LED thermal characteristics, beginning with the PN junction and extending to the ambient environment, must be well understood in order to ensure a safe, reliable design and satisfactory performance. There may be multiple thermal interfaces such as die attach or glue layer in the heat flow path, and their thickness and resistance can be difficult to control in manufacturing. Moreover the thermal interface between the LED package and the luminaire acts as a heat-sink from the LED’s perspective, further complicating the design challenge. Thermal resistance must be understood as early as possible in the prototype phase.