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junction temperature is the temperature at the LED die itself, the tiny chip that produces light.
It is the single most important factor in LED lifespan and output. Every degree above the rated maximum costs you lumens and hours of life. Thermal resistance is how you predict and control junction temperature.
What Is Thermal Resistance?
Thermal resistance (°C/W) measures how much temperature rises per watt of heat flowing through a material or component. It works like electrical resistance; heat flows from hot to cold, and thermal resistance determines how much temperature “drops” along the way.
The thermal path from the LED die to the ambient air has four links in series:
| Link | Symbol | What It Is | Typical Value |
|---|---|---|---|
| Junction to LED case | Rθ J-C | Thermal resistance of the LED junction to the outside of the case | 2–10 °C/W |
| LED case to the module bottom | Rθ C-B | Thermal resistance of the mounting base | 1–5 °C/W |
| Module bottom to heatsink | Rθ B-HS | Thermal resistance of the material use to fasten the module to the heatsink | 0.5–1 °C/W |
| Heatsink to ambient air | Rθ HS-A | Thermal resistance of the fastening material | 1.4–26 °C/W |
Total thermal resistance = Rθ J-C + Rθ C-B + Rθ B-HS + Rθ HS-A
Why This Matters
junction temperature = ambient temperature + (power × total thermal resistance)
For example, a single white Rebel ES LED at 700 mA dissipates ~1.3 W. With a heatsink rated at 15.5 °C/W and assuming ~7 °C/W for the LED package and interface:
Juntion temperature = 25°C + (1.3W × 15.5 °C/W) = 62°C
That’s well within safe limits. Now the same LED on a heatsink rated at 21 °C/W:
junction temperature = 25°C + (1.3W × 21 °C/W) = 70°C
Still safe, but warmer. At 40°C ambient (enclosed fixture in summer), that becomes 85°C — still acceptable, but getting close to the point where output starts to derate.
What Happens When junction temperature Is Too High
| junction temperature | Effect |
|---|---|
| 25°C (rated) | Full rated lumen output |
| 85°C | ~85–90% of rated output (typical 10–15% loss) |
| 120°C | ~70–75% of rated output, accelerated aging |
| 150°C (absolute max) | Risk of permanent damage |
The relationship is gradual — there is no cliff where the LED suddenly fails. But every 10°C increase roughly doubles the rate of lumen depreciation over time.
What You Can Control
You cannot change Rθ J-C or Rθ C-B; that’s the manufacturer’s domain. But you can control:
- Rθ C-B — Use high-performance thermal tape or Arctic Silver thermal epoxy. Ensure the module sits flat against the heatsink with no air gaps.
- Rθ HS-A — Choose a heatsink with a °C/W rating for your power dissipation. See our Heatsink Selection Guide.
- Ambient temperature — Ventilate the enclosure. Mount away from other heat sources.
Practical Rule of Thumb
For most applications, keep the heatsink surface temperature below 55°C (comfortable to touch briefly). If the heatsink is too hot to hold, the junction temperature is likely above 85°C, and the LED is significantly derated.
