Abstract:
The invention relates to a method for controlling a lighting system, wherein the lighting system comprises a plurality of light-emitting ceiling tiles and a control unit for controlling the plurality of light-emitting tiles. The method comprises the step of controlling the plurality of light-emitting tiles to provide a predetermined luminance contrast in the ceiling. The method further comprises the step of, for the predetermined luminance contrast, controlling the plurality of light-emitting tiles to provide a substantially uniform light distribution incident on a target surface such as a horizontal work plane. The method enables the lighting conditions in a room to be adjusted for improving visual comfort and illumination without affecting the illumination at a task level.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2014/061643, filed on Jun. 5, 2014, which claims the benefit of European Patent Application No. 13171167.3, filed on Jun. 10, 2013. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to lighting controls. In particular, the invention relates to an improved lighting system and to a method for controlling a lighting system comprising light-emitting tiles. 
     BACKGROUND OF THE INVENTION 
     The development of new and more energy-efficient illumination devices is one of the important technical challenges which society faces. Common technologies which are more energy-efficient than traditional lighting solutions are often based on light-emitting diodes (LEDs). Currently, the reduced cost and improved performance of LEDs enable their use for general illumination. The small size of LEDs allows for easy integration of LEDs into building materials and furniture. 
     Combining these trends, the general illumination of an indoor space may be provided by a luminous ceiling, rather than by discrete fixtures in a dark ceiling. Such diffuse, large-area and low-brightness ceiling sources provide comfortable lighting with low glare and almost no shadows, while creating an aesthetic, clean ceiling without disturbing fixtures. The recent development of acoustic absorbing ceiling tiles with embedded lighting enables a full coverage of the ceiling with light without deteriorating the acoustics in the room. 
     Daylight through windows results in lighting that differs from indoor lighting in intensity, in color temperature, and directionality (shadows and contrast). Conventional indoor lighting systems may be equipped with photo-sensors, such that the increase in illumination level caused by daylight may be compensated by dimming down the artificial lighting. Unwanted gradients in illumination level are compensated by changing the dimming level as a function of the distance to the window, or by using a control loop to adjust the local illumination level to a certain predefined value. The controls may also be linked to blinds to regulate the amount of incoming daylight. An example of a prior art indoor lighting system with daylight controls can be found in U.S. Pat. No. 7,190,126, which also further describes an office lighting system with controls that regulate the light flux as function of the amount of daylight incident in a room. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to enable improved performance and visual comfort for lighting systems comprising light-emitting tiles and methods for controlling such lighting systems. 
     According to a first aspect of the invention, these and other objectives are achieved through the use of a method for controlling a lighting system, wherein the lighting system comprises a plurality of light-emitting tiles arranged in a ceiling and a control unit for controlling the plurality of light-emitting tiles, and wherein the method comprising the steps of (i) controlling the plurality of light-emitting tiles to provide a predetermined luminance contrast in the ceiling, and (ii) for the predetermined luminance contrast, controlling the plurality of light-emitting tiles to provide a substantially uniform light distribution incident on a target surface, for example a horizontal work plane. 
     By lighting system it should be understood that the purpose of the method and lighting system is to provide lighting, and that the light-emitting tiles, which are commonly driven by LEDs are the main components providing the light. 
     Luminance should be understood as the flux density emitted in a certain direction. Hence, luminance contrast is how a difference in luminance between an area of higher luminance and an area of lower luminance is perceived from a distant point by an observer. Controlling the plurality of light-emitting tiles to provide a luminance contrast thus comprises controlling the plurality of light-emitting tiles to provide areas, in the ceiling, of higher relative luminance and areas of lower relative luminance. For diffuse sources and diffuse reflectors (Lambertian sources or Lambertian reflectors), the luminance is independent of the viewing direction. Hence, the luminance contrast is directly linked to the flux, which will be the same from all viewing directions. Advantageously, the light-emitting tiles will be emitting and reflecting diffuse light, and therefore the luminance contrast may be set without taking the relative position of an observer into account. 
     A substantially uniform light distribution incident on a target surface should be interpreted as a substantially uniform illuminance. The target surface may be a horizontal work plane that is oriented substantially parallel to the ceiling in which the light-emitting tiles are arranged and located at a certain distance from the ceiling. An example of a horizontal work plane could be a desk in an office, or any similar work area in an indoor environment. The target surface should be at a distance from the ceiling at least equal to 1.5 to 2 times the spacing of the ceiling tiles for a uniform illumination to be provided. The need and desire for a substantially uniform illuminance is due to regulations which prescribe a minimum average illuminance and illuminance uniformity on target surfaces such as work planes, typically 500 lux for an office environment. The regulations aim to provide a minimum level of visual comfort. The uniformity of the illuminance may depend on the distance. For example, a relatively high uniformity on a desk, whereas the uniformity is allowed to be lower in the corner of a room. It is important that the lighting system ensures illumination uniformity on a target surface such as a horizontal work plane. 
     The luminance distribution in a room should not contain too harsh contrasts. A luminous ceiling may provide a low luminance contrast in the ceiling, which would be expected to provide a visual comfort. However, many people experience a substantially homogenous luminous ceiling as visually discomforting due to the lack of a luminance contrast in the ceiling. 
     The present invention is based on the realization that by using distributed lighting, such as light-emitting ceiling tiles, the lighting conditions in a room may be adjusted to improve the visual comfort and illumination without affecting the illumination at a task level. The ceiling, where the light-emitting tiles are arranged, covers a large part of the visual field in an indoor area and the light-emitting tiles are controlled to provide a luminance contrast in the ceiling. The light-emitting tiles are also controlled to provide a substantially uniform light incident on a target surface, hence the luminance contrast in the ceiling will provide an improved visual comfort or aesthetic appreciation of the room without changing the illumination at a work plane level. Accordingly, an improved visual comfort can be achieved even if only a portion of the ceiling tiles in an indoor area are controllable, even though it may be desirable to provide a ceiling where all ceiling tiles are light-emitting tiles. 
     According to one embodiment of the invention, the luminance contrast may be achieved by reducing the luminance of a first subset of the plurality of light-emitting tiles compared to a second subset. Reducing the luminance of a first subset of the plurality of light-emitting tiles to provide a luminance contrast compared to a second subset will hence provide a visual comfort to an observer in the room. To reduce the luminance from a subset of light-emitting tiles, the amount of light produced by one or more light-emitting tiles comprised in the subset can be reduced. Alternatively one or more light-emitting tiles in the subset could be switched off. However, a light-emitting tile which is switched off will still provide some luminance from light reflected by that light-emitting tile. Hence the reflectivity of light-emitting tiles or even ordinary tiles comprised in the ceiling may also be tuned, in both reflectance and the amount of diffuse reflection. 
     According to another embodiment of the invention, the luminance contrast may be achieved by reducing the luminance of a first subset of the plurality of light-emitting tiles and by increasing the luminance of a second subset of the plurality of light-emitting tiles. By increasing the luminance of a second subset of light-emitting tiles and reducing the luminance of a first subset of light-emitting tiles, a luminance contrast is provided as the contrast in luminance between the two subsets. In particular this effect can advantageously be used to provide a large luminance contrast between two subsets of light-emitting tiles. 
     According to one embodiment of the invention, the plurality of light-emitting tiles may be controlled based on daylight conditions. Daylight conditions will influence the amount of light in an indoor area comprising daylight entrances such as windows, and hence influence the illuminance in an indoor area where a lighting system is present. The influence of daylight conditions on the illuminance will be used to reduce energy consumption of the lighting system, through reducing the flux from the plurality of light-emitting tiles. Furthermore, if daylight is incident on the target surface, a subset of light-emitting tiles illuminating an area where daylight is incident may be turned off. If the daylight conditions introduce visual discomfort, the plurality of light-emitting tiles can be controlled such that the luminance contrast from the light-emitting tiles reduces this visual discomfort. Accordingly the plurality of light-emitting tiles may be controlled based on a time of day or on a weather forecast. The information of the time of day or the weather forecast will further be used to either counter the effect of daylight conditions, or to further reduce the energy consumption of the lighting system. The time of day or the weather forecast could be incorporated indirectly through predetermined values, or read from a database or a similar store of information. Another possibility is to directly sense the time of day or the present weather conditions through sensor means. 
     According to another embodiment of the invention, the plurality of light-emitting tiles may be controlled based on daylight incident on a surface illuminated by the plurality of light-emitting tiles. To further enhance the visual comfort the plurality of light-emitting tiles will be controlled based on daylight incident on a surface illuminated by the plurality of light-emitting tiles. The luminance contrast provided by the light-emitting tiles will be controlled to provide a visual comfort, and a substantially uniform light distribution on the surface, in other words the surface where the daylight is incident will in high likelihood perceive a lower illuminance from the light-emitting ceiling tiles compared to other surfaces in the same plane which do not have daylight incident upon them. 
     In one embodiment of the invention, if daylight is incident on a surface illuminated by a first subset of the plurality of light-emitting tiles, the plurality of light-emitting tiles may further be controlled such that a luminance contrast provided by the first subset of light-emitting tiles is lower than a luminance contrast provided by a second subset of light-emitting tiles, where the second subset of light-emitting tiles illuminates a surface where substantially less daylight is incident. Incident daylight will introduce luminance contrasts in an indoor area. To be able to provide a visual comfort, the light-emitting tiles should provide a luminance contrast in a portion of the ceiling which is illuminating a surface where no daylight is incident, to balance the luminance contrast introduced by daylight. Thereby providing a comfortable luminance contrast in the entire ceiling through the luminance contrast introduced by daylight in combination with the luminance contrast provided by the light-emitting tiles acting to balance the daylight 
     According to one embodiment of the invention, the luminance of a first subset of light-emitting tiles may be controlled to be at least two times greater than a luminance of a second subset of light-emitting tiles adjacent to the first subset. A high luminance from a singular light-emitting tile may be perceived as disturbing and cause visual discomfort if adjacent ceiling tiles have a low luminance, just as a singular strong light bulb may be perceived. By providing a second subset of light-emitting tiles, adjacent to the first subset, which has a luminance at least two times lower, the visual discomfort will be avoided due to the lower luminance contrast between the two subsets, and further the luminance contrast between the second subset and adjacent light-emitting tiles which may be switched off, or adjacent conventional ceiling tiles. Accordingly, the luminance of a first subset of light-emitting tiles may be controlled to be between five and twenty times greater than a luminance of a second subset of light-emitting tiles adjacent to the first subset. 
     According to one embodiment the method may further comprise controlling the light-emitting tiles to change from a first predetermined luminance contrast pattern to a second predetermined luminance contrast pattern in the ceiling, and controlling the substantially uniform light distribution on the target surface to be the same for the first luminance pattern as for the second luminance pattern. By providing the possibility to form predetermined luminance patterns, an observer in the room can tailor the luminance contrast to his or her own liking, using different interesting luminance patterns while still maintaining the illuminance at the target surface (for example a work plane) substantially uniform. Furthermore a dynamic luminance contrast pattern can be introduced to create a more interesting appearance by varying the luminance contrast pattern. Moreover, the luminance contrast pattern may be adjusted to be more visually suitable for different activities, or to match the architecture or daylight condition in an indoor area. Accordingly, the method may further comprise controlling the light-emitting tiles to dynamically provide a plurality of predetermined luminance contrast patterns in the ceiling, and controlling the substantially uniform light distribution on the target surface to be the same for the plurality of predetermined luminance patterns. A plurality of predetermined luminance patterns could be cycled during a longer or shorter time period to further enhance the visual comfort in the room. For example, certain predetermined patterns could be used during the earlier hours in a workday and certain other patterns during the later hours, thereby providing the possibility to make people in the room more alert and sprightly through the cycling of luminance patterns in the ceiling. Furthermore, the contrast patters may be changed continuously to provide a ceiling which is dynamic while not influencing the target surface illumination. 
     According to a second aspect of the present invention, there is provided a lighting system comprising (i) a plurality of light-emitting tiles for being arranged in a ceiling, and (ii) a control unit arranged to control the plurality of light-emitting tiles, wherein the control unit is configured to control the plurality of light-emitting ceiling tiles to provide a predetermined luminance contrast in the ceiling, and for the predetermined luminance contrast, control the plurality of light-emitting of light-emitting tiles to provide a substantially uniform light distribution incident on a target surface. 
     According to one embodiment of the invention the lighting system further comprises a sensing unit arranged to sense lighting conditions within an indoor area, and configured to send a signal indicative of the lighting conditions to the control unit and the control unit is configured to control the plurality of light-emitting ceiling tiles to provide a predetermined luminance contrast and, for the predetermined luminance contrast, control the plurality of light-emitting tiles to provide a substantially uniform light distribution incident on a target surface based on the signal indicative of the lighting conditions. The sensing unit will be arranged such that it will sense lighting conditions within an indoor area where the lighting system is present. Advantageously, the control unit then controls the light-emitting tiles based on the signal from the sensing unit such that the light-emitting tiles provides a substantially uniform light distribution incident on the target surface. Furthermore, by lighting conditions, a number of parameters should be understood to be included such as flux, intensity, illuminance or luminance or lighting contrast, or hue, or color temperature, or influence of natural daylight through a window or a transparent ceiling of any sort. 
     According to another embodiment each of the plurality of light-emitting tiles may be individually controllable. By providing individually controllable light-emitting tiles more precise and exact control can be implemented. 
     In one embodiment of the invention, each of the light-emitting tiles may advantageously be adjacent to at least one other light-emitting tile. Thereby, for each light-emitting tile, a tunable luminance contrast can be provided between at least two tiles. A ceiling comprising only light-emitting ceiling tiles would naturally provide the highest degree of control of the luminance which can be provided. However, for practical reasons it may not always be possible to have a ceiling entirely made up of light-emitting ceiling tiles. Even so, various embodiment of the present invention may be used to an advantage. 
     Effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. For example the light-emitting tiles may be driven by different light generators such as e.g. lasers, flash lamps, Xenon lamps or even X-ray sources. The skilled person will realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. 
    
    
     
       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 an embodiment of the invention. 
         FIG. 1  is a perspective view of an indoor area where light-emitting tiles are arranged in the ceiling and controlled according to an embodiment of the invention; 
         FIG. 2  is a schematic side view of light-emitting tiles arranged in the ceiling and controlled according to an embodiment of the invention; 
         FIG. 3  is a schematic side view of light-emitting tiles arranged in the ceiling and controlled according to another embodiment of the invention; 
         FIG. 4  is a schematic side view of light-emitting tiles arranged in the ceiling and controlled according to an embodiment of the invention; and 
         FIG. 5  is a schematic side view of light-emitting tiles arranged in the ceiling and controlled according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the present detailed description, various embodiments of a lighting system according to the present invention are mainly discussed with reference to a lighting system comprising light-emitting tiles having light-emitting diodes (LEDs) as a light source. It should be noted that this by no means limits the scope of the invention, which is also applicable in other circumstances, for example for use with other types of light sources. Moreover the amount of light-emitting ceiling tiles shown in the enclosed drawings is only a schematic representation. In use, the arrangement, number, or percentage of light-emitting tiles in relation to the total number of ceiling tiles and other such details will be decided by each application. 
       FIG. 1  is a perspective view of an indoor area where a lighting system according to the present invention is installed in an indoor area. The lighting system comprises illuminating tiles  100  and non-illuminating tiles  108 , both arranged in a ceiling. The illuminating tiles  100  and non-illuminating tiles  108  may both be light-emitting tiles, wherein the non-illuminating tiles  108  have their luminance reduced or wherein they have been switched off. The combination of illuminating tiles  100  and non-illuminating tiles  108  provides a luminance contrast in the ceiling, which covers a large part of the visual field. The indoor area further has windows  102  providing an entrance for daylight, which may affect the lighting conditions and luminance contrast in the indoor area. 
     Now referring to  FIG. 2 , a schematic side view of an indoor area comprising an exemplary embodiment of the invention is shown. The indoor area comprises a window  102 , illuminating light-emitting tiles  100  and non-illuminating tiles  108 . The non-illuminating tiles  108  are light-emitting tiles which are turned off. If all tiles were illuminating the ceiling would only provide a uniform light-emitting surface, which as described earlier may be experienced by some as boring and visually uncomfortable. Hence to achieve the same lighting as if each tile was providing a luminance of 300 cd/m 2 , the illuminating light-emitting tiles in  FIG. 2  will provide a luminance of 600 cd/m 2  while the non-illuminating tiles  108  provide a contrast. However, any setting between zero and maximum illumination from the non-illuminating tiles  108  which is not equal to the illumination from the illuminating tiles  100  will provide a contrast in the ceiling and thereby a visual comfort. Furthermore, the light-emitting tiles may be controlled individually, which gives the largest flexibility in the lighting conditions which may be created, or they may be grouped into subsets comprising a row of tiles or part of a row, where each subset is controllable. The illuminating light-emitting tiles  100  and non-illuminating tiles  108  are controlled by the control unit (not shown) to provide a luminance contrast in the ceiling, thereby providing an improved visual comfort in the room which may improve the appreciation or contrast which matches the architecture, activity at hand or the daylight conditions. Furthermore the illuminating light-emitting tiles  100  and non-illuminating tiles  108  provide a substantially uniform light distribution incident on a target surface, in this case a horizontal work plane. The horizontal work plane should be understood to be parallel to the ceiling, and located at a certain distance from the ceiling. The horizontal work plane is illustrated in  FIG. 2  as the plane comprising the desks  200 . The luminance contrast in the ceiling can be adapted to provide visual comfort in the indoor area while the illuminance at a task level or work plane such as the horizontal work plane  200  is substantially uniform and fulfills regulations governing the requirements of work plane illumination. By adapting the luminance contrast in the ceiling, it should be understood that the control unit (not shown) is able to increase or decrease the luminance from the illuminating light-emitting tiles  100  as well as the luminance from the non-illuminating light-emitting tiles  108 . 
     In  FIG. 3 , a schematic side view of an indoor area comprising an exemplary embodiment of the invention is shown. The indoor area comprises a window  102 , illuminating light-emitting tiles  100 , non-illuminating tiles  108  and a work plane  200  to be illuminated. Compared to the embodiment shown in  FIG. 2 , a subset of the light-emitting tiles has their luminance reduced and can now be seen as non-illuminating tiles  108 . By reducing the illumination of a subset of light-emitting tiles, the luminance contrast in the ceiling may be adapted to provide an improved visual comfort, or to provide a light distribution incident on the horizontal work plane  200  such that the light distribution is uniform even under the presence of daylight through the window  102 . Furthermore, to balance the effect of daylight incident through a window, which may create sharp contrasts, a second subset of light-emitting tiles  300  can be controlled to provide a luminance contrast which is higher, through concentrating the light flux to one or several tiles, in the example in  FIG. 3  one tile with a luminance as high as 2000 cd/m 2 . The high light flux of the second subset of tiles  300  should then be compared to a luminance contrast provided by a first subset of light-emitting tiles which is lower, for example 300 cd/m 2 , where the first subset are directly illuminating a surface where daylight is incident. 
     Now referring to  FIG. 4 , a schematic side view of an indoor area comprising an exemplary embodiment of the invention is shown. The indoor area comprises a window  102 , illuminating light-emitting tiles  100 , non-illuminating tiles  108  and a work plane  200 . In the embodiment shown in  FIG. 4  there is a subset of light-emitting tiles  400  which are arranged farther away from the window  102 , and which are controlled to counter the effect of daylight incident through the window  102 . In comparison to the embodiment shown in  FIG. 3  the subset of light-emitting tiles  400  will provide the same luminance contrast as the other illuminating light-emitting tiles  100  while balancing the effect of daylight through the window  102 , and hence provide a constant illuminance at the desks  200 . 
       FIG. 5  shows a schematic side view of an indoor area comprising a window  102  and illuminating light-emitting tiles  500 ,  502  and non-illuminating light-emitting tiles  108 . A first subset of the light-emitting tiles comprises light-emitting tiles  500  having a higher luminance compared to a second subset of light-emitting tiles  502 , where the second subset is adjacent to the first subset. By providing a second subset adjacent to the first subset which has a lower luminance, a too high luminance contrast will be avoided and a visual comfort will be provided. Preferably the luminance of the first subset  500  is at least two times higher than the luminance of the second subset  502 , or even more preferably the luminance difference between the second subset and first subset can be between five and twenty times. Accordingly, the first subset can provide a luminance of 2000 cd/m 2 , the second subset can provide a luminance of 200 cd/m 2 , and the non-illuminating tiles typically have a luminance by reflection only of around 50 cd/m 2 . 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, some of the non-illuminating tiles in the embodiments shown in Figures may be conventional ceiling tiles. For example steps in the method could be implemented as software in the control unit or as preprogrammed electronic circuits. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.