Steradian Steradian Soul Category="Natural units"Category="SI derived units"Category="Units of measure"The steradian (symbol: sr) is the SI unit of Solid angle. The name is partly derived from the Greek stereos for solid.

The steradian is useful to distinguish between quantities of different nature but the same dimension.

In practice, the symbol sr is used where appropriate, but the derived unit "1" is generally omitted in combination with a numerical value. 1 sr = m2·m–2 = 1

The steradian is the 3 dimensional equivalent of the 2 dimensional Radian.

As an example, radiant intensity can be measured in watts per steradian (Wsr-1)

The steradian is defined as "the Solid angle subtended at the center of a Sphere of RADIUS r by a portion of the surface of the sphere having an area r 2." Since the surface area of this sphere is 4πr 2, then the definition implies that a sphere measures 4π steradians. A steradian can also be called a squared radian.

A steradian is also equal to the spherical area of a Polygon having an Angle excess of 1 radian, to 1/4π of a complete Sphere, or to (180/π)2 or 3282.80635 square degrees .

The steradian was formerly an SI supplementary unit, but this category was abolished from the SI in 1995.

SI multiples

MultipleNameSymbolMultipleNameSymbol
100steradiansr   
101decasteradian dasr10–1decisteradian dsr
102hectosteradian hsr10–2centisteradian csr
103kilosteradian ksr10–3millisteradian msr
106megasteradian Msr10–6microsteradian µsr
109gigasteradian Gsr10–9nanosteradian nsr
1012terasteradian Tsr10–12picosteradian psr
1015petasteradian Psr10–15femtosteradian fsr
1018exasteradian Esr10–18attosteradian asr
1021zettasteradian Zsr10–21zeptosteradian zsr
1024yottasteradianYsr10–24yoctosteradianysr