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Consider two bodies in space exchanging heat solely by radiation. Let's call them a star and a planet.

  1. If the star contains a heat source and the planet doesn't, it can be shown that temperature of the planet can't get higher than that of the star. This sounds perfectly intuitive and is explained e.g. in this what-if article: https://what-if.xkcd.com/145/
  2. Would the situation change if the source of the star's electromagnetic radiation wasn't its temperature, but it was produced by some other mechanism? For example, if its surface was covered in LEDs? Where I'm getting with this is a hypothetical situation where we have a cold body emitting light and another body without any heat source being warmed up by this light and eventually achieving higher temperature than the first body. Could this happen?

Another way to think about this is the difference bwtween a conventional light bulb and an LED. The light bulb produces light by heating up a wire. When we put the bulb as the only source of heat into a box with some other objects, the wire will be the hottest location. On the other hand, can you put an LED in a box using some clever arrangement of lenses or similar and heat up some other object to a temperature higher than the LED itself?

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    $\begingroup$ A good example: Microwave oven. $\endgroup$
    – fraxinus
    Feb 24, 2020 at 15:04
  • $\begingroup$ @fraxinus, I was going to say "a laser cutter", but yours is even better. $\endgroup$
    – Matthew
    Feb 24, 2020 at 17:42
  • $\begingroup$ @fraxinus Remember, the vital point in the "What-If" is "Lenses and mirrors are not adding energy, they are not working for free.". If you do manage to add energy, then yes you can make the target hotter. An oven is adding energy. $\endgroup$
    – jean
    Feb 24, 2020 at 18:19
  • $\begingroup$ @jean yep, so do the LEDs in OP. $\endgroup$
    – fraxinus
    Feb 24, 2020 at 19:35
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    $\begingroup$ Part of my confusion was whether visible light also carries heat, as opposed to just the infrared part of the spectrum (or better put, if it gets converted to heat when absorbed by an object). The microwave example illustrates nicely that electromagnetic radiation at all frequencies has some energy which must go somewhere when absorbed. $\endgroup$ Feb 24, 2020 at 20:11

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Yes, you can drive the temperature of some target above a source by using light.

Heat transfer is a process based on the temperature difference that always moves energy in a way to equilibrate temperatures. But we can move energy via other methods, and that energy transfer can be used to change the temperature of an object.

In fact, let's think about your conventional light bulb. How does it get so hot in the first place? We can use a room-temperature battery and some wires drive a flashlight bulb, getting it (much) hotter than the rest of the circuit. Energy is moving, just not via heat. Instead of electrical energy being the mechanism we could drive a shaft, or pull a chain, or send EM radiation.

It would be possible to use something like an LED (or better, a laser) to raise the temperature of a target via driven EM radiation. But this is not heat transfer. So your title of "LED heat radiation" does not describe what is happening.

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