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I dont know how people absorb infared vs concrete, but i do know that once it emits a photon of infared, that energy is gone from the system, regardless of where it is absorbed. Meaning you spend the exact same power heating nothing as heating the air, concrete, or people. As long as the heater is emitting, it is emitting the same amount always.

This parapraph began as a short tangent but now i have a longer question. I was going to explain that the energy delivered does chnage if you prevent the energy from escaping, but now im not sure exactly how. If you, say, reflect all the ir back at the emitter it heats up. Therefore, it emits more blackbody radiation. Im just gonna take a guess and say that blackbody radiation is not the mechanism by which quartz ir heaters operate though. Im assuming they operate closer to an LED, where they are doing fancy quantum magic to directly create the chosen photon and the chosen amplitude, which would be way more efficient. In that case, as the emitter heats up it would have a lower efficiency because the electrical resistance goes up, wasting power. It would also serve to expand the bandgap in the IRED, either reducing efficiency or pushing it into another frequency of light altogether.

However, all that "wasted energy" or "reduced efficiency" has another name... heat. And its a heater. So like.... net energy generation stays the same? I think my confusion is coming from not clearly defining what it actually is im trying to measure or where im taking that measurement.