One of the claims you'll hear about LEDs is that they don't radiate heat like a regular bulb does. Hot filament bulbs radiate in the same way black body radiators work, as the physicists say. They produce more infrared (heat) than visible light. Most of us have grabbed a hot light bulb before, so we don't need to be reminded!
source) But LEDs aren't black body radiators and aren't radiating by incandescence, so one of the common things you hear is that you don't have to be concerned about heat coming from the front of the bulb when you mount it. Writing in the trade magazine EDN, Ed Rodriguez tells of an investigation into heat coming from the front of an LED.
What you will further seem to know for sure is that the PN junction is the hottest point in an LED (or any other power semiconductor for that matter) with the LED mounting substrate and or heat sink being somewhat cooler. How much cooling is needed can be rather accurately calculated if you know a) the heat sink temperature (easy to determine) and b) the junction-to-case thermal resistance (easy to determine for the data sheet).He goes on to describe his surprise at burning himself on an LED he was working with. The junction was certainly around 40C, and that's just barely above body temperature - it shouldn't burn skin. So he measured with instrumentation and it read 125C! A sliver of wax formulated to melt a 70C melted instantly confirming the measurement.
You likely agree with what you have just read. Having been involved over several decades with silicon power semiconductor and power LEDs in one way or another I also would have agreed... until I did not.
To further determine if this perceived high temperature was a “phantom”, I proceeded to place over all devices at one time or another a small (1” by 1” by .032) piece of aluminum. Without exception, each became a hot plate, elevating the aluminum temperature (as measured by a thermocouple on the top illumination-immune side away from the light). Without exception, the temperature got to between 125-150ºC.The article is techno-geeky to the extreme, but I know some of my readers will read and enjoy. The bottom line is that despite the press, LEDs do generate infrared and radiate it out of the front of the light. When LED makers were struggling to reach 1W input power, it was less noticeable. With today's much higher powers, a few percent turning into infrared is noticeable. Heat can affect phosphor life and therefore the life of any bulbs you buy. And if you want to put an LED bulb in a sealed fixture, you should be aware of the heat. A sealed fixture designed for a large incandescent bulb should be fine, because the LED will still radiate less heat than it was designed for. But if you're starting from scratch, don't assume all the heat goes out of the metal base of the bulb or convenient heat sink.