Heat, infrared and the body's response
Radiant heat
Radiant heat is thermal energy travelling as electromagnetic radiation. Every object above absolute zero emits it, and it needs no air or contact to cross a gap. In a sauna it is the warmth you feel directly from a hot surface or an infrared emitter.
Also called thermal radiation, radiant heating
Heat moves in three ways: conduction through direct contact, convection through a moving fluid such as air or water, and radiation through electromagnetic waves. Radiant heat is the third. It is the reason you feel warmth from a fire across a room, or from the sun through cold air.
The physics in one line. How much an object radiates rises steeply with its absolute temperature (the Stefan-Boltzmann relationship), and the wavelength at which it radiates most strongly gets shorter as it gets hotter. A domestic infrared emitter is not hot enough to glow, so its output is invisible and lies in the infrared.
Radiation goes both ways. You are also a radiant heat source. Skin at about 33 °C radiates with a peak near 9.5 µm, which is why a person feels cold in a room with cold walls even when the air is warm — they are radiating to the walls faster than the walls radiate back.
In the sauna. On our Standard, Deluxe and Red Light ozone steam saunas, radiant far infrared and convective steam heat operate at the same time. They are different mechanisms reaching you by different routes, which is why a sauna with both feels warmer than its air temperature alone suggests.
Why it matters here
Radiant heat is why a far infrared sauna feels warm at an air temperature you would find comfortable in a room. It is the physics behind the far infrared feature across the current range.