wherein the shape of the diffusive light exit window is such that a height of the diffusive light exit window above the bottom portion is greater at the central portion than the peripheral portion.

The diffusive light exit window may be curved (e.g. of semi-spherical shape of bulb-shaped) and define an apex located above the central portion. This design lends itself favorably for use in a lighting device with a bulb-and-socket-factor.

In some embodiments, a reflectivity of the reflective inner surface of the light mixing chamber is at least 80%, preferably at least 85%, more preferably at least 88%, most preferably at least 90%. This may contribute to an efficient mixing of the first and second light. The reflective inner surface may advantageously be diffusively reflective (as this may improve the light mixing), however specular reflectivity (at least some) is also possible.

The absorbance of the diffusive light exit window for the first and second light may advantageously be at most 2%. An absorbance in this range further contributes to a high efficiency by enabling low light loss due to low light absorption during mixing and the light exiting through the diffusive light exit window.

In some embodiments, the lighting device comprises a plurality of second LEDs, wherein the plurality of first LEDs are arranged at an (average) first pitch (Pl); wherein the plurality of second LEDs are arranged at an (average) second pitch (P2); and wherein P2>2 P1.

In some embodiments, the plurality of first LEDs may comprise at least 20 LEDs, preferably at least 40 LEDs, more preferably at least 60 LEDs, most preferably at least 80 LEDs.

In some embodiments, the at least one second LED may comprise at least 2 LEDs, preferably at least 4 LEDs, more preferably at least 6 LEDs, most preferably at least 8 LEDs.

In some embodiments, the number of first LEDs may be at least two or at least three times the number of second LEDs.

In some embodiments, the plurality of first LEDs may be arranged in a pattern having a gradient e.g. depending on the path length and/or distance with respect to the at least one second LED.

In embodiments, the first light and/or the second light may be white light having a correlated color temperature in a range from 2000K to 6500K and/or a color renderings index of at least 80, preferably at least 85. Likewise, in an example, the mixed light may be white light having a correlated color temperature in a range from 2000K to 6500K and/or a color renderings index of at least 80, preferably at least 85.

The lighting device described in the above can be used in a large variety of lighting units, such as luminaires and lamps. Hence, according to a second aspect there is provided a luminaire comprising the lighting device according to the first aspect.

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 embodiments of the present invention.

Fig. 1 shows a lighting device according to a first embodiment.

Fig. 2 shows a lighting device according to a second embodiment.

Fig. 3 shows a lighting device according to a third embodiment.

Fig. 4 shows a lighting device according to a fourth embodiment.

Fig. 5 illustrates an example of the size difference between a first LED and a second LED.

DETAILED DESCRIPTION

Fig. 1 schematically shows a cross-sectional side view of a lighting device or luminaire 10 according to a first embodiment. The lighting device 10 comprises a light mixing chamber 12. The lighting device 10 further comprises, arranged within the light mixing chamber 12, a plurality of first LEDs 22 and a number of second LEDs 24. Fig. 1 shows two second LEDs 24, however this is merely an example and the lighting device 10 may comprise both fewer or more second LEDs, i.e. at least one. In general, it is contemplated that the number of first LEDs 22 may be greater than the number of second LEDs 24. The number of first LEDs 22 may for instance be at least two or at least three times the number of second LEDs 24. The lighting device 10 may for example comprise at least 20, at least 40 or at least 60 first LEDs 22, and at least 2, at least 4, at least 6, or at least 8 second LEDs 24. The first LEDs 22 may e.g. be arranged at an average first pitch (Pl) and the second LEDs 24 may be arranged at an average second pitch (P2), wherein P2>2 P1. That is, an average pitch of the second LEDs 24 may be at least 2 times the pitch of the first LEDs 22.

The first LEDs 22 are smaller than the second LEDs 24. For example, the first LEDs 22 are microLEDs or nanoLEDs and the second LEDs 24 are normal LEDs.

In Fig. 5 there is illustrated schematically a cross-sectional view of the respective die 221, 241 of a first LED 22 and a second LED 24. The second LED 24 has an LED die 241 having a total surface area of SA2. The die 241 of the second LED 24 also has a largest spatial extent of SE2, wherein SE2 is greater than or equal to 300 micrometers. The first LED 22, on the other hand, is much smaller, having a total surface area of SAI and a die 221 with a largest spatial extent of SEI, wherein SEI is less than or equal to 100 micrometers. A ratio R between the die surface areas SA2 and SAI is defined as R=SA2/SA1 wherein R is equal to or larger than 10. That is, the surface area SA2 of the second LED die 241 is at least 10 times greater than the surface area SAI of the first LED die 221.

In fig. 5 the die 221, 241 of the respective type of LEDs is illustrated as substantially square, however it is understood that the shape of the die 221, 241 for the respective LED types could be any shape, e.g. circular, rectangular or polygonal. Additionally, it is noted that the shape of the die 221, 241 of the respective LED types could be different form each other, for example the second LEDs 24 may have a rectangular or square shaped die 241 whereas the first LEDs 221 may have a polygonal or circular die 221.

The light mixing chamber 12 comprises a diffusive light exit window 20 (interchangeably “light exit window”). The light exit window 20 is configured to allow light to be emitted (i.e. transmitted) from the light mixing chamber 12. The light mixing chamber 12 further comprises a bottom portion 14, a top portion 18 surrounding the diffusive light exit window 20, and a circumferential sidewall structure 16 extending between the bottom portion 14 and the top portion 18.

The bottom portion 14, the sidewall structure 16 and the top portion 18 are each reflective and typically opaque / non-transparent portions of the light mixing chamber 12, and collectively define a reflective inner (i.e. interior or inwardly facing) surface 12a of the light mixing chamber 12. The light mixing chamber 12 is thus formed between the bottom portion 14, the sidewall structure 16, the top portion 18, and the diffusive light exit window 20. The light mixing chamber 12 may for example be formed with a width W and a height H such that an aspect ratio of the light mixing chamber 12 is at least 8.

The inner surface 12a has a higher reflectivity (typically considerably higher) than the diffusive light exit window 20. A reflectivity of the inner surface 12a may for instance be at least 80 %, at least 85 %, at least 88 % or, advantageously, at least 90 %, wherein it is to be understood that the reflectivity refers to average reflectivity within the wavelength ranges emitted by the first and second LEDs 22, 24. The bottom portion 14, the sidewall structure 16 and the top portion 18 may for instance be provided with a reflective coating, such as silver or another conventional reflective metal or non-metal coating. A reflectivity of the light exit window 20 (i.e. average reflectivity within the wavelength range of the first and second LEDs 22, 24) may be 28% or less, such as in a range from 15 to 25 %. A higher reflectivity value may improve the light mixing but reduce the output efficiency. The absorbance of the diffusive light exit window 20 within the wavelength ranges emitted by the first and second LEDs 22, 24 may be at most 2%. The light exit window 20 may for instance be formed of glass or a polymer. Light scattering particles of any conventional type may be incorporated to provide the light exit window 20 with the desired diffusive (light scattering) characteristics.

As per the illustrated embodiment, the bottom portion 14 and the light exit window 20 may have a flat (planar) shape and extend in parallel to each other. The sidewall structure 16 may as further shown extend in a direction substantially transverse to the bottom portion 14 and connect to the top portion 18, wherein the top portion 18 may protrude inwardly from the sidewall structure 16 (e.g. substantially transverse to the sidewall structure 16) and connect to the light exit window 20. The light mixing chamber 12 may be shaped in the form of a hollow rectangular box, wherein the sidewall structure 14 comprises four sidewall portions, arranged in the shape of a rectangle about a rectangular bottom portion 14 and top portion 18. Other shapes are however also possible, such as triangular or other polygonal shapes, as well as circular or oval shapes. Other configurations of the sidewall structure 14 than transversally oriented with respect to the bottom portion 14 are also possible, such as a sidewall structure sloping inwardly or outwardly along a direction towards the light exit window 20, or a sidewall structure with a non-planar inner surface.

The lighting device 10 is configured to emit a mixture of light through the light exit window 20. The mixture of light emitted through the light exit window 20 comprises light emitted from the first LEDs 22 (termed first light) and light emitted from the second LEDs 24 (termed second light).

The first LEDs 22 are arranged on the bottom portion 14, facing the light exit window 20. Meanwhile, the second LED2 are arranged on the sidewall structure 16. Hence, while the first LEDs 22 are arranged with their respective first dies (e.g. die 221 in Fig. 5) facing the light exit window 20, (i.e. such that their respective optical axes oriented towards the light exit window 20), the second LEDs 24 are arranged with their respective second dies (e.g. die 241) at an angle 0 of approximately 90 degrees with respect to the first dies (i.e. such that their respective optical axes oriented towards a portion of the sidewall structure 16 located at an opposite side of the light mixing chamber 12). The first LEDs 22 are hence arranged in a so-called direct configuration, while the second LEDs 24 are arranged in a so- called indirect configuration. More specifically, at least a majority of the first light is light propagating directly from the first LEDs 22 to the light exit window 20. Conversely, at least a majority of the second light is light propagating indirectly from the second LEDs 24 to the light exit window 20, i.e. light propagating via at least one portion of the reflective inner surface 12a of the light mixing chamber 12.

In Fig. 1, the path LI is a representative example of a direct first optical path (propagation path) of first direct light emitted by a first LED 22. The path L2 is a representative example of an indirect second optical path of second light emitted by a second LED 22, which propagates via a reflective surface portion of the bottom portion 14. While Fig. 1 only shows single first and second optical paths LI, L2, a plurality (uncountably many) of such first and second optical paths exist between each first LED 22 and the light exit window 20 and each second LED 24 and the light exit window 20. As may be appreciated, light emitted from a first LED 22 or second LED 24 and exiting the light mixing chamber 12 through the light exit window 20 may propagate along a plurality of different optical paths between the LED 22, 24 and the light exit window 20. By the direct and indirect configurations of the first LEDs 22 and the second LEDs 24, an average optical path length for the second light, calculated over the second plurality of optical paths, between each second LED 24 and the light exit window 20, exceeds an average optical path length for the directly propagating first light, calculated across the first plurality of optical paths, between each first LEDs 22 and the light exit window 20.

The average optical path length for the second light may be at least 1.5 times greater than the average optical path length for the second light, at least 1.7 times greater, at least 1.9 times greater, or at least 2 times greater, wherein a greater difference may contribute to further mixing, and further improved homogeneity of the second light, and hence for the total mixture of light output from the light mixing chamber 12. While reference in the above is made to “average optical path length”, the above relationships may in some implementations apply to each optical path length for the directly propagating first light and each optical path length for the second light.

Depending on the reflectivity of the light exit window 20, some fraction of the light emitted by the first and second LEDs 22, 24 and incident on the light exit window 20 will be reflected back into the light mixing chamber 20, one or more times, before being emitted from the light exit window 20. Hence, the mixture of light transmitted through the light exit window 20 may further comprise light from the first and second LEDs 22, 24 reflected within the light mixing chamber 20 multiple times before being transmitted. Fig. 2 schematically shows a cross-sectional side view of a lighting device 110 according to a second embodiment. The lighting device 110 corresponds to the lighting device 10 of Fig. 1, except in that the second LEDs 24 are arranged on the top portion 18 instead of on the bottom portion 14. The second LEDs 24 hence face the bottom portion 18. That is, the second LEDs 24 are arranged with their respective optical axes oriented towards the bottom portion 16 of the light mixing chamber 12. Compared to Fig. 1, this corresponds to an angle 0 of approximately 180 degrees between the orientation of the first dies of the first LEDs 22 and the second dies of the second LEDs 24. As indicated by the example optical path L2, the second LEDs 24 are thus, like in Fig. 1, arranged in an indirect configuration, such that an average optical path length for the second light between each second LED 24 and the light exit window 20, exceeds an average optical path length for the first light between each first LEDs 22 and the light exit window 20.

Fig. 3 schematically shows a cross-sectional side view of a lighting device 210 according to a third embodiment. The lighting device 210 corresponds to the lighting device 10 of Fig. 1 and 110 of Fig. 2, but differs in that both the first LEDs 22 and the second LEDs 24 are arranged in a direct configuration. As will be explained in the following, the lighting device 210 however still, like lighting devices 10 and 110, meet has as a feature that an average optical path length for the second light, between the at least one second LED 24 and the diffusive light exit window 20, exceeds an average optical path length for the first light, between the first LEDs 22 and the diffusive light exit window 20.

With reference to Fig. 3, the lighting device 210 comprises a light mixing chamber 212. The light mixing chamber 212 comprises a diffusive light exit window 220, corresponding to light exit window 20. The light mixing chamber 212, like the light mixing chamber 12, further comprises a bottom portion 14 and a circumferential sidewall structure 16. The sidewall structure 16 extends from the bottom portion 14 and to the diffusive light exit window 220, to surround the same. The light exit window 220 differs from the planar light exit window 20 of the light mixing chamber 20 by being shaped such that a (local) height (denoted h in Fig. 3) of the diffusive light exit window 220 above the bottom portion 14 is greater at a peripheral portion 14b than the central portion 14a. More specifically, the shape of the light exit window 220 is such that the height h is gradually reduced along a direction towards the central portion 14a of the light mixing chamber 212. In the depicted embodiment the light exit window 220 is shaped such that the height h decreases linearly towards the central portion 14a, however this is merely an example and other non-linear shapes are also possible. Furthermore, the first LEDs 22 are arranged on or at the central portion 14a (i.e. only at the central portion 14a) of the bottom portion 14 and the second LEDs 24 are arranged on or at the peripheral portion 14b (i.e. only at the peripheral portion 14b) of the bottom portion 14b. Thereby, a separation between each second LED 24 and the diffusive light exit window 220 (corresponding to h) exceeds a separation between each first LED 22 and the diffusive light exit window 220. As is readily apparent from the example optical paths LI and L2, an average optical path length for the second light, between the at least one second LED 24 and the diffusive light exit window 20, exceeds an average optical path length for the first light, between the first LEDs 22 and the diffusive light exit window 20.

Fig. 4 schematically shows a cross-sectional side view of a lighting device 310 according to a fourth embodiment. The lighting device 310 corresponds to the lighting device 210 in that in that both the first LEDs 22 and the second LEDs 24 are arranged in a direct configuration. The lighting device 310 also corresponds to the lighting device 210 in that it comprises a diffusive light exit window 320 with a non-planar shape. However, in contrast to the light exit window 220, the shape of the diffusive light exit window 320 is such that a height h of the diffusive light exit window 320 above the bottom portion 14 is greater at the central portion 14a than the peripheral portion 14b. More specifically, the diffusive light exit window 320 is curved (e.g. having a semi-spherical or bulb shape) and extends from the bottom portion 14 towards an apex located above the central portion 14a. The lighting device 310 further differs from the lighting device 210 in that the first LEDs 22 are arranged (only) on or at the central portion 14a and the second LEDs are arranged (only) on or at the peripheral portion 14b. Thereby, a separation between each second LED 24 and the diffusive light exit window 320 (corresponding to h) exceeds a separation between each first LED 22 and the diffusive light exit window 320. Accordingly, an average optical path length for the second light, between the at least one second LED 24 and the diffusive light exit window 20, exceeds an average optical path length for the first light, between the first LEDs 22 and the diffusive light exit window 320 (c.f. example optical paths LI and L2).

In the illustrated lighting device 310, the bulb-shaped light exit window 320 is arranged directly on the bottom portion 14, wherein the light mixing chamber 320 is formed exclusively between the bottom portion 14 and the light exit window 320. It is however also possible to combine a bulb-shaped light exit window 320 with a light mixing chamber further comprising a circumferential sidewall structure (corresponding to sidewall structure 16), wherein the light exit window 320 may be arranged on top of the sidewall structure. The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For instance, while in Fig. 1-2 the first and second LEDs 22, 24 are shown to be arranged such that the angle 0 is approximately 90 degrees and 180 degrees, respectively, between the first dies and the second dies, also other relative orientations / angles 0 are possible. For instance, the second LEDs may be arranged such that the angle 0 is at least 30 degrees, at least 60 degrees, or at least 80 degrees. For example, the second LEDs may be mounted in an inclined fashion on a sidewall structure transversally oriented with respect to the bottom portion (like the sidewall structure 16 in Fig. 1), or on an inwardly or outwardly sloping sidewall structure, as discussed above. Additionally, while not visible in the schematic side views of Fig. 1-2, the first LEDs 22 may be arranged in a two-dimensional array on the bottom portion 14 of the light mixing chamber 12, while the second LEDs 24 may be circumferentially distributed along the sidewall structure 16 (in Fig. 1) or the top portion 18 (in Fig. 2). Correspondingly, in the embodiments of Fig. 3-4, the first and second LEDs 22, 24 may each be arranged in a two-dimensional array on the bottom portion 14 of the respective light mixing chambers 212, 312. It is further contemplated that the first LEDs 22 may be arranged in a pattern having a gradient e.g. depending on the path length LI and/or distance with respect to the second LED(s) 24.

Claims

CLAIMS:

1. A lighting device (10) comprising: a light mixing chamber (12) comprising a reflective inner surface (12a) and a diffusive light exit window (20); a plurality of first LEDs (22) arranged in the light mixing chamber (12) on a bottom portion (14) thereof, wherein the plurality of first LEDs is configured to emit first light, and wherein each LED of the first LEDs comprises a first die (221) having a first surface area, SAI, wherein the first surface area SAI has a first largest spatial extent, SEI, that is less than or equal to 100 micrometers; and at least one second LED (24) arranged in the light mixing chamber (12), wherein the at least one second LED is configured to emit second light, and wherein each LED of the at least one second LED comprises a second die (241) having a second surface area, SA2, wherein the second surface area SA2 has a second largest spatial extent, SE2, that is greater than or equal to 300 micrometers; wherein a ratio, R, defined by SA2/SA1 is equal to or larger than 10; wherein the lighting device (10) is configured to emit a mixture of light through the diffusive light exit window (20), wherein the mixture of light comprises the first light emitted from the plurality of first LEDs (22) and the second light emitted from the at least one second LED (24); wherein at least a majority of the first light emitted from the plurality of first LEDs (22) defines directly propagating first light propagating directly to the diffusive light exit window (20), and the second light emitted from the at least one second LED (24) is propagating directly and/or indirectly to the diffusive light exit window (20); wherein the at least one second LED (24) is arranged such that an average optical path length (L2) for the second light, between the at least one second LED (24) and the diffusive light exit window (20), exceeds an average optical path length (LI) for the directly propagating first light, between the first LEDs (22) and the diffusive light exit window (20), and wherein a respective first die of each LED of the plurality of first LEDs (22) face the diffusive light exit window (20) and a respective second die of each LED of the at least one second LED (24) is arranged at an angle 0 different from zero with respect to the first dies, wherein preferably 0 is approximately 90 degrees and/or 180 degrees.

2. The lighting device (10) according to claim 1, wherein the average optical path length for the second light (L2) is at least 1.5 times greater than the average optical path length for the first light (LI).

3. The lighting device (10; 110) according to claim 1 or 2, wherein the at least one second LED (24) is arranged in the light mixing chamber (12) such that at least a majority of the second light emitted from the at least one second LED (24) propagates to the diffusive light exit window (20) via the reflective inner surface (12a) of the light mixing chamber (12).

4. The lighting device (10; 110) according to any one of the preceding claims, wherein the light mixing chamber (12) comprises a top portion (18) at least partly surrounding the diffusive light exit window (20) and facing the bottom portion (14), and a circumferential sidewall structure (16) extending between the bottom portion (14) and the top portion (18), and wherein the at least one second LED (24) is arranged on the sidewall structure (16) or on the top portion (18).

5. The lighting device (10) according to any one of claims 3-4, wherein: the diffusive light exit window (20) is substantially planar and substantially parallel to the bottom portion (14); and the aspect ratio of a width (W) and a height (H) of the light mixing chamber (12) is at least 8.

6. The lighting device (210; 310) according to any one of claims 1-2, wherein the at least one second LED (24) is arranged on the bottom portion (14) of the light mixing chamber (212; 312) and wherein a shape of the diffusive light exit window (220; 320) is such that a separation between each second LED (24) of the at least one second LED (24) and the diffusive light exit window (220; 320) exceeds a separation between each first LED (22) and the diffusive light exit window (220; 320).

7. The lighting device (210) according to claim 6, wherein the plurality of first LEDs (22) is arranged on a central portion (14a) of the bottom portion (14) and the at least one second LED (24) is arranged on a peripheral portion (14b) of the bottom portion (14b), and wherein the shape of the diffusive light exit window (220) is such that a height of the diffusive light exit window (220) above the bottom portion (14) is greater at the peripheral portion (14b) than the central portion (14a).

8. The lighting device (210) according to claim 7, wherein the light mixing chamber (212) comprises a circumferential sidewall structure (16) extending between the bottom portion (14) and the diffusive light exit window (220).

9. The lighting device (310) according to claim 8, wherein the at least one second LED (24) is arranged on a central portion (14a) of the bottom portion (14) and the plurality of first LEDs (22) is arranged on a peripheral portion (14b) of the bottom portion (14b), and wherein the shape of the diffusive light exit window (320) is such that a height of the diffusive light exit window (320) above the bottom portion (14) is greater at the central portion (14a) than the peripheral portion (14b).

10. The lighting device (310) according to claim 9, wherein the diffusive light exit window (320) is curved and defines an apex located above the central portion (14a).

11. The lighting device (10; 110; 210; 310) according to any one of the preceding claims, wherein a reflectivity of the diffusive light exit window is 28% or less, such as in a range from 15 to 25 %.

12. The lighting device (10; 110; 210; 310) according to any one of the preceding claims, wherein: the reflectivity of the reflective inner surface (14a; 212a; 312a) of the light mixing chamber (12; 212; 312) is at least 80%; and the absorbance of the diffusive light exit window for the first and second light is at most 2%.

13. The lighting device (10) according to any one of the preceding claims, wherein the lighting device (10) comprises a plurality of second LEDs (24), wherein the plurality of first LEDs are arranged at an average first pitch (Pl); the plurality of second LEDs are arranged at an average second pitch (P2); and wherein P2>2 P 1.

14. A luminaire comprising the lighting device (10, 110, 210, 310) according to any one of the preceding claims.