The steradian (symbol: sr) is the
SI unit of
solid angle. It is used to describe two-dimensional angular spans in three-
dimensional space, analogous to the way in which the
radian describes angles in a
plane. The name is derived from the
Greek stereos for "solid" and the
Latin radius for "ray, beam". The steradian is
dimensionless because 1 sr = m2·m-2 = 1. It is useful, however, to distinguish between dimensionless quantities of different nature, so in practice the symbol "sr" is used where appropriate, rather than the derived unit "1" or no unit at all. As an example,
radiant intensity can be measured in watts per steradian (W·sr-1).
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Unit of solid angle, i.e. in three dimensions.
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