Noun
steradian (plural steradians)
(geometry) In the International System of Units, the derived unit of solid angle; the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere that has area r. Symbol: sr
Definition Section of cone (1) and spherical cap (2) that subtend a solid angle of one steradian inside a sphere A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface. Source: Internet
Therefore one steradian corresponds to the plane (i.e. radian) angle of the cross-section of a simple cone subtending the plane angle 2θ, with θ given by: : This angle corresponds to the plane aperture angle of 2θ ≈ 1.144 rad or 65.54°. Source: Internet
For example, radiant intensity can be measured in watts per steradian (W·sr −1 ). Source: Internet