Patent Abstract:
The invention relates to an illumination device for embedding data symbols of a data signal into a luminance output of the illumination device. The device includes a LED comprising at least two segments which have a common electrode and are individually controllable. The LED is configured to generate the luminance output in response to a drive signal. The device further includes a controller configured for switching one of the segments on or off in response to the data signal to embed data symbols of the data signal into the light output of the device. One advantage of such an approach is that the proposed device is compatible with conventional LED drivers since no additional electronics for modulating the drive signal are necessary, which enables simple implementation and reduced costs.

Full Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate generally to the field of illumination systems, and, more specifically, to systems and methods for embedding data into the light output of such illumination systems. 
     BACKGROUND OF THE INVENTION 
     Visible light communications refer to communicating data via the light output produced by lighting sources. Such communications is a promising way of enabling localized wireless data exchange in the future because a wide unlicensed frequency band is available for this and because light emitting diodes (LEDs) used to illuminate a room or a space can be applied to provide the communications. Possibly every lighting source of the future could become a communications source. 
     One visible light communications technique is based on embedding data into the light output of an illumination device by modulating the light output of the illumination device in response to a data signal (such light output is sometimes referred to as “coded light” and abbreviated as “CL”). Preferably, the light output is modulated at a high frequency and/or using a special modulation scheme so that the modulation is invisible to human beings. 
     For the realization of visible light communication systems of this kind, illumination systems usually employ dedicated driver electronics to allow superimposing a data signal onto the LED driving signal.  FIG. 1  is a schematic illustration of such an illumination system  100 . As shown, the illumination system  100  includes a dedicated driver circuit  110  and a LED  120 , and is configured to generate a light output  125  according to light settings. The dedicated driver circuit  110  includes a drive signal generator  112  and a driver controller  114 . The illumination system  100  is configured to operate as follows. As shown in  FIG. 1 , the light settings for the illumination system  100  are provided to the drive signal generator  112 . The light settings indicate what the average light output  125  should be in terms, for example, of light power, e.g. defined in lumen, and color. The drive signal generator  112  translates the light settings into a drive signal (e.g., a drive current) for the LED  120  and provides the drive signal to the driver controller  114 . 
     The driver controller  114  is further configured to receive a signal  135  from a data source  130 . The signal  135  includes data bits to be embedded into the light output  125  of the LED  120 . The driver controller  114  is configured to modulate the drive signal to be applied to the LED  120  in response to the signal  135  in order to embed the data bits of the signal  135  into the light output  125 . Various techniques how the drive signal could be modulated in order to embed data into the light output of a light source are known to people skilled in the art (pulse width modulation, amplitude modulation, etc) and, therefore, are not described in further detail. 
     One problem with such an approach is that modifying a conventional LED driver to function as the dedicated driver circuit  110  described above is complicated and costly to implement. Therefore, what is needed in the art is a technique for embedding data into a light output of a LED that does not require modulating the drive signal applied to the LED. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a system and a method suitable for embedding data into the light output of a LED without modulating the drive signal provided to the LED. 
     According to one aspect of the invention, an illumination device for embedding one or more data symbols of a data signal into a luminance output of the illumination device is disclosed. The illumination device includes a LED comprising at least a first segment and a second segment. The first segment and the second segment have a common electrode and are individually controllable. The LED is configured to generate the luminance output in response to a drive signal. The illumination device further includes a controller configured for switching the second segment on or off in response to the data signal to embed the one or more data symbols of the data signal. 
     As used herein, the phrase “switch off a segment” [of a LED] refers to disrupting the drive signal provided to the segment. Similarly, the phrase “switch on a segment” [of a LED] refers to providing the drive signal to the segment. When a segment is switched off it does not generate light. When a segment is switched on, it generates light. The luminance output of the LED is a composition of the luminance outputs of each of the segments. 
     The present invention is based on the recognition that providing a LED separated into at least two segments having a common electrode and which are individually controllable (i.e. they can be individually switched on or off) allows varying the light output produced by the LED without having to change the drive signal supplied to the common electrode of the segments. When a drive signal is applied to the common electrode, switching off one of the segments results in the increase of the current density through the other segment which, at nominal operation, produces a degradation of the light output performance because the internal quantum efficiency (IQE) of the LED drops (this effect is commonly known as the “droop effect”). In turn, variations in the light output performance may be used to embed data symbols. In this manner, a conventional LED driver may be used to provide a drive signal to the common electrode of the two segments, while modulation of the light output is performed by switching one of the segments on and off using e.g. switches which are external to the LED driver. This approach provides an advantage over the prior art in that such a device is compatible with conventional LED drivers since no additional electronics for modulating the drive signal are necessary, which enables simple implementation and reduced costs. 
     As used herein, the term “nominal operation” is used to describe operation of a LED at such current density that desirably results in the maximum IQE of the LED. 
     The light sources described herein may comprise inorganic or organic light emitting diodes. Data embedded in the light output of the illumination system may comprise localized identification information of the light sources, their capabilities and/or settings, or other types of information related to the light sources. However, it should be noted that the illumination system is not necessarily applied for the purpose of illuminating a space or area but may also be applied for data communication as such. As an example, the illumination system may constitute an access point to a network. For such applications, at least part of the light output produced by the illumination system may lie outside of the visible spectrum (i.e., the light output of one of the light sources of the system may lie outside of the visible spectrum). 
     According to other aspects of the invention, a corresponding method for embedding one or more data symbols of a data signal into a luminance output of an illumination device as well as an illumination system comprising one or more illumination devices are provided. 
     Embodiments of claims  2 ,  3 ,  9 , and  10  provide ways to define a modulation depth for the illumination device. As used herein, the term “modulation depth” refers to a range of variation in the amplitude or intensity of the luminance output of the LED, where different levels in the amplitude or intensity correspond to different data bits encoded in the luminance output. 
     Embodiments of claims  4 ,  5 ,  11 , and  12  specify that the common electrode could be a cathode or an anode. 
     Embodiment of claims  6  and  13  provide an advantageous type of the LED to be employed in the illumination device. 
     Hereinafter, an embodiment of the invention will be described in further detail. It should be appreciated, however, that this embodiment may not be construed as limiting the scope of protection for the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an illumination system according to prior art; 
         FIG. 2  is a schematic illustration of an illumination system installed in a structure according to one embodiment of the present invention; 
         FIG. 3  is a schematic illustration of an illumination system according to one embodiment of the present invention; 
         FIG. 4  is a schematic illustration of implementation of a segmented LED approach according to one embodiment of the present invention; and 
         FIG. 5  is a schematic illustration of the droop effect. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. 
       FIG. 2  shows a structure  200 —in this case a room—with an installed illumination system  210 . The illumination system  210  comprises one or more of light sources  220  and one or more controllers (not shown in  FIG. 1 ) controlling the light sources  220 . When driven with an electrical signal, the light sources  220  illuminate parts of the structure  200 . The light sources  220  may comprise inorganic and/or organic light emitting devices. The illumination system  210  may further comprise a remote control  230  allowing a user to control the light sources  220 . 
       FIG. 3  is a schematic illustration of an illumination system  300  according to one embodiment of the present invention. The illumination system  300  may be used as the illumination system  210  in the structure  200  illustrated in  FIG. 2 . As shown, the illumination system  300  includes a LED  320  which includes at least two individually controllable segments having a common electrode, a LED driver  310  configured to provide a drive signal to the LED  320 , and a data source  330  configured to provide data to be embedded into the light output of the LED  320 . 
     The illumination system  300  is configured to operate as follows. As shown in  FIG. 3 , the light settings for the illumination system  300  are provided to the LED driver  310 . The light settings may be e.g. provided by a user via the remote control  230  or may be preprogrammed and provided from an external unit controlling the scene setting. Alternatively, the light settings may be preprogrammed and stored in a memory within the LED driver  310  or within the illumination system  300 . The LED driver  310  translates the light settings into a drive signal for the LED  320 . 
     In one embodiment, the LED driver  310  comprises a current source providing the drive signal in the form of a drive current. In such an embodiment, the LED  320  may be implemented as illustrated in  FIG. 4 . As shown in  FIG. 4 , the LED  320  includes an emitting portion  422  and a switching portion  424 . The emitting portion  422  is manufactured in such a way that n portions of the LED chip area can be partially isolated from the others, resulting in n segments, shown as D 1 , D 2 , . . . Dn, each of which is configured to emit light in response to the drive current. As used herein, n denotes any integer number equal or greater than 2. The segments D 1 , D 2 , . . . Dn have a common electrode. In  FIG. 4  the common electrode is shown to be an anode  426 , but, in other embodiments and with modifications to the circuit that will be apparent to the person skilled in the art, the common electrode could be a cathode. 
     The switching portion  424  includes (n−1) switches, shown in  FIG. 4  as S 2 ,. . . Sn, where each of the switches S 2 ,. . . Sn is used to switch on or off a corresponding segment D 2 ,. . . Dn of the emitting portion  422 . Thus, a switch S 2  corresponds to a segment D 2 , a switch S 3  corresponds to a segment D 3 , and so on. When a constant drive current, shown in  FIG. 4  as Idrv, is provided from the LED driver  310  and applied to the common electrode  426  and all of the switches S 2 ,. . . Sn are closed (i.e., the corresponding circuits are closed, the corresponding segments are switched on), the currents going through each of the segments, shown in  FIG. 4  as currents I 1 , I 2 , . . . In, cause the segments to emit light. The sum of light contributions from each emitting segment comprises the luminance output of the LED  320 . 
     As used herein, the phrase “constant drive signal” (which includes “constant drive current”) is used to reflect the fact that the drive signal is not modulated to embed data bits. This does not exclude drive signals consisting of pulses, as long as the pulses are not modulated to embed data signals, as was done in the prior art. 
     Since the total drive current provided by the LED driver  310  remains constant, if one of the switches S 2 , . . . Sn would become open (i.e., the corresponding segment D 2 , . . . Dn is switched off), the current density in the segments that remain switched on would increase. Driving with the LED  320  with a nominal operation current (nominal operation here refers to all segments on), the increase in the current density through a segment after switching at least one other segment off produces a degradation in the light output performance of the emitting segment due to the droop effect. This effect is illustrated with a curve  510  in  FIG. 5 , where the x-axis is used to show values of the drive current (in mA), the y-axis is used to show values of the wall-plug efficiency (in %) corresponding to a commercial LED device using approximately 1 mm 2  of active area. The right side of the curve  510  makes clear that increase in current results in decreased efficiency. 
     Therefore, at nominal operation, due to the droop effect, when one of the segments D 2 , . . . Dn is switched off, the light output  325  produced by the LED  320  would decrease as the current density through the other segments increase. In order to utilize this effect, as shown in  FIGS. 3 and 4 , the LED  320  further includes a controller  340 . The controller  340  is configured to receive a data signal  335  from a data source  330 . The signal  335  includes (at least) data bits to be embedded into the light output  325  of the LED  320 . In the present description, the symbols are referred to as bits. However, it should be recognized that whenever the word “bit” is used in the present application, a wider definition of a “symbol” applies which may also comprise multiple bits represented by a single symbol. For instance multi-level symbols, where not only 0 and 1 exist to embed data, but multiple discrete levels are defined to represent data. 
     The controller  340  is configured to switch segments D 2 , . . . Dn on or off in response to the signal  335  in order to embed the data bits of the signal  335  into the light output  325 . The amount of emitting area corresponding to each of the different segments defines the intensity levels of the light output modulation. The number of segments that can be switched on or off defines the number of modulation levels. For example, for a two level modulation (i.e. each bit to be embedded is either “1” or “0”), only two segments within the LED  320  are required—one segment which is always switched on and another segment which could be switched on or off to embed data bits. Referring to  FIG. 4 , such an embodiment corresponds to the emitting portion  422  comprising only two emitting segments, D 1  and D 2 . Continuing with this example, consider that decreasing the light output of the LED  320  by 10% can be resolved on the detecting side as a binary value of “0”. In such an exemplary embodiment, the size of the segment D 2  may be made to be approximately 10% of the total area of the emitting portion  422  and to embed a binary value of “0” from the signal  335 , the controller  340  would switch segment D 2  off (i.e., open the corresponding switch S 2 ). 
     Persons skilled in the art will recognize other methods for switching the segments on and off in dependence of the signal  335  to embed data into light output of the illumination system. For example, multi-level modulation of the light output could be implemented by employing and switching larger number of segments than two. The larger the number of levels is, the higher the bit rate can get. Thus, in another embodiment, the emitting portion  422  comprises the segments D 1 . . . Dn. In practice n is an integer between 3 and 10, more preferably between 5 and 8, such as 6 or 7. Switching the segments D 2  . . . Dn using the switches S 2  . . . Sn enables implementing data with multiple discrete levels in the light output  325  of LED  320 . In an embodiment the relative sizes A2 . . . An of the segments D 2  . . . Dn are all equal. In another embodiment the relative sizes A2 . . . An are related to each other in a predefined relationship such that they continuously increase/decrease. For instance, An−1=2×An, so that A2=2×A3=2×(2×A4)=2×(2×(2×A5)), etc. In another embodiment the segments D 2  . . . Dn are designed such that their nominal operation current densities relate to each other similarly as described for the sizes above. 
     In addition to operation in the mode where the controller  340  switches some of the segments on and off to embed data symbols of the data signal  335 , which could be referred to as a “transmission mode,” the LED  320  could also operate in DC mode as any other conventional LED device when switches S 2  -Sn remain in on-state. Namely, the current through the segments D 1 , D 2 , . . . Dn will flow uniformly provided that the on-resistance of the switches S 2 , . . . Sn is much lower than the dynamic resistance of the segments. 
     Furthermore, in other embodiments, the LED driver  310  may comprise a voltage source providing the drive signal in the form of a drive voltage. Persons skilled in the art will readily recognize how the discussions provided above could be modified to accommodate the voltage source LED driver. 
     One advantage of the present invention is that the drive signal provided by the LED driver to the LED does not need to be modulated to embed the data symbols because the data symbols are embedded via switching of the individual segments of the LED. As a result, conventional LED drivers may be employed, eliminating the need to include complicated and costly electronics capable of modulating the drive signal. 
     While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. For example, aspects of the present invention may be implemented in hardware or software or in a combination of hardware and software. Therefore, the scope of the present invention is determined by the claims that follow.

Technology Classification (CPC): 7