HUMAN ABSORPTION OF INFRARED RADIATION

All materials absorb heat differently according to the composition of the material itself plus its thickness. For an infrared heater to give a good feeling of comfort, efficient and natural heating, it is important to choose the right type of infrared heating technology to suit the purpose. The nature of the target material, the distance between the heater and the target and the duration of heat exposure is critical when selecting the type of an infrared heater.

HUMAN ABSORPTION

We, as humans, consist of approximately 80% water, therefore for comfort heating we want an infrared heater to be optimised with output in the wavelengths at which water absorbs the best and reflects the least. The picture below shows the relation between wavelength and absorption of infrared radiation for water. This means that IR-B and IR-C which occur at longer wavelengths above 2,0μm are better absorbed by human skin. This translates to a more efficient heating of humans via these long and medium wave bands. Of course, using a heater which delivers IR-A will still heat the skin, but will be less efficient as the skin does not absorb as well.

REFLECTIVITY

The other characteristics of human skin is that it can reflect certain wavelengths as a natural defence mechanism. Looking at the reflectance level for IR-A, -B and -C it shows that IR-A has a high reflectance while IR-B wavelengths and IR-C has a significant lower reflectance. This can be concluded as IR-A hits the skin with its high intensiveness, while the skin reflects most of the radiation away and that part will not cause any heat. It is just wasted energy. Meanwhile, the wavelengths IR-B and IR-C (mediumwave and longwave) will be highly absorbed and have a low reflectance of the skin. This is two very favourable characteristics for efficient comfort heating.

PENETRATION DEPTH

The third important characteristics of the three wavelengths are that they penetrate the skin in different depths. The picture shows that wavelengths in the shortwave range penetrate deep into the sub layers of the skin. Even though the skins own defence mechanism by less absorption high reflectance for shortwave radiation, the intensive radiation from shortwave heaters penetrates deep into the skins sub layers making it potentially more harmful to the skin ageing process. While the medium and longwave radiation wavelengths do not penetrate as deep and are therefore a safer less harmful to the skins ageing process.