Life on Earth is significantly influenced by the electromagnetic radiation of the sun. Its energy most probably stimulated physical/chemical processes and has been used by living organisms for further development during evolution. The radiation is generated by nuclear fusion in the core of the sun and emitted into space with a wide range of different wavelengths. The terrestrial spectrum of this radiation shows characteristic absorption bands caused by atmospheric gases, mainly by water vapor, carbon dioxide, and ozone. The water molecules are excited by the absorption of preferentially long-wavelength radiation, which leads to absorption bands in the spectral range of the IR-A and to the total elimination of IR-B and IR-C. The excitation causes vibrations and rotations of the water molecules, increasing their kinetic energy, i.e., heat. This effect is used in medicine for therapeutic heating of tissues. It is technically achieved by water-filtered IR-A (wIRA) radiatior. Its radiation is comparable to the IR-A part of the terrestrial solar spectrum, but its total irradiance in the IR-A is much higher than that of the sun and can be adapted to therapeutic needs.
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