Sun unit

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).
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The sun as a unit of solar energy concentration. 1 sun is equal to the intensity of sunlight on a perpendicular surface, whereas practical concentrators of solar energy reach 10,000 suns or more.The maximum theoretically possible is about 200,000 suns, that number being the reciprocal of the sun's disk as a fraction of the sphere. That is, if the entire sphere were as bright as the sun, which is achieved with optimal concentration.
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