Unlike incandescent tungsten-filament light bulbs, high-power LEDs do not radiate heat. Instead, LEDs conduct heat from their PN junction to the thermal slug on the LED package. Because the heat generated by LEDs is conducted, the heat has a longer, more expensive, path to the atmosphere. In an LED, the heat path includes the thermal impedances from the junction to the slug, the slug to the board, the board to the heatsink, and the heatsink to the atmosphere. The heat path for a tungsten bulb is almost straight into the atmosphere, starting with the thermal resistance from the filament to the glass and ending with the thermal resistance from the glass to the atmosphere.
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