WO2007004112A2

WO2007004112A2 - Method and control system for controlling the output of a led luminaire

Info

Publication number: WO2007004112A2
Application number: PCT/IB2006/052090
Authority: WO
Inventors: Eduard J. Meijer; Peter G. Steeneken; Ronald Dekker; Eugene Timmering; Matthias Wendt
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2005-06-30
Filing date: 2006-06-26
Publication date: 2007-01-11
Also published as: TW200715905A; WO2007004112A3

Abstract

The present invention relates to a method of controlling the output of a luminaire comprising an array of LEDs emitting light of at least one color. The array comprises single color LED groups, each consisting of at least one LED. The method comprises the following steps for each LED group: - spectrally bandpass filtering the light emitted by the LED group by means of a filter device, wherein a filter characteristic of said filter device has a peak; detecting the spectrally filtered light from said filter device and generating a response signal, wherein the level of said response signal is related to an amount of detected spectrally filtered light; and - controlling the light output of said LED group at least partially on the basis of said response signal, comprising maximizing the level of said response signal by spectrally shifting the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other. A control system for performing the method is also provided.

Description

Method and control system for controlling the output of a LED luminaire

The present invention relates to a method of controlling the output of a luminaire comprising an array of LEDs emitting light of at least one color, the array comprising single color LED groups, wherein each group consists of at least one LED.

Luminaires having arrays of colored light-emitting diodes (LEDs), also known as RGB LED luminaries, generate various colors of light which, when properly combined, produce white light. Other colors generated by an RGB combination are also preferred in some applications. RGB LED luminaries are used in, for example, LCD back- lighting, commercial- freezer lighting, and white light illumination.

Illumination by means of LED-based luminaires presents difficulties because the optical characteristics of individual RGB LEDs vary with temperature, forward current, and aging. In addition, the characteristics of individual LEDs that are meant to be equal vary as well. More particularly, they vary significantly from batch to batch for the same LED fabrication process and from manufacturer to manufacturer. Consequently, the quality of the light emitted from RGB LED luminaires can vary significantly, and the desired color and the required light intensity of the white light may not be obtained without a suitable light output control system.

US 6,630,801 discloses a LED luminaire including red, green, and blue LED light sources, each consisting of a plurality of LEDs driven by an independent driver. The light emitted from each LED light source is detected by a respective filtered photodiode and an unfiltered photodiode. The response signals are correlated to chromaticity coordinates for each LED light source. Forward currents driving the respective LED light sources are adjusted in accordance with differences between the chromaticity coordinates of each LED light source and corresponding coordinates of a desired mixed color light. While compensating the varying LED properties of the RGB LED luminaire to some extent, this method is unable to discriminate between spectral shifts, spectral broadening and intensity changes. It is an object of the present invention to provide a method and an apparatus for controlling the light output of a LED luminaire which alleviates the above-mentioned drawbacks of the prior art. According to the present invention, this object is achieved by a method as defined in claim 1 and by a control system as defined in claim 9.

The invention is based on the recognition that the use of a spectral filter, which is properly designed in relation to a predefined spectrum of a LED light source, provides the possibility of determining if an actually detected spectrum originating from the LED light source deviates in wavelength from the predefined LED spectrum. This knowledge is in turn useful for controlling the light output of the LED light source.

Thus, in accordance with an aspect of the present invention, a method of controlling the light output of a luminaire comprising an array of LEDs emitting light of at least one color, the array comprising single color LED groups, wherein each group consists of at least one LED comprises the following steps for each LED group: spectrally bandpass filtering the light emitted by the LED group by means of a filter device, wherein a filter characteristic of said filter device has a peak; detecting the spectrally filtered light from said filter device and generating a response signal, wherein the level of said response signal is related to an amount of detected spectrally filtered light; and controlling the light output of said LED group at least partially on the basis of said response signal, comprising maximizing the level of said response signal by spectrally shifting the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other. Since the filter used for spectrally filtering the emitted LED light has a passband character and a peak, it is possible to at least approximately align the peak of the spectrum of the LED light and the peak of the filter characteristic, which corresponds to the spectrum of the filtered light that has passed the filter. The maximum amount of light passed is obtained at, or in a relatively narrow range around, alignment. Thus, it is possible to keep the peak wavelength of the spectrum of the light emitted by the LED group at a desired wavelength or to determine the wavelength of the peak.

In accordance with an embodiment of the method as defined in claim 2, the maximizing step is performed by shifting the spectrum of the LED group. This can be done by adjusting a forward current of the LED group or by adjusting its temperature. These control options can also be combined.

In accordance with an embodiment of the method, as defined in claim 4, the maximizing operation is performed by tuning the filter device. In accordance with an embodiment of the method, as defined in claim 5, the filter device has a number of filters with different filter characteristics. The filter that has the highest response signal level is chosen for the maximizing operation. Thus, it is likely that the smallest shifting has to be made.

In accordance with an embodiment of the method as defined in claim 7, the total light output is detected additionally. Intensity changes are thereby more easily and accurately detected.

In accordance with another aspect of the present invention, a control system is provided for controlling the light output of a luminaire comprising an array of LEDs emitting light of at least one color, the array comprising single color LED groups, wherein each group consists of at least one LED. For each LED group, the system comprises: a spectral bandpass filter device arranged to receive the light emitted from the LED group, wherein a filter characteristic of said filter device has a peak; and a first photodetector optically connected with said filter device and arranged to detect spectrally filtered light, which has passed said filter device, and to generate a response signal, wherein the level of said response signal is related to an amount of the detected spectrally filtered light. Furthermore, the system comprises a control device which is connected with the first photodetector and is arranged to control the light output of said LED group at least partially on the basis of said response signal. The control device is therefore arranged to maximize the level of said response signal by spectrally shifting the spectrum of the light emitted by the LED group and the filter characteristic in relation to each other.

This system is arranged to perform the method described above and presents corresponding advantages.

These and other aspects, features, and advantages of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings,

Fig. 1 is a schematic block diagram of an embodiment of a control system according to the present invention; Figs. 2a, 2b, and 3 schematically show spectral diagrams illustrating different spectral situations that may occur and are processed in embodiments of a method according to the present invention.

Fig. 1 shows an embodiment of the control system for controlling the output of an RGB-based LED luminaire integrated in the luminaire 1. For reasons of simplicity, a basic structure with very few elements is shown. Thus, the luminaire 1 has one red, one green, and one blue LED group, or LED light source, 2-4. Each group 2-4 consists of one LED and is driven by a respective driver 5-7 of a driver device 8. The control system consists of a control device 9, two photodetectors 10, 11 for each LED group 2-4, and a spectral filter 13 for each LED group 2-4. For two of the LED groups 2-4, the photodetectors and filters are shown in broken lines only. It is assumed that each photodetector 10, 11 is provided with the appropriate amplification and signal conversion circuitry as is commonly known in the art. The photodetectors 10, 11 are photodiodes, but may also be other types of photodetecting elements, such as charge-coupled devices or phototransistors.

Primarily the structure and operation of the control of the red color will now be explained. The structure and operation is similar for the other colors. Each photodetector 10, 11 has an output which is connected to a corresponding input of the control device 9. The filter 13 is a narrow-band filter, and its filter characteristic SfI is schematically illustrated, for example, in Fig. 2a, in conjunction with the spectrum Sp of the LED 2. The filter 13 is arranged in front of a first photodetector 10 of the photodetectors 10, 11. A second photodetector 11 of the photodetectors 10, 11 receives unfiltered light from the red LED 2.

The control device 9 consists of a driver controller 16, a reference generator 17 and a user input unit 18. The user input unit 18 is connected to the reference generator 17, which in turn is connected to the driver controller 16. This control system operates as follows.

The first photodetector 10 applies a first response signal to the driver controller 16, and the second photodetector 11 applies a second response signal thereto. The levels of the response signals are related to the respective amount of light that reaches the respective photodetector 10, 11. Initially, the driver 5 for the red LED 2 receives a control signal from the driver controller 16, which control signal is generated on the basis of a reference signal received by the driver controller 16 from the reference generator 17. In turn, the reference signal is generated on the basis of input data, which is input by a user via the user input unit 18. Alternatively, this data is preprogrammed in the reference generator 17.

The input data is set in order to cause the red LED 2 to emit a predefined spectrum of the light. The predefined spectrum Sp, or more particularly the spectral density, of the light emitted from the LED 2 is illustrated in Fig. 2a. The input data is set on the basis of LED property data as defined by the manufacturer of the LED 2. A characteristic of the first filter is also illustrated in Fig. 2a, at SfI, as well as in Fig. 2b. The filter characteristic SfI is set in relation to the predefined spectrum Sp so that a peak wavelength of the filter characteristic SfI is aligned with a peak wavelength of the predefined spectrum Sp. In this way, the response signals, which correspond to the amount of light passing the filter 10, become useiul for detecting any deviations of the actually emitted spectrum from the predefined spectrum Sp. The bandwidth, or spectral width, of the filter characteristic SfI is smaller than the spectral width of the predefined spectrum Sp.

As explained above, due to variations and deviations caused by inaccuracy of the manufacturing process, operational conditions, etc., the spectrum that is actually generated by the red LED 2 often differs from the predefined spectrum to some extent. If the detected spectrum of the red LED 2 is spectrally shifted towards shorter wavelengths, as shown in Fig. 2b, the first response signal has a lower level than it would have if the spectrum was aligned with the filter characteristic SfI as in Fig. 2a. The driver controller 16 periodically checks whether the position of the LED light spectrum is optimal. This is done by means of adaptive control, wherein the driver controller 16 feeds control signals to the driver 5 which causes the driver 5 to feed the LED group 2 with drive signals that adjusts the spectrum of the LED light either towards longer or shorter wavelengths. The driver 5 preferably controls the forward current of the LED 2 in order to obtain the spectral shifts. If the forward current is increased, the spectrum of the LED 2 is shifted towards shorter wavelengths, and vice versa. Alternatively or additionally, the temperature of the LED may be changed by, for example, a cooling device such as a Peltier cooler, or a heating device. Meanwhile, the driver controller 16 detects the first response signal from the first photodetector 10. This adaptive control maximizes the first response signal level. When the maximum is found, the spectrum is aligned as accurately as possible with the filter characteristic. This alignment method is also called pinning.

Alternatively, the driver controller 16 keeps track of the first response signal level. When this level is offset from the value it had after the last pinning operation by more than a predefined extent, a new pinning operation will be performed. Otherwise, the pinning is a continuous process.

When the first filtered photodetector response signal is used, the control system is able to discriminate between a shift of the peak wavelength and an intensity change or a spectral broadening. More particularly, when the intensity changes, the control system is able to keep the spectrum aligned because the maximum is obtained at the same spectral position. A spectral broadening neither has an adverse effect on the control.

However, if it is desired to separately detect spectrally symmetrical deviations, the second, unfiltered photodetector 11 comes into use. This second photodetector 11 detects the total intensity of the light emitted from the red LED 2 and applies the second response signal to the control device 9. In accordance with another embodiment, a modified control program is implemented with this two-photodetector structure of the control system. On the basis of the two response signals, the driver controller is programmed to compromise, if necessary, between the spectral range and the intensity of the light emitted by the LED 2, because the intensity is not allowed to decrease below a lower limit, or to decrease for some other reason. Decreasing the drive current to the LED 2 causes a decrease in the light output and thus for some cases it might not be possible to fully adjust a shifted spectrum by decreasing the drive current because the overall intensity would become too low. A corresponding situation may be at hand at a very high drive current. In accordance with another embodiment, the filter device 13 is a Fabry-Perot etalon. Such a filter can be designed to have several different filter characteristics, as illustrated in Fig. 3. The spectrum Sa of the actually emitted LED light is shown by a dot- and-dash line. The filter characteristic that the spectrum should be aligned with is selectable by means of this Fabry-Perot filter 13. If the actual spectrum deviates to a large extent from the predefined one, the closest filter characteristic may be another one than the filter characteristic that should have been used considering the predefined spectrum. By choosing to align the spectrum with the closest characteristic, a smaller adjustment is needed than if the spectrum had been aligned with the filter characteristic pointed out beforehand.

In another embodiment, the spectrum of the LED 2 is not shifted, but the filter characteristic is. This is done by using a tunable filter as the filter device 13. Useful tunable filters are, for example, MEMS (Micro Electro Mechanical Systems)-based filters, and liquid crystal-based filters.

It should be noted that the above examples relate to an RGB luminaire, but the invention is equally useful for a single color luminaire. Embodiments of the control method and the control system according to the present invention have been described hereinbefore. These embodiments should be considered as non- limiting examples only. As will be evident to a skilled person, many modifications and alternative embodiments are possible within the scope of the invention. As has been explained with reference to the embodiments described hereinbefore, a single filter or a plurality of filters having a bandwidth that is narrower than the LED spectrum is used for keeping the LED spectrum in a desired spectral position, by compensating for processes that cause spectral shifts over time.

It is to be noted that for the purposes of this application, and in particular with regard to the appended claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and use of the indefinite article "a" or "an" does not exclude a plurality of elements or steps, which will be evident to a person skilled in the art.

Claims

CLAIMS:

1. A method of controlling the output of a luminaire comprising an array of

LEDs emitting light of at least one color, the array comprising single color LED groups, wherein each group consists of at least one LED, the method comprising the following steps for each LED group: - spectrally bandpass filtering the light emitted by the LED group by means of a filter device, wherein a filter characteristic of said filter device has a peak; detecting the spectrally filtered light from said filter device and generating a response signal, wherein the level of said response signal is related to an amount of detected spectrally filtered light; and - controlling the light output of said LED group at least partially on the basis of said response signal, comprising maximizing the level of said response signal by spectrally shifting the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other.

2. A method according to claim 1, wherein said spectral shifting of the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other is performed by spectrally shifting said spectrum of the light emitted by the LED group.

3. A method according to claim 2, wherein said spectral shifting of the spectrum of the light emitted by the LED group is performed by means of at least one of changing a forward current and changing the temperature of said LED group.

4. A method according to claim 1, 2 or 3, wherein said spectral shifting of the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other is performed by means of tuning the filter device.

5. A method according to any one of the preceding claims, wherein said filter device comprises a plurality of filters each having a bandpass filter characteristic with a peak, wherein said filter characteristics of the plurality of filters differ spectrally from each other, the method further comprising the steps of: detecting the spectrally filtered light from each one of said plurality of filters and generating a corresponding plurality of response signals; said controlling step further comprising: determining which response signal of said plurality of response signals has the highest level; using said highest level response signal for said maximizing operation.

6. A method according to any one of the preceding claims, wherein said filter device is a Fabry-Perot etalon.

7. A method according to any one of the preceding claims, further comprising detecting a total light output of the LED group.

8. A method according to any one of the preceding claims, wherein the filter characteristic of said filter device is narrower than a predefined spectrum of the LED group.

9. A control system for controlling the light output of a luminaire comprising an array of LEDs emitting light of at least one color, the array comprising single color LED groups, wherein each group consists of at least one LED, the control system comprising, for each LED group: a spectral bandpass filter device arranged to receive the light emitted from the

LED group, wherein a filter characteristic of said filter device has a peak; - a first photodetector optically connected with said filter device and arranged to detect spectrally filtered light, which has passed said filter device, and to generate a response signal, wherein the level of said response signal is related to an amount of the detected spectrally filtered light, wherein the system further comprises: - a control device connected with said first photodetector and arranged to control the light output of said LED group at least partially on the basis of said response signal, wherein said control device is arranged to maximize the level of said response signal by spectrally shifting the spectrum of the light emitted by the LED group and said filter characteristic in relation to each other.

10. A control system according to claim 9, further comprising, for each LED group, a second photodetector which is connected to the control device and is arranged to detect the unfiltered light emitted from the LED group.

11. A control system according to claim 9 or 10, wherein the filter characteristic of said filter device is narrower than a predefined spectrum of the LED group.