Radiant Therapy Saunas

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Infrared Performance Criteria

We evaluate the infrared performance of a sauna by focusing on the infrared emitters themselves. We factor in the emitter surface temperature, surface type, surface size and shape, and then cool down pattern for saunas that turn the emitters on/off during a typical sauna session, which most do. Some emitters have little thermal mass, so when the sauna cabin is warm and they turn off, they cool quickly and don’t emit much infrared while you sit in the relatively warm cabin.

Infrared Volume. Here we evaluate the irradiance, or “power” of the infrared at the surface. We look at the surface type and temperature, surface area, and true emission time (true constant emission vs. on/off). We then apply the Stefan-Boltzmann Law which basically states that as the temperature of a surface is increased, more of the energy output is in the form of radiant energy, in this case far infrared. For a sauna, ideally there should be a mixture of surface temperatures from 100 to 600 degrees.

Infrared Spectrum. Ideally you want a broad spectrum from 3 to 10+ microns. We calculate this by measuring the surface temperatures at various points of the infrared emitters throughout a typical 45-minute sauna, and then by knowing the material the emitter is made from and applying Planck’s Law we calculate the result. In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a hypothetical black body at a given temperature. Peak wavelength can be determined using Wien’s law, but sauna manufacturer’s that cite Wien’s law as sole proof of infrared performance are being evasive.

Infrared Effectiveness. Here we consider the radiant pattern of the emitters as well as distance from the body. Infrared diffuses as a function of the square of the distance, so that the intensity of infrared at two feet from the emitter has one fourth the intensity then at one foot from the emitter. If the emitter surface is too cool and too far away, it may heat the cabin via convection but your body will not be receiving much infrared.

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Quality Criteria

Type of Wood. Many species of wood in used in saunas, aspen is the most hypoallergenic followed by basswood. Cedar is still used by some manufactures and is a good choice (although expensive) unless you don’t like the scent of cedar indoors. Hemlock is used in many imported (Chinese) saunas and is a reasonable choice if the grain is tight and clear.

Build Quality. We consider the overall craftsmanship, fit and finish of the entire sauna as compared to a quality piece of furniture. Assembly should be fast and easy, with good instructions.

Comfort/Convenience Criteria

The sauna cabin should heat rapidly, but emitters should never feel too hot. The benches should be deep enough to allow for reclining. Some are reversible with one flat side and one contoured side. Lighting should be ample for reading, ideally adjustable.

Stefan-Boltzmann Law:
The Stefan-Boltzmann Law gives the total power radiated at a specific temperature from an infrared source. That is, for a given temperature the amount of infrared radiation emitted from a given source at all associated wavelengths.

Planck's Law
Planck's Law gives us the spectral distribution of radiation from a blackbody source. We can use this to approximate the spectral distribution using an average surface temperature and emissivity value.

Finding the Balance
In a nutshell, as the surface temperature of an emitter goes up the amount of infrared produced increases, but the wavelength becomes shorter.

Also, the infrared produced must be directed at the body. The fact is many saunas are not effective at focusing the infrared energy where it's needed.

Remember that infrared is light, and it behaves like light. Surface temperature and proximity to the body are vital. Surfaces that are further away have to be hotter than those closer or the infrared does not reach you. Similarly, the heater surface itself must be oriented towards the body or it does no good. Light does not bend to find you. So, for example, those large carbon saunas with the panels going to the ceiling waste most of the infrared into the cabin air since the heater surface is much higher than the body itself. The infrared comes off of the surface in a straight line -- and misses the body. This is basic science that cannot be overcome with marketing hype.