WO2020260204A1

WO2020260204A1 - A light emitting diode, led, based lighting device arranged for emitting a particular color of light, as well as a corresponding assembly and a method

Info

Publication number: WO2020260204A1
Application number: PCT/EP2020/067361
Authority: WO
Inventors: Joris Hubertus Antonius Hagelaar; Lucas Louis Marie VOGELS; Berend Jan Willem Ter Weeme
Original assignee: Signify Holding B.V.
Priority date: 2019-06-25
Filing date: 2020-06-22
Publication date: 2020-12-30

Abstract

A LED-based lighting device (21) arranged for emitting a particular color of light, wherein said device comprises a power supply unit (9) adapted to provide a DC bus voltage (7), a plurality of parallel cascaded LED channels (2, 3, 4, 5, 6), wherein each of said LED channels is connected to said DC bus voltage and comprises at least one colored LED and a current control element (R) for tuning a light intensity of light emitted by said corresponding at least one colored LED, a memory (22) comprising a relationship, for each channel, between DC bus voltages or currents flowing through said respective channel, and the light intensity emitted by said at least one corresponding colored LED of said respective channel, a controller (23) adapted to determine said DC bus voltage and/or said currents flowing through said respective channels and to control each of said LED channels for emitting said particular color of light taking into account said relationship.

Description

A LIGHT EMITTING DIODE, LED, BASED LIGHTING DEVICE ARRANGED FOR EMITTING A PARTICULAR COLOR OF LIGHT, AS WELL AS A CORRESPONDING ASSEMBLY AND A METHOD

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting and, more specifically, to a Light Emitting Diode, LED, based lighting device which is arranged to emit a particular color of light. The present invention further relates to an LED assembly as well as a method of operating the LED based lighting device.

BACKGROUND OF THE INVENTION

Lighting devices have been developed that make use of Light Emitting Diodes, LEDs, for a variety of lighting applications. Owing to their long lifetime and high energy efficiency, LED lamps are nowadays also designed for replacing traditional fluorescent lamps, i.e. for retrofit applications. For such an application, a retrofit LED lamp is typically adapted to fit into the socket of the respective lamp fixture to be retrofitted.

Moreover, since the maintenance of a lamp is typically conducted by a user, the retrofit LED lamp should ideally be readily operational with any type of suitable fixture without the need for re-wiring the fixture.

The present disclosure is related to multi-channel LED based lighting devices. Each channel may comprise a plurality of LED’s that are capable of emitting light at a particular color. For example a first channel may be directed to emit red colored light. A second channel may be directed to emit green colored light and a third channel may be directed to emit blue colored light.

In such lighting devices, a fixed voltage source may be used to power the LED’s in each of the channels. The current through each of the channels may be set in the factory by tuning a resistor which is placed in series with the LED’s of a particular channel. One of the downsides of such an approach is related to several disturbing factors such as voltage variations of the power source, cable lengths, i.e. impedances, interactions between channels which may cause errors for the targeted flux and color point.

More specifically, typically, there is a controller having a plurality of switches, wherein each of the switches is arranged to enable a particular channel. For example, a first switch may enable a red channel, a second switch may enable a green channel, a third switch may enable a blue channel, etc. The switches may be provided width Pulse Width

Modulation, PWM, signals having particular duty cycles. The frequency of the PWM signals should be chosen such that it exceeds the refresh rates of the human eye. This would prevent a user from seeing any flickering. By controlling the duty cycle, the contribution of each of the channels to the total amount of light emitted can be controlled, and thus also the color of the light that is emitted by the LED based lighting device.

Often, a user may express, or input, the color that he/she wants the LED based lighting device to emit. As mentioned above, the inventors have found that many disturbing factors may exist which prevents the use of static duty cycles for each of the PWM signals fed to the switches.

SUMMARY OF THE INVENTION

It would be advantageous to achieve a Light Emitting Diode, LED, based lighting device arranged for emitting a particular color of light, wherein the actual emitted color of light is close to the intended emitted color of light. As such, it would be

advantageous to achieve a more accurate LED based lighting device.

It would also be advantageous to achieve an LED assembly as well as a method of operating the LED based lighting device.

To better address one or more of these concerns, in a first aspect of the invention, a Light Emitting Diode, LED, based lighting device arranged for emitting a particular color of light is provided. The LED based lighting device comprises:

a power supply unit arranged for providing a Direct Current, DC, bus voltage for powering LED’s;

a plurality of parallel cascaded LED channels, wherein each of said LED channels is connected to said bus voltage and comprises at least one colored LED and a current control element for tuning a light intensity of light emitted by said corresponding at least one colored LED;

a memory comprising a relationship, for each channel, between bus voltages or currents flowing through said respective channel, and light intensity emitted by said at least one corresponding colored LED of said respective channel; a controller arranged for determining said DC bus voltage and/or said currents flowing through said respective channels and for controlling each of said LED channels for emitting said particular color of light taking into account said relationship.

It was the insight of the inventors that the light intensity of a channel is related to the current that flows through that particular channel. A current control element is present for assuring that a predefined amount of current flows through the channel. The value of the current control element may, however, be determined based on a nominal, i.e. standard, DC bus voltage. Variations that occur in the DC bus voltage are then not taken into account. These variations may thus result in different light intensities of a particular channel.

The current control element may be a tune resistor, for tuning the resistance value of the corresponding channel.

It is further noted that the forward voltages of the LED’s in each of the channels may differ. As such, red colored LED’s may have different forward voltages compared to green colored LED’s and compared to blue colored LED’s. It is thus likely that the current control element is different for each of the channels, and it is thus likely that variations in the DC voltage bus will have different impact on each of the channels.

The inventors have found that it may be beneficial if the controller, i.e. the one that controls each of the channels, takes the above mentioned aspects into account. More specifically, the controller may use with respect to either DC bus voltage to light intensity characteristics for each of the channels or may use current flowing through the channel to light intensity characteristics for each of the channels, for compensating aspects.

That is, the controller may use the above described information for controlling each of the channels to a particular color of light.

In accordance with the present disclosure, the power supply unit may be arranged for receiving a mains input supply voltage, for example 230Vac or anything alike, and may be arranged to convert that mains input supply voltage to a DC bus voltage for powering the LED’s in each of the channels.

In accordance with the present disclosure, the memory may be a Read Only Memory, ROM, Random Access Memory, RAM, a cache or anything alike.

In accordance with the present disclosure, the controller may, for example, be a microcontroller or any other control device such as a microprocessor, a field programmable gate array, FPGA, or anything alike. The microcontroller may, for example, receive the relevant input signals at some of the available input pins and may be provide output control signals at other available output pins. It is noted that the memory comprises a relationship, for each channel, between bus voltages or currents flowing through said respective channel, and light intensity emitted by said at least one corresponding colored LED of said respective channel. This is to be perceived broadly. Typically, the relationship is directed to the light outputted by a channel and the electrical characteristics of that channel. This may be expressed in several ways. For example, a bus voltage to current characteristic or anything alike. The relationship may thus also be provided indirectly as the current through a channel is indicative for the light emitted by that channel.

A colored LED is, in accordance with the present disclosure an LED that emits a particular color, for example white, blue, green, red, etc.

In an example, the controller is arranged for determining said currents flowing through said respective channels by:

measuring said DC bus voltage and calculating said currents by taking into account said measured DC bus voltage, nominal currents flowing through said channels and LED forward voltages of each of said LED’s in the channels.

It was found that the current control element is typically tuned for a particular DC bus voltage, for example 24 Volt DC. This further assumes that the forward voltages of the LED’s of a particular channel are known, or can be determined. The forward voltage of a LED is defined as the voltage drop of the LED in the channel. Using the above described information, it is possible to calculate the nominal current flowing through a particular channel.

The inventors have found that the current flowing through a channel is linearly dependent on the DC bus voltage. The current through a particular channel may then be calculated based on the difference between the measured DC bus voltage and the nominal DC voltage, i.e. the particular DC bus voltage for which the current control element was initially tuned. This aspect is described in more detail with respect to the accompanying figures.

The DC bus voltage may be the voltage at the output of the power supply unit, or may be voltage over a particular channel, or anything alike.

In a further example, said controller is further arranged for measuring said LED forward voltages.

In another example, the controller is arranged for determining currents flowing through said respective channels are determined by:

measuring said currents flowing through said respective channels for at least two different DC bus voltages, and measuring said DC bus voltage and determining said respective currents flowing through said respective channels by interpolating said measurements for said at least two different DC bus voltages.

This particular example is directed to the concept that, initially, currents are measured for at least two different DC bus voltages. This may, for example, be performed in the factory. This is provided by determining a straight line which flows through both measurement point. As such, the two measurement points are interpolated across the entire range. The interpolation is performed, preferably, in a linear manner.

In a further example, the controller is further arranged for measuring an environmental temperature, and wherein said controller is arranged controlling each of said LED channels for emitting said particular color of light taking into account said relationship as well as said temperature.

In a second aspect of the invention, there is provided an LED assembly, comprising:

an LED based lighting device in accordance with any of the previous claims, wherein said controller of said LED based lighting device is further arranged for wirelessly receiving a color set point being said particular color of light, and

a remote control unit comprising:

input means arranged for receiving said color set point manually inputted by a user, and

transmitting means arranged for wirelessly transmitting said inputted color set point to said controller.

It is noted that the advantages and definitions as disclosed with respect to the embodiments of the first aspect of the invention also correspond to the embodiments of the second aspect of the invention, being the LED assembly.

In an example, said remote control unit comprises:

a color sensor for measuring a color of light emitted by said LED based lighting device, and wherein said transmitting means are further arranged for transmitting said measured color of light to said LED based lighting device.

Preferably, the controller is further arranged for determining currents flowing through said respective channels determined by:

determining a contribution of each of said respective channels to said measured color of light. The inventors have found that the color of the light actually emitted by the LED based lighting device may be measured or determined. This may, for example, be determined by the remote control unit which is, preferably, in direct line of sight with the LED based lighting device.

The color of the light as measured may be fed back to the LED based lighting device, and the LED based lighting device, more particularly the controller, may use that particular information to determine the amount of light for each of the corresponding channels. As such, the controller is also able to correct for the light as emitted by the LED based lighting device based on the information as received from the remote control unit.

In a third aspect, there is provided a method of operating a Light Emitting Diode, LED, based lighting device in accordance with any of the claims 1- 5, wherein said method comprises the steps of:

providing, by said power supply unit, a DC bus voltage for powering LED’s; determining, by said controller, said DC bus voltage and/or said current flowing through said respective channels, and

controlling, by said controller, each of said LED channels for emitting said particular color of light taking into account said relationship in said memory.

It is noted that the advantages and definitions as disclosed with respect to the embodiments of the first and second aspect of the invention also correspond to the embodiments of the second aspect of the invention, being the method of operating the LED based lighting device.

In an example, the step of determining comprises:

measuring said DC bus voltage, and

calculating said currents by taking into account said measured DC bus voltage, nominal currents flowing through said channels and LED forward voltages of each of said LED’s in the channels.

In a further example, the controller is arranged for measuring said LED forward voltages.

In another example, the method further comprises the step of: determining said currents flowing through said respective channels for at least two different DC bus voltages,

and wherein said step of determining comprises: measuring said DC bus voltage and determining said respective currents flowing through said respective channels by interpolating said measurements for said at least two difference DC bus voltages.

In a further example, the method further comprises the step of: measuring an environmental temperature,

and wherein said step of controlling further comprises:

controlling each of said LED channels for emitting said particular color of light taking into account said relationship in said memory as well as said temperature.

In a fourth aspect of the present disclosure, there is provided a computer readable medium having instructions stored thereon which, when executed by a LED based lighting device, cause said LED based lighting device to implement a method in accordance with any of method examples as provided above.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an LED based lighting device in accordance with the prior art; Fig. 2 shows an LED based lighting device in accordance with the present disclosure;

Fig. 3 shows a flowchart of a method in accordance with the present disclosure.

DETAILED DESCRIPTION

Figure 1 shows an LED based lighting device 1 in accordance with the prior art.

Here, a power supply unit 9 is provided for generating the Direct Current, DC, bus voltage 7. The DC bus voltage 7 is typically about 24 Volts DC, but could range to any value. Usually, in order to prevent any hazardous situation, the DC bus voltage 7 is at least lower than about 50V DC. Electromagnetic Interference, EMI, filters may be placed close to the output of the power supply unit 9 for reducing any disturbances in the DC bus voltage 7.

In the present scenario, the LED based lighting device 1 comprises five channels as indicated with reference numerals 2, 3, 4, 5, 6. Each of the channels 2, 3, 4, 5, 6 is arranged for emitting light with a particular color. For example, the channel as indicated with reference numeral 2 is arranged for emitting red light, the channels as indicated with reference numeral 3 is arranged for emitting green light, the channel as indicated with reference numeral 4 is arranged for emitting blue light, the channel as indicated with reference numeral 5 is arranged for emitting flame white light and the channel as indicated with reference numeral 6 is arranged for emitting cool white light.

Each of the LED’s of the different channels 2, 3, 4, 5, 6, may have different current requirements and may have different forward voltages. A forward voltage of a LED is defined as the voltage drop over that specific LED.

To accomplish that, each of the channels 2, 3, 4, 5, 6 is equipped with a current control element for tuning the current going through the channel. Suppose the DC bus voltage is nominally 24VDC. The first channel, i.e. the one as indicated with reference numeral 2, may have six LED’s each having a forward voltage of 3VDC. This would accumulate to about 18VDC voltage drop over the LED’s. The remaining voltage, i.e.

24VDC - 18VDC is 6VDC is the voltage over the current control element. The resistor value may then be tuned to specify the current flowing through the channel.

A controller 8 may be present to control the channels 2, 3, 4, 5, 6. More specifically, the controller 8 may provide enablement signals to the corresponding switches of the channels 2, 3, 4, 5, 6, for either enabling or disabling the corresponding channels 2, 3, 4, 5, 6, for realizing a particular desired color of the total light emitted.

Figure 2 shows an LED based lighting device 21 in accordance with the present disclosure.

Here, the same reference numerals are used for aspects that have the same, or similar, function. The power supply unit 9 is arranged for providing the DC bus voltage, for example 12VDC or 24VDC. The power supply unit 9 may receive a power supply mains voltage, for example 230 VAC, and may be arranged to convert that power supply mains voltage to the DC bus voltage.

It is noted that, in accordance with the present disclosure, the power supply unit may operate in accordance with a voltage source, not a current source. As such, the power supply unit may be arranged to assure that the output voltage thereof is static, i.e. the DC bus voltage does not fluctuate too much over time.

The LED based lighting device 21 in accordance with the present disclosure may be used for a variety of purposes. For example, the LED based lighting device 21 may be used in an office setting, at home, in a warehouse, in a supermarket or anything alike. The LED based lighting device 21 may be used to enable a particular setting, for example a cool white or blue light which may be useful in an office setting for inducing energy of users in the office, or a warm white setting which may be useful in a home setting for creating a cosy setting, or anything alike.

The present disclosure is directed to a concept in which the desired color setting, for example a particular color set point, is also realized, as good as possible, by the LED based lighting device 21.

The inventors have noted that, often, the DC bus voltage of the led based lighting device has particular error margins. Ideally, a power supply unit 9 should provide for a DC bus voltage 7 which is predefined, and preconfigured, for example 24VDC. However, the output provided by the power supply unit 9 may be different, and may also change over time. The result there is that the voltage of each of the current control elements may differ, and thus also the current through each of the channels. The current through a particular channel may thus not be equal to the nominal current through that particular channel, which then again results in a color offset of the light emitted by the LED based lighting device.

The present disclosure provides for a solution to combat the above described effect.

In an example, the present disclosure discloses a measurement method to measure the currents through each channel 2, 3, 4, 5, 6. This measurement method may be predominantly intended to get a good estimation of the power load of the LED based lighting device 21. In case the power load exceeds a predefined value, the LED based lighting device may be dimmed.

The current control element placed in series with the LED’s can be replaced by a simple current source. But independent of such linear implementation, sufficient voltage overhead, i.e. DC bus voltage minus the forward voltages of the LED’s in a channel, may be required to maintain a constant current. Increased voltage overhead may result in improved robustness to changes, i.e. bin spread, voltage source spread, temperature effects of the LEDs. However, the inventors have thus found that the increased overhead directly affects the efficiency of this system.

This present disclosure is directed to the following problems. Improving color consistency of the LED based lighting device, becoming less dependent of the installed application and becoming less dependent of operating conditions. Improving the power estimation of the predicted load which will be applied, i.e. when a new color/flux setting is requested by the user. Including forward voltage dependent, hence temperature dependent, behavior improves the accuracy of the estimate. By improving the feed forward estimate also less overhead, for example to compensate for unknown parasitic effects, may be required and hence the efficiency may improve.

The LED based lighting device 21 further comprises a memory 22 comprising a relationship, for each channel 2, 3, 4, 5, 6, between

bus voltages or currents flowing through said respective channel, and light intensity emitted by said at least one corresponding colored LED of said respective channel.

The inventors have noted that it may be beneficial to have some data available that represents the relationship between the different DC bus voltages, for example DC bus voltages between 18VDC and 30VDC, and the actual light output by the different channels. There should thus be some sort of relationship between the DC bus voltage and the actual light that is outputted.

The above described relationship may, thus, also be presented by the actual current that flows through a particular channel. The current through a channel is determined by the current control elements value. The value is tuned to the nominal DC bus voltage. A deviating DC bus voltage will result in a deviated channel current.

Finally, the controller 23 may use the relationship for controlling any of the switches for enabling, and disabling, any of the channels 2, 3, 4, 5, 6. The controller 23 may control the switches based on a PWM signal, wherein the duty cycle of the PWM signal determined based on the relationship as described above.

In accordance with the present disclosure, a color algorithm may remain the same as before, but the inputs may be changed since the current dependency of the primary colors may be included to be able to compensate any color shift due to DC bus voltage differences in control. For this compensation, the flux vs. Vbus dependency may be needed. However, since this may require an optical measurement in a luminaire factory, it is easier to do this directly via the current. The present disclosure is, however, directed to both concepts. In this particular invention, multiple examples are provided how this can be implemented:

1) In the calibration data, next to the color point (x,y) and flux (Y), the voltage of the LED string, i.e. the aggregation of the forward voltages of all LED’s in a channel, may be added. This can then be used to calculate the deviation of the current from the nominal current 10 at 24V at Vx via the relation Ii/I0,I = (Vbus-VLED,i)/(24-VLED,i).

The above would not require a measurement in the factory. 2) As a variant of #1 above, instead of storing it in the calibration data, the voltage across the LEDs could also be measured. Advantage is a more accurate value of VLED,I, i.e. the forward voltage.

3) Another way could be to determine the current vs. bus voltage for each of the channels 2, 3, 4, 5, 6 by measuring the forward voltage of the LED string at different voltages. To keep it simple, it could be 2 different voltages after which a linear relation is made between V and I per primary color. This option would require an electrical calibration at the factory but is rather straightforward.

4) Finally, the measurement of the relationship between Ii/I0,I could be performed by the MCU on the control device. The contribution of the different channels may be measured by the unit itself and expressed in an equivalent resistor. This resistor in turn can be used to calculate the current per channel as a result of a change in the voltage.

For all the above methods, the ratio Ii/Io,I as a function of the voltage may be determined. The controller 23 may need to measure the voltage in order to be able for this ratio to be computed. Together the new - current correct - fluxes of the LED strings can be calculated and fed into the color algorithm. This would largely reduce the color deviation with a limited voltage head room.

In an example, said controller 23 is further arranged for measuring an environmental temperature, and wherein said controller is arranged for controlling each of said LED channels for emitting said particular color of light taking into account said current relationship as well as said temperature.

The procedure for extracting the temperature difference with respect to the reference may be done using the following steps:

Measure current during a fixed color point and flux. Knowing the current control element values, the difference in forward voltage can be extracted and the board temperature predicted.

With the forward voltage difference, a temperature difference with respect to the reference can be deduced:

where R is the total dynamic resistance of the LED string and Vi_ed,o is the LED string voltage at calibration temperature

With the temperature difference related to the reference situation, following corrections can be made for the temperature dependencies of the LEDs on x,y coordinates and the flux output L:

% actual— % calibration F Tjactual Tjcalibration ) * DxT y actual — y calibration F (Tj_act_uai Tj calibration ) * DyT

L (l - (l - HC) ^j’^actu“_s ^l ) actual

L calibration Tj calibration 25

(l - (1 - HC) 75 )

The reference levels can be pre-defmed in the factory or determined with a measurement in a defined situation/environment.

It is noted that the LED based lighting device 21 may comprise transmitting means 24 arranged for wirelessly transmitting said inputted color set point to said controller.

Figure 3 shows a flowchart 31 of a method in accordance with the present disclosure.

The method 31 is directed to the operating a Light Emitting Diode, LED, based lighting device in accordance with any of the examples as provided above, wherein said method comprises the steps of:

providing 32, by said power supply unit, a DC bus voltage for powering

LED’s;

determining 33, by said controller, said DC bus voltage and/or said current flowing through said respective channels, and

controlling 34, by said controller, each of said LED channels for emitting said particular color of light taking into account said relationship in said memory.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims, In the claims, the word“Comprising” does not exclude other elements or steps, and the indefinite article“a” or“an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. 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. A computer program may be stored/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 may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope thereof.

Claims

CLAIMS:

1. A Light Emitting Diode, LED, based lighting device (1) arranged for emitting a particular color of light, wherein said LED based lighting device (1) comprises:

a power supply unit (9) arranged to provide a Direct Current, DC, bus voltage for powering LED’s;

a plurality of parallel cascaded LED channels (2, 3, 4, 5, 6), wherein each of said LED channels (2, 3, 4, 5, 6) is connected to said bus voltage and wherein each of the LED channels (2, 3, 4, 5, 6) comprises:

- at least one colored LED; and

- a current control element (R) for tuning a light intensity of light emitted by the at least one colored LED;

a memory (22) comprising a relationship, for each LED channel (2, 3, 4, 5, 6), between:

the DC bus voltage or current flowing through said respective LED channel (2, 3, 4, 5, 6), and

light intensity emitted by said at least one colored LED of said respective LED channel (2, 3, 4, 5, 6);

a controller (8, 23) arranged to, based on the relationship, determine said DC bus voltage or said currents flowing through said respective LED channels (2, 3, 4, 5, 6) and for controlling each of said LED channels (2, 3, 4, 5, 6) such that the LED based lighting device (1) emits said particular color of light.

2. An LED based lighting device (I) in accordance with claim I, wherein said controller (8, 23) is arranged to determine said currents flowing through said respective LED channels (2, 3, 4, 5, 6) by:

measuring said DC bus voltage and calculating said currents by taking into account said measured DC bus voltage, nominal currents flowing through said LED channels (2, 3, 4, 5, 6) and LED forward voltages of each of said LED’s in the LED channels (2, 3, 4, 5, 6).

3. An LED based lighting device (1) in accordance with claim 2, wherein said controller (8, 23) is further arranged to measure said LED forward voltages.

4. An LED based lighting device (1) in accordance with claim 1, wherein said controller (8, 23) is arranged to determine the currents flowing through said respective LED channels (2, 3, 4, 5, 6) by:

measuring said currents flowing through said respective LED channels (2, 3,

4. 5, 6) for at least two different DC bus voltages, and

measuring said DC bus voltage and determining said respective currents flowing through said respective LED channels (2, 3, 4, 5, 6) by interpolating said

measurements for said at least two different DC bus voltages.

5. An LED based lighting device (1) in accordance with any of the previous claims, wherein said controller is further arranged to determine an environmental

temperature, and wherein said controller is arranged controlling each of said LED channels (2, 3, 4, 5, 6) for emitting said particular color of light taking into account said relationship as well as said temperature.

6. An LED assembly, comprising:

an LED based lighting device in accordance with any of the previous claims, wherein said controller (8, 23) of said LED based lighting device (1) is further arranged to wirelessly receive a color set point being said particular color of light, and

a remote control unit comprising:

input means arranged to receive said color set point manually inputted by a user, and

transmitting means (24) arranged to wirelessly transmit said inputted color set point to said controller.

7. An LED assembly in accordance with claim 6, wherein said remote control unit comprises:

a color sensor for measuring a color of light emitted by said LED based lighting device, and wherein said transmitting means are further arranged to transmit said measured color of light to said LED based lighting device.

8. An LED assembly in accordance with claim 7, wherein said controller (8, 23) is further arranged to determine the current flowing through the respective channel (2, 3, 4, 5, 6) by determining a contribution of each of said respective LED channels (2, 3, 4, 5, 6) to said measured color of light.

9. A method of operating a Light Emitting Diode, LED, based lighting, wherein said method comprises the steps of:

providing, by a power supply unit (9), a DC bus voltage for powering LED’s; determining, by a controller (8, 23), said DC bus voltage and/or a current flowing through a plurality of LED channels (2, 3, 4, 5, 6), and

controlling, by said controller (8, 23), each of said LED channels (2, 3, 4, 5, 6) for emitting said particular color of light taking into account said relationship in said memory (22).

10. A method in accordance with claim 9, wherein said step of determining comprises:

measuring said DC bus voltage, and

calculating said currents by taking into account said measured DC bus voltage, nominal currents flowing through said LED channels (2, 3, 4, 5, 6) and LED forward voltages of each of said LED’s in the LED channels (2, 3, 4, 5, 6).

11. A method in accordance with claim 10, wherein said controller (8, 23) is arranged to measure said LED forward voltages.

12. A method in accordance with any of the claims 9 - 11, wherein said method further comprises the step of:

determining said currents flowing through said respective LED channels (2, 3, 4, 5, 6) for at least two different DC bus voltages,

and wherein said step of determining comprises:

measurements for said at least two difference DC bus voltages.

13. A method in accordance with any of the claims 9 - 12, wherein said method further comprises the step of:

determining an environmental temperature,

and wherein said step of controlling further comprises:

- controlling each of said LED channels (2, 3, 4, 5, 6) for emitting said particular color of light taking into account said relationship in said memory (22) as well as said temperature.