WO2013072815A1

WO2013072815A1 - Dynamic texture effect

Info

Publication number: WO2013072815A1
Application number: PCT/IB2012/056233
Authority: WO
Inventors: Lucius Theodorus Vinkenvleugel; Denis Joseph Carel Van Oers; Denis Fournier; Dragan Sekulovski; Pascal Eric Jerome TERNACLE
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-11-16
Filing date: 2012-11-07
Publication date: 2013-05-23

Abstract

A controller (110) is provided for controlling at least two light sources (A, B) arranged for illuminating a surface (101), wherein the light sources are arranged at different positions relative to the surface. The controller is configured to obtain the illuminance on the surface. Furthermore, the controller is configured to vary at least one operating parameter (111) of the at least two light sources over time such that a rendition of the surface by means of the light from the at least two light sources varies over time, wherein the rendition is dependent on the texture of the surface. Furthermore, the controller is configured to control the at least one operating parameter such that the illuminance on the surface is maintained over time within a predetermined, bounded interval (120).

Description

DYNAMIC TEXTURE EFFECT

FIELD OF THE INVENTION

The present invention relates to the control of light sources.

BACKGROUND OF THE INVENTION

The use of artificial lighting to achieve practical or aesthetic effects is continuously increasing, and there are numerous examples of lighting systems adapted for offices, restaurants, museums, advertising boards, homes, shops, shop windows, and so on. In settings where items/objects are displayed for observers, e.g. shop windows for displaying merchandise to customers, museums where works of art are displayed to visitors, etc., the purpose of the lighting system may be to attract the attention of a customer/visitor to the specific item displayed. The lighting may be arranged as one or more lights (e.g. LEDs, spot lights, fluorescent lamps) with the aim to create a suitable luminous environment.

However, a problem with the present lighting arrangements is that the lighting used for this purpose is not properly adapted to the item which is to be displayed. More specifically, the present lighting arrangements do not provide satisfactory lighting with respect to the three-dimensional surface of the item, allowing the viewer to perceive or grasp its quality, texture, curvature, etc.

In view of this, there is a wish to provide lighting that emphasizes the surface of the item shown/displayed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lighting controller, a lighting control system and a method of controlling which achieve enhanced perception of the surface of an item. This and other objects are achieved by a controller, a lighting control system and a method having the features set forth in the independent claims. Preferred embodiments are defined in the dependent claims.

Therefore, according to a first aspect of the present invention, there is provided a controller for controlling at least two light sources arranged for illuminating a surface, wherein the light sources are arranged at different positions relative to the surface. The controller is configured to obtain the illuminance on the surface. Furthermore, the controller is configured to vary at least one operating parameter of the at least two light sources over time such that a rendition of the surface by means of the light from the at least two light sources varies over time, wherein the rendition is dependent on the texture of the surface. Furthermore, the controller is configured to control the at least one operating parameter such that the illuminance on the surface is maintained over time within a predetermined, bounded interval. Thus, the present invention is based on the idea of providing a controller which is able to vary the operating parameter(s) of the light source(s) over time such that a rendition of the surface, which rendition is dependent on the texture of the surface, varies over time, while still maintaining the illuminance on the surface within a predetermined, bounded interval. By the term "rendition" is meant here the

perception/apprehension of a viewer of the surface. Further, the controller may be adapted to control the operating parameter in a first light source which does not correspond to the controlled operating parameter in a second light source.

According to a second aspect of the present invention, there is provided a method of controlling at least two light sources arranged for illuminating a surface, wherein the light sources are arranged at different positions relative to the surface. The method comprises the step of obtaining the illuminance on the surface. Furthermore, the method comprises the step of varying at least one operating parameter of the at least two light sources over time such that a rendition of the surface by means of the light from the at least two light sources varies over time, wherein the rendition is dependent on the texture of the surface. Furthermore, the method comprises the step of controlling the at least one operating parameter such that the illuminance on the surface is maintained over time within a predetermined, bounded interval.

The present invention is advantageous in that the controller provides a rendition of the surface which varies over time, and as the rendition of the surface is dependent on the texture of the surface, a dynamic texture effect of the surface is thereby created. This is realized as the texture of the surface (e.g. fiber texture, texture patterns, folds, roughness, variation in depth, etc.) affects e.g. the shadows on the surface, and the texture of the surface furthermore affects other properties such as the light reflection, absorption, transmission, scattering, etc., of/from the surface. Hence, the controller's varying of the operating parameter(s) of the light source(s) over time and the differently positioned light sources lead to a varying rendition of the surface. In other words, as the light sources are arranged at different positions relative to the surface, each light source generates a different rendition of the surface due to the texture of the surface, and by varying operating parameter(s) of the light sources, a varying rendition of the surface is obtained. The varying rendition of the surface results, in its turn, in a dynamic texture effect of the surface, wherein the texture of the surface is emphasized and the perception of the surface is enhanced. As the dynamic texture effect emphasizes e.g. details of the surface, the present invention is further advantageous in that the perception of quality of the surface is augmented. Hence, the controller of the present invention helps create a dynamic texture effect on a surface of e.g. merchandise (clothing), a painting, a sculpture, etc., wherein the dynamic texture effect accentuates the surface quality and attractiveness.

The present invention is further advantageous in that the illuminance on the surface, which is maintained within a predetermined, bounded interval, leads to a dynamic texture effect of the surface which is subtle, attractive, unobtrusive and/or perceived as non- artificial. Furthermore, as the controller of the present invention is configured to maintain the illuminance on the surface constant or bounded over time, the attention of an observer is primarily drawn to the surface, not to the lighting from the light sources itself.

The controller is arranged for controlling at least two light sources. In other words, in a lighting arrangement of two or more light sources, the controller is arranged for controlling at least two light sources and maximally all of the light sources.

The at least two light sources are arranged for illuminating a surface, wherein the light sources are arranged at different positions relative to the surface. Hence, the two or more light sources are arranged at positions from where they provide different directions of incident light towards the surface. Consequently, properties of the surface, such as e.g. the texture, may e.g. lead to shadows pointing in different directions on the surface, reflections pointing away in different directions from the surface, etc.

The controller is configured to obtain the illuminance on the surface. The illuminance on the surface may be obtained e.g. from estimations/calculations and/or from measurements (e.g. by a sensor).

The controller is further configured to vary the operating parameter(s) of the light sources over time such that a rendition of the surface by means of the light from the light sources varies over time, wherein the rendition is dependent on the texture of the surface. By "operating parameter" is meant here a control signal for controlling substantially any property of the light source, e.g. luminous intensity, color, etc., wherein the variation of the operating parameters creates a dynamic texture effect on the surface. It will be appreciated that the controller may vary any operating parameter of any light source, e.g. a first operating parameter of a first light source, a first and/or a second operating parameter of a second light source, etc.

The controller is further configured to control the operating parameter(s) of the light sources such that the illuminance on the surface is maintained over time within a predetermined, bounded interval. Hence, the controller acts to maintain/preserve a substantially constant illuminance on the surface, wherein the two or more light sources are arranged to illuminate the surface.

According to an embodiment of the present invention, the controller may further comprise an illuminance sensor arranged for measuring the illuminance on the surface, such that the illuminance is obtained as a measured illuminance. In other words, in the present embodiment, the illuminance on the surface is measured by an illuminance sensor (hereafter: sensor) connected to or included in the controller, and the controller is configured to control the operating parameter(s) of the light sources, based on the measured illuminance. The present embodiment is advantageous in that the controller may even further improve its control of the operating parameters such that a desired/predetermined rendition of the surface is achieved. This is realized in that the sensor provides feedback to the controller, and the controller may thus vary the operating parameters such that a desired illuminance is reached/maintained which corresponds to a desired/predetermined rendition of the surface. The present embodiment is further advantageous in that the controller may control the illuminance on the surface to an even greater extent such that the illuminance is maintained over time within a predetermined, bounded interval. This is realized in that the sensor provides feedback to the controller e.g. to increase, decrease or maintain one or more operating parameters of the light from the light sources for the purpose of maintaining the illuminance within the mentioned interval.

According to an embodiment of the present invention, the sensor may further be arranged for measuring a spatial distribution of the illuminance on the surface. By the term "spatial distribution" is meant here a distribution of the illuminance on the surface, which surface may in turn be arranged in three dimensions. For example, the controller may measure a distribution of the illuminance on a surface which is e.g. wrinkled, folded, undulated, etc. Furthermore, the spatial distribution may for example be relatively high on a portion of the surface exposed to the light from the light sources and relatively low on a portion of the surface comprising shadows. The present embodiment is advantageous in that the controller may even further improve the variation of the operating parameter(s) such that a texture effect of the surface, which corresponds even more to the preferred one, is obtained. For example, if the illuminance of a portion of the surface is considered too high and the illuminance of another portion of the surface is considered too low, the controller may vary the operating parameters e.g. by decreasing the difference in brightness between the light sources such that a more even illuminance is provided over the surface.

According to an embodiment of the present invention, the sensor may further be arranged for measuring the illuminance at at least two spatially separated points of the surface. An advantage of the present embodiment is that the controller, by measuring the illuminance at two or more points on the surface by means of the sensor, may estimate a spatial average of the illuminance on the surface. This even further contributes to the control of the operating parameters, such that the illuminance on the surface is maintained over time within a predetermined, bounded interval.

According to an embodiment of the present invention, the sensor may further be arranged for measuring a spatial average of the illuminance over at least one sub-area of the surface. An advantage of the present embodiment is that the controller, based on the illuminance(s) of the sub-area(s), may even further improve the estimate of the spatial average of the illuminance on the surface. Consequently, the controller may even further improve the control of the operating parameters such that the illuminance on the surface is maintained over time within a predetermined, bounded interval.

According to an embodiment of the present invention, the illuminance on the surface may be obtained as an estimated illuminance based on the respective contributions of the at least one operating parameter of the at least two light sources and, possibly, on a known or predicted transfer function or the like. In other words, the illuminance on the surface may be estimated/predicted based on the contributions of the operating parameter(s) (e.g. luminous intensity, color, etc.). As the illuminance on the surface is estimated, the present embodiment is advantageous in that no additional sensors are needed for measuring the illuminance on the surface. Alternatively, the estimate of the illuminance may further be based on measurement data from an existing sensor, such that the estimate is even further improved. In the case that a sensor is provided, an estimate based on sensor measurements may especially be advantageous in the event of a dysfunction or breakdown of the sensor.

According to an embodiment of the present invention, the controller may further be configured to vary the at least one operating parameter such that the at least one operating parameter of the at least two light sources varies periodically over time. By "periodically" is meant here that the variation of the operating parameter is such that the operating parameter follows a recurring pattern over time. An advantage of the present embodiment is that the periodic variation of the operating parameter(s) over time provides an even more attractive rendition of the surface, wherein properties such as e.g. shadows and reflections vary periodically. The present embodiment is further advantageous in that the periodicity of the operating parameter(s) of the light sources even further contributes to lighting that causes the attention of an observer to be primarily drawn to the illuminated surface instead of to the lighting itself. According to an embodiment of the present invention, the operating parameter of the at least two light sources may vary sinusoidally over time. Here, the controller is configured to provide a smooth, billowing light which even further contributes to an attractive rendition of the surface. Moreover, the subtle and unobtrusive illumination of the embodiment consequently leads to even more enhanced attention of an observer towards the surface, instead of to the lighting itself.

According to an embodiment of the present invention, the controller may further be configured to control the at least one operating parameter such that the at least two light sources are symmetrically shifted in phase and vary with the same frequency and amplitude. For example, if there are n light sources, then the total light from the light sources yields /? -phase illumination, wherein the phase shift between adjacent periods of the light sources is 360%. The present embodiment is advantageous in that the symmetry in the variation of the operating parameter(s) provides a symmetric rendition of the surface over time, which even further enhances the attractiveness of the rendition of the surface.

Furthermore, the present embodiment is advantageous in that the controller may more easily maintain the illuminance on the surface over time within a predetermined, bounded interval, and thereby bring out a subtle and unobtrusive texture effect from the surface.

According to an embodiment of the present invention, the controller may further comprise a first subcontroUer configured for controlling at least one of the at least two light sources to define a periodic variation of the at least one operating parameter. The controller may further comprise a second subcontroUer configured to control the at least one operating parameter of at least one of the at least two light sources other than the at least one light source defining a periodic variation of the at least one operating parameter, such that the illumination on the surface is maintained over time within the predetermined, bounded interval. In other words, one or more light sources may be controlled (or set) by the first subcontroUer to a periodic variation (e.g. a sinusoidal function) of the operating parameter(s), wherein the operating parameter(s) of the other light sources may be controlled dependent on the previously mentioned operating parameter(s). An advantage of the present embodiment is that not all of the light sources need to be actively controlled, e.g. for the purpose of maintaining the illuminance on the surface over time within a predetermined, bounded interval. Instead, the second subcontroller may control a (small) subset of the light sources, which results in a more convenient arrangement for the control of the light sources.

According to an embodiment of the present invention, the controller may further be configured to control the at least one operating parameter of all the light sources. As all light sources are controlled, wherein the light sources are arranged at relative positions to the surface, the present embodiment is advantageous in that an even more

preferred/desired rendition of the surface may be provided. It is also possible to estimate the illuminance on the surface very accurately even in the absence of a sensor, since only natural light and other background sources (which may be assumed to vary at different frequencies) contribute in addition to the controlled light sources.

According to an embodiment of the present invention, the at least one operating parameter may be luminous intensity, color point, wavelength spectrum, or a combination thereof. In other words, the operating parameter(s) may be substantially any parameter(s) of the light of the light sources which may be varied to create a varying rendition of the surface, and thereby, a dynamic texture effect. The parameters such as luminous intensity, color point, wavelength spectrum, etc., may contribute to different aspects of the rendition of the surface. For example, a variation in luminous intensity may create varying shadows on the surface (i.e. a high luminous intensity may create a distinct shadow), a different color point may diffract the light differently (e.g. blue and red light), etc. Thus, the present embodiment is advantageous in that the variation of the operating parameters may provide an even more attractive rendition of the surface.

According to an embodiment of the present invention, there is provided a lighting control system for controlling the illuminance on a surface, comprising at least two light sources arranged at different positions relative to the surface, and a controller as defined in any one of the preceding embodiments, arranged to control the at least two light sources.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 is a schematic illustration of a lighting system for controlling the illuminance on a surface; Figs. 2a-c are diagrams of the illuminance on the surface as a function of time; and

Figs. 3-4 are schematic illustrations of exemplifying lighting systems for controlling the illuminance on a surface.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, the present invention is described with reference to a controller for controlling at least two light sources arranged for illuminating a surface.

Fig. 1 is a schematic illustration of a lighting system 100 for controlling the illuminance on a surface 101. The lighting system 100 comprises two light sources A and B which are arranged at different positions relative to the surface 101. Here, the light sources A and B are positioned in the ceiling of a room and at a relatively large distance from each other, wherein the light sources A and B are arranged to illuminate the surface 101 in an obliquely downward direction. The light sources A and B are further arranged such that the angle between the incident lights from the light sources A and B is relatively large. It will be appreciated that substantially any other arrangement of the light sources A and B at different positions is feasible, and further, that any number of light sources exceeding two, as shown in this figure, may be realized.

A controller 110 is provided in the ceiling for controlling the light sources A and B. The controller 110 may be connected to the light sources A and B by wire, or alternatively, by a wireless connection. It may be noted that the arrangement of the controller 110 in the ceiling serves merely as an example, and that the controller 110 may alternatively be provided substantially anywhere for controlling the light sources A and B.

The controller 110 is configured to obtain the illuminance on the surface 110, wherein the controller 110 may obtain the illuminance by estimation/calculation/prediction (e.g. by estimating the illuminance on the surface 101 as a function of the luminous intensity of the light sources A and B, the distance from the light sources A and B to the surface 101, etc.), or as a measurement from e.g. a sensor, measuring the illuminance on the surface 101.

The controller 110 is configured to vary at least one operating parameter 111 of the light sources A and B arranged for illuminating the surface 101, wherein the operating parameter 1 11 may be e.g. luminous intensity, color point, wavelength spectrum, or a combination thereof. By variation of the operating parameter 111 , the rendition of the surface 101 by means of the light from the light sources A and B varies over time. The rendition of the surface 101 is dependent on the texture of the surface 101, wherein the texture comprises characteristics/properties of the surface 101. It will be appreciated that the surface 101 may be a surface 101 of substantially any item or object, e.g. a piece of clothing, a canvas of a painting, the hood of a car, etc. For example, if the surface 101 comprises a material such as cloth, fabric, felt, or the like (e.g. in a piece of clothing), the texture of the surface 101 may represent properties such as e.g. fiber texture, fiber pattern, roughness, smoothness, folds, variation in depth, perforations, etc.

In the embodiment of Fig. 1, the controller 110 varies the operating

parameter(s) 111 of the light sources A and B such that the rendition of the surface 101 (here, a piece of clothing) varies over time. By the variation of the rendition of the surface 101, wherein the rendition is dependent on the texture of the clothing, the controller 110 creates a dynamic texture effect which emphasizes the perception of the surface 101 by an observer.

In Fig. 1, the light sources A and B are arranged such that the angle between the incident lights from light source A and light source B is relatively large. In this embodiment, the variation of the controller 110 of the operating parameters 111 of the light sources A and B over time leads to a rendition of the surface 101 which may vary

significantly over time, e.g. due to large variations of the shadows on the surface 101. In connection herewith, the controller 110 may further be configured to control the operating parameter 111 such that the dynamic texture effect is neither too small to be visible, nor too large to distract an observer.

Fig. 2a is a diagram of the illuminance E_v of the light from the light sources A and B on the surface 101 as a function of time t. In this embodiment, the controller 110 is configured to vary the operating parameter 111 such that the operating parameter 111 of the light sources A and B varies periodically over time.

At time t_{l s} the contribution to the illuminance on the surface 101 comes merely from the light source A, as the operating parameter 111 of the light source A generates a high illuminance on the surface 101, whereas the operating parameter 111 of the light source B does not contribute to the illuminance on the surface 101 (here, light source B is turned off). With respect to the arrangement in Fig. 1, the illuminance on the surface at ti is provided by the light from light source A, i.e. coming from the upper left hand side direction, from the viewpoint of an observer. Here, the rendition of the surface 101 is dependent on the light (e.g. operating parameters 111 and direction of the light) from the light source A and the texture of the surface 101, which texture provides a texture effect due to e.g. shadows, reflections, etc., on the surface 101. At time t₂, the contribution to the illuminance on the surface 101 by light source A is decreased and that by light source B is increased. Here, the operating

parameters 111 of the light sources A and B contribute equally to the illuminance on the surface 101. With respect to the arrangement in Fig. 1, the illuminance on the surface at t₂ is thus provided by the light from both light sources A and B, and the rendition of the surface 101 is dependent on the texture of the surface 101, which e.g. may render shadows and reflect light in different directions.

At time t3, the contribution to the illuminance on the surface 101 comes merely from the light source B, wherein the operating parameter 111 of the light source B generates a high illuminance on the surface 101, whereas the operating parameter 111 of the light source A does not contribute to the illuminance on the surface 101 (here, light source A is turned off). From the viewpoint of an observer, the illuminance on the surface 101 at t3 is provided by the light from light source B coming from the upper right hand side direction. Here, the rendition of the surface 101 is dependent on the light (e.g. operating parameters 111 and direction of the light) from light source B and the texture of the surface 101, which texture provides a texture effect due to e.g. shadows, reflections, etc., on the surface 101. At time t₄, only light source A contributes to the illuminance on the surface 101, whereas light source B is turned off.

The variation of the operating parameters 111 in Fig. 2a exhibits a somewhat truncated periodical profile, wherein the maximum and minimum contribution of illuminance on the surface 101 is maintained for a time interval 122. Furthermore, the operating parameters 111 may be symmetrically shifted in phase and vary with the same frequency and amplitude, wherein the embodiment in Figs. 1 and 2 provides two-phase lighting where the operating parameters 111 of the light sources A and B are shifted 180°.

It will be appreciated that the minimum contribution of illuminance on the surface 101 from each of the light sources A and B may be different from zero. For example, instead of light source A being turned off at t_ls it may still be turned on during the time interval 122 and contribute to the illuminance on the surface 101.

Fig. 2b shows an alternative variation of the operating parameters 1 11 of the light sources A and B. Here, the phase-shifted light sources A and B alternately exhibit a step-wise variation, wherein the operating parameters 111 of the light sources A and B change instantly from a turned-off state to a fully turned-on state of the light sources A and B, retain the maximum illuminance on the surface 101 during a time interval 122, then immediately change from the fully turned-on state to the turned-off state, retain the minimum illuminance during the time interval 122, and thereafter cyclically repeat the profile.

Analogously, in Fig. 2c, the operating parameters 111 of the phase-shifted light sources A and B vary cyclically, but here the profile of the variation of the operating parameters 111 is saw-tooth shaped.

It will be appreciated that the profiles of the operating parameters 111 as shown in Figs. 2a-c are provided as examples, and that substantially any other profile of the operating parameters 111 of the light sources A and B (or from more light sources) may be feasible, e.g. a sinusoidal variation.

The described periodical variation of the operating parameter 111 of the light sources A and B by the controller 110 provides a rendition of the surface 101 which varies over time. As the texture of the surface 101 (e.g. fiber texture) thus creates shadows, reflections etc., which vary over time (e.g. in direction, intensity, etc.), the controller 110 provides a dynamic texture effect on the surface 101.

The controller 110 is further configured to control the operating parameter 111 such that the illuminance on the surface 101 is maintained over time within a predetermined, bounded interval. In Fig. 2, the interval 120 is represented by dashed lines which over time define a constant interval and within which the illuminance on the surface 101 is maintained. For example, at t_{l s} t₂, and t₃, the contribution to obtain total illuminance within interval 120 comes from light source(s) A, A and B, and B, respectively. In Fig. 2, the controller is configured to maintain the illuminance on the surface 101 substantially constant at level 121 over time, which is indicated as a solid line in the middle of the interval 120. In other words, the controller 110 synchronizes light sources A and B to obtain the maintained illuminance. The controller 110 thus provides illuminance on the surface 101 which renders a subtle, attractive and unobtrusive texture effect.

The substantially constant illuminance on the surface 101 further results in the attention of an observer being primarily drawn to the surface 101 instead of to the lighting itself coming from the light sources A and B. Furthermore, the light sources A and B may be arranged/mounted such that an observer will not actually see the light sources A and B, but merely the dynamic texture effect that they provide on the surface 101. In other words, the light sources A and B (or more light sources) may be arranged in such a way that an observer may not be able to determine the number of light sources which illuminate the surface 101, resulting in an effect which even further emphasizes that the attention of an observer is drawn to the surface 101 and not to the lighting arrangement itself. The controller 110 may further be configured to actively control one or more light sources (e.g. light source A), whereas the remaining one or more light sources (e.g. light source B) is controlled to have a fixed operating parameter cycle. For example, the controller 110 may be configured to actively control light source A (e.g. based on the illuminance which is measured by illuminance sensor 301), whereas the light source B is set to a fixed cycle. Moreover, apart from the operation of the controller 110, the light sources A and B may be arranged to communicate directly with each other, such that e.g.

synchronization, tuning, etc., of the operating parameters 111 may be even further improved.

Fig. 3 is a schematic illustration of an exemplifying lighting system 300 for controlling the illuminance on a surface 101. Here, the controller 110 further comprises an illuminance sensor 301 arranged for measuring the illuminance on the surface 101, such that the illuminance is obtained as a measured illuminance. It will be appreciated that the illuminance sensor 301 may be arranged substantially anywhere, e.g. adjacent one of the light sources A or B, adjacent the surface 101, etc. Furthermore, there may be more than one illuminance sensor 301.

The illuminance sensor 301 may be arranged to measure different aspects of the illuminance on the surface 101. For example, the illuminance sensor 301 may measure the spatial distribution of the illuminance on the surface 101, the spatial average of the illuminance over one or more sub-areas of the surface 101, etc. The illuminance sensor 301 may alternatively be a camera which images the illuminance on the surface 101 and/or the texture effect (due to shadows, reflections, etc, as described earlier) on the surface 101.

Fig. 4 is a schematic illustration of an exemplifying lighting system wherein the light sources A and B are arranged such that the angle between the incident lights from light source A and light source B is relatively small. In this embodiment, the variation of the operating parameters 111 of the light sources A and B over time by the controller 110 leads to a rendition of the surface 101 which only slightly varies over time, leading to a smaller and more subtle texture effect as compared to the texture effect obtained in the embodiment of Fig. 1. However, the controller 110 may further be configured to control the operating parameter 111 such that the dynamic texture effect is not too small to be visible.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent to those skilled in the art. After studying this description, the described embodiments are therefore not to be construed as limiting the scope of the invention, which is defined only by the appended claims. For example, the number of light sources may be different from that shown in Fig. 1 and/or Fig. 3. Furthermore, the distances between the light sources A and B may vary, as well as the distance between the light sources and the surface 101. Moreover, the light sources A and B may alternatively be arranged on the floor and direct their respective light beams obliquely upwards towards the surface 101.

Claims

CLAIMS:

1. A controller (110) for controlling at least two light sources (A, B) arranged for illuminating a surface (101), wherein said at least two light sources are arranged at different positions relative to said surface, said controller being configured to:

obtain the illuminance on said surface,

vary at least one operating parameter (111) of said at least two light sources over time such that a rendition of the surface by means of the light from said at least two light sources varies over time, wherein said rendition is dependent on the texture of said surface, and

control said at least one operating parameter such that said illuminance on said surface is maintained over time within a predetermined, bounded interval (120).

2. The controller as claimed in claim 1, further comprising an illuminance sensor (301) arranged for measuring the illuminance on said surface, such that the

illuminance is obtained as a measured illuminance.

3. The controller as claimed in claim 2, wherein said illuminance sensor is further arranged to measure a spatial distribution of the illuminance on said surface.

4. The controller as claimed in claim 2, wherein said illuminance sensor is further arranged for measuring said illuminance at at least two spatially separated points of said surface.

5. The controller as claimed in any one of claims 2 to 4, wherein said illuminance sensor is further arranged for measuring a spatial average of said illuminance over at least one sub-area of said surface.

6. The controller as claimed in claim 1, wherein the illuminance on said surface is obtained as an estimated illuminance based on respective contributions of said at least one operating parameter of said at least two light sources.

7. The controller as claimed in claim 1, wherein said controller is further configured to vary said at least one operating parameter such that said at least one operating parameter of said at least two light sources varies periodically over time.

8. The controller as claimed in claim 7, wherein said controller is further configured to vary said at least one operating parameter such that said at least one operating parameter of said at least two light sources varies sinusoidally over time.

9. The controller as claimed in claim 1, wherein said controller is further configured to control said at least one operating parameter such that said at least one operating parameter for said at least two light sources is symmetrically shifted in phase and varies with the same frequency and amplitude.

10. The controller as claimed in claim 1, comprising:

a first subcontroller configured for controlling at least one of said at least two light sources to define a periodic variation of said at least one operating parameter, and

a second subcontroller configured to control said at least one operating parameter of at least one of said at least two light sources, other than said at least one light source defining a periodic variation of said at least one operating parameter, such that said illumination on said surface is maintained over time within said predetermined, bounded interval.

11. The controller as claimed in claim 1 , wherein said controller is further configured to control said at least one operating parameter of all said light sources.

12. The controller as claimed in claim 1, wherein said at least one operating parameter is luminous intensity, color point, wavelength spectrum, or a combination thereof.

13. A lighting control system (100) for controlling the illuminance on a surface, comprising

at least two light sources arranged at different positions relative to said surface, and a controller as claimed in claim 1 , arranged to control said at least two light sources.

14. A method of controlling at least two light sources (A, B) arranged for illuminating a surface (101), wherein said at least two light sources are arranged at different positions relative to said surface, comprising the steps of:

obtaining the illuminance on said surface,

varying at least one operating parameter (111) of said at least two light sources over time such that a rendition of the surface by means of the light from said at least two light sources varies over time, wherein said rendition is dependent on the texture of said surface, and

controlling said at least one operating parameter such that said illuminance on said surface is maintained over time within a predetermined, bounded interval (120).