CONNECTIONS

FIELD OF INVENTION

The field of the invention relates to a luminaire system and method for gauging the reliability of connections, and in particular connections interconnecting a plurality of LEDs.

BACKGROUND

Typically, electrical circuits in luminaires, especially outdoor luminaires, go through numerous thermal cycles depending on the daily cycle of night and day, the consecutive lighting sequences, and/or other causes having internal and external origins. These electrical circuits are generally made of different materials having different thermal parameters. Thus, thermal stress is applied at the boundaries between these different materials, causing aging of the connections between the different electrical components comprised in the electrical circuits, and potentially leading to a failure of the electrical circuits. According to existing methods, in order to analyze this type of aging, the connections, such as solder joints, have to be analyzed e.g. by cutting the solder joint and taking microscopic images of the solder joint. Such methods require the luminaire to be opened and damaged and cannot be used in practice. Therefore, a system is needed to prevent and/or predict such failures of the electrical circuits in a luminaire caused by the repeated thermal cycles.

It is further noted that generally it is difficult to predict the lifetime of the connections of a luminaire as the aging will be largely dependent on its use which is often not known in advance. Indeed, the operation of the luminaire may be dependent on sensed data, user settings, etc. It is known to provide an electrical circuit with a dedicated component failing upon thermal stress representative of the thermal stress applied to the electrical circuit. However, such an electrical circuit has a chance of failing at the same time as the dedicated component, which is an unwanted situation in a luminaire which is required to perform an uninterrupted service once installed.

Hence, there is a need for a luminaire system and method capable of gauging the reliability of its electrical connections in order to prevent failure and/or predict aging.

SUMMARY

The object of embodiments of the invention is to provide a luminaire system and method capable of gauging the reliability or“health” of the connections used to interconnect a plurality of LEDs of a luminaire system in a simple manner which can be easily implemented and does not require a physical analysis of the connections.

According to a first aspect of the invention there is provided a luminaire system including a plurality of LEDs interconnected via connections. The luminaire system comprises a voltage measurement unit and a gauging means. The voltage measurement unit is configured for measuring a first voltage value across at least one interconnected LED of the plurality of interconnected LEDs at a first time within a predetermined time period, preferably within 10 minutes, from a thermal event occurring to the plurality of LEDs. The voltage measurement unit is further configured for measuring a second voltage value across the at least one LED of the plurality of interconnected LEDs at a second time different from the first time, said second time being within the

predetermined time period. The gauging means is configured for gauging the reliability of the connections interconnecting the plurality of LEDs based on the first voltage value and the second voltage value.

Embodiments of the invention are based inter alia on the inventive insight that the voltage across at least one interconnected LED after a thermal event will be influenced by the status of the connections, and in particular by the status of the solder joints. The inventors have discovered that, in particular before reaching steady state, the voltage will vary more or less depending on the status of the connections, and in particular on the status of the solder joints. Connections which have aged will present typically higher deviations from an ideal voltage curve in function of time after occurrence of a thermal event. By measuring at least two voltage values at at least two different moments in time within a predetermined time period, preferably within 10 minutes, from a thermal event occurring to the plurality of LEDs, the reliability or status of the connections can be judged in a simple non-invasive manner.

More particularly, when a constant current is applied during a normal operation of the luminaire system, e.g. the luminaire system is switched on to illuminate its environment, the plurality of LEDs may be subject to a transitory phase during which voltage across the plurality of LEDs rises until reaching a substantially constant voltage corresponding to a thermal steady state. This transitory phase is particularly targeted by the present invention since it is during this phase that differences between thermal properties of materials composing the connections

interconnecting the plurality of LEDs are the most apparent. During this transitory phase, cracks in the connections will cause the voltage across the plurality of LEDs to be chopped, or to present jumps in values, instead of presenting a smooth curve with time. It is the detection and the characteristics of these jumps that will indicate the presence of cracks and allow gauging the reliability of the connections interconnecting the plurality of LEDs. For example, when switching on the luminaire system during normal operation and applying a constant current intensity to the plurality of LEDs, voltage across the plurality of LEDs may ideally smoothly decrease with time from a high value to a low value. So if a voltage value is higher than another voltage value closer in time to the thermal event, it may be an indication that cracks are present in the connections. It is to be noted that equivalent observations on the voltage across the plurality of LEDs may be made during a drop in temperature during normal operation of the luminaire system, e.g. the luminaire system is switched off and a constant current value (zero) is applied to the plurality of LEDs.

It can be noted that the measuring is not limited to only two measurements of voltage values. In an exemplary embodiment, more than two measurements may be performed. Depending on the measurements considered among the performed measurement, the gauging may lead to an incorrect reliability not representative of the connections interconnecting the plurality of LEDs. As such, measuring more than two measurements may increase the accuracy of the gauged reliability by allowing to choose voltage values more representative of the connections interconnecting the plurality of LEDs. Additionally or alternatively, the measuring and gauging may be performed several times within the predetermined period for obtaining comparative values of the gauged reliabilities and selecting the most representative gauged reliability among the gauged reliabilities. In a typical embodiment, the plurality of LEDs are soldered to a Printed Circuit Board, PCB, via a plurality of solder joints, said plurality of solder joints being part of the connections

interconnecting the plurality of LEDs. The PCB may be a metal core PCB, MC PCB. Typically, the LEDs are soldered to the copper paths on the PCB. During its lifetime the PCB is subject to multiple thermal cycles. This causes cracks in the solder joints which are typically a weak point of the connections. In exemplary embodiments, the solder joints are located next to a ceramic portion of the LED chips causing thermal expansions differences resulting in cracks. This problem has increased in recent years as solder joints no longer contain lead, and are as a consequence less flexible. The inventors have discovered that the amount of cracks is correlated to the voltage variations measured across at least one interconnected LED upon occurrence of a thermal event, and typically before steady state is reached.

Preferably, the predetermined time period is within the time period between the time of the thermal event occurring to the plurality of LEDs (ti) and the time when the plurality of LEDs soldered to the PCB has reached a thermal steady state. The thermal steady state may be defined as the state in which the average temperature value of the plurality of LEDs varies by less than 5%, preferably less than 2%, over time. By measuring within such a predetermined time period, it can be guaranteed that the measured voltages will be representative for the status of the connections.

Indeed, as observed above, it is at the inception of the thermal event that the differences in thermal properties between different materials will be the most apparent. It is thus during this transitory phase until thermal steady state is reached that the presence of cracks in the connections will have the highest impact on voltage measurements across the at least one interconnected LED.

Embodiments of the invention will be particularly useful in outdoor or industrial luminaires. Outdoor luminaires are luminaires which are installed on roads, tunnels, industrial plants, stadiums, airports, harbors, rail stations, campuses, parks, cycle paths, pedestrian paths or in pedestrian zones, for example, and which can be used notably for the lighting of an outdoor area, such as roads and residential areas in the public domain, private parking areas and access roads to private building infrastructures, etc. Industrial luminaires can be used e.g. in warehouses, industry halls, etc. In a preferred embodiment, the luminaire system comprises a lighting driver connected to the plurality of LEDs, and the voltage measurement unit and/or the gauging means is included in the lighting driver. The voltage measurement unit may be an extra unit added to the luminaire head for the specific purpose of the present invention, or may already be included in the luminaire head, for example in a lighting driver connected to the plurality of LEDs. By using a voltage measurement unit included in the lighting driver, existing systems can be readily adapted to measure in the predetermined time period and to implement the invention. The gauging means may be included partially or totally in the lighting driver. The gauging means may be composed of one element, or may be distributed over a plurality of elements, locally and/or remotely.

In a preferred embodiment, the plurality of LEDs is mounted in series and comprises at least four LEDs in series, preferably at least eight LEDs in series, more preferably at least sixteen LEDs in series, and preferably the at least one LED comprises at least two LEDs connected in series. In such embodiments, it may be preferred to measure the voltage across at least two, or at least three LEDs, or across all of the LEDs connected in series. In that way, the reliability of the

measurements may be increased. Indeed, when measured across a plurality of LEDs in series, or preferably across all of the plurality of LEDs connected in series, the cumulative consequences of the presence of cracks in the connections interconnecting the plurality of LEDs may have a bigger impact of the voltage measurements than if measured across only one interconnected LED since more and/or larger jumps in voltage values may be measured with time until thermal steady state is reached. So, gauging the reliability of the connections may be rendered easier.

In addition or alternatively, the at least one LED of the plurality of interconnected LEDs comprises at least one most thermally stressed LED among the plurality of interconnected LEDs.

In another exemplary embodiment, the at least one LED of the plurality of interconnected LEDs comprises a representative LED of the plurality of LEDs. The representative LED may be defined as a LED presenting a behavior according to the expected average behavior of a LED subject to similar conditions.

In an exemplary embodiment, the thermal event occurring to the plurality of LEDs is one of: a powering of the plurality of LEDs, a change in the lighting regime of the plurality of LEDs, a change in the cooling regime of the plurality of LEDs whilst the LEDs are in operation, an interruption in the powering of the plurality of LEDs. Especially, the powering of the plurality of LEDs will be a suitable thermal event as this is a common event which takes places in ah luminaires. In other words, the thermal event may correspond to an event causing an increase or a decrease in temperature to the plurality of interconnected LEDs. As such, the increase in temperature may be caused, for example, by the powering on of the plurality of interconnected LEDs, or by a change from a first lighting regime to a second lighting regime demanding a higher light output from the plurality of interconnected LEDs. The decrease in temperature may be caused, for example, by the start of an active cooling of the plurality of LEDs, e.g. the activation of a cooling fan, to dissipate the generated heat.

In an exemplary embodiment, the gauging means is further configured for obtaining a

predetermined voltage value associated to a time within the predetermined time period, and for gauging the reliability of the connections interconnecting the plurality of LEDs further based on the predetermined voltage value. The predetermined voltage value may be a theoretical, or ideal, voltage value. Gauging the reliability of the connections interconnecting the plurality of LEDs based on the predetermined voltage value in addition to the first and second voltage values allows increasing the precision of the gauged reliability. In an exemplary embodiment, the predetermined voltage value may correspond to a voltage value associated to the same time at which the first or second voltage value has been measured. In another exemplary embodiment, the predetermined voltage value may correspond to a voltage value associated to a different time than the first or second time.

In an exemplary embodiment, the gauging means is further configured for: if the first voltage value is at a prior time with respect to the second voltage value, and if the first voltage value is lower than the second voltage value, and if the thermal event causes an increase in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or

if the first voltage value is at a subsequent time with respect to the second voltage value, and if the first voltage value is higher than the second voltage value, and if the thermal event causes an increase in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or

if the first voltage value is at a prior time with respect to the second voltage value, and if the first voltage value is higher than the second voltage value, and if the thermal event causes a decrease in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or if the first voltage value is at a subsequent time with respect to the second voltage value, and if the first voltage value is lower than the second voltage value, and if the thermal event causes a decrease in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system.

The expected result of the comparison between the first and second voltage value may be different whether the triggering thermal event causes an increase or a decrease in temperature of the plurality of interconnected LEDs. In an exemplary embodiment, there is an increase in temperature of the plurality of interconnected LEDs and an anomaly in the luminaire system may be determined by a voltage value higher than another voltage value measured prior in time. In another exemplary embodiment, there is a decrease in temperature of the plurality of interconnected LEDs and an anomaly in the luminaire system may be determined by a voltage value lower than another voltage value measured prior in time.

Additionally, the luminaire system may comprise a temperature measurement unit. The temperature measurement unit may be used to obtain one or more temperature values of a LED of the plurality of LEDs in order to determine whether the thermal event corresponds to an increase or a decrease in temperature to correlate with the anomaly detection based on the comparison between the first and second voltage values.

In other embodiments, the increase or the decrease in temperature may be determined by the gauging means based on a detected type of thermal event. For example, the gauging means may detect a change in the provision of current to the plurality of LEDs or a change in an active cooling unit of the plurality of LEDs.

In an exemplary embodiment, the voltage measurement unit is further configured for measuring a plurality of voltage values, preferably at least four voltage values, more preferably at least ten voltage values, across the at least one LED of the plurality of interconnected LEDs during a monitoring time period, said monitoring time period being within the predetermined time period after the thermal event occurring to the plurality of LEDs. Preferably, the gauging means is then configured for gauging the reliability of the connections interconnecting the plurality of LEDs based on the plurality of voltage values, and in particular differences in the values between the plurality of voltage values measured during the monitoring time period. By increasing the number of the measurements, the reliability of the gauging can be further improved. It is to be noted that the monitoring time period may correspond to the whole time period between the occurrence of the thermal event and the time at which the steady state is reached. In an embodiment, the voltage measurement unit may be configured for measuring the plurality of voltage values repetitively and at regular time intervals during the monitoring time period.

Alternatively or additionally, the voltage measurement unit may be configured for measuring a voltage value of the plurality of voltage values on request.

In an exemplary embodiment, the gauging means is further configured for obtaining at least one other voltage value based on the plurality of voltage values measured during the monitoring time period, and for gauging the reliability of the connections interconnecting the plurality of LEDs further based on the another voltage value. The at least one other voltage value obtained by the gauging means may be one or more of: an average voltage value of the plurality of voltage values measured during the monitoring time period, the highest voltage value among the plurality of voltage values measured during the monitoring time period, the lowest voltage value among the plurality of voltage values measured during the monitoring time period. Using such one or more other voltage values derived from the plurality of voltage values measured during the monitoring time period, may further facilitate the processing and the gauging of the status of the connections.

Additionally, the plurality of voltage values measured during the monitoring time period MT may be processed in various manners in order to improve the accuracy of the gauged reliability of the connections interconnecting the plurality of LEDs. The plurality of voltage values may be processed by frequency analysis, and/or the number of times the slope sign changes between successive measured points may be counted, and/or the range of variations between successively measured voltage values may be statistically analyzed.

For example, the voltage may be measured in a continuous manner during the monitoring time period MT. The measurement may be implemented during a normal operation of the luminaire system across at least one LED of a plurality of LEDs connected in series, preferably across all of the plurality of LEDs connected in series. The values measured may then be processed by applying a second derivative to the voltage values with respect to time. The second derivative may be indicative of changes in the slope of the voltage values curve with respect to time, e.g. changes originating from jumps in voltage values caused by cracks in the solder joints of the plurality of LEDs. Based on this second derivative the reliability of the connections may be gauged. According to an exemplary embodiment, the gauging means is further configured for sending a warning signal, said warning signal being sent upon gauging that the reliability of the connections interconnecting the plurality of LEDs is below a predetermined reliability level. The warning signal may be sent wirelessly or in a wired manner locally or to a remote device. The warning signal may be e.g. any one or more of: a coded message, a luminous signal, a text signal, a sound signal.

According to an exemplary embodiment, the gauging means is further configured for saving the gauged reliability in a memory unit; and, during another subsequent thermal event occurring to the plurality of LEDs, gauging another reliability of the connections interconnecting the plurality of LEDs, obtaining the saved prior gauged reliability from the memory unit, and predicting a failure of the connections interconnecting the plurality of LEDs based on the saved prior gauged reliability and the another gauged reliability. More in particular, the gauging results may be stored on a regular basis, e.g. at least once a month, in order to determine the development of the status of the connections in function of time. In that manner, it will be possible to predict a failure in a more accurate manner. The memory unit may be included in a lighting driver.

According to an exemplary embodiment, the gauging means is further configured for determining a predictive maintenance time based on the saved prior gauged reliability and the another gauged reliability. The gauging means may be configured for sending said predictive maintenance time in the shape of a health indicator illustrating the health of the connections interconnecting the plurality of LEDs. A good health of the connections interconnecting the plurality of LEDs may be interpreted as connections having high reliability and not needing maintenance within a relatively short time. A bad health of the connections interconnecting the plurality of LEDs may be interpreted as connections having low reliability and needing maintenance shortly or immediately. The predictive maintenance time may be further determined based on accumulated reliabilities histories from other similarly implemented connections interconnecting the plurality of LEDs, e.g. plurality of LEDs included in similar luminaire systems, and/or on data sheets of the elements composing the connections interconnecting the plurality of LEDs.

According to an exemplary embodiment, the luminaire system further comprises a temperature measurement unit, wherein the temperature measurement unit is configured for measuring at least one temperature value related to the at least one LED of the plurality of LEDs at a time within the predetermined time period, and wherein the gauging means is further configured for gauging the reliability of the connections interconnecting the plurality of LEDs further based on the at least one temperature value.

This is based on the insight that, upon occurrence of a thermal event, after a certain period of time, temperature in the proximity of the plurality of LEDs may stabilize, and the temperature reaches a thermal steady state. The time at which the thermal steady state of the connections interconnecting the plurality of LEDs is reached may correspond to a stabilized voltage value measured across the at least one LED of the plurality of LEDs by the voltage measurement unit and to a stabilized temperature measured by the temperature measurement unit. Thus, temperature values and voltage values measured may have correlated behaviors: when the temperature value increases, the voltage value should normally decrease. By measuring the temperature, it can be verified that the thermal event has effectively occurred and that the voltage measurements are being performed during a period in which the temperature changes significantly.

According to an exemplary embodiment, the voltage measurement unit and the temperature measurement unit are configured for, simultaneously, measuring a voltage value across the at least one LED of the plurality of LEDs and measuring a temperature value related to the at least one LED of the plurality of LEDs, respectively.

According to an exemplary embodiment, the temperature measurement unit is further configured for measuring a first temperature value at a first time within the predetermined time period and a second temperature value at a second time within the predetermined time period, and for determining the occurrence of the thermal event based on the first temperature value and the second temperature value. The determination of the occurrence of the thermal event may then trigger the measurement of voltage values by the voltage measurement unit.

According to a second aspect of the invention, there is provided a method of gauging a reliability of connections interconnecting a plurality of LEDs included in a luminaire, comprising the steps of:

- measuring a first voltage value across at least one LED of the plurality of interconnected LEDs at a first time; wherein the first time is within a predetermined time period, preferably within 10 minutes, from a thermal event occurring to the plurality of LEDs; - measuring a second voltage value across the at least one LED of the plurality of LEDs at a second time different from the first time, said second time being within the predetermined time period; and

- gauging the reliability of the connections interconnecting the plurality of LEDs based on the first voltage value and the second voltage value.

The technical advantages and preferred features of embodiments of the luminaire system apply mutatis mutandis for the embodiments of the method.

Preferably, the plurality of LEDs are soldered to a PCB via a plurality of solder joints; and the predetermined time period is smaller than the time period needed for the plurality of LEDs soldered to the PCB to reach a thermal steady state after the thermal event occurring to the plurality of LEDs.

According to a preferred embodiment, the plurality of LEDs is mounted in series and comprises at least four LEDs in series, preferably at least eight LEDs in series, more preferably at least sixteen LEDs in series, and wherein the at least one LED comprises at least two LEDs connected in series.

According to an exemplary embodiment, the method further comprises: if the first voltage value is at a prior time with respect to the second voltage value, and if the first voltage value is lower than the second voltage value, and if the thermal event causes an increase in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or

if the first voltage value is at a subsequent time with respect to the second voltage value, and if the first voltage value is higher than the second voltage value, and if the thermal event causes an increase in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or

if the first voltage value is at a prior time with respect to the second voltage value, and if the first voltage value is higher than the second voltage value, and if the thermal event causes a decrease in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system; or if the first voltage value is at a subsequent time with respect to the second voltage value, and if the first voltage value is lower than the second voltage value, and if the thermal event causes a decrease in temperature of the plurality of LEDs, determining a presence of an anomaly in the luminaire system.

According to an exemplary embodiment, the method further comprises measuring a plurality of voltage values, preferably at least four voltage values, more preferably at least ten voltage values, across the at least one LED of the plurality of interconnected LEDs during a monitoring time period, said monitoring time period being within the predetermined time period after the thermal event occurring to the plurality of LEDs. The gauging of the reliability of the connections interconnecting the plurality of LEDs may then be further based on the differences in the values between the plurality of voltage values measured during the monitoring time period.

According to an exemplary embodiment, the method further comprises obtaining at least one other voltage value based on the plurality of voltage values measured during the monitoring time period; and gauging the reliability of the connections interconnecting the plurality of LEDs further based on the at least one other voltage value.

According to an exemplary embodiment, the method further comprises sending a warning signal to a receiving unit, said warning signal being sent upon gauging that the reliability of the connections interconnecting the plurality of LEDs is below a predetermined reliability level.

According to an exemplary embodiment, the method further comprises saving the gauged reliability in a memory unit; and during another subsequent thermal event occurring to the plurality of LEDs, gauging another reliability of the connections interconnecting the plurality of LEDs, obtaining the saved prior gauged reliability from the memory unit, and predicting a failure of the connections interconnecting the plurality of LEDs based on the saved prior gauged reliability and the another gauged reliability.

According to an exemplary embodiment, the method further comprises determining a predictive maintenance time based on the saved prior gauged reliability and the another gauged reliability. According to an exemplary embodiment, the method further comprises measuring at least one temperature value related to the at least one LED of the plurality of LEDs at a time within the predetermined time period; and gauging the reliability of the connections interconnecting the plurality of LEDs further based on the at least one temperature value.

According to an exemplary embodiment, the method further comprises, simultaneously, measuring a voltage value across the at least one LED of the plurality of LEDs and measuring a temperature value related to the at least one LED of the plurality of LEDs, respectively.

According to an exemplary embodiment, the method further comprises measuring a first temperature value at a first time within the predetermined time period and a second temperature value at a second time within the predetermined time period, and determining the occurrence of the thermal event based on the first temperature value and the second temperature value.

BRIEF DESCRIPTION OF THE FIGURES

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing exemplary embodiments of the invention. Like numbers refer to like features throughout the drawings.

Figure 1 illustrates schematically an exemplary embodiment of a luminaire system;

Figure 2 shows a time graph of voltage values as anticipated for a luminaire system;

Figure 3 illustrates schematically another exemplary embodiment of a luminaire system;

Figure 4 shows another time graph of voltage values and a time graph of temperature values as anticipated for a luminaire system.

DESCRIPTION OF EMBODIMENTS Figure 1 illustrates schematically an exemplary embodiment of a luminaire system, preferably an outdoor luminaire system, according to the present invention. The luminaire system includes a plurality of LEDs 11 interconnected via connections, and comprises a voltage measurement unit 20 and a gauging means 30.

The plurality of LEDs 11 may be arranged on a mounting substrate provided to a luminaire head of a luminaire. The mounting substrate may be a printed circuit board (PCB), e.g. a metal- core PCB, with the plurality of LEDs 11 disposed thereon. The mounting substrate may be in thermal contact with a support made of a thermally conductive material, preferably from a metal, more preferably from aluminum. Multiple LEDs of the plurality of LEDs 11 may be grouped, e.g. in a multi-chip of LEDs. The plurality of LEDs 11 may be arranged without a determined pattern or may describe an array, e.g. an array of a plurality of rows by a plurality of columns. The size of the array may be designed depending on the intended use of the luminaire which includes the plurality of LEDs 11, e.g. walk path illumination, large road, park, etc.

The plurality of LEDs 11 may be interconnected via connections. The connections may be any one of: wires, electrical tracks, electrical connectors, solder joints. The plurality of LEDs 11 may be mounted in parallel and/or in series, and soldered to the PCB by a plurality of solder joints. In the exemplary embodiment of Fig.1 , the plurality of LEDs 11 comprises at least 8 LEDs in series. In another embodiment, the plurality of LEDs 11 may comprise at least 4 LEDs in series, preferably at least 8 LEDs in series, more preferably at least 16 LEDs in series. Additionally, the plurality of LEDs 11 may comprise a plurality of LED strings mounted in parallel, each of the plurality of LED strings comprising a plurality of LEDs mounted in series.

The voltage measurement unit 20 is configured for obtaining a voltage value across at least one LED 10 of the plurality of interconnected LEDs 11. In the embodiment of Fig.l, the voltage measurement unit 20 is connected for obtaining a voltage value across four LEDs 10 mounted in series. In another embodiment, the voltage measurement unit 20 may be configured for measuring a voltage value across all the LEDs 10 of the plurality of LEDs 11. The at least one LED 10 may be or comprise the most thermally stressed LED among the plurality of LEDs 11. In another exemplary embodiment, the at least one LED 10 of the plurality of interconnected LEDs comprises a representative LED of the plurality of LEDs 11. The representative LED may be defined as a LED presenting a behavior according to the expected average behavior of a LED subject to similar conditions. The voltage measurement unit 20 may be an extra unit added to the luminaire head for the specific purpose of the present invention, or may already be included in the luminaire head as part of an already present component, e.g. included in a lighting driver connected to the plurality of LEDs 11. The gauging means 30 may be connected to the voltage measurement unit 20 wirelessly or in a wired manner. In one embodiment, the gauging means 30 may be comprised at least partially locally in a processing means within the luminaire. In addition or alternatively, the gauging means 30 may be comprised at least partially in a remote device. In an exemplary embodiment, the gauging means 30 may already be included in the luminaire head as part of an already present component, e.g. included in a lighting driver connected to the plurality of LEDs 11.

The voltage values measured by the voltage measurement unit 20 may be sent to or queried by the gauging means 30 locally or partially remotely. The voltage values may be sent from an emitting interface wirelessly or in a wired way to a receiving interface, e.g. a wireless or wired digital communication interface in the luminaire.

Based on the first voltage value and the second voltage value received by the gauging means 30, a reliability of the connections interconnecting the plurality of LEDs 11 is gauged. By reliability, it is meant the electrical connecting stability of the connections interconnecting the plurality of LEDs 11 , said stability originating from the status or“health” of the different elements composing the connections. Aspects of the gauging by the gauging means will be described in more detail below with reference to Fig.2.

The luminaire system may also comprise a memory unit 40. The memory unit 40 may be located locally or remotely with respect to the luminaire. In an exemplary embodiment, the memory unit 40 may already be included in the luminaire head as part of an already present component, e.g. included in a lighting driver connected to the plurality of LEDs 11.

The memory unit 40 may be connected to the voltage measurement unit 20 and/or to the gauging means 30. In one embodiment, the memory unit 40 is connected to the gauging means 30 only, and the gauging means 30 is configured for saving the gauged reliability in the memory unit 40. In another embodiment, the memory unit 40 may be connected to the gauging means 30 and to the voltage measurement unit 20. In the latter case, the voltage measurement unit 20 may save the measured voltage values in the memory unit 20 and, at a later time for example, the gauging means 30 may obtain the measured voltage values from the memory unit 20. After gauging the reliability, the gauging means 30 may save the gauged reliability in the memory unit 40.

The saved prior gauged reliability in the memory unit 40 may be obtained by the gauging means 30; another reliability may be gauged by the gauging means 30; and the gauging means may use the saved prior gauged reliability and the another gauged reliability for predicting a failure of the connections interconnecting the plurality of LEDs 11. In this manner, by anticipating a failure based on the reliability history of the connections interconnecting the plurality of LEDs 11 , maintenance may be performed ahead of the failure and the luminaire may continue providing its services without an unexpected interruption.

Additionally, based on the saved prior gauged reliabilities history in the memory unit 40, the gauging means 30 may determine a predictive maintenance time. The gauging means 30 may be configured for sending said predictive maintenance time in the shape of a health indicator illustrating the health of the connections interconnecting the plurality of LEDs 10. A good health of the connections interconnecting the plurality of LEDs 10 may be interpreted as connections having high reliability and not needing maintenance within a relatively short time. A bad health of the connections interconnecting the plurality of LEDs 10 may be interpreted as connections having low reliability and needing maintenance shortly or immediately. The predictive maintenance time may be further determined based on accumulated reliabilities histories from other similarly implemented connections interconnecting the plurality of LEDs 10 and/or on data sheets of the elements composing the connections interconnecting the plurality of LEDs 10.

The gauging means 30, upon gauging that the reliability of the connections interconnecting the plurality of LEDs 10, may be configured for sending a warning signal WS if the gauged reliability is below a predetermined reliability level. The warning signal may be sent wirelessly or in a wired manner locally or to a remote device. The warning signal WS may be any one of: a luminous signal, a text signal, a sound signal.

Figure 2 shows a time graph of voltage values as anticipated for a luminaire system according to the present invention. The luminaire system 100 considered may be similar to the luminaire system 100 illustrated in Fig.l.

When employing a plurality of LEDs 11 included in a luminaire, sudden changes may occur in the power provided to the plurality of LEDs 11. The power provided may change suddenly over time depending, for example, on a received lighting data. The lighting data may comprise instructions to reach a desired lighting intensity from the luminaire which may cause a sudden change in provided power. Powering ON of the plurality of LEDs 11 may also be the cause of a sudden power change provided to the plurality of LEDs 11. It is this latter case that will be described for ease of understanding with reference to Fig.2 in the following. Flowever the skilled person will understand that any sudden increase or decrease in the power provided to the plurality of LEDs may be applicable to the below described embodiment, mutatis mutandis.

In the embodiment of Fig.2, the plurality of LEDs 11 included in the luminaire system 100 is powered ON at an initial time t,. Two voltage curves are represented in the graph in function of time: V _r (t) and V _th (t). The second curve, V _th , may be considered as the theoretical, or ideal, curve of the voltage values across the at least one LED 10 of the plurality of LEDs 11 when powering the plurality of LEDs 11. At t,, the voltage is at a high initial value V;. It then decreases until it reaches a steady state at time t _s with a low voltage value of V _s .

The high voltage value V; may be attributed to the occurrence of a thermal event to plurality of LEDs 11. A thermal event occurring to the plurality of LEDs 11 may be defined as an event causing an increase, or a decrease, of the temperature of the plurality of LEDs 11 at a rate at least twice faster than the normal increase, or decrease, rate of ambient temperature. Indeed, when powered, the plurality of interconnected LEDs 11 presents some resistance causing a sudden increase in temperature. Increase in temperature may also come from some heat dissipation due to thermal losses in the conversion of electrical energy to light emission. As time passes, the increase in temperature of the plurality of interconnected LEDs 11 gets stabilized until reaching a thermal steady state, and the voltage decreases in value from a high V; to a low V _s .

In other words, the thermal event may correspond to an event causing an increase or a decrease in temperature to the plurality of interconnected LEDs 11. As such, the increase in temperature may be caused, for example, by the powering on of the plurality of interconnected LEDs 11 , or by a change from a first lighting regime to a second lighting regime demanding a higher light output from the plurality of interconnected LEDs 11. The decrease in temperature may be caused, for example, by the start of an active cooling of the plurality of LEDs 11 , e.g. the activation of a cooling fan, to dissipate the generated heat.

In theory, or during the first thermal cycle of the plurality of interconnected LEDs 11 , the curve V _th is smooth. After a certain number of thermal cycles, the real voltage of the plurality of interconnected LEDs 11 may be as shown by the curve V _r . The curve V _r may have an overall decreasing behavior similar to V _th , but the curve is noisy and may present values rapidly varying in function of time with peaks and valleys. The steady state when powering the plurality of LEDs 11 may be reached in a matter of minutes.

The voltage measurement unit 20 configured for measuring voltage values may measure voltage values within a predetermined time period PT. The predetermined time period PT in the embodiment of Fig.2 corresponds to the time between the powering time, t,, and the steady state time, t _s . In another embodiment, the predetermined time period PT may be smaller than the time between t; and t _s .

The voltage measurement unit 20 is configured for measuring a first voltage value V _! at a first time t _Vi within the predetermined time period PT, and for measuring a second voltage value V _2M at a second time t _V2 within the predetermined time period PT. t _Vi may be prior or subsequent to the second time t _V2 , subsequent in the embodiment of Fig.2. As can be seen on the theoretical curve V _th , the voltage value at t _Vi is smaller than the voltage value at t _V2 . However, on the real curve V _r , the voltage value V _! may be higher than V _2M .

Based on the first and second voltage values measured V _l V _2M the gauging means 30 is configured for gauging the reliability of the connections interconnecting the plurality of LEDs 10. The gauging may be based on one or more of the following parameters: the voltage difference between values V _! and V _2M , the time difference between t _Vi and t _V2 , the slope between the points Ovi _> V and (t _V2 , V _2M ).

Predetermined voltage values may be stored in a memory. The predetermined voltage values may correspond to a theoretical or a calibrated voltage value expected to be measured at a specific time. In the embodiment of Fig.2, the time t _V 2 may correspond to a measured voltage value of V _2\i, and to a predetermined voltage value of V _2P . The gauging means 30 may obtain a predetermined voltage value, and gauge the reliability of the connections interconnecting the plurality of LEDs 11 further based on the obtained predetermined voltage value. The obtained predetermined voltage value may be the voltage value at a time, within the predetermined time period PT, corresponding or not to a time at which a voltage value has been measured by the voltage measurement unit 20.

In the embodiment of Fig.2, the predetermined voltage value V _2P and the measured voltage value V _2M are both associated to the same time t _V2 . Therefore, it is expected that V _2P and V _2M are equal. Additionally, it may be expected to have a predetermined voltage value difference between V _2M and V _{| ,} and/or between V _2P and VV The gauged reliability may be based on the comparison of V _2P and V _2M , the comparison of V _! and V _2M , and the comparison of V _! and V _2P . In another exemplary embodiment, the predetermined voltage value may be associated to a time different than tyi and t _V2 . It may be expected to have predetermined voltage value differences between each of the first voltage value, second voltage value, and predetermined voltage value. The gauged reliability may be based on the comparison between the first and second voltage values, the first and predetermined voltage values, and the second and predetermined voltage values.

Additionally, the voltage measurement unit 20 may be configured for measuring a plurality of voltage values across the at least one LED 10 of the plurality of LEDs 11 during a monitoring time period MT within the predetermined time period PT. In one embodiment, the voltage measurement unit 20 may be configured for measuring voltage values at regular intervals during the monitoring time period MT, e.g. 2 or 3 measurements per second. Alternatively or additionally, the monitoring time period MT may be triggered based on the voltage difference between V _j and

V ₂ M·

The gauging means 30 may be configured for obtaining another voltage value based on the plurality of voltage values measured during the monitoring time period MT, and for gauging the reliability of the connections interconnecting the plurality of LEDs 11 further based on the obtained another voltage value. The skilled person will understand the different methods and operations that can be applied to the plurality of voltage values measured during the monitoring time period MT in order to obtain the most representative, accurate, or relevant another voltage value. The another voltage value may be obtained by, for example, averaging the plurality of measured voltage values, selecting the highest voltage value among the plurality of measured voltage values, selecting the lowest voltage value among the plurality of measured voltage values. More than one another voltage value may be obtained by the gauging means.

The plurality of voltage values measured during the monitoring time period MT may be processed in various manners in order to improve the accuracy of the gauged reliability of the connections interconnecting the plurality of LEDs 11. The plurality of voltage values may be processed by frequency analysis, and/or the number of times the slope sign changes between successive measured points may be counted, and/or the range of variations between successively measured voltage values may be statistically analyzed.

The gauging means 30 may be configured for sending a warning signal WS if the gauged reliability is below a predetermined reliability level. The predetermined reliability level may be based, for example, on a tolerance between the measured voltage values Vi,V _2M> based on a predetermined voltage value V _2P , and/or based on various levels processed based on the theoretical curve V _th .

Figure 3 illustrates schematically another exemplary embodiment of a luminaire system according to the present invention. The luminaire system includes a plurality of LEDs 11 interconnected via connections, and comprises a voltage measurement unit 20, a gauging means 30, a memory unit 40, and a temperature measurement unit 50. Embodiments of the luminaire system 100 described with reference to Fig.1 , although omitted for convenience of description, are applicable to the embodiment of Fig.3.

In the embodiment of Fig.3, the plurality of LEDs 11 comprises at least 4 LEDs 11 mounted in series. The voltage measurement unit 20 is connected across one LED 10 of the plurality of interconnected LEDs. The one LED 10 may be the most thermally stressed LED among the plurality of LEDs 11. In another exemplary embodiment, the at least one LED 10 of the plurality of interconnected LEDs comprises a representative LED of the plurality of LEDs 11. The representative LED may be defined as a LED presenting a behavior according to the expected average behavior of a LED subject to similar conditions.

The voltage measurement unit 20 is configured for measuring voltage values across the at least one LED 10 within a predetermined time period PT. The predetermined time period PT may be shorter than a period of time lasting between a time t, when a thermal event occurs to the plurality of LEDs 11 and a time t _s when the plurality of LEDs 11 has reached a thermal steady state. The gauging means 30 may gauge a reliability of the connections interconnecting the plurality of LEDs 11 based on the measured voltage values. The gauging means 30 may also be connected to a memory unit 40. The gauging means 30 may save the gauged reliability and, when a subsequent thermal event occurs to the plurality of LEDs 11 , obtain the saved prior gauged reliability from the memory unit 50, gauge another reliability of the connections interconnecting the plurality of LEDs 11 , and predict a failure of the connections interconnecting the plurality of LEDs 11 based on the saved prior gauged reliability and the another gauged reliability.

The gauging means 30 may also be connected to the temperature measurement unit 50.

The temperature measurement unit 50 is configured for measuring at least one temperature value related to the at least one LED 10 of the plurality of LEDs 11. The temperature measurement may be performed e.g. in proximity to a LED 10 across which the voltage value is measured, in proximity to a LED 11 different from the at least one LED 10 across which the voltage value is measured, at a component interconnected with the plurality of LEDs 11 , and/or in proximity of a surface onto which the plurality of LEDs 11 is mounted. The gauging means 30 is configured for gauging the reliability of the connections interconnecting the plurality of LEDs 10 further based on the at least one temperature value measured by the temperature measurement unit 50. In the embodiment of Fig.3, the temperature measurement unit 50 is configured for measuring the at least one temperature value at proximity of a LED 11 away from the LED 10 across which the voltage measurement unit 20 measures voltage values.

The voltage measurement unit 20 and the temperature measurement unit 50 may be configured for, simultaneously, measuring a voltage value across the at least one LED 10 of the plurality of LEDs 11 and measuring a temperature value related to the at least one LED 10 of the plurality of LEDs 11 , respectively. Then the gauging means 30 may gauge the reliability of the connections of the plurality of interconnected LEDs 11 further based on the voltage values and temperature values measured. Alternatively or additionally, the temperature measurement unit 50 may be configured for measuring temperature values within the predetermined time PT independently from the voltage measurement unit 20.

In still another embodiment, the temperature measurement unit 50 may be connected to the voltage measurement unit 20. The temperature measurement unit 50 may be configured for measuring a first temperature value at a first time, a second temperature value at a second time, and determining the occurrence of a thermal event to the plurality of interconnected FEDs 11 based on the first temperature value and the second temperature value. The determination of the occurrence of the thermal event may trigger the measurement of voltage values by the voltage measurement unit 20.

Figure 4 shows another time graph of voltage values and a time graph of temperature values as anticipated for a luminaire system according to the present invention. Embodiments of the luminaire system 100 described with reference to Fig.2, although omitted for convenience of description, are applicable to the embodiment of Fig.4.

In the embodiment of Fig.4, the plurality of FEDs 11 included in the luminaire system 100 is powered at an initial time f. Two voltage curves are represented in the upper graph in function of time: V _r (t) and V _th (t). The second curve, V _th , may be considered as the theoretical, or ideal, curve of the voltage values across the at least one FED 10 of the plurality of FEDs 11 when powering the plurality of FEDs 11. At t,, the voltage is at a high initial value Vi. It then decreases until it reaches a steady state at time t _s with a low voltage value of V _s .

One temperature curve is represented in the lower graph in function of time. Upon powering at the initial time t,, the plurality of interconnected FEDs 11 warms up from a low starting temperature, the ambient temperature in the embodiment of Fig.4, until it reaches a steady state at a high temperature T _s .

However, the skilled person will understand that any sudden increase or decrease in the power provided to the plurality of FEDs 11 may be applicable to the below described embodiment, mutatis mutandis. Additionally, the skilled person will understand that any sudden increase or decrease in the temperature value in the proximity of the plurality of FEDs 11 may be applicable to the below described embodiment, mutatis mutandis.

The high voltage value V; may be attributed to the occurrence of a thermal event to plurality of FEDs 11. A thermal event occurring to the plurality of FEDs 11 may be defined as an event causing an increase, or a decrease, of the temperature of the plurality of FEDs 11 at a rate at least twice faster than the normal increase, or decrease, rate of ambient temperature. After a certain period of time, the temperature in the proximity of the plurality of LEDs 11 may stabilize, and the temperature reaches a thermal steady state at a time t _s . The time t _s at which the thermal steady state of the connections interconnecting the plurality of LEDs 11 is reached may correspond to a stabilized voltage value V _s measured across the at least one LED 10 of the plurality of LEDs 11 by the voltage measurement unit 20. Thus, temperature values and voltage values measured may have correlated behaviors: when the temperature value increases, the voltage value decreases.

At least a first temperature value T _| and a second temperature value T ₂ may be measured at different times t _T1 and t _T2 , respectively, within the predetermined time. Based on the slope between the at least two points (t _T1 , T _L) and (t _T2 , T ₂ ), an occurrence of a thermal event may be determined by the temperature measurement unit 50. Upon determination of the occurrence of the thermal event to the plurality of interconnected LEDs 11 , the measurements of a first voltage value V _! at a first time tyi and of a second voltage value V ₂ at a second time t _V2 may be triggered. Additionally, another first temperature value T _Vi and another second temperature value T _V2 may be measured, said another first and second temperature values T _Vi , T _V2 being measured at t _Vi and t _V2 . The gauged reliability of the connections interconnecting the plurality of LEDs 11 may be based on the voltage values measured and the temperature values measured, and/or any obtained processed values thereof.

Whilst the principles of the invention have been set out above in connection with specific embodiments, it is to be understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.