As of today, there exist no widely accepted international standard for high power LED testing. Many vendors follow the JEDEC JESD51 series of standards for obtaining thermal metrics of their LEDs, but in calculating the thermal resistance they do not count with the emitted light energy. On the other hand, when the light output of an LED is measured following the recommendations of the CIE 127-2007 document, thermal aspects are rarely considered properly. To overcome these problems the best practice would be to combine the recommendations of the above international standards. This way the real thermal resistance of power LEDs can be identified. Knowing this, light output metrics of power LEDs can also be obtained as function of the real operating temperature of LED chips' pn-junctions. This way design for the LEDs' actual 'hot lumens' becomes possible...