Steradian
A unit of solidangle measure in the International System of Units (SI)
The steradian is a unit of solidangle measure in the International System of Units (SI). It is defined as
 the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere’s radius.
The complete surface area of a sphere is 4π times the square of its radius and
 the total solid angle about a point is equal to 4π steradians.
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