Heat, infrared and the body's response
Far infrared (FIR)
Far infrared (FIR) is long-wavelength infrared radiation, in sauna use roughly 3 to 100 micrometres. Water absorbs it so strongly that almost all of its energy is deposited in the outermost fraction of a millimetre of skin, warming you directly rather than heating the air.
Also called FIR, far infra-red, IR-C, long-wave infrared
Where the band sits. The boundary between mid and far infrared is a matter of convention rather than physics. Under ISO 20473 the 3 to 50 micrometre (µm) range is mid-infrared and far infrared starts at 50 µm. Under the photobiological CIE scheme, everything from 3 µm to 1 millimetre is IR-C, and it is IR-C that the sauna industry calls far infrared. The 12 µm figure sits inside that industry usage.
How it deposits energy. Skin is mostly water, and water absorbs extremely strongly at these wavelengths. Far infrared energy is absorbed within roughly the outer tenth of a millimetre of skin — the epidermis and the very top of the dermis. Deeper tissue then warms by ordinary conduction and by blood carrying the heat inward. It is a real and effective way of warming a person, from the skin inwards. Thoroughly warming the skin at a comfortable air temperature is exactly what the technology is for.
The emitters fitted. The manufacturer figures for our far infrared emitters are an operating temperature of 900 °C, a maximum surface rating of 64 kW/m², and a heat-up from 0 to 600 °C in two minutes. Those describe the element rather than the skin: what reaches the user is radiant energy across a gap, moderated by the sauna’s temperature control.
Where the research is. Our far infrared studies page collects the literature and links on to its sources.
Why it matters here
12 micron far infrared is a headline feature of the Standard, Deluxe and Red Light saunas. Knowing that it warms you from the skin inwards tells you what the feature actually does for you in a session.