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There are many infrared heaters on the market, that I would put in 3 categories, filament in quartz tube, filament in ceramic, and carbon film (for flat panel heaters). They all claim to be "infrared heaters" however, as far as I can tell, they all just create heat from electricity and radiate that heat, therefore, they just have a regular thermal radiation spectrum, a blackbody spectrum.

The blackbody spectrum is rather spread out, especially in lower temperatures...So how can these heaters possibly have a narrower band to enable them to claim "near IR radiation" or "far ir radiation" type ? Is it all marketing ?

Manu de Hanoi
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The heater is often so hot that it glows red, so it also radiates some visible light. And with fine enough instruments one could also detect some emitted microwave noise. But the main part of the radiated energy is at the wavelength range we call infrared. That's more than 0.7 μm but less than 1 mm.

You can verify that claim by using some free online blackbody radiation calculator.

Let the temperature of the radiator be say 800 kelvin. Calculate the band radiance for ex. for wavelength range from 0.7 to 20 μm. You'll see that the result doesn't change radically if you expand the wavelength range to contain also visible light and let the longest wavelength be say 200 micrometers.

The air attenuates infrared due molecular resonances. That means the air gets warmer due the IR radiation which is absorbed due the low transmittance of the air. See Infrared Window to see where the high and low transmittance (=transparent for IR) wavelength ranges are.

limited air transmittance probably isn't especially remarkable if the distance is short, for ex. only 1 meter.

Fred
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  • thanks for your answer, in the case of the ceramic heaters, isnt the ceramic temperature what matters for the emission of IR and not the filement temperature ?As I understand the cermaic will absorb all the radiation from the filament, get heated then emit. In other words, no matter the type of ceramic, as long as it's opaque, then it will emit the blackbody spectrum? And therefore the ceramic's emission absobtion bands play little to no role in the EM spectrum emitted ? – Manu de Hanoi Jul 11 '22 at 17:20
  • There can be high reflection back inwards at some wavelengths (=low surface emissivity) Unfortunately I do not know enough of ceramic heater materials to be give any facts of any specific material. –  Jul 11 '22 at 18:06
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Each type of heater has a different surface area from which it radiates the rated heat output. This means that to develop the rated power with that surface area requires a different surface temperature for each different radiating area.

So a radiant quartz tube with a tungsten wire inside rated at 500 watts will be typically glowing red or orange (500 watts through the small surface area of the tungsten wire) whereas the carbon film panel heater will not be glowing (500 watts through a large surface area of many square inches).

This means each different heat source in this case will be operating at a different temperature, and each will be producing a blackbody spectrum with a peak somewhere in the IR range- but with a different spectral peak corresponding to that particular temperature.

niels nielsen
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  • Surface area aside, the choice of the material has little to do with the spectrum right ? So you always get a blackbody spectrum, isn't it ? – Manu de Hanoi Jul 12 '22 at 04:13
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    yes, for "ordinary" materials and relatively low temperatures where the material is not being ionized! – niels nielsen Jul 12 '22 at 04:33