The present invention relates to a method of operating a LED based light source, i.e. a light source which comprises at least one LED. Conventionally, such a light source is operated by supplying power to the light source whenever illumination is required. The level of power that is supplied is normally constant, unless dimming of the light source is required, in which case less power will be supplied or the power will be intermittently applied by a pulse width modulation or equivalent dimming strategy .

A problem of LED based light sources is that the amount of light produced by the

LEDs in the light source decreases over time, while at the same time the reduction in light emission is converted to an increasing amount heat production in the LED. This phenomenon is known as lumen depreciation. For most applications, it is important to be able to rely on a certain level of light output of the light source over a prolonged period of time. This is expressed in terms of lumen maintenance.

Lumen maintenance compares the amount of light produced from a light source when it is brand new to the amount of light output at a specific time in the future. For instance, if a light source produced 1000 lumens of light when it was brand new and now produces 700 lumens of light after 30,000 hours, then it would have lumen maintenance of 70% at 30,000 hours. Lumen maintenance is often specified as L50, L70, L80, or L90. In each case, L stands for lumen maintenance and the number is the percentage of light output remaining.

The appropriate lumen maintenance target is usually based on the application and the requirements set forth by users of the light source. Since the human eye generally cannot detect a change in light output until there has been 30% depreciation, L70 is often established as the target for an application. With state of the art LEDs, it generally takes tens of thousands of hours before the light source will lose 30% of its initial light output.

Lumen depreciation can be depicted in a logarithmic graph which will typically resemble the example shown in Figure 1. The relative intensity of the light source over time is represented by a downwardly sloping curve R. In this example the lumen maintenance target is shown as L80, corresponding with a 20% depreciation over time. In the illustrated example this level is maintained up to an operating time of about 80,000 hours.

However, when a user wishes to have a light source with a guaranteed minimum level of lumen maintenance over time, the power supply must be designed for the nominal light output of 100%. This has for its result that up to 20% excess power is supplied to the light source over its entire lifetime, leading to a substantial waste of energy. The area between the curve R and the horizontal line representing 100% relative intensity represents the amount of power that is unnecessarily converter into light just in order to maintain a minimum specified lumen output at the end-of-life criterion of a fitting design.

The invention has for its object to provide a method of operating a LED based light source more efficiently. In accordance with the invention, this is achieved by supplying power to the light source whenever illumination is required, wherein a condition of the at least one LED is monitored and an amount of power supplied to the light source is controlled on the basis of the monitored condition. The invention is based on the insight that instead of supplying a constant overdose of power over the lifetime of the light source in order to guarantee a certain level of performance at the end of the lifetime, it is much more efficient to supply exactly the required amount of power, based on condition monitoring. A further advantage of the invention is that especially in the first part of their lifetime the temperature of the LEDs will remain lower, since no excess power is supplied then. This lower temperature - in comparison to the conventional method - results in reduced pace of aging and a therefore in a longer total lifetime.

Preferably, the monitored condition is a light emitting efficiency of the at least one LED, and the amount of power supplied to the light source is controlled such as to maintain a total amount of light output by the light source at a substantially constant level for a predetermined period of time. This leads to very efficient operation of the light source. In that connection, the light emitting efficiency of the LEDs is defined as the amount of light that is output when a specified amount of power is supplied (expressed in lm/Watt).

When the light emitting efficiency decreases with increasing age of the at least one

LED, the amount of power supplied to the light source is preferably increased over time to compensate for the decreasing efficiency. This increase in power, especially towards the end of the lifetime of the LEDs, is less than the power that is wasted by the conventional technique of supplying a constant excess power over the lifetime of the LEDs to maintain their lumen at a specified level at end of the lifetime. Moreover, the continuous improvement in the efficiency of LEDs means that LED drivers, i.e. power supplies, will generally be somewhat over-dimensioned by the reduced need for power to achieve the same level of lumen output, so that existing drivers can indeed supply the increased power that is eventually needed to compensate for the lumen depreciation of its attached LED.

The amount of power supplied may easily be controlled by controlling a current supplied to the at least one LED.

In order to avoid wasting energy on LEDs that have reached the end of their useful life, operation of the light source may be discontinued when a predetermined boundary condition is met. Such a boundary condition may e.g. be a maximum amount of power that is available for the light source or a maximum allowable temperature of the LEDs. In order to facilitate the creation of a maintenance and replacement schedule, a remaining lifetime of the light source may be predicted on the basis of the monitored condition.

The invention further has for its object to provide a lighting device including a LED based light source which is more efficient than conventional lighting devices. This is accomplished in accordance with the invention by a lighting device comprising a light source including at least one LED, a power supply and a controller, wherein the controller is adapted to monitor a condition of the at least one LED and to control the power supply on the basis of the monitored condition.

In a preferred embodiment of the lighting device, the controller is adapted to monitor a light emitting efficiency of the at least one LED, and to control the power supply such as to maintain a total amount of light output by the light source at a substantially constant level.

When the light emitting efficiency decreases with increasing age of the at least one LED, the controller is preferably adapted to control the power supply such as to increase over time the amount of power supplied to the light source.

Advantageously, the controller may be adapted to control a current supplied by the power supply.

In order to provide sufficient scope for increasing the amount of power supplied over time, the power supply preferably has a nominal power output which is greater than a nominal power consumption of the light source.

In a preferred embodiment of the lighting device, the controller is adapted to generate a warning or to tune down the power supply when a predetermined boundary condition is met. In this way a user is provided with a clear indication that the light source has reached the end of its useful life.

The controller may advantageously be adapted to predict a remaining lifetime of the light source on the basis of the monitored condition.

In a preferred embodiment, the controller may be adapted to monitor a duration of use of the at least one LED and to compare this to a predetermined maximum duration of use to predict the remaining lifetime of the light source.

In order to arrive at a realistic prediction of the remaining lifetime, the controller may be adapted to convert the monitored duration of use to an equivalent duration of use of the at least one LED under full load so as to allow the remaining lifetime to be predicted on the basis of standard LED ageing data.

In a preferred embodiment of the lighting device, the controller may be adapted to determine a temperature of the light source on the basis of a measurement of a temperature of the power supply. The temperature of the light source is an important indicator for its condition, and measuring the temperature of the power supply is relatively straightforward. In yet another embodiment, the lighting device may further comprise input means and output means, and the controller may be arranged to receive information regarding ageing predictions of the at least one LED and/or to send information regarding the monitored condition of the at least one LED. In this way communication with the lighting device may be established. Such communication may be either wireless or through dedicated wires, e.g. similar to the so- called DALI system which is used in maintenance of lighting systems in large buildings.

In order to provide optimum control of the lighting device, the light source may include a plurality of LEDs connected in a circuit comprising a plurality of parallel strings, each said string including at least one LED and at least one trimmable element, wherein a characteristic value of at least some of the trimmable elements may have been individually adjusted to equalize currents flowing through the strings.

The present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.

Figure 1 illustrates a conventional method of operating a LED-based light source. Figure 2 illustrates a method of operating a LED-based light source in accordance with the invention.

Figure 3 shows a first embodiment of a lighting device in accordance with the invention.

Figure 4 shows a driver for a lighting device in accordance with a second embodiment of the invention.

Figure 5 illustrates how the lumen output of the light source is tuned down at the end of its lifetime.

One way of carrying out the method of the invention is illustrated in Figure 2. The downwardly sloping curve R again represents the theoretic value of the relative intensity of the LEDs as a function of operating time. This curve is based on statistics provided by a LED manufacturer. The dots A on the downward sloping curve represent measurements of the actual relative intensity, which are performed as part of the condition monitoring that is an important feature of the present invention. Based on the actually measured values and on stored information regarding the curve R, a determination can be made of the point on the curve R which represents the current condition of the LEDs. This determination also allows a prediction to be made of the remaining lifetime of the LED based light source, again using the stored information about the statistical course and length of the curve.

Alternatively or additionally, the determination can be made on the basis of the duration of use, measured in hours that the LEDs emit light. In that case a measurement of elapsed time should be compensated for factors which may influence the operational life of the light source. For instance, the standard ageing curve of a LED is based on measurements of LEDs in labs when operating at full load under controlled conditions. However, when a LED in actual operation is dimmed, its temperature will drop and therefore it will age more slowly, so its movement along the curve is not as fast as predicted. On the other hand, actual operating conditions for the LED may be more adverse than the controlled conditions in the lab, which will result in increased lumen depreciation, i.e. faster movement down the curve. After having been corrected for these influences, the measured time and the position on the curve can be stored.

By mirroring the curve with respect to a horizontal line representing the required lumen maintenance level, it can be determined how much additional power has to be supplied in order to compensate for the lumen depreciation. This additional power that is required will increase with age of the LEDs. The total amount of additional power is represented by the area between the upwardly sloping mirrored curve M and the horizontal line representing the nominally required relative intensity of the light. This area is much smaller than the area between the 80% and 100% lines shown in Figure 1 , which represents the total excess power supplied constantly over the lifetime of the light source in the conventional method.

Eventually, a point will be reached where it is no longer possible to maintain the lumen of the light source at the required level. This is because a predetermined boundary condition will be met or exceeded.

For instance, the power rating of the driver may be insufficient to further increase the power supplied to the light source to keep compensating for the lumen depreciation. This boundary condition is illustrated by the dashed line B at the top of Figure 2. Alternatively, it may be judged that the efforts needed for lumen maintenance are no longer economically viable, for instance in view of the rising energy cost or because new and improved light sources are available. It is also conceivable that the supply of additional power may result in the DC current supplied to the LEDs reaching values that exceed the absolute maximum ratings of DC power into the LEDs. Under those circumstances lumen maintenance will be discontinued. And finally, a boundary condition may be imposed by the temperature of the LEDs or the driver, which can be measured as a junction or solder temperature. When the LEDs become too hot, they will age and deteriorate more rapidly.

When the boundary condition is met and compensation is discontinued, the lumen output of the light source will normally decrease according to a curve R' that follows the original depreciation curve R. Alternatively, a warning signal may be generated to indicate the end of the lifetime of the light source. For instance, the amount of power supplied to the light source may be reduced so that its output will fall to a much lower level, e.g. 10% of the initial intensity, as shown by line L in Figure 5. Such a low intensity is a clear visual indication that the light source should be replaced, but is still sufficiently high for the light source to provide basic illumination. The above method can be performed in a lighting device which includes one or more LEDs 1 , 1 ' , ... , l ⁿ , a power supply 7 and a controller 8. The LEDs 1 , 1 ' , ... , l ⁿ form part of a light source 2, while the power supply 7 and the controller 8 together form a driver 3 (Figure 3). The LEDs 1 , 1 ' , ... , l ⁿ in the light source 2 are arranged in a plurality of parallel strings 4, 4' , ... , 4 ^k . Each string 4, 4' , ... 4 ^k includes a plurality of LEDs 1 , 1 ' , ... , l ⁿ arranged in series, as well as a trimmable elements 13, 13', ..., 13 ^k connected in series with the LEDS 1— l ⁿ at the negative end 6 of the string 4, 4' , ... , 4 ^k . The trimmable element 13 may be a suitable resistor, e.g. a surface mounted device chip resistor, either thick film or thin film, which may be trimmed using a laser trimming machine. Trimming of the trimmable resistor elements 13 is done in the way described in the applicant's co-pending European patent application EP 2 699 056 A2. This results in strings 4 of LEDs 1 - 1° having properties which are known and predictable to a high degree. In this manner the DC current per LED 1— l ⁿ is substantially constant, or the spread in DC current values is very narrow, e.g. in the order of 1 percent or less. The temperatures of all the LEDs 1 - 1° will also be maintained within a narrow range of e.g. 1°C Also the light emitting efficiency of the actively trimmed LEDs 1— l ⁿ is known within narrow margins, so that the controller may precisely determine the lifetime and lumen depreciation of the light source 2.

The controller 8 may be adapted to control a current I supplied by the power supply 7 in such manner that the total amount of light or lumen output by the light source 2 is kept substantially constant. As an additional function, the controller 8 may be adapted to predict the remaining lifetime of the light source 2 on the basis of the monitored condition. This can be done on the basis of the temperature measurement, which indicates the current position of the light source 2 on the depreciation curve R. The depreciation curve R may be stored in a memory 11 which may be read by the controller 8 and in which the controller 8 may store additional data.

A particularly simple form of control is achieved, when the controller is adapted to determine the temperature T _LED of the light source on the basis of a measurement of the temperature T _DRIVER of the power supply 7 or driver 3. To this end a temperature sensor 9 is arranged in the driver 3, which provides temperature data to the controller 8 (Figure 4). This indirect measurement of T _LED is possible because the configuration of the lighting device, i.e. the position of the LEDs 1 - l ⁿ with respect to the driver is known, as well as the nominal temperature of the LEDs 1 - 1° and the driver 3. By relating these temperatures to the ambient temperature T _AMB> which is measured by an external temperature sensor 17, the controller 8 can derive the temperature T _LED of the LEDs 1 - l ⁿ by measuring the temperature T _DRIVER of the driver 3 and the ambient temperature T _AMB . This derived value of the LED temperature T _LED can then be used to determine both the power that must be supplied to maintain the lumen at the required level and the remaining lifetime of the light source 2. The controller 8 will keep a record of the number of operating hours. The hours are counted by a clock 12 and the record is stored in the memory 11. The number of operating hours may be corrected for dimming level (since dimming will lead to reduced LED temperatures and prolong the lifetime), ambient temperature T _AMB , driver temperature T _DRRVER (and LED temperature T _LED derived therefrom), and output current and voltage. This later parameter is measured by a power meter 14 in the driver 3. Information about the actual level of illumination provided by the light source 2 is supplied by an external light sensor 16 , which is also connected to the controller 8. In this way the controller 8 is always able to determine the current position of the light source 2 on the depreciation curve R and the remaining lifetime.

The driver 3 further includes input and output means 15 which allow the controller

8 to communicate with a user. The I/O means 15 may include a transmitter and a receiver, which may be arranged for wireless transmission or for transmission via a data line 19. Through the I/O means 15 the controller may output data regarding the use and condition of the light source 2 and its remaining life time. On the other hand, data may be send to the controller 8, e.g. if continued testing of similar LEDs has led to a revision of the depreciation curve R. Such a revised curve may then be uploaded to the driver 3 and stored in the memory 11.

Although the driver 3 may include all the additional functions described above, it is also conceivable that a conventional lighting device, which includes a classic driver, is retrofitted with additional electronics to perform the method of the invention. These additional electronics may then be arranged in a separate control module 18, which may control the conventional driver 3 in a master-slave arrangement as shown in Figure 3.

Although the inventive method can be practiced with any LED based light source, it will produce particularly good results if the properties of the LEDs are known and predictable to a high degree. In the illustrated embodiment this is done by using LED strings which have been actively trimmed. However, other ways of providing substantially identical LEDs, e.g. through binning within narrow boundaries, may also be considered.

The method and apparatus described above allow a light source to be operated much more efficiently than would be possible when using the conventional method of supplying a constant excess amount of power. Moreover, the lifetime of the light source may be prolonged by the lower operating temperatures of the LEDs which are the result of the lower power supply rating.

Although the invention has been illustrated by way of an example, it will be clear that it is not limited thereto. The scope of the invention is defined solely by the following claims.