Steradian
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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 partly derived from the Greek stereos for "solid".
The steradian is dimensionless, since 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).
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 r2." Since the surface area of this sphere is 4πr2, then the definition implies that a sphere measures 4π steradians. By the same argument, the maximum solid angle that can be subtended at any point is 4π sr. 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.cs:Steradián da:Steradian de:Steradiant es:Estereorradián eo:Steradiano fr:Stéradian ko:스테라디안 it:Steradiante he:סטרדיאן ms:Steradian nl:Steradiaal ja:ステラジアン no:Steradian nn:Steradian pl:Steradian pt:Esferorradiano ru:Стерадиан sk:Steradián sr:Стерадијан fi:Steradiaani sv:Steradian uk:Стерадіан zh:球面度