NL2019630B1

NL2019630B1 - LED driver for VLC

Info

Publication number: NL2019630B1
Application number: NL2019630A
Authority: NL
Inventors: Saes Marc; Johanna Elisabeth Otten Andy
Original assignee: Eldolab Holding Bv
Priority date: 2017-09-26
Filing date: 2017-09-26
Publication date: 2019-04-03

Abstract

An LED driver for driving a plurality of LEDs or LED groups to transmit light in a VLC mode is described, wherein the LED driver comprises a control unit that is configured to: 5 - receive, at an input terminal, a set-point representing a desired illumination characteristic and data that is to be transmitted using VLC; - determine a current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as 10 perceived by a sensor to represent a variable length VLC code representing the data, the VLC code comprising code words, a code word comprising a plurality of symbols; - generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code; and 15 - upon receipt of a further set-point representing a different illumination characteristic during the transmission of the VLC code, determine an adjusted current profile or profiles for one or more of the plurality of LEDs or LED groups based on the different illumination characteristic, whereby the current profile is only adjusted for some, not all, symbols of a next code word to be transmitted; 20 - generate one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles, and - output, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles.

Description

FIELD OF THE INVENTION

The invention relates to the field of light applications, in particular to LED-based lighting applications that are applied for visual light communication (VLC).

BACKGROUND OF THE INVENTION

The present invention relates to visible light communication (VLC) as may be applied by LED based lighting applications. In general, communication by means of visual light may be realized in various manner, using different modulation schemes to code the information that is to be communicated. LED based lighting applications in general allow for a high quality illumination experience, e.g. having variable color functionality and dimming functionality. In addition, there are additional demands that have to be met, e.g. with respect to flicker. Such a combination of demands may not always be met. In particular, it may be difficult to maintain a broadcasting using VLC while an illumination set-point is adjusted. Or, it may be difficult to maintain a broadcasting using VLC substantially flicker-free. It is an object of the present invention to provide in control methods for LED based lighting applications which provide in an improved realization of the various demands.

SUMMARY OF THE INVENTION

It would be desirable to provide in an LED driver enabling to more accurately operate in a VLC mode during a transient such as an illumination characteristic transient. It would also be desirable to provide in an LED driver enabling to maintain operating in a VLC mode during various circumstances.

To address these concerns, in a first aspect of the invention, an LED driver for driving a plurality of LEDs or LED groups is provided, the LED driver being configured to drive the plurality of LEDs or LED groups to transmit light in a VLC mode, wherein the LED driver comprises a control unit that is configured to:

- receive, at an input terminal, a set-point representing a desired illumination characteristic;

- determine a current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as perceived by a sensor to represent a variable length VLC code comprising code words, a code word comprising a plurality of symbols;

- generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code; and

- upon receipt of a further set-point representing a different illumination characteristic during the transmission of the VLC code, determine an adjusted current profile or profiles for one or more of the plurality of LEDs or LED groups based on the different illumination characteristic, whereby the current profile is only adjusted for some, not all, symbols of a next code word to be transmitted;

- generate one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles, and

- output, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles.

In accordance with the first aspect of the present invention, an LED driver is provided for driving a plurality of LEDs or LED groups. In an embodiment, the LEDs or LED groups may e.g. have different illumination characteristics such as a different color.

As an example, the plurality of LEDs or LED groups may comprises one or more warm white LEDs constituting a first group and one or more cold white LEDs constituting a second group.

In an embodiment, the LED driver comprises one or more power converter for supplying power to the LEDs or LED groups. Such power converters may e.g. comprise switched mode power converters such as Buck or Boost converters which may be configured to generate a controlled output current in accordance with a control signal as received from a control unit of the LED driver. Such a control unit may e.g. comprise one or more processors or microcontrollers or the like, an input terminal for receiving input signals representing a desired illumination characteristic to be generated by the plurality of LEDs or LED groups and an output terminal for outputting control signals for a power converter of the LED driver.

In an embodiment, the control unit may further be configured to receive data or information that is to be transmitted by means of VLC by the LEDs or LED groups. Such data or information may also be received via the input terminal or may be received via a dedicated data terminal of the control unit of the LED driver.

In accordance with the present invention, the LED driver is configured to drive the LEDs so as to generate a variable length VLC code, whereby code words, representing the data or information that is to be transmitted, are transmitted by means of a modulation of the overall intensity of the LEDs or LED groups. More details on such a code are provided below.

In order to generate such a VLC code, the control unit of the LED driver according to the first aspect of the present invention is configured to determine, e.g. based on a set-point representing a desired illumination, a required current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile, i.e. the combination of the current profiles of the plurality of LEDs or LED groups, comprises a modulation in order for a corresponding intensity modulation as perceived by a sensor to represent a variable length VLC code representing the data. The control unit may thus determine current profiles for the LEDs or LED groups, such that, when these current profiles are applied, an intensity modulation is obtained which can be perceived by a sensor. Further, in an embodiment, the current profiles are determined in such manner, that, on average, the illumination as generated by the LEDs or LED groups corresponds to the desired illumination characteristic. Note that, in general, by ensuring that the modulation occurs on a comparatively high frequency, e.g. above 250 or 300 Hz, the modulation will not be noticed by an observer, rather, the observer will perceive the mixed, i.e. the averaged intensity of the LEDs or LED groups. In accordance with the present invention, the control unit of the LED driver according to the first aspect of the present invention is further configured to generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code. These control signals may e.g. be provided to the one or more power converters of the LED driver.

In accordance with the present invention, the control unit of the LED driver is further configured to ensure that a transmission of a VLC code, e.g. representing data or information, is not interrupted or disturbed, when an illumination characteristic of the LEDs or LED groups is altered or to be altered. In particular, in an embodiment of the present invention, the control unit of the LED driver according to the present invention is configured to, upon receipt of a further set-point representing a different, desired illumination characteristic, only gradually implement this new illumination characteristic when a VLC code is transmitted.

In an embodiment, such a gradual implementation is realized by determining an adjusted current profile for one or more of the LEDs or LED groups, based on the different desired illumination characteristic, whereby only some of the symbols in a next code word that is to be transmitted take account of the new illumination characteristic. As such, a transition from a first illumination characteristic, e.g. represented by a first desired color and intensity of the LEDs or groups of LEDs to a second illumination characteristic, e.g. represented by a second desired color and intensity of the LEDs or LED groups is introduced in multiple steps.

As an example, the control unit of the LED driver may e.g. be configured to:

- in a first step, determine current profiles for transmitting a code word comprising multiple symbols based in the first illumination characteristic;

- in a second step, determine one or more adjusted current profiles for transmitting a second code word comprising multiple symbols based on the second illumination characteristic, whereby the some, but not all symbols of the second code word take the second illumination characteristic into account;

- in third step, determine one or more further adjusted current profiles for transmitting a third code word comprising multiple symbols based on the second illumination characteristic, whereby the further adjusted current profiles result, when applied to the LEDs or LED groups, in the second illumination characteristic.

In an alternative embodiment, rather than only applying an adjusted current profile to only some of the symbols of a next code word that is to be transmitted, a desired transition between a first illumination set-point or characteristic and a second illumination set-point or characteristic can be gradually introduced by applying one or more intermediate current profiles to a sequence of code words.

In such arrangement, a control unit as applied in an LED driver according to the present invention may be configured to, after receiving a first illumination characteristic and determining the corresponding current profiles to realize this illumination characteristic:

- receive a further set-point representing a different second illumination characteristic;

- determine a further current profile for one or more of the plurality of LEDs to generate the different second illumination characteristic represented by the further set-point,

- determine one or more intermediate current profiles for one or more of the plurality of

LEDs, and

- determine output control signals for the LED driver to sequentially drive the plurality of

LEDs or LED groups according to:

- the current profiles when transmitting a first code word;

- the one or more intermediate current profiles, when transmitting a respective one or more further code words, and

- the further current profiles when transmitting a yet further code word.

In an embodiment, the intermediate current profiles may e.g. represent illumination characteristics that are combinations of the first and second illumination characteristics. As an example, the first illumination set-point may e.g. be to operate a first LED group at 50% and the second LED group at 50%, whereas the second illumination set-point may e.g. be to operate a first LED group at 70% and the second LED group at 30%.

In such example, the control unit may e.g. be configured to determine and sequentially apply:

- current profiles corresponding to operate a first LED group at 50% and the second LED group at 50% when transmitting a first code word;

- current profiles corresponding to operate a first LED group at 60% and the second LED group at 40% when transmitting a second code word;

- current profiles corresponding to operate a first LED group at 70% and the second LED group at 30% when transmitting a third code word.

In such embodiment, the control unit may thus implement one or more intermediate illumination set-point, e.g. obtained by linear interpolation between a first set-point and a second set-point, thereby obtaining a gradual transition between two set-points, during a VLC code transmission.

In an embodiment of the present invention, the control unit as applied in the LED driver according to the present invention is configured to substantially maintain a modulation depth during the operation in VLC mode when an illumination characteristic is adjusted.

In an embodiment, the frequency content of the current profiles as applied during the transition from a first illumination set-point to a second illumination set-point is kept above a predetermined value. In particular, in an embodiment, the adjusted current profiles or intermediate current profiles are constructed to have a lowest frequency that is equal to or higher than a lowest frequency of the current profiles.

In accordance with a second aspect of the present invention, there is provided a visual light communication method for a lighting system comprising an LED driver and a plurality of LEDs or LED groups, the method comprising the steps of:

- receiving a set-point representing a desired illumination characteristic and data that is to be transmitted using VLC; and

- determining a current profile for each of the plurality of LEDs or LED groups based on the desired illumination characteristic represented by the set-point, whereby an envelope of a combined current profile of the current profiles is modulated in order for a corresponding intensity variation as perceived by a sensor to represent a variable length VLC code representing the data, the VLC code comprising one or more code words, a code word comprising a plurality of symbols, whereby each of the plurality of symbols is characterized by a respective predetermined period, the combined current profile having a high current period and a low current period during each predetermined period, and wherein the combined current profile comprises, during the high current period or the low current period of at least one of the predetermined periods characterizing a symbol, a current increment during a current increment period that is less than the high current period or the low current period.

Within the meaning of the present invention, a period or predetermined period refers to a duration in the time domain, i.e. a length or portion of time.

Within the meaning of the present invention, a current increment refers to an incremental increase or decrease of a supplied current to an LED or LED group during a specified period, whereby incremental increase or decrease refers to a transition between two consecutive available current levels.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a current profile representing symbols to be used in a variable length VLC method.

Figure 2 depicts a detailed current profile of a symbol to be used in a variable length VLC method.

Figure 3 and 4 depict two possible current profiles for a first and second LED or LED group representing symbols to be used in a variable length VLC method.

Figures 5 and 6 depict current profiles for a sequence of two code words to be transmitted using VLC, the second code word having an amended profile.

Figures 7 and 8 depict alternative current profiles representing symbols to be used in a variable length VLC method.

Figures 9 and 10 depict current profiles as can be applied by an LED driver according to the present invention during operation in VLC mode.

Figure 11 schematically depicts an LED driver according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a current profile which can be applied by an LED driver for transmitting a code or code word using VLC (visible light communication) in accordance with an aspect of the present invention. In general, a VLC scheme applies a plurality of different symbols, e.g. 4-10, which can be combined into code words, each code word consisting of a combination of a predefined set of symbols. In such a VLC scheme, each symbol is represented by a particular current modulation, which may result in a modulated intensity when applied, which intensity variation may be detected by a sensor. Known examples of such modulation are on-off keying (OOK) and pulse position modulation (PPM). In the current profile as shown, a code word or code that is to be communicated consists of a combination of multiple symbols, whereby the symbols that are used have a different length. In the example as shown, 4 symbols SO - S3 are shown, each having a different duration or period TO - T3. In the example as shown, the current profile of each symbol has a first period, also referred to as the high current period, during which the current is comparatively high, followed by the second period, also referred to as the low current period, during which the current is comparatively low. As a result, a receiver having a light sensor may be capable, based on the intensity variations observed, to determine the instants tO -13 when the symbols start and end and thus, based on the periods between these instants, identify the symbols and thus the code word that is transmitted. In particular, a receiver capable of determining the duration between two consecutive instants at which the intensity increases, would be capable to identify the symbols, based on the determined duration. In order to ensure proper detection of the instants that define the length of the symbols, a sufficient increase in intensity should be applied. In the present invention, the difference between the current at the end of a symbol and the current at the beginning of a next symbol may be referred to as the modulation depth MD. It may be a measure indicative of the ease or accuracy with which the instants tO -13 may be determined. In case the modulation depth MD is too small, it may be difficult for a receiver or sensor to notice the intensity change.

In a practical implementation, periods TO - T3 of the symbols may e.g. be in a range between 200 psec and 500 psec, whereas the duration or period of a code word is e.g. smaller than or equal to 3 ms or 3.33 ms.

With respect to the implementation of a VLC scheme, it may further be important to point out that, in general, the instants at which the current level may be adjusted are fixed, e.g. fixed relative to a clock frequency. As an example, the clock frequency can be such that a new setpoint can only be applied upon expiring of a minimal predetermined period. This minimal period may e.g. be referred to as the incremental period, or clock period, W (see Figure 2) and may be in a range between 20 psec and 30 psec, e.g. 26 psec. As such, the length or period of the particular symbols SO - S3 as used in the coding will typically be a multiple of the incremental period W, e.g. SO = 10 x W, SI = 14 x W, S2 = 18 x W, S3 = 22 x W. In addition, the amplitude of the current I as shown in the current profile will typically also be adjustable in a discrete manner, rather than a continuous manner. In particular, as shown in Figure 2, the smallest adjustment that can be applied to the current is the distance between two adjacent horizontal dotted lines, this smallest current increment being referred to, in the present invention, as the incremental current step ICS. Or, phrased differently, an incremental increase or decrease of a supplied current to an LED or LED group refers to a transition between two consecutive available current levels.

Figure 2 schematically shows a current profile CP representing a particular symbol, the symbol being represented or identifiable by means of the period T of the symbol. In Figure 2, the vertical dotted lines indicate instants at which the current I can be changed, the horizontal dotted lines indicate possible amplitudes of the current I.

In such an arrangement, an increase or decrease of the current I can only occur along the dotted lines.

In case a current profile as shown in Figure 1 or 2 is used to transmit information, it will be clear that this current profile will correspond to a certain illumination intensity. In order to avoid the modulation of the current to be perceived by an observer, the periods of the different symbols or the overall period of a code word should be selected to be comparatively low, such that the frequency content of the current profile is comparatively high. As an example, the periods of the symbols used can be selected in such manner that the overall length or period of a code word is less than or equal to a predetermined value, e.g. 3.3 msec.

In addition to a desired intensity, an illumination set-point may further include a desired color to be generated by the light source, the light source e.g. comprising a plurality of LEDs emitting light of a different frequency or frequency spectrum. The color of the illumination as perceived by an observer can be adjusted by adjusting the relative intensity of the plurality of LEDs applied.

In case various LEDs of different color are applied in an LED fixture, VLC may still be implemented in a similar manner as described above. In particular, when differently colored LEDs, or at least some of the LEDs, are provided with a modulated current, the envelope of the overall current profile can be used to represent the desired symbols forming a code word. In such case, the overall, or combined current profile will result in an overall intensity variation which may be perceived by an optical sensor. In order to realize this, a synchronization of the modulation applied to the different LEDs may be required.

Figure 3 schematically shows current profiles applied to a first and a second LED and the combined current profile. As shown, the current profile of the first LED, II, is a continuous current with amplitude 11, the current profile of the second LED, 12 is a modulated current. Combined, the envelope of the current profile resembles the current profile of Figure 1; i.e. the combined current profile may be considered to represent the required current modulation forbroadcasting four symbols SO, SI, S2, S3. Note that, in line with Figure 2, the current amplitude for both the first LED and the second LED are modulated in accordance with discrete values, as are the instants where the current amplitude can be adjusted. In case the depicted current profiles are applied in a first and a second LED, a sensor observing the light generated by the combined current profile will be able to determine the instants at which the amplitude of the current envelope changes (i.e. increase from a value II to a value II + 12, and thus the symbols that are communicated via the modulation.

As will be understood by the skilled person, the color and intensity set-point as obtained by the current profiles shown in Figure 3, may be obtained using alternative current modulations. Figure 4 schematically shows an alternative manner to arrive at the same combined current envelope using alternative modulations of the currents II and 12, both the alternative modulation of Figure 4 and the modulation shown in Figure 3 resulting in the same intensity and color experience for an observer. In this respect, it is assumed that there is a linear relationship between the current and the intensity.

With respect to realizing a particular illumination characteristic or set-point, i.e. characterized by both a color set-point and an intensity set-point, it can be pointed out that, depending on the period or duration of a symbol or code word, the realization of a desired intensity and color may be realized per symbol, per code word or per multiple code word.

With respect to the sensing of the VLC, it can however be pointed out that, although the modulation depth of the current modulations shown in Figures 3 and 4 is the same, there may be a preferred manner to modulate the currents II and 12, e.g. to take into account that a sensor or detector that is used to detect the intensity changes may have a different sensitivity for different wavelengths.

During the transmission via VLC, there are a large number of requirements that need to be met in high end illumination systems. As mentioned above, in order to realize the VLC, a sufficient modulation depth is required. Further, in order avoid flicker, which could cause nausea for certain observers, the frequency content of the intensity modulations should be sufficiently high. In addition, during the VLC, a desired illumination and color set-point should be preserved.

It would also be preferred that the mentioned requirements are maintained also during transients. Within the meaning of the present invention, transients may e.g. include a transition between a first illumination set-point and a second illumination set-point, an illumination set-point e.g. being characterized by a desired illumination intensity and an illumination color. During such a transition, it would be desirable to maintain the requirements, e.g. w.r.t. modulation depth, for using VLC, such that a transition between a first illumination set-point and a second illumination set-point can be performed without interruption of the VLC, i.e. the visible light communication. In addition, such a transient should be flicker-free, i.e. the transient should not be visible A transition for a non-VLC mode to a VLC mode or vice versa may also be considered a transient.

In an embodiment of the present invention, a transition from a first illumination set-point to a second illumination set-point, during VLC is performed by gradually adjusting the current profile, in particular by only adjusting, when a next code word is transmitted, the current profile of only some of the symbols that make up the code word, i.e. not all the symbols, in a first step, upon receipt of a set-point representing a different illumination characteristic as represented by the current profile, only the current profile of some of the symbols. In a second step, the current profile of other symbols may be adjusted, to arrive more closely to the desired second illumination set-point.

By doing so, a gradual transition from a first illumination set-point to a second set-point is realized, thereby maintaining an accurate VLC operation.

In an embodiment of the present invention, such a gradual adjustment is realized by means of transitioning via one or more intermediate current profiles.

Such embodiment may e.g. be realized by means of a control unit which may be applied to control an LED driver configured to drive a plurality of LEDs, whereby the control unit is configured to:

- generate a current profile for each of the plurality of LEDs to generate the desired illumination characteristic represented by a received set-point, whereby an envelope of the combined current profile is modulated in order for a corresponding intensity variation as perceived by a sensor to represent a variable length VLC code comprising code words, a code word comprising a plurality of symbols, and wherein the control unit is further configure to:

- receive a further set-point representing a different desired illumination characteristic as represented by the set-point, determine a further current profile for one or more of the plurality of LEDs to generate the different desired illumination characteristic represented by the further set-point, and

LEDs, and

- output control signals for the LED driver to sequentially drive the plurality of LEDs according to:

- the current profiles when transmitting a first code word;

- the further current profiles when transmitting a yet further code word. By controlling the plurality of LEDs in such manner, a gradual transition form a current profile to a further current profile is realized via one or more intermediate current profiles, whereby the current profile is adjusted per code word that is to be transmitted. By applying such a gradual transition between a first illumination set-point and a second illumination setpoint, the transmission of VLC can more easily or more accurately sustained.

In an embodiment, care is taken that the modulation depth is not decreased during the transition from a first intensity/color set-point to a second intensity/color set-point.

In an embodiment, the current profile is adjusted proportional for each of the LEDs of the plurality of LEDs. By doing so, the color set-point may be maintained during the transient.

In the current profiles in Figures 3 and 4, one can recognize that the current may be adjusted at only predetermined instants spaced apart by incremental period W, with predetermined amounts, spaced apart by the incremental current step ICS . The smallest current increment that can be applied to a current profile is thus to increase or decrease the current with the incremental current step ICS during a period equal to the incremental period W. This smallest increment is referred to as the minimal period current increment.

Figure 5 schematically shows the current profiles for a first code word (consisting of symbols SO,S1,S2,S3), followed by a current profile for transmitting the same code word, whereby a positive minimal period current increment M+ is applied to the second profile, in particular at the start of the symbol S2.

In order to gradually decrease the intensity, in an embodiment of the present invention, the current profile of a next to be transmitted code word is adjusted by applying a negative minimal period current increment. Such an embodiment is shown in Figure 6. Figure 6 schematically shows the current profiles for a first code word (consisting of symbols SO,S1,S2,S3), followed by a current profile for transmitting the same code word, whereby a negative minimal period current increment M- is applied to the second profile, in particular at the end of the high current portion of symbol S2.

In an embodiment of the present invention, the current profile of the plurality of LEDs is adjusted by only one minimal period current increment per code word that is transmitted.

In an embodiment of the present invention, a symbol is characterized by its length or duration in time, or period. In the VLC code as applied in the present invention, a symbol may thus be characterized or identified by having a predetermined period or length in time. When a sequence of symbols is to be transmitted, the end of a symbol, corresponding to the start of a next symbol, is triggered by a step (either positive or negative) function. Such a step function may be realized in various manners.In the embodiments as shown in Figures 2,3,4, the step function is realized by applying a substantial increase in the total current profile at the start of a next symbol, thus indicating the end of a symbol. In order to realize this, the symbols as shown in Figures 2,3,4 each start with a comparatively high total current portion HCP, total current referring to the summed current of all the LED currents, and end with a comparatively low total current portion LCP. Phrased differently, each of the plurality of symbols that is to be transmitted can be characterized by a respective predetermined period, e.g. period SO, SI, S2, S3. In order for a sensor to identify these symbols, the combined current profile of the plurality of LEDs of LED groups applied is modulated in such manner that during each of the predetermined periods, the combined current profile has a high current period, e.g. the high total current portion HCP, and a low current period, e.g. the low total current portion LCP.

In an embodiment, the total current or combined current of the current profiles, or the envelope of the combined current is such that the total or combined current during the high current periods is substantially the same for all symbols, or all words that are to be transmitted and is such that the total or combined current during the low current periods is substantially the same for all symbols, or all words that are to be transmitted.

Alternatively, the high current portions of the total or combined current may be different for different symbols in a code word. Similarly, the low current portions of the total or combined current may be different for different symbols in a code word.

In the embodiments as shown in Figures 2, 3, 4, a transition from a high current period to a low current period within the predetermined period representing a symbol or from a low current period to a high current period within the predetermined period representing a symbol can be considered a step function. Alternatively, a more gradually transition may be applied as well. During such a more gradual transition, the transition from a high current level to a low current level can e.g. take place over multiple incremental periods or clock periods, such as the period W shown in Figure 2.

In the embodiments shown in Figures 2,3,4, the end of a symbol can be detected as the occurrence of a positive step, i.e. an increase of the total current. In particular, referring to Figure 3, the total current as applied to the plurality of LEDs increases from a comparatively low value (value II) to a comparatively high value (value 11+12) at the end of the symbols SO,

SI, S2, S3. The low total current value and the high total current value should be selected in such manner that the associated intensity variation is sufficiently large that it can be detected by a sensor that is to receive the VLC.

Alternatively, one could trigger the end of a symbol by a negative step function, i.e. by applying a substantial decrease in the current symbol at the end of a symbol. Figure 7 schematically illustrates different symbols S4, S5, S6, S7 that are represented in this manner. In the embodiment as shown, the end of the symbols S4, S5, S6 and S7 corresponds to the instants where the current 12 is reduced to zero, i.e. at instants tl, t2, t3 and t4.

At these instants, there will be an important decrease in the intensity of the illumination as provided by the LEDs, such decrease may be detected by a sensor, thus enabling the determination of the duration or periods of the different symbols.

With respect to adjusting of an intensity and/or color set-point during operation in a VLC mode, i.e. during visible light communication, it can be pointed out that such an adjustment may also be realized by adjusting the relative duration of the high current portion HCP and the low current portion LCP.

This is schematically illustrated in Figure 8. Figure 8 schematically illustrates the same symbols S4, S5, S6, S7 of Figure 7, whereby, compared to Figure 7, the HCP of both symbols S4 and S7 is decreased.

Increasing or decreasing the relative width or duration of the high current portion or period and the low current portion or period thus provides in an additional way, compared to increasing or decreasing the current amplitude, of adjusting an intensity of a light source, during the operation in VLC mode.

With respect to increasing or decreasing an amplitude of the current, e.g. either current 11 or 12 during a part of the HCP, as illustrated in Figures 5 and 6, it can be pointed out that it may be advantageous, for the symbols that start with a HCP, to implement a positive current increment, such as the positive minimal period current increment M+ shown in Figure 5, at the beginning of the HCP of a symbol and to implement a negative current increment, such as the negative minimal period current increment M- shown in Figure 6, at the end of the HCP of a symbol.

In case of the application of the positive minimal period current increment M+ shown in Figure 5 at the beginning of the symbol SI, this increment result in an increase in the modulation depth MD for this symbol. As can be seen, the modulation depth MD for symbol

SI is larger than then the modulation depth MD for symbol SO. Applying a negative minimal period current increment M-, as shown in Figure 6, at the end of the HCP of a symbol enables to avoid that the modulation depth would become smaller.

As indicated above, in order to adjust an intensity of the light emitted by the LEDs or LED groups, the current profiles can be adjusted, either by adjusting an amplitude of the current, e.g. an amplitude of a current profile, or by adjusting a width of a portion of the current profile. In an embodiment, the current profiles are adjusted such that the frequency content of the profile is not lowered, i.e. care is taken that no lower frequencies are introduced by determining the adjusted or intermediate current profiles. When considering this, it may e.g. be preferred to introduce one or more current increments to a current profile, rather than widening or narrowing an HCP of a current profile.

The application of current increments or the application of gradual transitions between current levels or combined or total current levels enables to adjust or control the frequency content of the current profile or total current profile as applied. Controlling the frequency content of the applied current profile enables to control the occurrence of flicker as observed by a human observer. According to a second aspect of the present invention, there is thus provided a method of transmitting data using a variable length VLC code, wherein these current increments or gradual transitions are implemented.

In an embodiment, the present invention provides in an improved method of transmitting data using a variable length VLC code in that in enables to mitigate the occurrence of flicker, in particular during a VLC transmission, by the application of current increments to the current profiles representing the symbols that are transmitted, or the application of gradual transitions between current levels or combined or total current levels.

As illustrated in Figure 2, data or information may be coded into an array of symbols, whereby a particular combination of symbols may be construed as words. In the variable length VLC code according to the present invention, symbols are identifiable, as illustrated in Figure 2, by their length, whereby the beginning and ending of symbols may be noticed by a (steep) increase or decrease of the generated intensity of the plurality of LEDs applied, the combined current profile being representative of the generated intensity.

In the method according to the present invention, particular measures may be taken to ensure that the transmission of the VLC code is satisfactory, under various circumstances, e.g. various illumination set-points, whereby illumination may refer to either color or intensity or both. As indicated above, in order to identify the VLC code that is transmitted, intensity variations of the light as perceived by a sensor, e.g. a sensor in a user equipment such as a smartphone, are relied upon. Such variations, which can e.g. be quantified by the aforementioned modulation depth, should be sufficiently high in order for a processing unit, e.g. a processing unit of a user equipment that needs to receive the VLC signal, to determine the beginning and end of the various symbols that are transmitted. Typically, such a user equipment, e.g. a smartphone or the like, would comprise a sensor that is configured to detect the illumination that is transmitted by the plurality of LEDs and a processing unit configured to process an illumination signal received form the sensor, the illumination signal representing the detected illumination. Based on the illumination signal, the processing unit may determine the beginning and end of the various symbols that are transmitted. This can e.g. be done using threshold detection but can also be done using fast fourier transform. Such processing of a received sensor signal can e.g. comprise the filtering out of a DC level of the illumination signal and perform a processing of the remaining filtered (AC) signal.

In an embodiment, the envelope of the current profile of the different LED groups is such that it comprises a modulation that can be detected by a user equipment comprising a sensor and a processing unit and is sufficiently large so as to determine which symbol it represents. In an embodiment, rather than each symbol having an HCP and an LCP, each with a substantially constant current, a current increment is applied, either at the HCP or at the LCP or both. Such a current increment can be applied on each symbol of a word or on only a subset of symbols of a word, e.g. on only one symbol.

In an embodiment, the current increment is applied to the longest symbol.

Figure 9 schematically depicts various possible modulations as can be applied in a method according to the present invention. The possible modulations as shown can be considered modifications to the modulation as shown in Figure 2.

In the current profile shown in Figure 9 (a), a positive current increment PCI is applied in the high current portion HCP of the current profile CP.

In the current profile shown in Figure 9 (b), a positive PCI and a negative current increment NCI are applied.

In the current profile shown in Figure 9 (c), a negative current increment NCI is applied and a gradual transition between the HCP and the LCP via two intermediate current levels Iml and Im2.

In an embodiment, the current modulation is realized with only one LED group of the plurality of LED groups. Phrased differently, in such embodiment, the modulation depth may entirely be attributed to a transition of a current supplied to one of the LEDs or LED groups from a comparatively low current (e.g. zero)to a comparatively high current, while the currents supplied to the other LED or LED groups are kept substantially constant.

Such a situation is e.g. depicted in Figure 3, whereby the modulation depth is entirely determined by the modulation of the current 12.

In an embodiment of the present invention, it is assumed that the currents as supplied to the plurality of LEDs or LED groups may be independently controlled. Such an arrangement may e.g. allow to supply a first LED group with a modulated current, e.g. switching between a high level and a low level, and to supply a second LED group with a substantially constant current.

Such an embodiment may e.g. be implemented in case the desired intensity is comparatively high. In such case, the required modulation depth for a successful VLC operation may be realized with only one LED or LED group of the plurality of LEDs or LED groups.

As such, in an embodiment of the present invention, the modulation applied to the current envelope representing the combined current profiles of the LEDs or LED groups is realized with only one LED or LED group of the plurality of LEDs or LED groups, in case the desired intensity is above a predefined threshold.

In such a situation, additional criteria such as efficiency or an accurate representation of a desired color or avoiding the occurrence of flicker may be considered as well, in addition to the requirement of enabling the operation in VLC mode.

As such, in an embodiment, of the present invention, current increments as e.g. shown in Figures 9 a-c are applied to the current profiles to obtain a desired color set-point, in case the desired intensity, e.g. derived from a received illumination set-point, is above a predefined value or threshold.

In an embodiment of the present invention, a desired color set-point or intensity set-point need not be realized for each symbol of a code word separately; rather, it may be sufficient that the current profiles as applied for transmitting a code word, i.e. a combination of a plurality of symbols, are such that a desired color set-point or intensity set-point is realized. By doing so, a desired color set-point or intensity set-point can be more accurately obtained. I.e. a higher resolution w.r.t. color and/or intensity can be realized. In such embodiment, the use of one or more current increments to one or more symbols of a code word that is to be transmitted further increases the possibilities to generate a desired color set-point or intensity set-point.

In an embodiment, different current increments are applied to different symbols of a word. As an example, a positive current increment may be applied on the first symbol, whereas a negative current increment can be applied on a second symbol of a word.

The current increment may be applied on only one current or current profile or on multiple currents or current profiles.

In order to ensure a proper detection or demodulation of the VLC signal, the modulation depth should be sufficiently large. In general, a sufficiently large modulation depth may be realized comparatively easy when the intensity set-point is rather high.

At comparatively low intensity set-points, the modulation depth may e.g. be realized by providing multiple currents or current profiles with an HCP followed by an LCP. In an embodiment, the transitions from an HCP to an LCP for the multiple currents may be synchronized.

Figure 10 schematically shows a current profile of a first current 11, e.g. supplied to a first LED or LED group and a current profile of a second current 12, e.g. supplied to a second LED or LED group and the envelope of the resulting current Itot. In an embodiment, the first LED or LED group may have different characteristics that the second LED or LED groups, e.g. a different color spectrum. Each of the currents II and 12 has a profile with a high current portion HCP followed by a low current portion LCP, in a period T, e.g. representing a particular symbol in a variable length VLC code. As a result, each current may be considered to have a modulation depth, MD1, resp. MD2. The resulting current Itot will have the combined modulation depth MD, which may e.g. be sufficiently large for a user equipment, in particular a sensor of a user equipment, to detect the intensity variation associated with the current variation, so as to determine the length, duration or period T.

In this respect, it can be noted that, in general, the conversion from current to light and the sensitivity of the human eye (or of an illumination sensor) will not be equal for all types of LED and all colors (spectra). These differences may e.g. be taken into account when determining the current profiles applied for the different LED groups. In order to implement this, an LED driver according to the present invention may be provided with a sensitivity characteristic or function of one or more illumination sensors, the characteristic indicating the sensor’s sensitivity as a function of frequency. Using such information, it may e.g. be advantageously to implement the largest modulation step in a current for an LED or LED group having a color spectrum for which the sensor is the most sensitive.

By applying current transitions (e.g. MD1, MD2) in multiple current profiles, whereby the summed transition serves as an overall modulation depth (MD), operating in VLC mode may even be possible at comparatively low intensities. Note that, in order to ensure proper operation in VLC mode, i.e. ensuring a sufficiently large modulation depth MD, one may need to make concessions with respect to other criteria that may need to be met, e.g. a desired color set-point.

Generalizing this, the skilled person may understand that, apart from the realization of a particular illumination set-point, e.g. specified by a desired intensity and color, other criteria or requirements may be desired as well. Such criteria may e.g. include the avoidance of flicker, driver efficiency, etc. In case a VLC code is to be transmitted by modulating the intensity of the generated light, the realization of the various requirements may come into conflict with each other. Phrased differently, not all required or desired characteristics may be realized; a selection or prioritizing of the goals to be achieved may be required.

As an example, it may be difficult to realize a desired modulation depth when the desired intensity of the generated light drops below a certain value. In such an embodiment, a desired color set-point is only approximated when the desired intensity is below a certain, predetermined or predefined threshold and operating in VLC mode is required.

On the other hand, in case the desired intensity is comparatively high, it may be comparatively easy to realize a desired modulation depth, i.e. the desired or required illumination depth may be realized in different manners. In such circumstances one may e.g. use this operational freedom to further optimize the operation of e.g. the LED driver supplying the power to the LEDs or LED groups. In an embodiment, such an optimization may include optimizing the operation of the LED driver towards efficiency. Alternatively, the operation of the LED driver may be optimized towards the most accurate realization of a desired color set-point.

In accordance with an aspect of the present invention, the modulation scheme of the current or currents supplied to a plurality of LEDs or LED groups is such that priority is given in attaining the visible light communication, while mitigating the occurrence of flicker. In such embodiment, the desired illumination set-point, e.g. characterized by an intensity and a color, may not be accurately generated, but only approximated. In such situation, priority may e.g.

be given to generating light with the desired intensity combined with a current modulation with a sufficient modulation depth. In order to generate this sufficient modulation depth, concessions with respect to color may have to be made.

In an embodiment, the present invention provides in an LED driver that is configured to perform the above described methods of operating a plurality of LEDs or LED groups, in order to transmit a VLC code.

In such embodiment, the LED driver may comprise a control unit to receive an illumination set-point and data to be transmitted. Such a control unit may further be configured to determine the required current profiles and generate control signals for a power converter of the LED driver, in order to generate the required current profiles.

In an embodiment, such an LED driver may be configured to operate in three different operating modes, depending on an intensity set-point.

In a first operating mode, characterized by the intensity set-point being below a first predetermined value or threshold, priority is given to obtaining a required modulation depth for transmitting a VLC code, irrespective of a desired color set-point or other criteria.

In a second operating mode, characterized by the intensity set-point being above a second predetermined value or threshold, the LED driver may be configured to consider a further criterion to be met, apart from the illumination set-point. Such a criterion may e.g. be an optimization of the efficiency of the LED driver, or an optimization w.r.t. flicker; i.e. optimizing the current profiles of the LEDs or LED groups such that flicker is minimized. When operating in the second operating mode, the LED driver, in particular the control unit of the LED driver, may e.g. be configured to apply a plurality of current increments to the plurality of symbols that form a code word, thereby increasing a frequency content of the current profile or current profiles. Such measures have been found to mitigate or avoid the occurrence of flicker. The application of one or more current increments may further enable to accurately obtain the desired color set-point. At the same time, the current profiles may be selected so as to optimize efficiency of the LED driver. An optimization of the efficiency may e.g. be realized by considering one or more of the following:

an overall efficiency characteristic of the LED driver driving the plurality of LEDs or LED groups;

switching losses in the switches controlling the current through the LEDs or LED groups;

illumination characteristics of the LEDs or LED groups (i.e. current vs. illumination for the LEDs or LED groups.

In a third operating mode, characterized by the intensity being in between the first threshold and the second threshold, the LED driver may e.g. be configured to focus of obtained the desired illumination set-point and to avoid or mitigate flicker during VLC transmission, rather than optimizing the efficiency.

The LED driver according to the present invention may be configured to be combined with an LED fixture comprising LEDs or LED groups for realizing desired illumination setpoints.

Examples of such LED fixtures may e.g. comprise LEDs or LED groups emitting different colors or different color spectra. Such fixtures may e.g. include RGBW (red, green, blue, white) or RGBA (red, green blue, amber), RGBWA (red, green, blue, white, amber), RGB UV (red, green, blue, ultra-violet) LED groups, or combinations thereof.

LED fixtures may also comprise different types of white LEDs, e.g. a combination of WW (warm white) LEDs and CW (cold white) LEDs.

In an embodiment, an LED driver according to the present invention comprises one or more power converters, e.g. switched mode power converters such as Buck or Boost converters. Such an LED driver is schematically shown in Figure 11. Figure 11 schematically shows an LED driver 710 according to the present invention. The LED driver may e.g. be connected to a light engine 720 comprising two LED groups 722 and 724. In the embodiment as shown, the LED driver 710 may be configured to output three currents la, lb, Ic, at respective output terminals 710.1, 710.2 and 710.3, the output currents being provided by three power converters, e.g. switched mode power converters, 750.1, 750.2 and 750.3 of the LED driver. The LED driver 710 further comprises a control unit 730 which may be configured to provide, at an output terminal 730.2, control signals 730.3 to control the power converters of the LED driver, the control signal e.g. representing the desired current profiles. The control unit 730 further comprises an input terminal 730.1 configured to receive an input signal representative of a desired illumination set-point and data that is to be transmitted via VLC. In the embodiment as shown, the output terminals 710.1, 710.2 of the LED driver 710 may be connected to the input terminals 720.1, 720.2 of the light engine 720 to power the LED groups 724 and 722. As such, in the embodiment as shown, currents la and lb of the LED driver may be provided, to LED groups 722 and 724 of the light engine.

As required, detailed embodiments of the present invention are disclosed herein;

By means of the present invention, a desired illumination set-point (e.g. defined by a color and intensity set-point) can be realised, even during the application of a VLC communication. The particular application of current increments on one or more of the symbols that are to be transmitted enable an accurate realisation of a desired illumination set-point, despite limitations w.r.t. available current values or available instants to adjust the current. By realising a desired illumination set-point when considering an entire code word, rather than considering each symbol separately, an improved resolution, with respect to intensity and color is realised. The application of the one or more current increments further enables to avoid or mitigate the adverse effects of flicker to the human observer.

However, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of several items recited in the claims.

The terms program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution on a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CONCLUSIONS

A visible light communication method for a lighting system comprising an LED driver and a plurality of LEDs or LED groups, the method comprising the steps of:

- receiving a set point representing a desired lighting characteristic and data to be sent using VLC; and

- determining a current profile for each of the plurality of LEDs or LED groups based on the desired illumination characteristic represented by the set point, wherein an envelope of a combined current profile of the current profiles is modulated such that a corresponding intensity variation as observed by a sensor is a VLC code with variable length representing the data, the VLC code comprising one or more code words, a code word comprising a plurality of symbols, each of the plurality of symbols being characterized by a respective predetermined period, the combined current profile having a high current period and comprises a low current period during each predetermined period, and wherein the combined current profile during the high current period or the low current period of at least one of the predetermined periods characterizing a symbol comprises at least one current increment during a current increment period d ie smaller than the high flow period or the low flow period.

The method of claim 1, wherein the at least one current increment is applied to one or more current profiles.

The method of claim 1 or 2, wherein a difference between an average current during the high current period and an average current during the low current period is above a predetermined value.

The method of any one of claims 1-3, wherein a positive current increment is applied to a subset of the symbols of a code word.

The method of any one of claims 1-4, wherein a positive current increment is applied to a subset of the symbols and a negative current increment is applied to another subset of the code word symbols.

The method of any one of claims 1 to 5, wherein the flow increment starts at the start of a symbol.

The method of any one of claims 1 to 6, wherein the current increment is applied symmetrically about a central position of the high current period or the low current period.

The method of any one of claims 1 to 7, wherein the high flow period continues the low flow period.

The method of any one of claims 1 to 8, wherein a transition between the high flow period and the low flow period lasts longer than a predetermined period.

The method of claim 9, wherein the predetermined period is an incremental period associated with a clock frequency.

The method of any one of claims 1 to 10, wherein the current increment corresponds to a minimum period of current increment.

A LED driver for controlling a plurality of LEDs or LED groups to transmit light in a VLC mode, the LED driver comprising a control unit adapted to perform the method according to any of claims 1 to 11.

13. LED driver for controlling a plurality of LEDs or LED groups to send light in a VLC mode, the LED driver comprising a control unit that is arranged to:

- at an input terminal, receive a set point representing a desired lighting characteristic and data to be sent by VLC;

- determine a current profile for each of the plurality of LEDs or LED groups based on the desired illumination characteristic represented by the set point, wherein an envelope of a combined current profile of the current profiles is modulated such that a corresponding intensity variation as detected by a sensor has a VLC code with variable length representing the data, the VLC code comprising one or more code words, a code word comprising a plurality of symbols, each of the plurality of symbols being characterized by a respective predetermined period, the combined flow profile having a high flow period and a low flow period during each predetermined period, and wherein the combined flow profile during the high flow period or the low flow period of at least one of the predetermined periods characterized by a symbol comprises at least one flow increment during a flow increment period that includes clay ner than the high flow period or the low flow period;

- generate one or more control signals to control the multiple LEDs in accordance with the current profiles so that the multiple LEDs or LED groups send the VLC code.

14. An LED driver for controlling a plurality of LEDs or LED groups to emit light in a VLC mode, the LED driver comprising a control unit adapted to:

- receiving, at an input terminal, a set point representing a desired exposure characteristic, and data to be transmitted using VLC;

- determining a current profile for each of the plurality of LEDs or LED groups to generate the desired exposure characteristic, wherein an envelope of the combined current profile comprises a modulation such that a corresponding intensity modulation, as observed by a sensor, represents a variable length VLC code of the data, wherein the VLC code comprises code words, a code word comprises a plurality of symbols;

- generating one or more control signals to control the multiple LEDs according to the current profiles, so that the multiple LEDs or LED groups emit the VLC code; and

- after receiving a further set point representing a different exposure characteristic during the transmission of the VLC code, determining an adjusted current profile or profiles for one or more of the multiple LEDs or LED groups based on the different exposure characteristic, the current profile corrected only for some, not all, symbols of a subsequent code word to be transmitted;

- generating one or more control signals to control the multiple LEDs or LED groups according to the adjusted current profile or profiles, and

- outputting, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adapted current profile or profiles.

The LED driver of claim 14, wherein the LED driver is adapted to adjust a current for any LED of the plurality of LEDs or LED groups to multiple discrete values at multiple discrete times.

The LED driver according to claim 15, wherein the LED driver is adapted to adjust the current profile of only one symbol of a subsequent code word with a minimum current increment per time point.

The LED driver according to claim 14 or 15, wherein each symbol is characterized by a predetermined period and at least two different intensities applied during the predetermined period.

The LED driver of claim 17, wherein each symbol is characterized by applying a relatively high intensity during a first portion of the predetermined period, followed by applying a relatively low intensity during a second portion of the predetermined period.

The LED driver according to any one of the preceding claims, wherein the LED driver comprises one or more switched power converters, arranged for:

- receiving the one or more control signals from the control unit and

- generating output currents for the multiple LEDs or LED groups according to the determined current profiles or adapted current profile or profiles.

20. A control unit for controlling an LED driver to control multiple LEDs to emit light in a VLC mode, the control unit comprises:

- an input terminal adapted to receive a set point representing a desired exposure characteristic and data to be transmitted using VLC; and

- an output terminal arranged for outputting one or more control signals for controlling the LED control, the control unit being arranged for:

- determining a current profile for each of the plurality of LEDs around the desired exposure characteristic represented by the set point, wherein an envelope of the combined current profile is modulated such that a corresponding intensity variation as perceived by a sensor represents a variable length VLC code, comprising code words, code word comprising several symbols,

- receiving a further set point representing a different desired illumination characteristic, determining a further current profile for one or more of the multiple LEDs on the basis of the different desired illumination characteristic, and

- determining one or more intermediate current profiles for one or more of the multiple LEDs and

- determining output control signals for the LED control to successively control the multiple LEDs or LED groups according to:

- the flow profiles when sending a first code word;

the one or more intermediate stream profiles, when transmitting a respective one or more further code words, and

- the further flow profile when sending a further code word.

The LED driver of claim 20, wherein each symbol is characterized by a predetermined period and at least two different intensities applied during the predetermined period.

The LED driver of claim 21, wherein each symbol is characterized by applying a relatively high intensity during a first portion of the predetermined period, followed by applying a relatively low intensity during a second portion of the predetermined period.

A visible-light communication method for a lighting system comprising an LED driver and a plurality of LEDs or LED groups, the method comprising the steps of:

- receiving a set point representing a desired exposure characteristic and data to be sent with VLC; and

- determining a current profile for each of the plurality of LEDs to generate the desired exposure characteristic as proposed by the set point, wherein an envelope of the combined current profile is modulated such that a corresponding intensity variation as perceived by a sensor represents a variable length VLC code representing the represents data, the VLC code comprising code words, a code word comprising a plurality of symbols.

The method of claim 23, wherein each of the plurality of symbols is characterized by a predetermined period.

The method of claim 23 or 24, further comprising the steps of:

- receiving a further set point representing a different desired exposure characteristic,

- determining a further current profile for one or more of the plurality of LEDs to generate the different desired illumination characteristic represented by the further set point, and

- determining one or more intermediate current profiles for one or more of the multiple LEDs and determining output control signals for the LED driver to successively control the multiple LEDs or LED groups according to:

- the flow profiles when sending a first code word;

- the further flow profile when sending a further code word.

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