FIELD OF THE INVENTION

The present invention relates to a lighting module comprising a control circuitry configured to control the operating current between a minimum operating current and a maximum operating current based on a predetermined lumen output and a lumen depreciation.

BACKGROUND OF THE INVENTION

Solid state lighting (SSL) light sources are increasingly used in various lighting applications such as, for example, luminaires. A difficulty with SSL light sources is that the lumen output, or brightness, of an SSL light source may vary in response to changing the die temperature, aging and other factors, despite a constant operating current. To reduce variations in light output and maintain lumen output, modern lighting modules that use SSL light sources, such as the DLM Lexel from Philips, controls the lumen output to a

predetermined value during the lifetime of the lighting module by adjusting the operating current.

An example of an arrangement that reduces the variations in light output can be found in US 2008/0238340, disclosing a method to generate a predetermined brightness for a light emitting semiconductor element by applying a test voltage to the light emitting element, determining a corresponding test current through the light emitting element, and determining an operating current adjustment dependent on the determined test current and the applied test voltage.

The lighting modules are typically delivered to OEM luminaire producers who design the lighting modules into their luminaires. However, as the operating current that is required to maintain lumen output tends to increase during the lifetime of the lighting module, a luminaire using the lighting module may suffer from overheating, and increased electromagnetic interference as the lighting module ages. Thus, there seem to be a need for lighting module that enables an improved luminaire design. SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem, and to provide a lighting module that enables an improved luminaire design.

According to an aspect of the invention, this and other objects are achieved by a lighting module comprising: at least one light emitting device; power supply circuitry to supply an operating current to the at least one light emitting device; and control circuitry configured to control the operating current between a minimum operating current and a maximum operating current based on a predetermined lumen output and a lumen depreciation of said at least one light emitting device, wherein the control circuitry is capable of allowing the maximum operating current independently of the predetermined lumen output and the lumen depreciation of the at least one light emitting device.

The present invention is based on the understanding that by providing a lighting module with control circuitry that allows a maximum operating current to be supplied to the at least one light emitting device independently of the predetermined lumen output and the lumen depreciation, the OEM luminaire producers can easily test their luminaires using the highest power that is achieved over the lifetime of the lighting module. This enables an improved luminaire design as the cooling capacity can be adapted to cope with the heat developed for the higher power required in an aged SSL light source.

Furthermore, the approbation tests (such as harmonics, power factor, temperature, EMI, etc) can be performed at the correct input power levels.

The term lumen output (or light output) should here be understood as the quantity of light that leaves a light source, such as the lighting module. Further, the term lumen depreciation here refers to a reduction in lumen output of the lighting module (for a constant operating current) that may occur, for example, as the light emitting devices in the lighting module ages, or as temperature of the light emitting devices is increased. (Light emitting devices such as light emitting diodes may have a temperature dependence such that the light output at higher temperature is less. This can be compensated by increasing the operating current.) Further, the term minimum operating current is here intended to indicate the lowest operating current that may be used for achieving the predetermined lumen output during the lifetime of the lighting module (i.e. typically the operating current used for a new lighting module). Furthermore, the maximum operating current is here intended to indicate the highest operating current that might be required for achieving the predetermined lumen output during the lifetime of the lighting module (i.e. typically the operating current used for a lighting module near end-of-life). The lighting module may further comprise a light sensor, such as a photo diode, for measuring a lumen output from the at least one light emitting device, wherein the lumen depreciation is indicated by the measured lumen output. This enables a feedback loop where the operating current is adjusted based on the actual lumen output to achieve the predetermined lumen output.

According to an alternative embodiment, the lumen depreciation may be estimated from a burning time of the lighting module. The relationship between the lumen depreciation and the burning time may be known from prior experiments and accessible to the control circuitry e.g. in the form of a look-up table or a mathematical function. It is noted that the estimation of the lumen depreciation may be explicit or implicit. An example of the latter would be the use of a function that describes the operating current required to achieve the predetermined output as a function of burning time. An advantage with an approach where the lumen depreciation is estimated from the burning time is that no light sensor is required thereby enabling a cost-efficient solution.

The at least one light emitting device may be a solid state lighting (SSL) light source, such as a light emitting diode (LED), an organic light emitting diode (OLED), or a polymer light emitting diode (PLED).

Further, the control circuitry may be a programmable device that includes computer executable code that controls operation of the programmable device for controlling lumen output of light emitted by the lighting module.

The control device may further comprise an interface that allows an external device (or control unit) to communicate with the control circuitry. Thus, the control circuitry may receive instructions to supply the maximum operating current independently of the predetermined lumen output and the lumen depreciation. For example, a computer that controls the lighting module during testing may trigger a supply of the maximum operating current. The interface may be a digital interface, such as, e.g. DALI (Digital Addressable Lighting Interface), RDM (Remote Device Management), and DMX (Digital Multiplex).

Furthermore, the control circuitry may be configured to enable a supply of the maximum operating current (independently of the predetermined lumen output and the lumen depreciation) to be triggered by means of a connector or a switch (e.g. a micro switch). As the supply of the maximum operating current can be triggered manually, this arrangement does not require an external device (e.g. a computer) which is configured to trigger the supply of the maximum operating current. Thus, the lighting module can be used with existing test equipment. It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 schematically illustrates a lighting module according to an embodiment of the invention;

Fig. 2a schematically illustrates how the lumen output of a lighting module is reduced as the LEDs in the lighting module age;

Fig. 2b schematically illustrates how the operating current must be increased to maintain a constant lumen output as the LED in the lighting module age;

Fig. 3 is a schematic block diagram illustrating a method to maintain a constant lumen output;

Fig.4 is a schematic block diagram illustrating a test procedure for testing a luminaire provided with a lighting module.

DETAILED DESCRIPTION

Referring now to the drawings and to Fig. 1 in particular, there is depicted a lighting module 100 according to an embodiment of the invention.

The lighting module 100 is here an LED module 100 comprising an array of light emitting diodes (LEDs) 102, power supply circuitry 104 comprising an LED driver for each LED, and control circuitry 106 configured in electrical connection with the power supply circuit 104. The control circuitry 106 is here a programmable device (e.g. a microprocessor, microcontroller, or programmable digital signal processor) that includes computer executable code that controls operation of the programmable device. Furthermore, the control circuitry is here provided with a digital interface 107, such as DALI or

DMX/RDM , enabling connection of the control circuitry to an external device (e.g. a computer used for testing the lighting module). Further, the lighting module here also includes a light sensor 108, such as a photo receiver diode, for measuring the lumen output (or luminous flux) of the LEDs 102. Optionally, the lighting module may also include a heat sink to provide enhanced thermal dissipation, and beam shaping optics (e.g. lenses and reflectors). In operation, the power supply circuitry 104 provides input power to the lighting module by supplying an operating current to each of the LEDs 102 whereby light is emitted by the LEDs 102. However, the lumen output, or brightness, of the lighting module tends to decrease (for a constant operating current) as the LEDs ages. This lumen

depreciation appears from

Fig. 2a, which illustrates that to achieve a lumen output L, a new lighting module (t=0h) requires a current I _¾ whereas a lighting module with a burning time of e.g. 10 000 hours (t=10 OOOh) requires a current L, where Io<Ii.

To be able to compensate for the lumen depreciation, and maintain a constant lumen output throughout the lifetime of the LED module, the initial operating current (i.e. the operating current for a new LED) is below the maximum operating current of the LED. Then, as the LED ages the operating current is increased to maintain a constant lumen output.

Fig. 2b schematically illustrates an example of how the operating current must be increased from a minimum operating current, I _m i _n , for a new LED module, to a maximum operating current, I _max ,at the end-of-life of the lighting module in order to maintain a constant lumen output as the LEDs in the lighting module age.

A method to maintain a constant lumen output is schematically illustrated by the block diagram in Fig. 3. The method is here described in relation to the LED module 100 in Fig. l.

In step 301, the control circuitry 106 receives a measurement from the light sensor 108 indicating the total lumen output of the light emitted by the LEDs 102 in the lighting module 100. The measured lumen output is then compared, in step 302, to a predetermined value indicating a desired lumen output. Then, in step 303, the control circuitry 106 controls the power circuitry 104, to adjust the operating currents to the LEDs 102 to reduce any difference between the measured lumen output and the desired lumen output. Thus, the method provides a feedback loop that controls the operating current to each of the LEDs such that the total lumen output of the lighting module is maintained at a desired lumen output, e.g. 1000 Lumen.

Moreover, according to the invention, it is possible to supply the maximum operating current to the LEDs independently of the predetermined lumen output and the lumen depreciation as indicated by step 304. Here this is achieved by overriding the feedback loop. An override of the feedback loop can be triggered, for example, by means of a predetermined command which is received by the control circuitry from an external device via the digital interface 107. However, according to an alternative embodiment, it may also be triggered by means of a connector, or (micro-)switch.

As is recognized by a person skilled in the art, an override of the feedback loop can be achieved in various ways. One way would be to set the predetermined value which indicates a desired lumen output to a value higher than the maximum lumen output that can be achieved by the lighting module. Another way would be to disable the light sensor 108 (or reduce the measured value) such that the measured lumen output stays below the desired lumen output no matter how high the lumen output actually is. Either way the feedback loop will be tricked into increasing the operating current to a maximum value. Yet another alternative would be to utilize a separate circuit to override the feedback loop, i.e. to provide a first circuit that implements the feedback loop, and a second circuit that by-pass the feedback loop and always supplies maximum operating current.

Fig. 4 schematically illustrates a test procedure for evaluating the performance of a luminaire designed to use the above described lighting module. A more detailed description of such a test procedure can be found in ASSIST recommends,

"Recommendations for Testing and Evaluating White LED Light Engines and Integrated LED Lamps Used in Decorative Lighting Luminaires " Volume 4, Issue 1 May 2008 which is hereby incorporated by reference.

In step 401, the LED module performance is characterized as a function of temperature. This can be done by arranging the LED module in a test chamber provided with a heater. As the test chamber provides a controlled ambient temperature in the chamber, the LED module can be tested for various temperatures. For each temperature for which the LED module is tested a set of photometric and electric quantities are measured such as luminous flux, spectral power distribution, voltage, current, active and apparent power, power factor total harmonic distortion.

From the data collected a set of characteristics can be developed that may be useful when designing the luminaire, such as luminous flux as a function of LED

temperature; luminous efficacy as a function of LED temperature; chromaticity as a function of LED temperature; correlated color temperature as a function of LED temperature; general color rendering index as a function of LED temperature.

Next, in step 402, the LED temperature is determined as the LED module is operating inside the luminaire.

Then the luminaire performance is evaluated in step 403. Knowing the LED temperature for the luminaire type from step 402, the luminaire performance can be evaluated by estimating, the luminous flux, luminous efficacy, CIE x,y, CCT, and CRI for various temperatures from the set of characteristics found in step 401. Typically, the above procedure includes a worst possible application scenario, where the luminaire is tested for the worst case temperatures. This may be useful in deciding whether additional thermal management is needed to further improve the performance of the luminaire.

The above test procedure may preferably be performed twice. The first time the test procedure is performed the feedback loop is activated, to determine the performance of the lighting module and luminaire at the start of its life (i.e. for a relatively low operating current). The second time the test procedure is performed, the feedback loop is disabled, and the maximum operating current is supplied (independently of the predetermined lumen output and the lumen depreciation) to simulate end-of-life of the lighting module and verify whether the optical performance, the heat dissipation, and the electromagnetic interference (EMI) is still within limits. Thus, even though the luminaire is tested with a new LED module, it is possible to perform the test using the highest power that is achieved over the lifetime of the LED module, thereby enabling an improved luminaire design.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, although the invention has here been described using LEDs as source of illumination, other SSL (solid state lighting) light sources, such as organic light emitting diodes (OLEDs), or polymer light emitting diodes (PLEDs), may also be used. Furthermore, in some applications (such as linear light sources) where the sensor does not get light from all LEDs, more than one light sensor may be used, and the average sensor reading may be used for the feedback.

Further, instead of using a light sensor and a feed-back loop, it is possible to predict lumen depreciation and maintain a predetermined lumen output with a feed-forward procedure. For example, software may be used to count the burning hours of the lighting module, whereby the lumen depreciation can be estimated based on a predetermined relationship between the lumen depreciation and the burning time. The operating current may thus be controlled to reduce any difference between an estimated lumen output (that has been corrected for lumen depreciation) and the predetermined desired lumen output.