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Some background: I am curious if infrared lamps used in hatcheries and bathroom fixtures offer any advantage over ordinary incandescent bulbs. A 40 watt bulb produces the same amount of energy, regardless of spectrum, of course. I understand blackbody radiation and surface emissivity. There is already a forum question on whether or not infrared bulbs produce more heat. (154723 Answer: not a lot more.) My question is whether they are more effective for warming "everyday" materials like carpets, walls, and skin.

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At room temperature, blackbody radiation is below visible light wavelengths.

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This means that the quantum mechanical emission wavelengths are of the order of infrared radiation and below. Absorption and emission coefficients are equal, so these are also the absorption wavelengths.

Emitting most of the energy in the visible light, the em waves will need multiple scatterings before the em energy is brought down to infrared wavelengths; these will be absorbed into vibrational levels and seen in the temperature of the solids in the room, in a statistical way of energy degradation in air and walls etc.

Radiating the energy in the infrared is more efficient in doing this transfer fast. The cycle of heating solids with the convection of heated air is skipped, and the effect of heating is felt immediately.

In a sense it is a time and comfort feeling effect. Infrared heating has immediate effect and the human body also feels immediately warmer. At the equillibrium state it should not make a difference if the energy dissipated by the lamps, visible/infrared, is the same in watts, because the visible will finally degrade to the infrared ( unless it leaves from windows and other transparencies like open doors with the air, the infrared heats directly where it hits).

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  • $\begingroup$ Thank you. The situation I had in mind was using it to heat a telescope in drafty planetarium like enclosure. The air escapes quickly, so the equilibrium point is whatever 40 watts adds to entire atmosphere. There is a weak analogy to mismatch in a radio receiver. If the input impedance of the receiver is not matched to the antenna, the energy is reflected rather than absorbed. By analogy it is as if IR is a better impedance match. Heating the telescope is intended prevent dew and icing, not necessarily for comfort of the observer :( Thanks again. $\endgroup$ – Jim De Camp Feb 13 '18 at 12:06

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