Lighting device for use as an artificial skylight

Abstract

The invention relates to a lighting device (1000) having a housing (1100) with an interior space (1110) that is bounded by a back surface (1120), a front surface (1130), and a wall surface (1140), wherein the back surface (1120) and the wall surface (1140) are diffusely reflective, and wherein the front surface (1130) has a light exit window (1131). The lighting device (1000) further has a first light source (1210) for emitting a first light output into the interior space (1110), the first light output having a first correlated color temperature in a range of 2,000 K to 10,000 K or the first light output being blue. The first light source (1210) is provided at a location between the light exit window (1131) and the wall surface (1140). In a direction from the light exit window (1131) to the back surface (1120), the back surface (1120) and the light exit window (1131) are located at a first separation distance from each other. In the same direction, the light exit window (1131) has a projected area on the back surface (1120). The separation distance is smaller than a largest window dimension of the light exit window, and the projected area of the light exit window (1131) is smaller than the back surface (1120). In the lighting device (1000), one or more of the following applies: (i) the wall surface (1140) is separated from the light exit window (1131) by a second separation distance, a ratio of the second separation distance and the first separation distance being at least four, and (ii) at least where the wall surface (1140) connects to the back surface (1120), the wall surface (1140) has an arc-shaped cross section in a plane perpendicular to the light exit window (1131). The lighting device (1000) can be used as artificial skylight to provide an enhanced experience of being exposed to natural daylight conditions.

Description

[0001] Lighting device for use as an artificial skylight

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a lighting device for use as an artificial skylight.

[0004] BACKGROUND OF THE INVENTION

[0005] The beneficial properties of daylight on human and animal well-being are well documented. However, an increasing proportion of the population spends most of its time indoors.

[0006] Although skylights and windows offer the opportunity to provide daylight to an indoor environment, these can only be placed at or near the boundaries of a building and cannot typically illuminate a center of the building.

[0007] Instead, an artificial skylight may be used to simulate natural daylight and create a sense of open space in an indoor environment. Such devices can be installed in homes, offices, hospitals, schools, or any other indoor space that lacks natural light. They typically use LED technology to mimic the color and intensity of daylight, and some even have the ability' to adjust their brightness and color temperature to match the time of day or weather conditions. Artificial skylight devices have many benefits, including improving mood, productivity , and overall well-being.

[0008] However, it has been recognized that the beneficial effect of artificial skylights is negatively impacted if the artificial skylight has an unnatural appearance, such that a viewer of the artificial skylight does not perceive it to be or resemble a true skylight.

[0009] SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide an improved artificial skylight that is perceived, by a viewer, to be more similar to a true skylight.

[0011] In a first aspect, the invention provides a lighting device having a housing with an interior space. The interior space is bounded by a back surface, a front surface opposite the back surface, and a wall surface separating the back surface and the front surface.

[0012] The back surface and the wall surface of the interior space are diffusely reflective. Each of the back surface, the front surface, and the wall surface may have a reflectivity of at least 80 %, such as at least 90 %. The latter results in an improved efficiency, which is due to the fact that in the proposed architecture, light will typically be reflected several times before it exits the lighting device. With a reflectivity of at least 80 %, any losses are limited, and thus the efficiency is relatively high.

[0013] The front surface of the interior space has a light exit window. The light exit window may have any suitable shape, such as a circular shape or a polygonal shape. An example of a polygonal shape is a rectangular shape, such as a square shape.

[0014] Irrespective of its shape, the light exit window will always have a largest window dimension. The largest window^ dimension is the largest of the mutually perpendicular length and width dimensions of the smallest rectangle that encloses the light exit window.

[0015] The lighting device further has a first light source for emitting a first light output into the interior space. The first light output has a first correlated color temperature in a range of 2,000 K to 10,000 K.

[0016] The first light source may comprise a plurality of light-emitting elements, such as light-emitting diodes. Together, the light-emitting elements are arranged to provide the first light output, but each light-emitting element may be individually addressable and / or may be color controllable.

[0017] The range of correlated color temperatures from 2,000 K to 10,000 K covers the ranges corresponding to sunset / sunrise (from 2,000 K to 3,500 K, also known as warm white light), to daylight (from 4,000 K to 6,000 K, also knowm as cool white light), and to overcast sky (from 6,500 K to 8,000 K).

[0018] By being arranged to emit a first light output having a first correlated color temperature in a range of 2,000 K to 10,000 K, the first light source of the lighting device can mimic natural sunlight conditions, ranging from sunset / sunrise to daylight and to overcast sky conditions. In other words, the first light source is arranged to emit artificial sunlight.

[0019] Alternatively, the first light output emitted by the first light source is blue. A light output is considered blue when at least 90 % of the spectral distribution of the light output is within a wavelength range of about 380 nm to about 500 nm.

[0020] The first light source is provided at a location between the light exit window and the wall surface. Preferably, the first light source is provided at the front surface of the interior space. In a direction from the light exit window to the back surface, the back surface and the light exit window are located at a first separation distance from each other. The first separation distance is smaller than the largest window dimension of the light exit window.

[0021] Also in a direction from the light exit window to the back surface, the light exit window has a projected area on the back surface. The projected area of the light exit window is smaller than the back surface of the interior space.

[0022] In the lighting device, one or more of two configurations apply.

[0023] In a first configuration, the wall surface is separated from the light exit window by a second separation distance, wherein a ratio of the second separation distance and the first separation distance is at least four, such as at least five, or at least ten.

[0024] In a second configuration, at least where the wall surface connects to the back surface, the wall surface has an arc-shaped cross section in a plane perpendicular to the light exit window. In this configuration the transition between the back surface and the wall surface is smoothly curved. In other words, at least the part of the wall surface that is closer to the back surface than to the front surface is smoothly curved.

[0025] For the second situation, the cross section may have the shape of a circular arc with a radius of curvature, wherein a ratio of the radius of curvature and the first separation distance is at least 0.3, such as at least 0.5 times the first separation distance. The radius of curvature may be equal to the first separation distance, or it may be larger than the first separation distance.

[0026] The first light source is provided in the interior space at a location between the light exit window and the wall surface. When the lighting device is in operation, the first light output that is emitted by the first light source into the interior space is diffusely reflected by the front surface, the back surface, and the wall surface of the interior space, and subsequently exits the interior space via the light exit window.

[0027] Each of the above two situations corresponds to a configuration that allows an observer to look into the interior space of the lighting device through the light exit window from many different viewing directions while giving the observer the impression that the diffusely reflective back surface of the interior space extends indefinitely behind the light exit window. In turn, this enhances the impression of being exposed to natural daylight conditions.

[0028] As said, the first light source is provided in the interior space at a location between the light exit window and the wall surface. The first light source may be screened from direct view through the light exit window. In other words, the first light source may be positioned in the lighting device such that there is no direct line of sight through the light exit w indow towards the first light source. This further enhances the impression of being exposed to natural daylight conditions.

[0029] For example, the first light source may be provided on the front surface for emitting the first light output towards the back surface.

[0030] The lighting device may further have a rim that surrounds the light exit window and that extends into the interior space. The rim then forms an edge, and the first light source can be located behind this edge.

[0031] When the first light source has a first light source height, and the rim has a rim height (the height being a dimension in a direction perpendicular to the light exit window) the rim height may be at least equal to the first light source height so that the first light source is shielded from direct view by the rim. The rim height may be 50 % or less of the first separation distance, such as 40 % or less, or 30 % or less.

[0032] In a direction from the light exit window to the back surface, the back surface and the light exit window are located at a first separation distance from each other.

[0033] The first separation distance may be equal to or less than 30 centimeters, such as equal to or less than 25 centimeters, or equal to or less than 22 centimeters, or equal to or less than 20 centimeters. Such a maximum separation distance gives sufficient space for each of the aforementioned two situations to ensure that the lighting device can easily be integrated into a suspended ceiling.

[0034] The first separation distance may be equal to or more than 3 centimeters, such as equal to or more than 5 centimeters, or equal to or more than 7 centimeters, or equal to or more than 10 centimeters. Such a minimum separation distance gives sufficient space for each of the aforementioned two situations to create the impression of an indefinitely extending diffusely reflective back surface for a relatively large range of possible viewing directions.

[0035] In one or more planes perpendicular to the light exit window, preferably in any such plane, the interior space of the lighting device may have a cross section with a convex shape.

[0036] Where the back surface connects to the wall surface, the aforementioned convex shape may have a right angle or an obtuse angle.

[0037] The aforementioned convex shape may be a rectangle, which is a convenient shape for an interior space of a lighting device. In this case, the second separation distance may be at least 15 centimeters, such as at least 20 cm, again for creating the impression of an indefinitely extending diffusely reflective back surface for a relatively large range of possible viewing directions. Furthermore, the second separation distance may be at most 60 centimeters, such as at most 50 cm, so that the lighting device may be easily implemented in a dropped ceiling.

[0038] The aforementioned convex shape may also be stadium. A stadium, being a type of oval, is a geometric shape constructed of a rectangle with semicircles at a pair of opposite sides. Such a shape may also be referred to as a pill shape, a discorectangle, an obround, or a sausage shape. This is a convenient shape for an interior space of a lighting device.

[0039] The lighting device has a first light source for emitting a first light output into the interior space. The first light output has a first correlated color temperature in a range of 2,000 K to 10,000 K. Alternatively, the first light output is blue.

[0040] The first correlated color temperature of the first light output emitted by the first light source may be more than 4,000 K, such as at least 6,000 K. In this case, the lighting device may further have a second light source for emitting a second light output into the interior space, the second light output having a second correlated color temperature of 4,000 K or less, and a controller for individually controlling the first light source and the second light source. This would allow the overall light output of the lighting device to have a correlated color temperature that can be tuned from one corresponding to sunset / sunrise conditions (or warm white) to one corresponding to daylight conditions (or cool white) and / or overcast sky conditions.

[0041] The lighting device according to the first aspect of the invention may be an artificial skylight that can be integrated into a ceiling of a space, such as a residential space, an office space, a retail space, a hospitality space, or a healthcare space.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, which are not necessarily to scale, and in which corresponding reference symbols indicate corresponding parts.

[0044] Fig. 1 shows a lighting device in a cross sectional view.

[0045] Fig. 2 shows the lighting device of Figure 1 being used as ceiling luminaire, mounted to a surface of a ceiling.

[0046] Fig. 3 shows a lighting device in a cross sectional view. Fig. 4 shows the lighting device of Figure 3 being used as ceiling luminaire, mounted to a surface of a ceiling.

[0047] Figs. 5(a) and 5(b) show close-ups of the situation of Figure 4.

[0048] Figs. 6(a) to 6(d) show cross sectional views of a lighting device.

[0049] Fig. 7 shows a ceiling lighting system installed in a space, wherein the lighting system comprises multiple lighting devices.

[0050] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] Figure 1 shows a lighting device 1000 in a cross sectional view.

[0052] The lighting device 1000 has a housing 1100 with an interior space 1110. The interior space 1110 is bounded by a back surface 1120, a front surface 1130, and a wall surface 1140. The front surface 1130 is located opposite the back surface 1120, and the wall surface 1140 separates back surface 1120 and the front surface 1130.

[0053] The back surface 1120 and the front surface 1130 are both diffusely reflective.

[0054] The front surface 1130 has a light exit window 1131 with a largest window dimension w. In the lighting device 1000, the light exit window 1131 has a rectangular shape, but other shapes would also be possible.

[0055] The lighting device 1000 of Figure 1 has a first light source 1210. The first light source 1210 has a plurality of light-emitting diodes 1211, positioned on the front surface 1130 and on opposite sides of the light exit window 1131. This is just an example. The light source does not have to be positioned on the front surface 1130, and it may have any configuration. It may be a single light-emiting element, it may comprise multiple lightemiting elements on the same side of the light exit window 1131, as long as the light source is provided at a location between the light exit window 1131 and the wall surface 1140.

[0056] The first light source 1210 is arranged to emit a first light output into the interior space 1110. The first light output has a first correlated color temperature in a range of 2,000 K to 10,000 K. Alternatively, the first light source may be arranged to emit a first light output that is blue.

[0057] In a direction from the light exit window 1131 to the back surface 1120, the back surface 1120 and the light exit window 1131 are located at a first separation distance d1from each other. The separation distance d is smaller than the largest window dimension w of the light exit window 1131.

[0058] In the lighting device 1000 of Figure 1, the first separation distance d±is 10 centimeters. The first separation distance d may alternatively have a different value, such as equal to or less than 30 centimeters, or as equal to or less than 20 centimeters, or equal to or less than 5 centimeters., as long as it is smaller than the largest window dimension w of the light exit window 1131.

[0059] Also in a direction from the light exit window 1131 to the back surface 1120, the light exit window 1131 has a projected area Apon the back surface 1120. The projected area Apof the light exit window 1131 is smaller than the back surface 1120. In other words, the back surface 1120 extends beyond the projected area Apof the light exit window 1131, preferably in all directions.

[0060] Regarding the location of the light exit window 1131, as is the case for lighting device 1000 shown in Figure 1, it would typically be provided centrally in the lighting device 1000, and such that the projected area of the light exit window 1131 on the back surface 1120 is concentric therewith.

[0061] In Figure 1, the cross sectional view of the lighting device 1000 is made in a plane perpendicular to the light exit window 1131. In this view, the interior space 1110 has a cross section with a convex shape. In fact, in any plane perpendicular to the light exit window 1131, the interior space 1110 has a cross section with a convex shape.

[0062] In Figure 1, the cross sectional shape of the interior space 1110 is a stadium, which is a geometric shape constructed of a rectangle with semicircles at a pair of opposite sides. A stadium is a specific example of a convex shape wherein, where the back surface 1120 connects to the wall surface 1140, there is a circular arc with a radius of curvature r.

[0063] In Figure 1, the radius of curvature r is 0.5 times the first separation distance dvFor the purpose of the invention, and in case the lighting device is similar to what is illustrated in Figure 1, the radius of curvature r should be at least 0.3 times the first separation distance d1.

[0064] The lighting device 1000 of Figure 1 further has a rim 1300 that surrounds the light exit window 1131. The rim 1300 also extends into the interior space 1110, thereby forming an edge behind which the light-emitting diodes 121 1 of the first light source 1210 are located.

[0065] When height is defined as being a dimension in a direction perpendicular to the light exit window 1131, the first light source 1210 has a first light source height, and the rim 1300 has a rim height. In the configuration of Figure 1 , the rim height is about equal to the first light source height, and approximately 50 % of the first separation distance dr. The rim ensures that the first light source 1210 is shielded from being directly visible when looking through the light exit window 1131.

[0066] Figure 2 shows the lighting device 1000 of Figure 1 being used as ceiling luminaire, mounted to a surface of a ceiling. Alternatively, it may also be integrated into the ceiling, so that the light exit window is flush with the surface of the ceiling. The latter can for example easily be done when the ceiling is a dropped ceiling.

[0067] In Figure 2, P denotes a plane parallel to the ceiling on which a viewing point of an observer may be located. When the ceiling is of an average height (2,70 meters), and when the observer is a person of average height (1,70 meters), the plane P would be located substantially 1 meter below the surface of the ceiling.

[0068] From any viewing point of plane P, all direct lines of sight passing through the light exit window 1131 will either be incident on the back surface 1120 or on the wall surface 1140. In Figure 2, this is schematically shown for two distinct viewing points on plane P.

[0069] The back surface 1120 and the wall surface 1140 are both diffusely reflective. Furthermore, where the back surface 1120 connects to the wall surface 1140, the convex cross sectional shape of the interior space 1110 has a circular arc with a radius of curvature of at least 0.3 times the first separation distance. This effectively means that the back surface 1120 and the wall surface 1140 appear as a continuous surface with no visible boundary in between. This seemingly continuous surface appears to extend indefinitely behind the light exit window 1131.

[0070] Because the first light output emitted by the first light source 1210, and which diffusely reflects from the back surface 1120 and the wall surface 1140 through the light exit window 1131, mimics natural sunlight conditions, a seemingly continuous surface appearing to extend indefinitely behind the light exit window 1131 gives the impression of looking through a window to the outside environment while being exposed to natural daylight conditions.

[0071] Figure 3 shows a lighting device 2000, similar to the lighting device 1000 of Figure 1, but now with a different shape.

[0072] Where in Figure 1 the cross sectional shape of the interior space 1110 is a stadium, in Figure 3 the cross sectional shape of the interior space 1110 is a rectangle. A rectangle is a specific example of a convex shape wherein, where the back surface 1120 connects to the wall surface 1140, there is a right angle.

[0073] In the lighting device 1000 of Figure 3, the wall surface 1140 is separated from the light exit window 1131 by a second separation distance d2. The second separation distance d2is three times the first separation distance d^. The ratio of the second separation distance d2and the first separation distance d may also have a different value, as long as it is at least two. In other words, for the purpose of the invention, and in case the lighting device is similar to what is illustrated in Figure 2, the wall surface 1140 must be separated from the light exit window 1131 by a second separation distance d2of at least twice the first separation distance d .

[0074] Regarding the location of the light exit window 1131, as is the case for lighting device 1000 shown in Figure 2, it would typically be provided centrally in the lighting device 1000, and such that the projected area of the light exit window 1131 on the back surface 1 120 is concentric therewith.

[0075] Similar to what Figure 2 shows for the lighting device 1000 of Figure 1, Figure 4 shows the lighting device 1000 of Figure 3 being used as ceiling luminaire, mounted to a surface of a ceiling. Again, P denotes a plane parallel to the ceiling on which a viewing point of an observer may be located. When the ceiling is of an average height (2,70 meters), and when the observer is a person of average height (1,70 meters), the plane P would be located substantially 1 meter below the surface of the ceiling.

[0076] In Figure 4, direct lines of sight passing through the light exit window 1131 are shown for a viewing point on plane P. For this viewing point, all direct lines of sight are incident on the back surface 1120.

[0077] Figures 5(a) and 5(b) show close-ups of the situation of Figure 4.

[0078] Figure 5(a) shows viewing point 510 that is located on plane P. Figure 5(a) also shows the two extreme direct lines of sight 511 and 512, respectively, that can be drawn from the viewing point 510 through the light exit window 1131.

[0079] When these two extreme direct lines of sight 511 and 512 are incident on the back surface 1120, all direct lines of sight coming from the viewing point 710 will be incident on the back surface 1120, and an impression of an indefinitely extending diffusely reflective back surface 1120 is provided.

[0080] The front surface 1130 essentially acts as a kind of mask for the direct lines of sight coming from a viewing point.

[0081] From any viewing point, the direct line of sight through the light exit window 1131 with the largest angle relative to the normal of the light exit window 1131, determines how far an observer can look into the lighting device 1000. For viewing point 510 shown in Figure 5(a), this would be the direct line of sight 511. The direct line of sight 511 passes through the light exit window 1131 immediately adjacent to an edge of the light exit window 1131 that is located a distance d2from the wall surface 1140, d2being the second separation distance separating the wall surface 1140 from the light exit window 1131.

[0082] In general, the larger the second separation distance d2. the higher the chance that the direct line of sight 511 will be incident on the back surface 1120.

[0083] In Figure 5(a), the light exit window 1131 is located centrally in the front surface 1130. But irrespective of the location of the light exit window 1131 in the front surface 1130, and irrespective of the shapes of both the light exit window 1131 and the front surface 1130, there will always be a shortest distance separating the wall surface 1140 from the light exit window 1131. In Figure 5(a), this shortest distance would be the second separation distance d2.

[0084] The second separation distance d2determines the maximum distance of a viewing point from which any direct light of sight through the light exit window 1131 would still be incident on the back surface 1120. Hence, it determines the maximum distance of a viewing point from which the impression of an indefinitely extending diffusely reflective back surface 1120 is provided.

[0085] The second separation distance d2and the first separation distance d together constitute the legs of a right triangle.

[0086] The aforementioned right triangle with legs d2and d2is similar to a right triangle with legs h and p, h denoting a distance of a view plane P from the front surface 1130, and p denoting a distance of viewing point 520 on the view plane P from the edge of the light exit window 1131.

[0087] In Figure 5(b), the aforementioned similar right triangles are show n as hatched areas

[0088] When the luminaire 1000 is used as a ceiling luminaire in a space with a standard ceiling height (2,70 meters), the viewing points of an observer of average height (1,70 meters) will be on a plane that is located about 1 meter below the front surface 1102 of the housing 1100.

[0089] When the second separation distance d2would be at least four times as large as the first separation distance d±, (i.e., d2 / d-:. > 4), the aforementioned observer would always have direct lines of sight to the back surface 1120 when within a distance of four meters from the edge of the light exit window 1131. For a typical indoor setting, such as an office space, the above situation would mean that an observer of average height just about always gets the impression of a diffusely reflecting back surface that extends indefinitely behind the light exit window.

[0090] To ensure that the desired impression of an indefinitely extending diffusely reflective back surface is provided under an even broader range of circumstances, the second separation distance d2may be at least five times as large as the first separation distance d±.

[0091] For a typical ceiling luminaire, the light exit window 1131 may be centrally located in the front surface 1130, and it may preferably have a largest window dimension of 20 centimeters or less. Furthermore, to allow easy integration into a dropped ceiling, the ceiling luminaire preferably has a length and width of not more than 60 centimeters. Under these boundary conditions, the second separation distance d2will be at least 20 centimeters.

[0092] To ensure that an observer of average height would practically always have direct lines of sight to the back surface 1120 when within a distance of four meters from the edge of the light exit window 1131, the first separation distance d should be 5 centimeters or less.

[0093] Under (a) to (d), Figure 6 shows several examples of lighting devices 1000 in a cross sectional view, the cross section being in a plane perpendicular to the light exit window 1 131.

[0094] The views of Figures 6(a) and 6(b) are similar to those of Figures 1 and 3, respectively. In the view of Figure 6(a), the interior space of the lighting device 1000 has a cross section with a rectangular shape. In the view of Figure 6(b), the interior space of the lighting device 1000 has a cross section that is shaped as a stadium.

[0095] In the view of Figure 6(b), the wall surface 1140 has an arc-shaped cross section. In the views of Figures 6(c) and 6(d), the wall surface 1140 also has an arc-shaped cross section, but different to that of Figure 6(b).

[0096] In Figure 6(b), the wall surface 1140 has a semicircular cross section In Figure 6(b), the wall surface 1140 has a cross section that is shaped a circular arc, while in Figure 6(d), it is shaped as a parabolic arc.

[0097] In each of Figures 6(b), 6(c), and 6(d), the transition between the back surface 1120 and the wall surface 1140 is smoothly curved.

[0098] In Figure 6(d), the transition between the front surface 1130 and the wall surface 1140 is also smoothly curved, while in each of Figures 6(c) and 6(d) the w all surface 1140 connects to the front surface 1130 under an angle. To provide the impression that the back surface 1120 extends indefinitely behind the light exit window 1131, to thereby enhance the impression of being exposed to natural daylight conditions, it is not important how the wall surface 1140 connects to the front surface 1130, provided that the lighting device 100 is designed such that when it is mounted to a ceiling, this transition is not directly visible through the light exit window 1131.

[0099] The lighting devices 1000 shown in the figures are all suitable for use as an artificial skylight.

[0100] A plurality of such artificial skylights may together be part of a lighting system, that can be installed in an indoor space, such as a residential space, an office space, a retail space, a hospitality space, or a healthcare space.

[0101] Figure 7 shows a lighting system 2000 comprising a plurality of lighting devices 1000, each of which being an artificial skylight.

[0102] It should be noted that the embodiments described herein illustrate rather than limit the invention, and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims.

[0103] The various aspects discussed herein 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.

[0104] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0105] 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.

[0106] The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

131. A lighting device (1000) having a housing (1100) with an interior space (1110) that is bounded by a back surface (1120), a front surface (1130) opposite the back surface (1120), and a wall surface (1140) separating the back surface (1120) and the front surface (1130), the back surface (1120) and the wall surface (1140) being diffusely reflective, the front surface (1130) having a light exit window (1131) with a largest window dimension, wherein the lighting device (1000) further has a first light source (1210) for emitting a first light output into the interior space (1110), the first light output having a first correlated color temperature in a range of 2,000 K to 10,000 K or the first light output being blue, wherein the first light source (1210) is provided at a location between the light exit window (1131) and the wall surface (1140), and wherein, in a direction from the light exit window (1131) to the back surface (1120): the back surface (1120) and the light exit window (1131) are located at a first separation distance from each other, and the light exit window (1131) has a projected area on the back surface (1120), wherein the first separation distance is smaller than the largest window dimension, and the projected area of the light exit window (1131) is smaller than the back surface (1120), and wherein each of the following applies: the wall surface (1140) is separated from the light exit window (1131) by a second separation distance, a ratio of the second separation distance and the first separation distance being at least four, and at least where the wall surface (1140) connects to the back surface (1120), the wall surface (1140) has an arc-shaped cross section in a plane perpendicular to the light exit window (1131).

2. The lighting device (1000) according to claim 1, wherein the first light source is screened from direct view through the light exit window.

3. The lighting device (1000) according to claim 2, wherein the first light source(1210) is provided on the front surface (1130) for emitting the first light output towards the back surface (1120).

4. The lighting device (1000) according to any one of claims 2 and 3, wherein the lighting device (1000) has a rim (1300) surrounding the light exit window (1131) and extending into the interior space (1110) to form an edge behind which the first light source (1210) is located.

5. The lighting device (1000) according to any one of the preceding claims, wherein the first separation distance is in a range from 3 to 30 centimeters.

6. The lighting device (1000) according to any one of the preceding claims, wherein the second separation distance is in range from 15 to 60 centimeters.

7. The lighting device (1000) according to any one of the preceding claims, wherein the second separation distance is at least 2.5 times the first separation distance.

8. The lighting device (1000) according to any one of the preceding claims, wherein the radius of curvature is equal to or larger than the first separation distance.

9. The lighting device (1000) according to any one of the preceding claims, wherein the first correlated color temperature is at least 6,000 K.

10. The lighting device (1000) according to claim 9, wherein the first correlated color temperature is more than 4,000 K, wherein the lighting device (1000) has a second light source for emitting a second light output into the interior space (1110), the second light output having a second correlated color temperature of at most 4,000 K, and wherein the lighting device (1000) has a controller for individually controlling the first light source (1210) and the second light source.

11. The lighting device (1000) according to any one of the preceding claims, wherein the light exit window (1131) has a circular shape or a polygonal shape.

12. The lighting device (1000) according to any one of the preceding claims, wherein the convex shape is a stadium.

13. The lighting device (1000) according to any one of the preceding claims, wherein each of the back surface (1120), the front surface (1130), and the wall surface (1140) has a reflectivity of at least 80 %.

14. The lighting device (1000) according to any one of the preceding claims, wherein the lighting device (1000) is an artificial skylight.

15. A lighting system (2000) comprising a plurality of lighting devices (1000) according to claim 14.

Images