Rationale for choosing case temperature (Section 5.2.2)
LED lumen degradation and color change over time is a temperature dependent behavior. In general, the higher the junction temperature, the faster the lumen depreciation is. The highest or maximum junction temperature is determined and recommended by the LED manufacturer based on the technologies used in the LED package. For LED users, the way in which the LEDs are designed into the automotive lighting application is based on the maximum junction temperature. Instead of setting a universal case temperature (Ts) to be tested, it is more practical for the LED producers and users, depending on LEDs and the applications, to choose a specific case temperature for a specific type of LED to be tested and reported. In the current practice, some LED manufacturers provide LED characteristics relating to case temperature, others refer to junction temperature. Because only case temperature can be directly measured, therefore it is recommended that the case temperature Ts is set, depending on the LED manufacturer’s specification, either to a temperature such that the junction temperature is within 10 °C below the maximum junction temperature claimed by the LED producers. or if the manufacturer only specifies case temperature, then it can be tested within 10 °C below the maximum case temperature.
Rationale for lumen and color maintenance testing duration (Section 5.5.4.1)
The exterior automotive lighting usage in the US for the life of the vehicle is referred to the reports UMTRI-2008-14 and UMTRI-2008-48 where a 95 percentile duty cycle was assumed which is more appropriate than average usage. In the reports, the annual usage of DRL is presented to be 382.0 h, and for the other (non-DRL) longest use lamps 107.1 h. Based on this information, the life of the vehicle lighting is estimated. If the LEDs used are expected to last the life of vehicle, the lumen maintenance test duration is recommended for 6000 h for DRL and 2000 h for other lamps.