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24-26 September 2008, Rome, Italy(RSPD) of the LED, or one may try to measure thetotal radiometric flux Φ e of the LED (using a photodetector having a flat spectral response). In practicalrealisations the second approach is more feasible. Oneshould be careful when selecting these methods:whatever type of total flux is measured, themeasurement should be calibrated to a standard LEDwhich possesses a similar relative spectral distributionas the LED under test. Photometric detectors are lesssensitive at the edges of the visible spectrum (since theV(λ) function vanishes both at the blue and red end ofthe visible spectrum). In case of a flat response(radiometric detectors) the physical limits come fromthe spectral distribution of the sensitivity of thephotoelement in use. In case of Si based detectors, cutoffof the detector starts at blue. Another aspect toconsider is the accuracy of the total flux measurementsystem used. When implementing the total(radiometric) flux measurement of LEDs, the relevantrecommendations of the CIE must be followed [9]. Anearly solution for correcting the LEDs' dissipationwith the emitted radiometric flux was suggested intwo papers by Farkas et al. [10], [11]. At LEDmanufacturers photometric measurement systems areanyway available – but maybe not in the same labwhere thermal characterisation is performed, this wayhampering a tight integration of the two measurements(thermal + radiometric).• How to measure the case temperature? Our opinion isthat power LEDs must be characterised on a coldplate.Then, though we do not have the casetemperature itself, we have quite accurate informationabout the cold-plate temperature. The interfacial layerbetween the cold-plate and the heat-slug should beaddressed. Detailed information about the junction-toambientheat-flow path can be extracted from thermaltransient measurements – providing an option toaddress issue of the interfacial thermal resistance aswell as allowing calculation of the junctiontemperature based on the known referencetemperature.• How to tackle multi-sources? See e.g. papers byZhang and Treurniet [12], Treurniet and Lammens[13] and Poppe et al. [14]. We propose to use the Z th /R th matrix for all sources – as discussed or suggestedin these references. One has to note that we canmeasure the elements of the R th / Z th matrices only ifaccess to every individual LED chip is provided – i.e.if the LED based system was also designed forthermal testability. If there is no individual(preferably 4-wire) access to all the LED chips of thedevice under test, we can measure only an overallaverage junction temperature and we run into theproblem of the 'ensemble' junction temperature and'ensemble' thermal resistance problem as discussedalready in relation to the junction temperature.IV. CONCLUSIONSAs highlighted in this paper, generating fair and usefuldata sheet information for power LEDs is not astraightforward matter. Though a vast amount of experiencewas already gained about thermal standardisation ofconventional (silicon) devices, power LEDs raise a couple ofnew questions. Especially the measurement side of LEDthermal characterisation poses new challenges. To answerthese questions, LED manufacturers, LED end users, testingand modelling experts need to take a joint action (see Part I).At this point in time it seems that next to standardisedmetrics and better defined contents of LED data sheets, alsomulti domain models (electrical, thermal, optical) would berequired to enable a fair comparison of competing productsand facilitate a reliable design of LED based lightingapplications.ACKNOWLEDGEMENTSThe authors highly appreciate the constructive commentsby Rudi Hechfellner, Quint van Voorst Vader and Joan Yufrom Philips Lumileds. Thanks are due to Gábor Molnárfrom MicReD for efficiency measurements.REFERENCES[1] O. Ueda: “Reliability Issues in III-V Compound SemiconductorDevices: Optical Devices and GaAs-based HBTs,” MicroelectronicsReliability, Vol. 39, pp. 1839-1855, 1999.[2] H. Kim, H. Yang, C. Huh, S.-W. Kim, S.-J. Park and H. Hwang:“Electromigration-induced Failure of GaN Multi-quantum WellLight Emitting Diode,” Electron. 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Grabner-Meyer , LED Data Sheet Comparison, LED-Professional Review, September 2007[9] CIE 127:2007 document: “Measurement of LEDs”[10] G. Farkas, Q. van Voorst Vader, A. Poppe, Gy. Bognár: "ThermalInvestigation of High Power Optical Devices by Transient Testing",IEEE Trans on Components and Packaging Technologies, Vol. 28,no. 1, (March 2005), pp. 45-50[11] G. Farkas, A. Poppe, J. Schanda, K. Muray: "Complexcharacterization of power LED-s: simultaneous measurement ofphotometric/radiometric and thermal properties", Proc. CIE LEDConference, 2004 Tokyo, CIE x026:2004, pp. 92-95[12] Zhang L., Treurniet T., On The Challenges of ThermalCharacterization of High-Power, High-Brightness LED Packages,ElectronicsCooling Vol.14, no.2, 2008[13] Treurniet, T., Lammens, V., "Thermal Management in ColorVariable Multi-Chip LED Modules", Proceedings of the XXII-ndSEMI-THERM Symposium, San Jose, Calif., 2006, pp. 186-190©EDA Publishing/THERMINIC 2008 218ISBN: 978-2-35500-008-9