A METHOD AND AN ELECTRONIC DEVICE FOR IMPROVING THE OPTICAL UNIFORMITY OF TILED OLED LIGHTING SOURCES

FIELD OF THE INVENTION

This invention relates to organic light emitting diodes, in particular to

OLED lighting sources constructed from multiple OLED tiles.

BACKGROUND OF THE INVENTION

Organic Light Emitting diodes (OLED) devices are comprised of two electrodes and an organic light emitting layer. The organic layer is disposed between the two electrodes. One electrode is the anode and the other electrode is the cathode. The organic layer is structured such that when the anode has a voltage bias that is sufficiently positive relative to the cathode, holes are injected from the anode and electrons are injected from the cathode. The necessary voltage bias depends upon the materials used for the organic layers. The holes and electrons recombine within the organic layer to induce an exited state in a molecule comprising the organic layer. Light is emitted during the process of excited molecules relaxing to their ground state. The anode is typically manufactured from a high work function material such as a Transparent Conducting Oxide (TCO), and the cathode is typically manufactured from a highly reflecting material such as aluminum or silver. However, there exist many different electrode designs which allow light to exit the cathode, the anode, or through both the cathode and the anode. The organic layer can be comprised of a single organic film, or it can be comprised of a stack of multiple organic films. OLED devices are useful as indicators and displays can be constructed from patterned arrays of OLED devices.

Large area OLED lighting sources for general illumination can be constructed from a plurality of smaller OLED tiles. The OLED tiles can be arranged in matrix form. This is known as OLED tiling and may have several advantages over a single monolithic large area OLED lighting source such as: the production yield is increased significantly, the power loss can be reduced by series connection, the fault

tolerance of the lighting device is increased, and the geometric appearance of the tiled OLED lamp can be easily customized because OLED tiles which are shaped, such as: strips, square of different aspects ratios and etc., can be used.

While OLED tiling has clearly several advantages, a major unresolved problem is the varying electro -optical properties of the individual tiles. Due to manufacturing tolerances, variations of the brightness as a function of current or voltage are typical even for OLED tiles from the same production batch. For lighting sources that have only one OLED tile, the brightness variation of an OLED device itself is less critical. For tiled OLED lighting sources, the inter tile variations of optical properties are easily noticed by a human observer. For example, the human eye is able to detect small variations in brightness when two OLED tiles are in close proximity. The trivial solution to avoid such abrupt brightness changes in a tiled OLED array is to use only tiles with similar properties. This however is expensive because it requires a time consuming selection process. US patent application 2005/0134525 Al describes a system and teaches a method for controlling and calibrating large, tiled OLED emissive displays. This is however not applicable to OLED lighting sources.

SUMMARY OF THE INVENTION

The invention provides for a method, an electronic device, a tiled OLED lighting source, and an OLED kit. Embodiments of the invention are given in the dependent claims.

Embodiments of the invention address the aforementioned problems by using the driver electronics for the OLED lighting source to adjust the electrical power to every single OLED tile of the arrangement so the variation of at least one optical property of the OLED lighting source is minimized. Optical properties of OLED tiles are defined herein as photometric, radiometric, or spectral properties of the light emitted from an OLED tile. As the power supplied to an OLED tile varies, the optical property will change.

An OLED tile is defined herein as an OLED device which is placed in a pattern with other OLED devices. The terms OLED device and OLED tile can be used

interchangeably.

Examples of photometric quantities are luminous energy, luminous flux, luminous intensity, luminance, illuminance, luminous emittance, and luminous efficacy.

Photometric quantities account for the varying sensitivity of the human eye to different wavelengths of visible light. Minimizing the variance of one or more photometric quantities is advantageous, because the light sources will be perceived as having a uniform brightness.

Examples of radiometric properties are radiant energy, radiant flux, radiant intensity, radiance, irradiance, radiant exitance, radiant emittance, radiosity, spectral radiance, and spectral irradiance. Making the radiometric properties more uniform is useful when the amount of energy from a lighting source needs to be uniform such as a light source for photolithography.

The spectral properties of the light emitted from OLED tiles can be measured with a spectrometer. For lighting sources the important quantity is the perceived color of the lighting source. OLED tiles can be constructed where the color of the light emitted changes as the applied current or voltage changes. It is also possible to construct multiple layered OLED tiles, where each layer of the OLED tile produces light with different spectral emissions. By controlling the electrical power to the different layers, the color of the OLED tile can be controlled. It is understood that the OLED tiles described herein can also refer to OLED tiles with multiple layers.

Electro-Optical properties are defined herein as the relationship between the electrical power applied to an OLED tile and the resulting optical property. Electro- Optical properties are typically expressed in terms of an optical property as a function of the applied voltage and/or current. An example would be the luminous emittance as a function of the applied current. Electro-Optical properties are intrinsic properties of each OLED tile, and they vary from tile to tile even within the same manufacturing batch. The control electronics of embodiments of the invention compensate for inter tile variations of the electro -optical properties to increase the uniformity of the resulting optical properties.

The amplitude of the voltage or the current applied to an OLED tile can be controlled. This affects the amount of light produced and in some cases the spectral

content (color) of the light. The OLED tiles can also be switched on and off repeatedly at a rate higher than the persistence of vision. This changes the perception of the brightness of the tile. Either or both of these techniques can be used to adjust the optical properties of the OLED tiles. Using both has the advantage that more than one quantity can be controlled. An example would be the independent control of the luminance and the color of the OLED tile. With multilayered OLED tiles, each of the individual layers can have its current or voltage amplitude or voltage controlled in addition to each layer being switched on and off at an independent rate and/or an independent duty cycle. For this case, there is more than one solution for a desired luminance and color. In one embodiment, a primary aspect of the invention is an automated procedure that determines

1. The brightness level for every single tile in an OLED array

2. the averaged brightness level for the plurality of tiles

3. adjusting the driving current for every single tile so that the brightness deviation from the averaged level is minimum

This procedure can be replicated for other or multiple optical properties of the OLED tiles.

Embodiments of the invention provide for a method for improving the uniformity of at least one optical property of a tiled OLED lighting source. Tiled OLED lighting sources comprise at least two OLED tiles. The method comprises several steps. In the first step, electrical power is applied to the OLED tiles with a power providing means, the power providing means comprises a control means that is adapted for controlling electrical power to each of the OLED tiles. Controlling the electrical power to each of the OLED tiles is defined herein as meaning controlling the voltage, the current, or the current and the voltage to each of the OLED tiles. It is also understood that controlling the electrical power to each of the OLED can also be pulsing the electrical power to the OLEDs at a specific frequency and duty cycle. In the case of multi-layered OLED tiles, controlling the electrical power is understood to be controlling the power to each layer. In the next step, at least one optical property of each OLED tile is measured as a function of electrical power applied to the OLED tile. This is done to determine at least one electro -optical property of each OLED tile. Finally, the control

means are modified using the electric optical properties for compensating the variation of the electric optical properties on the uniformity of the electrical properties of the OLED tiles. This method has the advantage of being able to make one or more optical properties of OLED lighting source more uniform. Even within a single manufacturing run there can be variations in the electro -optical properties. When OLED tiles are in close proximity, it is very easy for the human eye to detect differences in such properties as the brightness or the color. By modifying the control means, the differences within the electro -optical properties of the individual OLED tiles can be compensated for. The method of compensating depends upon the optical property or optical properties that are being measured. If the luminance or brightness is being measured, very typically the luminance is proportional to the applied current. In this case only one pair of optical and electrical measurements is needed. However, if different types of OLED devices are being used, for instance one where the color depends upon the current and the brightness also depends upon the current, then a more complicated function of the optical property depending on the electrical power may need to be measured. The measurements needed to compensate for multi layered OLED devices can also be very complicated.

The control means can be implemented in a variety of ways. It can be a simple analogue circuit, or it could be a more complicated computer or microprocessor controlled system.

In another embodiment, one of the optical properties that is measured is the luminance. This is an advantage because the luminance is the brightness that is perceived. The human eye is very sensitive to changes of brightness particularly when you have large tiles adjacent to each other. It is very easy for the human eye to detect differences in the luminance. The advantage is that if one of the optical properties is the luminance, then the method can compensate for differences in the luminance between the different OLED tiles and the lamp will have a much more uniform appearance to the human eye.

In another embodiment one of the optical properties is the color. It is an advantage to compensate for the color, because for lamps one of the features that the human eye notices is also the color. If two OLED tiles are adjacent to each other and they have different spectral or color properties this would be very easy to notice.

Compensating for variations in color gives a tiled OLED lighting source a much more uniform and pleasing appearance.

In another embodiment, the electrical power to each of the OLED tiles is controlled by adjusting either the current or the voltage applied to each of the OLED tiles. Diodes have a current-voltage characteristic. It is customary to talk about using the current to control a diode or an OLED type device, but an equivalent way is also to control the voltage. An advantage of controlling the current or the voltage applied to each of the OLED tiles is that this can be compared to measured optical properties and a functional relationship between the optical property or properties can be constructed. This allows the compensation for the variation of the optical properties.

In another embodiment the electrical power to each of the OLED tiles is controlled by repeatedly switching on and off the OLED tiles. The switching of the OLED tiles is performed faster than the persistence of vision. This is an advantage, because when they are switched faster than the persistence of vision, then this can be used to reduce the perceived brightness of the OLED tile. It would be possible to adjust the brightness by just adjusting the current, but some OLED tiles have a variation in color depending upon the current also. By adjusting the duty cycle of the power applied to the OLED tile this allows the brightness and then the color could possibly be controlled by controlling the current. This scheme is also advantageous, because only a single, constant current source is needed. This is more cost effective than having multiple current or voltage sources which are adjustable.

In another aspect the invention provides for an electronic device which is used for powering a tiled OLED lighting device. The OLED lighting device comprises at least two OLED tiles. The electronic device comprises a power providing means adapted for providing electrical power to each of the OLED tiles and a power providing means comprising a control means for adjusting the electrical power. This is used for compensating the effect of the variation of at least one electro -optical property of the OLED tiles. This is useful for improving the uniformity of at least one optical property of the OLED lighting device. This electronic device has the advantage that it has a means for compensating for the variation of the electrical optical properties of the OLED tiles. This is advantageous, because compensating for the electro -optical properties of the OLED device allows for the measured or perceived optical property of the OLED

lighting device to be made more uniform.

In another embodiment, the optical properties compensated for by the electronic device compensates are the luminance and/or the color. Compensating for the luminance is an advantage, because this is equivalent to what we perceive as brightness. When lamps are adjacent to each other differences in brightness can be very disturbing or easily noticed by people. Compensating for this makes the lamp more aesthetically pleasing. It is also the same for the color or the spectral composition of a light coming from the lamp. The human eye will very easily notice small variations in the color of the individual OLED tiles which make up the OLED lighting device. In another aspect the power providing means comprises a switching means adapted for switching the OLED tile on and off repeatedly at a frequency faster than the persistence of vision. The advantage of switching the OLED tiles on and off faster than the persistence of vision has already been discussed. The power providing means is adapted for switching the power to each OLED tile at an independent frequency and/or independent duty cycle. The advantage to this is that the perceived brightness is able to be changed. When the duty cycle is altered, the fraction of the time that the OLED tile is lit changes.

In another embodiment the electronic device has a power providing means which comprises a set of power converters adapted for powering the OLED tiles. There is a power converter for each of the OLED tiles, and the power converters are adapted for either adjusting the current or the voltage applied to each OLED tile. This is advantageous, because this allows compensating for the electro-optical properties of the OLED tiles. The advantage of this has been mentioned previously.

In another embodiment the electronic devices control means is an analogue electronic circuit. For many OLED devices, the optical property electro control is the brightness or the luminance. For many of these devices the luminance is simply proportional to the current. This means that the method of compensating for the variation of this electro -optical property is relatively simple. This could be accomplished by an operator adjusting something as simple as a trimpot on the power supply. Analogue circuits can also be used for compensating for more complicated OLED devices such as devices that also have a control for the color.

In another embodiment the electronic device has a control means which

comprises a logic circuit. The electronic devices further comprises a lookup table. The lookup table is adapted for providing values usable by the control means for compensating for the variation of the electro -optical properties of the OLED tiles. This can be implemented with a microcontroller or it can also be implemented with a computer. Using a lookup table is very advantageous, because the electro -optical properties of the individual OLED tiles can be measured in advance, and then can be used to construct a lookup table which is used to compensate for variations of these electro -optical properties. When the lamp is then operated, the optical properties of interest can be made uniform. Using a lookup table allows for very complicated compensation schemes. If one is going to compensate for both the brightness and the color of an OLED tile, the lookup table can contain information about the necessary current, the necessary duty cycle of the power applied to the OLED tile, or both. In another embodiment the control means is a logic circuit and the electronic device further comprises a trained software module. The trained software module provides values usable by the control mean to compensate for the variation of the electro -optical properties of the OLED tiles. The trained software module can be implemented with a neural network. Other possible examples of trainable software modules are modules constructed using one of the following techniques: fuzzy logic, Bayesian analysis, principal component analysis, perception learning algorithm, Linear discriminant analysis, regression analysis, ANOVA, factor analysis, and associative memory regression analysis. The relationship between the current and the color or other optical properties of interest can be quite complicated. A trainable software module would allow for these complicated relationships to be controlled in an efficient manner. This is also an advantage, because the manufacturer can measure the electro -optical properties of the OLED tiles and use this to train the software module and simply install the software module in the logic circuit. The trainable software module can be connected with a control system that is operable for adjusting the electrical power to the OLED tiles. This can be used for training the software module and it could also be used in conjunction with the control system to adjust the optical properties when the OLED lighting system is in operation.

In another embodiment, the electronic device is further comprised of a selection means for selecting at least one desired optical value for the optical properties

of OLED lighting source. Additionally the control means is adapted for minimizing the variation of the OLED tiles from the desired optical values. This is an advantage because this allows the lighting source to be dimmed, brightened, or to have a change in the color of the lights. In general, adding a selection means allows the adjustment of the optical properties of the light.

In another aspect the invention provides for a tiled OLED lighting source comprising two or more OLED tiles and the electronic device that was previously described when the two or more OLED tiles are connected to the power providing means. This arrangement is advantageous, because this is essentially an OLED lamp, comprised of OLED tiles and electronics which are able to compensate for the variations of the electro -optical properties of the individual OLED tiles.

In another aspect the invention provides for an OLED lighting kit comprising two or more OLED tiles and the electronic device that was previously described. This is an advantage because the OLED lighting source as described previously may not be constructed when it is delivered to the customer. For example very large OLED tiles could be used to replace ceiling tiles in a room and then the ceiling tiles could be used for lighting. The OLED tiles would be installed in the ceiling first, and then they would be connected to the electronics.

BRIEF DISCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

Fig. 1 Flow diagram of the method presented in claim 1,

Fig. 2 Diagram showing an embodiment of a tiled OLED lighting device,

Fig. 3 Flow diagram for an embodiment of a method improving the uniformity the luminous intensity by, modulating the current to the OLED tiles,

Fig. 4 schematic diagram of an embodiment of a tiled OLED lighting source which modulates the current to each

OLED tile,

Fig. 5 schematic diagram of an embodiment of a tiled OLED lighting source which controls the amplitude of the electrical power to each OLED tile, Fig. 6 idealized schematic diagram of an embodiment of a tiled

OLED lighting source which modulates the current to each OLED tile.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows an embodiment of a method for improving uniformity of at least one optical property of a tiled OLED lighting source. The method consists of applying electrical power to OLED tiles 100, measuring at least one optical property of each OLED tile 102 and then finally modifying the control means 104. The optical properties of the OLED tiles are measured as a function of voltage and/or current applied to the OLED tiles. This is used to determine the electro -optical properties of each of the OLED tiles. Once the electro -optical properties of the OLED tiles have been determined, they can be used to modify the control means. The electro -optical properties of the OLED tiles are intrinsic properties of the tiles, whereas the optical properties of the tiles change as the current or voltage through a particular tile changes. For example, the brightness typically increases as the current is increased through an OLED tile. Modifying the control means based on the measured electro -optical properties enables the optical property of the lighting source to become more uniform.

Figure 2 shows an embodiment of an exemplary tiled OLED lighting device 200. The OLED lighting device comprises a frame 202 and OLED tiles 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4 which are arranged in a rectangular pattern. A tiled OLED lighting source comprises a tiled OLED lighting device 200 and driving electronics adapted powering the OLED tiles 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4. The OLED tiles are labeled 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, and 3.4. The number before the decimal point indicates the row and after the decimal point indicates the column. These can also be used as coordinates in an array which is used to represent the OLED tile 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2,

3.3, 3.4. Optical properties can be physically measured and then placed into rows and columns of an array and into matrix form. The tiled OLED lighting device 200 also shows electrical connections 204.

Arranging the measured optical properties in a matrix enables the calculation of factors for correcting for variations in the electro -optical properties. As an example, given an tiled OLED device 200 constructed of 3x4 OLED tiles 1.1, 1.2, 1.3,

1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4 (as is shown in figure 2) and assuming a varying current efficacy cb _n , _m for the individual tiles (n,m) . The efficacies are represented in a matrix cb of size S=Nx M, with N=3, M=A, £=12:

The general equation for the averaged current efficacy cbav is

N M cbav :=

N-M y i- L y* * ti. m a = i m = 1

Applied to the example gives an averaged efficacy level of

cbav = 42 1

A

Assuming a desired luminous intensity of LI=400cd with an averaged efficacy of 42. led/ A the required driving current is Iav=400cd/(42.1cd/A)=9.5A. The resulting luminous intensity per tile is then LItile _K>m =Iav*cb _K>m :

To minimize the brightness variation the current I _{κ m} for every single is adapted to the tile efficacy :

I _K , _m =Iav*cbav/cb _K , _m , so that the matrix of driving current becomes

10 10.5 ".3 10 \

I = 9.S S.9 9 3 9.8 A I 9.3 10.5 9.5 9,8 /

The resulting local OLED tile 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4 brightness is then of course constant: Bopt _κ>m =cd _κ , _m * I _κ , _m , ; The result is a homogenized OLED lighting source.

The technical realization requires the measurement of the OLED tile 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4 efficacy and the control of the average OLED tile 1.1, 1.2, 1.3, 1.4, 2.1, 2.2, 2.3, 2.4, 3.1, 3.2, 3.3, 3.4 current. The tile efficacy can be measured repeatedly during operation of the lighting device e.g. via photo (flux) sensor or alternatively during assembly of the lighting device at the manufacturers site. The measured tile efficacy is stored in a Look Up Table (LUT) if it is measured once or continuously monitored so that it can be used to modify the average driving current. The procedure explained above is summarized in figure 3.

Figure 3 shows a method for modulating the current to OLED tiles for improving the uniformity of at least one optical property. The method consists of using stored or measured current efficiency measurements 300 for each tile. Using a reference value 302 for the desired luminous intensity these two values are then used to calculate 304 a reference value for driving the current. The reference values for driving the current are then stored 306 and the values for the driving current Irf _n 308 are placed into memory. To operate the lamps an initial value for Irf _n 316 is used. This is used to start the modulator 314 and then the current to each of the lamps is modulated 312 by the controller and while the lamp is operating values from the memory 308 are read from memory 310 and used to adjust the modulation rate 312 or duty cycle for controlling the current through the OLED tiles. In order to realize the method shown in figure 3, a current modulator is required which adapts the individual tile current to the calculated reference value. An example is shown in Figure 4.

Figure 4 shows an embodiment of a tiled OLED lighting source with an arbitrary number of OLED tiles 408, 410, 412. The OLED lighting source consists of a power source 400 which is used to power a power converter 402. The power converter

402 then delivers power to both the tiled OLED device 406 and the controller 404. The tiled OLED device consists of N different OLED tiles 408, 410, 412. OLED tile 1 408 is shown, OLED tile 2 410 is shown and the last OLED tile number N 412 is shown. This diagram represents a circuit for an arbitrary number of OLED tiles 408, 410, 412. The OLED tiles 408, 410, 412 are connected in series with lines for connecting to Field Effect Transistor (FET) 426, 428, 430 switches.

The controller 404 comprises a reference value 414 and a lookup table (LUT) 416 that are connected to the modulator control 418. The modulator control is connected to the power converter 402. The reference value 414 is a value which is given to the modulator control 418 that is used to set desired optical properties of the tiled OLED device 406. The LUT 416 contains values that are used by the modulator control 418 for the purpose of properly controlling the power delivered to each OLED tile 408,410, 412. The modulator control 418 is connected to JV different switch control units 420, 422, 424. The switch control units 420, 422, 424 are used to control FETs 426, 428, 430. There is a switch control unit 420, 422, 424 and a FET 426, 428, 430 corresponding to each OLED tile 408,410, 412. The FET 426, 428, 430 is connected in parallel with its OLED tile 408,410, 412. FET 1 426 is connected in parallel with OLED tile 1 408. FET 2 428 is connected in parallel with OLED tile 410 and so on. The last FET N 430 is connected in parallel with OLED tile number N 412. Each of the FET 426, 428, 430 and OLED tile 408,410, 412 pairs are connected to each other in series. When one of the FETs is activated, the current flows through the FET 426, 428, 430 and not through the OLED tile 408,410, 412. As a result the FETs 426, 428, 430 can be used to switch the OLED tiles 408,410, 412 on and off. The modulator control is able to control the apparent brightness by switching the OLED tiles 408,410, 412 rapidly on and off. The circuit and control arrangement described in this embodiment has the advantage that a single current source is used and then the switches are simply used to modulate whether the OLED tile 408,410, 412 is on or off at a certain time. The brightness is controlled by modulating the duty cycle of OLED tile 408,410, 412 at a rate faster than the persistence of vision. The system consists of a power source 400 delivering a constant average current to a tiled OLED device 406 where all tiles are connected in series. In parallel to each OLED tile 408,410, 412 bypass elements (FET switches) 426, 428, 430 are

arranged so that by closing the bypass element 426, 428, 430 the average current of the respecting tile can be changed. The current flowing through each OLED tile 408,410, 412 is proportional to a duty cycle dn, where dn=100% corresponds to a maximum current flow, i.e. the bypass element 426, 428, 430 is always open, while dn=0% corresponds to a bypass element 426, 428, 430 is always closed, so that no current is flowing through an arbitrary OLED tile 408,410, 412 labeled n. By varying the duty cycle dn for each OLED tile 408,410, 412 n, for a given switching period T _n , the average current flowing through each OLED tile 408,410, 412 can be changed. The effective current is Iavn=Iav*dn/100 where lav is driving current delivered by the power source. The driving current itself is determined by a reference value 414 which is either built in by design or adjustable e.g. via a control or lighting network interface.

Figure 5 shows a different embodiment of the invention. In this embodiment, the OLED tiles 510, 512, 514 are controlled by independent power converters 502, 504, 506. A power converter 502, 504, 506 is defined herein as a controller adapted for regulating the current or voltage applied to an OLED tile 510, 512, 514, cyclically switching the electrical power applied to the OLED tile 510, 512, 514 on and off, or both. There is a main power source 500 which supplies power converters 502, 504, 506 to an arbitrary number of JV different power converters 502, 504 and 506. Only power converter 1 502, power converter 2 504 and the last power converter number N 506 are shown in this diagram. There is a tiled OLED device 508 which comprises an arbitrary number of JV different OLED tiles 510, 512, 514. Power converter 1 502 is connected to OLED tile 1 510, power converter 2 504 is connected to OLED tile 2 512 and the last power converter, power converter N 506 is connected to OLED tile number N 514. There is a master controller 516 which is used for controlling each of the power converters 502, 504, 506. The master controller 516 uses a reference value 518 with a lookup table 520 to calculate the proper power that should be delivered to each OLED tile 510, 512, 514. The master controller 516 is connected to each of the power converters 502, 504, 506 by a communication line 522. The communication line 522 allows the master controller 516 to control the electrical power applied to each OLED tile 510, 512, 514. The communication line 522 can be either analogue or digital. Figure 6 shows an embodiment of a generalization of the OLED lighting source architecture which modulates the current to control the brightness of OLED tiles

610, 612, 614. This embodiment consists of a power source 600 which provides a constant current. This is connected to a power bus 602. The power bus 602 is connected to TV different current modulators 604, 606, 608, current modulator 1 604, current modulator 2 606 and current modulator N 608. The power bus 602 is connected such that each current modulator 604, 606, 608 receives the same current. This can be implemented by connecting the current modulators 604, 606, 608 in a series circuit. There is an OLED tile 510, 512, 514 corresponding to each of the current modulators 604, 606, 608. Current modulator 1 604 is connected to OLED tile 1 610, current modulator 2 606 is connected to OLED tile 2 612, current modulator 608 is connected to OLED tile N 614. To control the duty cycle, there are N different reference values 616. The reference values 616 are communicated with each of the current modulator over N different communication lines 618. Again, these communication lines 618 can be implemented as either analogue lines or as digital signals. For a digital signal the communication can be over N different communication lines 618 or it can be multiplexed on one or more communication lines 618.

A power supply architecture which allows controlling the tile currents individually is shown in figure 5. Here a master slave concept is applied, where the slave power converters 502, 504, 506 generate the individual tile currents while the master controller 516 calculates and communicates the reference values 518 to the slave power converters 502, 504, 506. The power supply architectures of figures 4 and 5 can be generalized as shown in figure 6 comprising:

• A power source 400, 402, 500, 600

• A set of current modulators 426, 428, 430, 502, 504, 506, 604, 606, 608

• A set of current reference values 414, 518, 616 The current modulators 426, 428, 430, 502, 504, 506, 604, 606, 608 adjust the current delivered to the individual OLED tiles 408, 410, 412, 510, 512, 514, 610, 612, 614 according to a set of reference values 302, 414, 518, 616 which are calculated according to the method of figure 3. The reference values 302, 414, 518, 616, can be either stored in memory 308, stored as a LUT 416, 520, implemented in hardware, and/or calculated continuously. The reference values can be implemented in hardware by using a trimmable resistance.

LIST OF REFERENCE NUMERALS: