Sun unit

Not to be confused with Sun (unit).

Not to be confused with the traditional Japanese unit of length, see cun (length)

The sun unit (abbreviated SU) is a measure of sun intensity weighted for skin damage related to the UV index.

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).

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.