What is water-filtered infrared-A (wIRA)? Why wIRA?
The experience of the pleasant heat of the sun in mode- rate climatic zones arises from the filtering of the heat radiation of the sun by water vapor in the atmosphere of the earth , , , see Figure 1. Mankind has de- veloped in the evolution under the influence of this water- filtered heat radiation of the sun . In contrast to this in the desert the sun is stinging and burning, as the water vapor is missing there in the atmosphere of the earth. The filter effect of water decreases those parts of infrared radiation (most parts of infrared-B and -C and the absorp- tion bands of water within infrared-A), which would cause – by reacting with water molecules in the skin – only a thermal load to the surface of the skin , , , see Figure 1.
Technically water-filtered infrared-A (wIRA) is produced in special radiators, whose whole incoherent broad-band radiation of a 3000 Kelvin halogen bulb is passed through a cuvette, containing water, which absorbs or decreases the described undesired wavelengths within infrared , , , see Figure 2: an example of the resulting spec- trum with visible light (VIS) and water-filtered infrared-A (wIRA) is shown in Figure 3.
Within infrared the remaining wIRA (within 780-1400 nm) mainly consists of radiation with good penetration proper- ties into tissue and therefore allows – compared to un- filtered heat radiation of conventional infrared bulbs with large amounts of infrared-B (defined as 1400-3000 nm) and -C (defined as 3000-1,000,000 nm) – a multiple energy transfer into tissue without irritating the skin (high energy transfer with limited temperature increase), similar to sun heat radiation in moderate climatic zones , . Based on the water-filtering typical wIRA radiators emit almost no infrared-B and -C radiation and have a de- creased irradiance of the absorption bands of water within infrared-A. In contrast to the sun typical wIRA radi- ators emit no ultraviolet (UV) radiation and the amount of infrared-A radiation in relation to the amount of visible light (380-780 nm) is emphasized (depending on the fil- tering of the visible light, approximately 75% of the radi- ation is water-filtered infrared-A), see Figure 4.
The water-filtering leads to high penetration properties with a low thermal load to the surface of the skin, see Figure 5. This is the basis that wIRA is able to essentially improve even energy-related specific factors of tissue metabolism, especially in regeneration or impaired con- ditions like wounds. Compared to other infrared radiation sources, there is typically no sense of discomfort or burning during irradiation with wIRA when applying an appropriate irradiance.
Within the spectra of infrared-A and water-filtered infra- red-A radiation effects especially of the energy-rich wavelengths near to visible light – approximately 780- 1000 nm (800-900 nm , , , 800 nm , 820 nm , , , 830 nm ) – have been described both in vitro and in vivo, and these wavelengths seem to represent the clinically most important part within in- frared-A and wIRA , see as well section about non- thermal and non-thermic effects below.
For special purposes, like photodynamic therapy (PDT), the filtering of the visible light can be adapted to special recommendations, see section perspectives in .
Figure 1: Spectral solar irradiance outside the atmosphere and at sea level,
in both cases with the sun at the zenith and for a mean Earth-sun separation. Shaded areas indicate absorption at sea level due
to the atmospheric constituents shown (from , adapted from ).
For comparison of Figures 1, 3 and 4: 1000 W m–2 µm–1 = 100 mW cm–2 µm–1 = 1 mW cm–2 (10 nm)–1
Figure 2: Cross-section of a water-filtered infrared-A radiator (Hydrosun, Müllheim, Germany)
The whole incoherent broad-band radiation of a 3000 Kelvin halogen bulb is passed through a cuvette, containing water, which absorbs or decreases the undesired wavelengths within infrared (most parts of infrared-B and -C and the absorption bands of water within infrared-A). The water is hermetically sealed within the cuvette. A fan provides air cooling of the cuvette to prevent
the water from boiling.
Figure 3: Spectral irradiance of a water-filtered infrared-A radiator
(Hydrosun® radiator 501 with 10 mm water cuvette and orange filter OG590) at approximately 210 mW/cm² (= 2.1 x 10³ W/m²)
total irradiance (from ); (visible light (VIS): 380-780 nm; infrared-A (IR-A): 780-1400 nm; infrared-B (IR-B): 1400-3000 nm)
Figure 4: Comparison of the spectra of the sun at sea level and of a water-filtered infrared-A radiator
Spectral solar irradiance at sea level (with the sun at the zenith and for a mean Earth-sun separation) as in Fig. 1 (adapted from ) and spectral irradiance of a water-filtered infrared-A radiator (Hydrosun® radiator 501 with 10 mm water cuvette and orange
filter OG590) at approximately 210 mW/cm² (= 2.1 x 10³ W/m²) total irradiance as in Fig. 3 (from ).
The spectrum of the sun at sea level includes ultraviolet radiation (UV, <400 nm), visible light (VIS, 380-780 nm), and infrared
radiation (IR, >780 nm). The spectrum of the water-filtered infrared-A radiator includes only visible light (VIS) and infrared radiation
(IR); the visible part depends on the used color filter; the wIRA radiator does not emit ultraviolet radiation (UV).
Both spectra show the decreased irradiance of the absorption bands of water.
Figure 5: Comparison of irradiation with water-filtered infrared-A and with conventional infrared
Thermographical comparison of skin surface temperatures in the lumbar region 12 minutes after beginning of irradiation with water-filtered infrared-A (left) and conventional infrared (right) with the same irradiance: the skin surface temperature is higher in case of irradiation with conventional infrared (presented in the thermography), while temperature in 1 cm depth of tissue is higher when irradiating with water-filtered infrared-A (from ). So water-filtered infrared-A presents a high tissue penetration
combined with a low thermal load to the skin surface.