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LEDs are key elements in modern, energy efficient lighting solutions as well as impose some issues from thermal point of view, since light output and reliability both depend on LEDspsila junction temperature. A comprehensive and accurate measurement method is required and demanded by several leader LED manufacturers. Failing a proper combined thermal and radiometric/photometric characterization of LED light sources it is impossible to fulfill the reliability prescriptions for LEDs and to trust the lifetime estimation given in LED datasheets. Light output of LEDs is typically measured in integrating spheres. A key element in such a total flux measurement setup is the appropriate set of standard LEDs which are both current and temperature stabilized and are accompanied with certificate values of their own total flux traceable to primary etalons of national measurement laboratories. So far there are hardly any such standard LEDs available for the high power range. In this paper we describe the design of such a device (having 5 colors) and describe a modification of the substitution type total flux measurement method which is suitable for an automated, comprehensive measurement of LEDs over a wide range of operating conditions.