Junction Temperature (Tj)

Junction temperature is the temperature at the LED chip’s semiconductor junction — the single variable that most decides how long an LED fixture actually lasts. Every LM-80 lumen-maintenance dataset is measured at specific junction temperatures; run the chip hotter than the datasheet case and depreciation accelerates, colour shifts, and the projected L70 life shortens dramatically. Tj cannot be measured directly in a finished luminaire, so designers control it through the thermal path: die-cast aluminium housings, fin area, thermal interface materials and driving the LED below its maximum current. This is why two fixtures using the same LED chip can have very different real-world lifetimes — the one with the better heat sink runs a cooler junction, and why continuous high ambient temperatures matter when selecting fixtures for steel mills or desert sites.