Lighting device for use as an artificial skylight

Abstract

The invention relates to a lighting device (1000) having a light source (1200) with a light emitting surface (1210) that is arranged to emit light towards a transparent light exit window (1110). The light emitting surface (1210) and the light exit window (1110) are located at a separation distance from each other, and the light exit window (1110) has a projected area on the light emitting surface (1210) that is smaller than the light emitting surface (1210). The light emitting surface (1210) has an inner light emitting surface area (1220) enclosed by an outer light emitting surface area (1230). The inner light emitting surface area (1220) is arranged to emit a first light output 1221), and the outer light emitting surface area (1230) is arranged to emit a second light output (1231). The first light output (1221) has a first correlated color temperature in a range of 2,000 K to 8,000 K, and the second light output (1231) has a second correlated color temperature that is different from the first correlated color temperature, or the second light output (1231) is blue. 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 a light exit window, and a light source with a light emitting surface that is arranged to emit light towards the light exit window. The light emitting surface is diffusely reflective, and the light exit window is transparent. In a direction from the light exit window to the light emitting surface, (i) the light emitting surface and the light exit window are located at a separation distance from each other, and (ii) the light exit window has a projected area on the light emitting surface.

[0012] The aforementioned direction from the light exit window to the light emitting surface is a direction that is perpendicular to the light exit window, or, in other words, parallel to a surface normal of the light exit window.

[0013] In the lighting device according to the first aspect of the invention, the projected area of the light exit window is smaller than the light emitting surface.

[0014] Furthermore, the light emitting surface of the light source has an inner light emitting surface area enclosed by an outer light emitting surface area. The inner light emitting surface area is arranged to emit a first light output, and the outer light emitting surface area is arranged to emit a second light output. The second light output is different from the first light output.

[0015] The first light output has a first correlated color temperature in a range of 2,000 K to 8,000 K.

[0016] The second light output has a second correlated color temperature that is different from the first correlated color temperature, or the second light output is blue.

[0017] In operation, the lighting device emits a device light output that comprises the first light output and the second light output, wherein the second light output is different from the first light output.

[0018] The device light output may have a color rendering index (CRI) of at least 80, such as at least 85.

[0019] Because the light exit window is located at a certain distance from the light emitting surface, and because the projected area of the light exit window is smaller than the light emitting surface, the light exit window acts as a beam shaping element so that the output of the light emitting surface will only be provided in a limited angular range and not in all directions.

[0020] Because the light emitting surface has an inner light emitting surface area enclosed by an outer light emitting surface area, the first light output of the inner light emitting surface will be provided in a first angular range, and the second light output of the outer light emitting surface in a second angular range, wherein the first angular range is smaller than and completely encompassed by the second angular range.

[0021] The range of correlated color temperatures from 2,000 K to 8,000 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 known as cool white light), and to overcast sky (from 6,500 K to 8,000 K).

[0022] By being diffusely reflective, and by being arranged to emit a first light output of a first correlated color temperature in a range of 2,000 K to 8,000 K, the inner light emitting surface area can mimic natural sunlight conditions, ranging from sunset / sunrise to daylight and to overcast sky conditions. In other words, the inner light emitting surface area is arranged to emit artificial sunlight.

[0023] In operation, the lighting device emits artificial sunlight in a first angular range. The lighting device can therefore operate as an artificial skylight through which artificial sunlight can be observed under certain viewing angles.

[0024] Outside of these viewing angles, the second light output emitted by the outer light emitting surface can be observed. In other words, the second light output can be seen at angles where no artificial sunlight can be observed.

[0025] The second light output emitted by the outer light emitting surface area has a second correlated color temperature that is higher by at least 1,000 K (such as by at least 2,000 K) than the first correlated color temperature of the first light output emitted by the inner light emitting surface area.

[0026] Alternatively, the second light output can be 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.

[0027] The above ensures that there is sufficient contrast between the first light output and the second light output, thereby further enhancing the experience of being exposed to natural daylight conditions.

[0028] The second light output emitted by the outer light emitting surface area can have a second correlated color temperature in a range of 7,000 K to 15,000 K, such as in a range of 9,000 K to 12,000 K. This range of correlated color temperatures corresponds to blue sky conditions. In this case, artificial sunlight conditions are provided in a first angular range, outside of which blue sky conditions are provided, thereby further enhancing the experience of being exposed to natural daylight conditions.

[0029] The inner light emitting surface area and the outer light emitting surface area can be separated by a sharp boundary. This has the advantage that there is an abrupt change from the first type of light (z.e., the first light output emitted by the inner light emitting surface area) to the second type of light (z.e., the second light output emitted by the outer light emitting surface area, which is different from the first light output), which creates the impression of a sun and a blue sky being clearly visible, thereby mimicking clear sky conditions.

[0030] A sharp boundary may be a discontinuity, which can for example be made by means of a reflector that separates two light mixing chambers, one of which at least partly constitutes the inner light emitting surface area, while the other at least partly constitutes the outer light emitting surface area.

[0031] An example of a reflector that can form a discontinuity, and thereby a sharp boundary, is a frame. For example, at the transition of the inner light emitting surface area to the outer light emitting surface area, the inner light emitting surface area may have an inner light emitting module and the outer light emitting surface area may have an outer light emitting module, wherein the inner light emitting module and the outer light emitting module are not optically coupled. The frame may then be formed by the interface between the inner light emitting module and the outer light module.

[0032] The first correlated color temperature may gradually increase in a direction from a center of the inner light emitting surface area to the outer light emitting surface area. This has the advantage that an effect may be obtained that better simulates non-clear sky conditions, such as hazed sky conditions.

[0033] The housing of the lighting device may have a back surface and an opposite front surface, separated from each other by one or more side surfaces, wherein the light exit window is provided in the front surface. The light source would then be provided next to the back surface, with the light emitting surface facing the light exit window.

[0034] Ideally, the lighting device has a configuration that allows an observer to look into it through the light exit window from any viewing point and always see the light emitting surface. This would give the observer the impression that the light emitting surface extends indefinitely behind the light exit window.

[0035] From a given viewing point, direct lines of sight can be drawn through the light exit window. When all of these direct lines of sight intersect the light emitting surface, the latter appears to extend indefinitely behind the light exit window from that viewing point.

[0036] Irrespective of the location of the light exit window in the front surface of the housing, and irrespective of the shapes of both the light exit window and the front surface, there will always be a shortest distance from an edge of the light exit window to a side surface of the housing. The aforementioned shortest distance from an edge of the light exit window to a side surface of the housing determines the maximum distance within which the impression of an indefinitely extending light emitting surface may be provided.

[0037] When this shortest distance is at least four times as large as the separation distance between the light emitting surface and the light exit window, an observer of average height (1,70 meters) would practically always get an impression of a light emitting surface that extends indefinitely behind the light exit window when the lighting device is used as a ceiling luminaire in a typical indoor space with average ceiling height (2,70 meters).

[0038] For example, when the lighting device has a separation distance of 10 centimeters, and a shortest distance from an edge of the light exit window to a side surface of the housing of 20 centimeters, and when the light emitting surface extends up to the side surface of the housing, the latter appears to extend indefinitely behind the light exit window for any viewing point within about 2 meters from the lighting device.

[0039] When instead the separation distance would be decreased from 10 centimeters to 5 centimeter, the light emitting surface appears to extend indefinitely behind the light exit window for any viewing point within about 4 meters from the lighting device.

[0040] Lighting devices preferably have a shape and size that allows easy integration into a dropped ceiling. A dropped ceiling typically has a T-grid construction with cells of 60- by-60 centimeters. To allow easy installation in such a T-grid construction, a lighting device should fit in such a cell. When the lighting device has a rectangular shape, the length and width should both be less than 60 centimeters.

[0041] Irrespective of the actual shape of the lighting device, to allow easy integration into a dropped ceiling it preferably has an equivalent diameter (measured in a direction perpendicular to the separation distance) of not more than 76 centimeters, which would mean that the light emitting surface has an equivalent diameter of about 75 centimeters or less.

[0042] The term “diameter” refers to any straight line segment that passes through the center of a circle and whose endpoints lie on the circle. The diameter of a circle is equal to the circumference of the circle divided by pi.

[0043] To extend the concept of a diameter to all geometric shapes, one may use the term “equivalent diameter”. The equivalent diameter of any shape is equal to the circumference of the shape divided by pi. For example, a square with sides of 60 centimeters has an equivalent diameter of about 76 centimeters.

[0044] Furthermore, and again to allow easy integration into a dropped ceiling, lighting devices preferably have a height of not more than 20 centimeters. For the lighting device of the invention this would mean that it preferably has a separation distance of 20 centimeters or less.

[0045] To properly function as a ceiling luminaire, the light exit window should have a certain minimum size. For example, the equivalent diameter of the light exit window is at least 10 centimeters, such as 20 centimeters.

[0046] For a lighting device having a light emitting surface with an equivalent diameter of 75 centimeters, and a light exit window with an equivalent diameter of 20 centimeters, centrally located in a front surface of the housing, the shortest distance from an edge of the light exit window to a side surface of the housing will be about 20 centimeters. When the light emitting surface would extend over approximately the entire back surface of the housing, it appears to extend indefinitely behind the light exit window for any viewing point within about 4 meters from the lighting device when the separation distance is about 5 centimeters.

[0047] This distance can be extended by reducing the separation distance between the light emitting surface and the light exit window, and / or by reducing the equivalent diameter of the light exit window.

[0048] Besides giving an observer the impression that the light emitting surface extends indefinitely behind the light exit window, it may further be preferred that the first light output emitted by the inner light emitting surface area (z.e., the artificial sunlight) is provided in a relatively restricted angular range, outside of which only the second light output emitted by the outer light emitting surface area is provided. Particularly when the second light output is for mimicking blue sky conditions, this further enhances the impression of being exposed to natural daylight conditions.

[0049] The angular range wherein the first light output emitted by the inner light emitting surface area is provided depends on (i) the size of the inner light emitting surface area, (ii) the size of the light exit window, (iii) the separation distance between the inner light emitting surface area and the light exit window, and (iv) the location of the light exit window relative to the inner light emitting surface area.

[0050] Regarding the location of the light exit window, it would typically be provided centrally in the lighting device, and such that the projected area of the light exit window on the light emitting surface is concentric with the inner light emitting surface area.

[0051] When the projected area of the light exit window on the light emitting surface is concentric with the inner light emitting surface area, three situations can be distinguished: (i) the projected area of the light exit window is equal to the inner light emitting surface area, (ii) the projected area of the light exit window is smaller than the inner light emitting surface area, and (iii) the projected area of the light exit window is larger than the inner light emitting surface area.

[0052] In each of the above situations, the first light output emitted by the inner light emitting surface area has a cut-off angle 0 (relative to the normal of the light exit window) according to the following equation:

[0053] In the above equation, a denotes the equivalent diameter of the inner light emitting surface area, w denotes the equivalent diameter of the light exit window, and s denotes the separation distance between the light emitting surface and the light exit window.

[0054] From the above equation it is clear that, when the projected area of the light exit window on the light emitting surface is concentric with the inner light emitting surface area, the cut-off angle 0 depends on a ratio of the sum of (i) the equivalent diameter w of the light exit window and (ii) the equivalent diameter a of the inner light emitting surface, and the separation distance s.

[0055] It may be preferred to limit the cut-off angle 0 so that the first light output emitted by the inner light emitting surface area can only be observed up to certain viewing angles relative to the normal of the light exit window.

[0056] Table 1 gives the upper limits of the ratio (a + w / s for various values of the cut-off angle 0.

[0057] Table 1: Upper limits for the ratio (a + w) / s as a function of cut-off angle 0 (in degrees, relative to the normal of the light exit window).

[0058] Table 2 gives the upper limits for the cut-off angle 6 for various values of the ratio (a + w) / s.

[0059] Table 2: Upper limits for the cut-off angle 6 (in degrees, relative to the normal of the light exit window) as a function of the ratio (a + w / s.

[0060] For example, when the ratio (a + w / s is 4 or less, the cut-off angle 0 is 63 degrees or less; when the ratio is 3 or less, the cut-off angle 0 is 56 degrees or less; when the ratio is 2 or less, the cut-off angle 0 is 45 degrees or less; and when the ratio is 1 or less, the cut-off angle 0 is 27 degrees or less. When the lighting device is intended to be used as a ceiling luminaire that can be integrated into a dropped ceiling, it preferably has an equivalent diameter of 76 centimeters or less, which means that the light emitting surface area preferably has an equivalent diameter about 75 centimeters or less.

[0061] For a lighting device that is intended to be used as a ceiling luminaire, the separation distance is preferably in a range of 4 to 20 centimeters.

[0062] Furthermore, the light exit window preferably has a certain minimum size, such as an equivalent diameter of at least 10 centimeters. At the same time, and to allow the light emitting surface to extend sufficiently far away from the projected area of the light exit window, the latter preferably has an equivalent diameter of not more than 20 centimeters. In other words, the equivalent diameter of the light exit window is preferably in a range of 10 to 20 centimeters.

[0063] In the lighting device of the invention, the light emitting surface of the light source may be a light outcoupling surface of a planar light guide.

[0064] In the lighting device of the invention, the light emitting surface of the light source may have a reflection in a range from 30 % to 70 % for light emitted by the light source. Additionally or alternatively, it may have an absorption of 5 % or less for light emitted by the light source.

[0065] The lighting device of the invention may further comprise (i) a controller for individually controlling the first light output and the second light output, and (ii) a signaling device for providing a control signal, wherein the signaling device is one or more of a sensor, a clock module and a user interface. The controller may then be configured to individually control the first light output and the second light output based on the control signal that it receives from the signaling device. For example, the lighting device may comprise a first light source for emitting the first light output and a second light source for emitting the second light output. The controller may then be functionally coupled to the first and second light sources so as to control each independently of the other, to thereby adjust a lighting parameter of their respective light outputs, such as a luminous flux, a luminous intensity, a point in a chromaticity diagram (in other words, a color point), or a correlated color temperature.

[0066] The first light output emitted by the inner light emitting surface area has a first luminous flux, and the second light output emitted by the outer light emitting surface area has a second luminous flux. In an operational mode of the lighting device, a ratio of the first luminous flux and the second luminous flux may be at least 3. In other words, the lighting device may be arranged to be operated in a mode wherein the luminous flux of the first light output is at least three times that of the second light output. Hereby, the lighting device may be better suitable for use in office lighting and retail lighting.

[0067] The lighting device of the invention may be an artificial skylight.

[0068] A plurality of such artificial skylights may together be part of a lighting system. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] 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.

[0070] Fig. 1 shows a lighting device in a cross sectional view and a front view, respectively.

[0071] Fig. 2 shows three lighting devices in a cross sectional view.

[0072] Fig. 3 shows a lighting device in a cross sectional view, illustrating first and second light outputs.

[0073] Fig. 4 shows a lighting device used as ceiling luminaire, illustrating regions below the luminaire where different lighting conditions apply.

[0074] Fig. 5 shows a lighting device in a cross sectional view, illustrating various cut-off angles.

[0075] Fig. 6 shows a lighting device used as ceiling luminaire.

[0076] Fig. 7 shows a close-up of Figure 6.

[0077] Fig. 8 shows a ceiling lighting system installed in a space, wherein the lighting system comprises multiple lighting devices.

[0078] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0079] Under (a) and (b), Figure 1 shows a lighting device 1000 in a cross sectional view and a front view, respectively.

[0080] The lighting device 1000 has a housing 1100 with a back surface 1101 and an opposite front surface 1102, separated from each other by a side surface 1103.

[0081] The lighting device 1000 also has a transparent light exit window 1110 that is provided in the front surface 1102 of the housing 1100.

[0082] The lighting device 1000 further has a light source 1200 that is provided next to the back surface 1101 of the housing 1100, wherein the light source 1200 has a light emitting surface 1210 that faces the light exit window 1110, so that it is arranged to emit light towards the light exit window 1110.

[0083] The light source 1200 may comprise an array of light-emitting elements, such as light-emitting diodes (LEDs), which array is provided opposite from the light emit window 1110.

[0084] Alternatively, the light source 1200 may comprises a light guide, such as a light guide plate, located opposite from the light exit window 1110, wherein the light emitting surface 1200 is a light outcoupling surface of the light guide. In this case, the light source 1200 further comprises one or more light-emitting elements, such as LEDs, for emitting light into the light guide, for example via one of its edge surfaces.

[0085] The light emitting surface 1210 of the light source 1200 is diffusely reflective.

[0086] To ensure a sufficiently high efficiency, the light emitting surface 1210 may have a reflection in a range from 30 % to 70 %, and / or an absorption of 5 % or less, for light emitted by the light source 1200.

[0087] The light emitting surface 1210 may extend up to the side surface 1103 of the housing 1100, as is the case for lighting device 1000 shown in Figure 1, but there may also still be a space separating the light emitting surface from the housing.

[0088] In a direction from the light exit window 1110 to the light emitting surface 1210, the light emitting surface 1210 and the light exit window 1110 are located at a separation distance s from each other.

[0089] In the same direction, the light exit window 1110 has a projected area Apon the light emitting surface 1210. The projected area Apof the light exit window 1110 is smaller than the light emitting surface 1210.

[0090] The light emitting surface 1210 has an inner light emitting surface area 1220 and an outer light emitting surface area 1230, wherein the outer light emitting surface area 1230 encloses the inner light emitting surface area 1220.

[0091] The inner light emitting surface area 1220 is arranged to emit a first light output.

[0092] The outer light emitting surface area 1230 is arranged to emit a second light output, which is different from the first light output.

[0093] The first light output emitted by the inner light emitting surface area 1220 has a first correlated color temperature in a range of 2,000 K to 8,000 K.

[0094] The first light output may have substantially the same first correlated color temperature over the full inner light emitting surface area 1220.

[0095] Alternatively, the first correlated color temperature may change in a direction from a center of the inner light emitting surface area 1220 to the outer light emitting surface area 1230. For example, there may be a gradual increase of the first correlated color temperature in this direction, so that the first correlated color temperature is lower in the center of the inner light emitting surface area 1220 than at the edge of the light emitting surface area 1220. For example, the inner light emitting surface area 1220 may have a first subsection wherein the first correlated color temperature is in a range of 4,000 K to 6,000 K, and a second subsection enclosing the first subsection wherein the first correlated color temperature is in a range of 6,500 K to 8,000 K.

[0096] The second light output emitted by the outer light emitting surface area 1230 has a second correlated color temperature that is different from the first correlated color temperature. Alternatively, the second light output emitted by the outer light emitting surface area 1230 is blue.

[0097] An inner light emitting surface area that emits a first light output with a first correlated color temperature in a range of 2,000 K to 8,000 K, corresponding to artificial sunlight conditions, and an outer light emitting surface area that encloses the inner light emitting surface area and that emits a second light output that either has a different correlated color temperature or that is blue provides an enhanced experience of being exposed to natural daylight conditions.

[0098] When the second light output emitted by the outer light emitting surface area 1230 has a second correlated color temperature that is different from the first correlated color temperature, the difference between the first and second correlated color temperatures may be at least 1,000 K, such as at least 2,000 K, or at least 3,000 K. Hereby, the contrast between the first light output and the second light output is sufficiently high to even further enhance the experience of being exposed to natural daylight conditions.

[0099] Furthermore, while the first correlated color temperature is in a range of 2,000 K to 8,000 K, the second correlated color temperature may be in a range of 7,000 K to 15,000 K, such as in a range of 9,000 K to 12,000 K. Now, the artificial sunlight conditions provided by the inner light emitting surface area are supplemented with blue sky conditions provided by the enclosing outer light emitting surface area, thereby further enhancing the experience of being exposed to natural daylight conditions.

[0100] When the second light output emitted by the outer light emitting surface area 1230 is blue, at least 90 % of the spectral distribution of the second light output is within a wavelength range of about 380 nm to about 500 nm.

[0101] In the cross sectional view under (a) of Figure 1, the lighting device 1000 has a rectangular shape, but other shapes would also be possible.

[0102] In the front view under (b) of Figure 1, the lighting device 1000 has a square shape, and the light exit window 1110 has a circular shape. For each of the lighting device and the light exit window, other shapes would also be possible. The lighting device and the light exit window may have a different shape (such as is the case under (b) of Figure 1), but they may also have the same shape.

[0103] In the lighting device 1000 shown in Figure 1, the projected area Apof the light exit window 1110 is concentric with the inner light emitting surface area 1220.

[0104] Furthermore, the light exit window 1110 and the inner light emitting surface area 1220 have substantially the same equivalent diameter. This may also be referred to as the light exit window and the inner light emitting surface area being concentric.

[0105] When the light exit window and the inner light emitting surface are concentric, three situations can be distinguished: (i) the light exit window and the inner light emitting surface area have substantially the same equivalent diameter, (ii) the equivalent diameter of the light exit window is smaller than that of the inner light emitting surface area, and (iii) the equivalent diameter of the light exit window is larger than that of the inner light emitting surface area.

[0106] These three situations are illustrated in Figure 2, under (a), (b), and (c) respectively. In Figure 2, w denotes the equivalent diameter of the light exit window 1110, a denotes the equivalent diameter of the inner light emitting surface area 1220, and s denotes the separation distance between the light emitting surface 1210 and the light exit window 1110.

[0107] In each of the lighting devices 1000 shown in Figure 2, the inner light emitting surface area 1220 and the outer light emitting surface area 1230 are separated by a sharp boundary.

[0108] The presence of a sharp boundary separating the inner light emitting surface area 1220 and the outer light emitting surface area 1230 has the effect that on the border between the inner light emitting surface area 1220 and the outer light emitting surface area 1230, there is an abrupt transition from the first light output to the second light output.

[0109] Each of the lighting devices 1000 shown in Figure 2 comprises a frame 1240 for creating the sharp boundary.

[0110] The frame 1240 is formed by an interface between two light emitting modules, one being an inner light emitting module (i.e., a light emitting module comprised in the inner light emitting surface area 1220) and the other being an outer light emitting module (i.e., a light emitting module comprised in the outer light emitting surface area 1230). At the interface, the inner and outer light emitting modules are not optically coupled. Alternatively, a sharp boundary may also be created by any other suitable measure.

[0111] For the purpose of the invention, it is not required that there is an abrupt transition from the first light output to the second light output. Instead, the inner light emitting surface area 1220 and the outer light emitting surface area 1220 may also be designed to provide a gradual transition from the first light output to the second light output.

[0112] Figure 3 shows a lighting device 1000 wherein the inner light emitting surface area 1220 and the light exit window 1110 are concentric, and wherein the inner light emitting surface area 1220 has a larger equivalent diameter than the light exit window 1110.

[0113] Under (a), Figure 3 shows the first light output 1221 emitted by the inner light emitting surface area 1220.

[0114] Under (b), Figure 3 shows the second light output 1231 emitted by the outer light emitting surface area 1230.

[0115] From Figure 3 it is clear that there is a first region where only the first light output 1221 can be observed, a second region where only the second light output 1231 can be observed, and an intermediate region between the first and second regions where both the first light output 1221 and the second light output 1231 can be observed.

[0116] The first light output 1221 has a first correlated color temperature in a range of 2,000 K to 8,000 K. This range 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 known as cool white light), and to overcast sky (from 6,500 K to 8,000 K). Consequently, the first light output 1221 corresponds to sunlight conditions.

[0117] The second light output 1231 has a second correlated color temperature in a range of 9,000 K to 12,000 K.

[0118] The correlated color temperature range of 7,000 K to 15,000 K corresponds to blue sky conditions, so the second light output 1231 resembles that of a blue sky.

[0119] For the purpose of the invention, it is not required that the second light output has a second correlated color temperature in a range of 7,000 K to 15,000 K, such as in a range of 9,000 K to 12,000 K, as long as it has a second correlated color temperature that is higher than the first correlated color temperature by at least 1,000 K.

[0120] Alternatively, the second light output may be blue, meaning that at least 90 % of the spectral distribution of the second light output is within a wavelength range of about 380 nm to about 500 nm. When an observer would move from first a location in the first region, where only the first light output 1221 can be observed, to a second location in the second region, where only the second light output 1231 can be observed, he or she moves from a location where artificial sunlight conditions are provided to a location where artificial blue sky conditions are provided, thereby getting the impression of being exposed to natural daylight conditions provided through a skylight.

[0121] Figure 4 shows a situation wherein the lighting device 1000 is used as a ceiling luminaire. In the lighting device 1000, the light exit window and the inner light emitting surface area are concentric, the latter having a larger equivalent diameter than the former and being enclosed by an outer light emitting surface area.

[0122] In the first region I, only the first light output emitted by the inner light emitting surface area can be observed, and the second region II, only the second light output emitted by the outer light emitting surface area can be observed. In the intermediate region III, between the first region I and the second region II, the first and second light output can both be observed.

[0123] Figure 5 shows the lighting device 1000, wherein the light exit window 1110 and the inner light emitting surface area 1220 are concentric, and wherein the latter has a larger equivalent diameter than the former.

[0124] As illustrated in Figure 5, the first light output emitted by the inner light emitting surface area 1220 has a cut-off angle 0 relative to the normal of the light exit window 1110, for which the following holds, wherein w and a are the equivalent diameters of the light exit window and the inner light emitting surface area, respectively, and s is the separation distance between them: a + w tan 0 = — — — 2s

[0125] The above applies both when the light exit window and the inner light emitting surface area have a different equivalent diameter (a w) and when they have the same equivalent diameter (a = w).

[0126] Figure 5 further illustrates that the first region, wherein only the first light output emitted by the inner light emitting surface area 1220 can be observed, has a cut-off angle (p with respect to the normal of the light exit window 1110, for which the following holds: \a — w| tan (p = — - -

[0127] When the inner light emitting surface area and the light exit window have substantially the same equivalent diameter (a = w), the cut-off angle (p will be substantially zero. This essentially means that there is no first region wherein only the first light output emitted by the inner light emitting surface area 1220 can be observed.

[0128] In other words, when it is desired that there is a first region wherein only the first light output emitted by the inner light emitting surface area 1220 can be observed, the inner light emitting surface area and the light exit window must have different equivalent diameters.

[0129] The extent to which the inner light emitting surface area and the light exit window must have different equivalent diameters to achieve a certain minimum cut-off angle (p depends on the separation distance between them.

[0130] It may be desired to achieve a minimum cut-off angle (p of 20 degrees. When the lighting device is used as a ceiling luminaire on a ceiling of average ceiling height (2,70 meters), this would correspond to a first region with a radius of at least about 1 meter on the floor right below the device. A first region with a radius of at least about 2 meters can be obtained for a minimum cut-off angle (p of 36 degrees.

[0131] To achieve a minimum cut-off angle (p of 20 degrees for a separation distance of 10 centimeters, the difference in equivalent diameter of the inner light emitting surface area and the light exit window must at least be about 7 centimeters. The same can be achieved for a separation distance of 5 centimeters when the difference in equivalent diameter of the inner light emitting surface area and the light exit window is at least about 3,5 centimeters.

[0132] To achieve a minimum cut-off angle (p of 36 degrees for a separation distance of 10 centimeters, the difference in equivalent diameter of the inner light emitting surface area and the light exit window must at least be about 14 centimeters. The same can be achieved for a separation distance of 5 centimeters when the difference in equivalent diameter of the inner light emitting surface area and the light exit window is at least about 7 centimeters.

[0133] Figure 6 shows the lighting device 1000 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.

[0134] In Figure 6, 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.

[0135] Ideally, the lighting device 1000 has a configuration that allows an observer to look into it through the light exit window 1110 from any viewing point on plane P and always see the light emitting surface 1210. This would give the observer the impression that the light emitting surface 1210 extends indefinitely behind the light exit window 1110.

[0136] In practice, an observer will only get the impression of an indefinitely extending light emitting surface when the observer is within a certain distance from the lighting device. Beyond that distance, there will be a position where the observer has a direct line of sight to the side of the luminaire where the light emitting surface terminates.

[0137] Figures 7(a) and 7(b) show close-ups of the situation of Figure 6.

[0138] Figure 7(a) shows viewing point 710 that is located on plane P. Figure 7(a) also shows the two extreme direct lines of sight 711 and 712, respectively, that can be drawn from the viewing point 710 through the light exit window 1110.

[0139] When these two extreme direct lines of sight 711 and 712 are incident on the back surface 1101 of the housing 1100, all direct lines of sight coming from the viewing point 710 will be incident on the back surface 1101. In case the light emitting surface extends up to the side surface 1103 of the housing 1100, all direct lines of sight coming from the viewing point 710 will be incident on the light emitting surface, and an impression of an indefinitely extending light emitting surface is provided.

[0140] The front surface 1102 of the housing 1100 essentially acts as a kind of mask for the direct lines of sight coming from a viewing point.

[0141] From any viewing point, the direct line of sight through the light exit window with the largest angle relative to the normal of the light exit window, determines how far an observer can look into the luminaire. For viewing point 710 shown in Figure 7(a), this would be the direct line of sight 711.

[0142] The direct line of sight 711 passes through the light exit window 1110 immediately adjacent to an edge of the light exit window 1110 that is located a distance y from the side surface 1103 of the housing 1100. In general, the larger the distance y, the higher the chance that the direct line of sight 711 will be incident on the back surface 1101 of the housing 1100.

[0143] In Figure 7(a), the light exit window 1110 is located centrally in the front surface 1102 of the housing 1100. But irrespective of the location of the light exit window 1110 in the front surface 1102 of the housing 1100, and irrespective of the shapes of both the light exit window 1110 and the front surface 1102, there will always be a shortest distance from the edge of the light exit window 1110 to the side surface 1103 of the housing 1101. In Figure 7(a), this shortest distance would be the distance y.

[0144] The aforementioned shortest distance from the edge of the light exit window 1110 to the side surface 1103 of the housing 1100 determines the maximum distance of a viewing point from which any direct light of sight through the light exit window would still be incident on the back surface 1101 of the housing 1100. In case the light emitting surface extends up to the side surface 1103 of the housing 1100, it also determines the maximum distance of a viewing point from which the impression of an indefinitely extending light emitting surface is provided.

[0145] The shortest distance from the edge of the light exit window 1110 to the side surface 1103, together with the separation distance between the light emitting surface and the light exit window 1110, constitute the legs of a right triangle.

[0146] The aforementioned right triangle with legs s and y is similar to a right triangle with legs h and p, h denoting a distance of a view plane P from the front surface 1102 of the housing 1100, and p denoting a distance of viewing point 720 on the view plane P from the edge of the light exit window 1110.

[0147] In Figure 7(b), the aforementioned similar right triangles are shown as hatched areas.

[0148] 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.

[0149] When the shortest distance from the edge of the light exit window 1110 to the side surface 1103 would be at least four times as large as the separation distance between the light emitting surface and the light exit window 1110 (z.e., y / s > 4), the aforementioned observer would always have direct lines of sight to the back surface 1101 when within a distance of four meters from the edge of the light exit window 1110. 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 light emitting surface that extends indefinitely behind the light exit window.

[0150] To ensure that the desired impression of an indefinitely extending light emitting surface is provided under an even broader range of circumstances, the shortest distance from the edge of the light exit window to the side surface may be at least five times as large as the separation distance between the light emitting surface and the light exit window.

[0151] For a typical ceiling luminaire, the light exit window 1110 may be centrally located in the front surface 1102 of the housing 1100, and it may preferably have an equivalent diameter 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 shortest distance from the edge of the light exit window 1110 to the side surface 1103 of the housing 1100 may be at least 20 centimeters.

[0152] To ensure that an observer of average height would practically always have direct lines of sight to the back surface 1101 when within a distance of four meters from the edge of the light exit window 1110, the separation distance between the light emitting surface and the light exit window 1110 should be 5 centimeters or less.

[0153] As said, the lighting device 1000 shown in Figure 1 has an inner light emitting surface area 1220 that is arranged to emit a first light output with a first correlated color temperature in a range of 2,000 K to 8,000 K, and an outer light emitting surface area 1230 that is arranged to emit a second light output with a second correlated color temperature that is different from the first correlated color temperature. In an alternative configuration, the second light output emitted by the outer light emitting surface area 1230 is blue.

[0154] Both configurations enable the lighting device 1000 to be suitable for use as an artificial skylight.

[0155] 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.

[0156] Figure 8 shows a lighting system 2000 comprising a plurality of lighting devices 1000, each of which being an artificial skylight.

[0157] In a lighting system that comprises a plurality of artificial skylights, each artificial skylight may have its own local controller, or the lighting system may have a central controller for controlling the artificial skylights, either as a group, or independently from each other.

[0158] 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.

[0159] 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.

[0160] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0161] 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.

[0162] The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A lighting device (1000) having a housing (1100) with a light exit window (1110), and a light source (1200) with a light emitting surface (1210) that is arranged to emit light towards the light exit window (1110), wherein the light emitting surface (1210) is diffusely reflective, and the light exit window (1110) is transparent, wherein, in a direction from the light exit window (1110) to the light emitting surface (1210): the light emitting surface (1210) and the light exit window (1110) are located at a separation distance from each other, and the light exit window (1110) has a projected area on the light emitting surface (1210), wherein the projected area of the light exit window (1110) is smaller than the light emitting surface (1210), wherein the light emitting surface (1210) has an inner light emitting surface area (1220) enclosed by an outer light emitting surface area (1230), wherein the inner light emitting surface area (1220) is arranged to emit a first light output (1221), and the outer light emitting surface area (1230) is arranged to emit a second light output (1231) different from the first light output (1221), wherein the first light output (1221) has a first correlated color temperature in a range of 2,000 K to 8,000 K, and wherein: the second light output (1231) has a second correlated color temperature that is higher than the first correlated color temperature by at least 1,000 K, or the second light output (1231) is blue.

2. The lighting device (1000) according to claim 1, wherein the second correlated color temperature is in a range of 7,000 K to 15,000 K.

3. The lighting device (1000) according to any one of the preceding claims, wherein the inner light emitting surface area (1220) and the outer light emitting surface area (1230) are separated by a sharp boundary.

4. The lighting device (1000) according to claim 3, wherein the lighting device (1000) comprises a frame (1240) for creating the sharp boundary.

5. The lighting device (1000) according to any one of the preceding claims, wherein the first correlated color temperature gradually increases in a direction from a center of the inner light emitting surface area (1220) to the outer light emitting surface area (1230).

6. The lighting device (1000) according to any one of the preceding claims, wherein the housing (1100) has a back surface (1101) and an opposite front surface (1102), separated from each other by a side surface (1103), wherein the light exit window (1110) is provided in the front surface (1102), and wherein a shortest distance from an edge of the light exit window (1110) to the side surface (1103) is at least four times as large as the separation distance between the light emitting surface (1210) and the light exit window (1110).

7. The lighting device (1000) according to any one of the preceding claims, wherein the light emitting surface (1210) of the light source (1200) is a light outcoupling surface of a planar light guide.

8. The lighting device (1000) according to any one of the preceding claims, wherein, for light emitted by the light source (1200), the light emitting surface (1210) has a reflection in a range from 30 % to 70 %, and an absorption of 5 % or less.

9. The lighting device (1000) according to any one of the preceding claims, wherein, in a direction perpendicular to the separation distance, the lighting device (1000) has an equivalent diameter of 76 centimeters or less.

10. The lighting device (1000) according to claim 9, wherein the separation distance between the light emitting surface (1210) and the light exit window (1110) is in a range from 4 to 20 centimeters, and wherein each of the light exit window (1110) and the inner light emitting surface area (1220) has an equivalent diameter in a range from 10 centimeters to 40 centimeters.

11. The lighting device (1000) according to any one of the preceding claims, wherein the projected area of the light exit window (1110) on the light emitting surface (1210) is concentric with the inner light emitting surface area (1220), and wherein a ratio(a + w) / s is 3 or less, w denoting an equivalent diameter of the light exit window (1110), adenoting an equivalent diameter of the inner light emitting surface (1220), and s denoting the separation distance.

12. The lighting device (1000) according to any one of the preceding claims, wherein the lighting device (1000) further comprises (i) a controller for individually controlling the first light output (1221) and the second light output (1231), and (ii) a signaling device for providing a control signal, wherein the signaling device is one or more of a sensor, a clock module and a user interface, and wherein the controller is configured to individually control the first light output (1221) and the second light output (1231) based on the control signal.

13. The lighting device (1000) according to any one of the preceding claims, wherein the first light output has a first luminous flux, and the second light output has a second luminous flux, and wherein, in an operational mode of the lighting device (1000), a ratio of the first luminous flux and the second luminous flux is at least 3.

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.

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