Applications of infrared radiation

An infrared lamp

The most powerful natural source of infrared radiation is the sun. Even in antiquity, the sun's "thermal radiation" was used to relieve a variety of complaints. By virtue of its beneficial effects, artificially generated infrared radiation therefore enjoys widespread applications in medicine and the wellness sector.

Effects

When infrared radiation strikes biological tissue, it causes molecules to vibrate, producing heat and causing the temperature to rise. As human tissue is largely made up of water, the absorption capacity of water for the various wavelengths of incident infrared radiation plays a key role in determining the penetration depth and effects of the radiation.

Penetration depth

However, water has an absorption minimum in the red region of visible light as well as in the adjacent infrared region of the spectrum (short-wavelength IR-A). This explains the relatively large penetration depth of IR-A radiation (780–1,400 nanometres), which can penetrate up to some 5 millimetres into the skin, allowing it to reach the hypodermis and act on it directly. In general, the shorter the wavelength of infrared radiation, the greater the penetration depth. IR-C (3,000 nanometres – 1 millimetre) and IR-B (1,400–3,000 nanometres) are absorbed in the upper layer of skin, the epidermis, which is the only layer on which they have a direct effect. The thermal effects of IR-A radiation are spread over a larger volume than that of IR-B and IR-C radiation, but indirect heat conduction allows the temperature increase to affect deeper layers as well.

The therapeutic effects of heat

Infrared radiation can promote local blood circulation and reduce muscle tension. Examples of traditional medical applications of infrared radiation include the relief of muscle pain and tension, as well as the treatment of autoimmune diseases or wound-healing disorders. However, the question of whether it is sensible to treat an illness or complaint with heat, or whether it may in fact be harmful to do so, must always be assessed by a doctor on a case-by-case basis.

Excess temperatures are harmful

Adverse effects are particularly likely if the temperature increase and exposure time exceed critical limits. Excessive exposure can result in damage or even burns. In general, thermal burden can lead to disturbances in the heat balance of the entire organism.

The eyes are particularly sensitive to thermal effects. Suitable protective goggles can protect the eyes against excessive exposure to infrared radiation.

The body’s warning and protection system

Heat and pain receptors are primarily located in the upper layer of the dermis, just beneath the epidermis, and act as the body's warning and protection system. They are less sensitive to IR-A radiation than to IR-C and IR-B radiation, which acts near the surface and on a smaller volume. This might well be the therapeutic intention in medical applications that are designed to produce greater warming of deeper regions. However, this factor must be taken into account in any application of infrared radiation.