Sun unit

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The sun unit (abbreviated SU) is a measure of sun intensity weighted for skin damage related to the UV index. In accordance with the International System of Units, the units for the intensity of solar radiation, or any of its biologically active components, are watts per square metre (W/m²), or joules per square metre (J/m²) (WHO 1979). Wavelength (lambda, λ) is expressed in nanometres (1×10⁻⁹ m). For photobiological research, the term UV fluence F is recommended instead of UV dose (Harm 1980) and its units are watts per hour per square metre (W/(h·m²)). Fluence is quantitatively the product of radiation intensity I and exposure time T (F = IT). Another photobiological unit of recent origin is the sun unit (SU), related to the erythemal effect of UV-B radiation (WHO 1979).

[edit] Basics of solar radiation

Solar radiation sustains life on earth and determines climate. The energy flow within the sun results in a surface temperature of around 5800 K, so the spectrum of the radiation from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in the cool peripheral solar gas (Fraunhofer lines).

[edit] Solar constant and "sun value"

The irradiance of the sun on the outer atmosphere when the sun and earth are spaced at 1 AU—the mean earth/sun distance of 149,597,870 km—is called the solar constant. Currently accepted values are about 1360 W/m² (the NASA value given in ASTM E 490-73a is 1353 ±21 W/m²). The World Metrological Organization (WMO) promotes a value of 1367 W/m². The solar constant is the total integrated irradiance over the entire spectrum (the area under the curve in Fig. 1 plus the 3.7% at shorter and longer wavelengths. The irradiance falling on the earth's atmosphere changes over a year by about 6.6% due to the variation in the earth/sun distance. Solar activity variations cause irradiance changes of up to 1%. For a solar simulator, it is convenient to describe the irradiance of the simulator in “suns.” One “sun” is equivalent to irradiance of one solar constant.