NOT ALL INFRARED WAVELENGTHS DELIVER COMFORTABLE HEAT
Humans and animals are exposed to infrared almost every day via the sun, and have evolved special mechanisms within their skin to maximize benefit from this source.
Infrared radiation causes a heating effect when it strikes a surface, in the case of comfort heating this is normally the skin. However not all infrared radiation is converted to heat. All infrared radiation can be
The amount of each depends on the surface and material characteristics. Infrared radiation that is reflected will not be converted into heat, whereas Infrared radiation that is absorbed is converted to heat directly. Infrared radiation that is transmitted may be partly converted to heat depending on the thickness of the bodies and objects, and how much radiation can pass through.
Infrared, invisible light spectrum, heat radiation...
Shorter wavelengths are usually more transmissive and therefore penetrative; while medium and long wave infrared energy usually penetrate less, therefore converting most of their energy to heat in the surface region. With regard to the human body, absorption is highly dependent on the skin characteristics. Skin absorbs principally due to its 80% or so water content, therefore it has an absorption spectrum like water.
Three biologically significant bands, IR-A and IR-B have lower absorption compared with IR-C. This means that IR-C and IR-B which occur at longer wavelengths are better absorbed by human skin.
This means that of all the infrared radiation striking the skin, a large proportion of the IR-A (shortwave radiation) will be reflected and therefore will not cause heating. The infrared in the longer wavelengths assist favorably with high absorption and low reflectance; two essential properties required for effective heating.
Biologically significant IR-C has high absorption, low reflectivity and low transmissivity, which are the benefit of heating the skin; ultimately transmits the heat to the body’s interior by diffusion into the cooler internal tissue. It is also the intrinsically safer waveband which is why comfort IR usually aims at the longer wavelengths (comfortably in the IR-B and IR-C)
Infrared radiation is absorbed, transmitted or reflected depending on the characteristics of the surface. In the case of humans or animals, this surface is the skin. The skin covering the human body has evolved absorption, transmission and reflective characteristics probably as a result of exposure to the sun. Natural defense characteristics have evolved such as favorable absorption of long wave IR and high reflectivity of short wave IR. In addition recent studies find that shortwave IR (IR-A) is damaging to the skin resulting in premature aging. Infrared in the bands IR-B and IR-C do not have such harmful effects and should be favored.
Please see the following link to learn more on this topic: "Preferred wavelengths for comfort heating “. by Dr Gerard McGranaghan, Ceramicx Ltd.