Steradian

The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. The symbol is sr.

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." It is also equal to the spherical area of a polygon having an angle excess of 1 radian, to 1/4&pi of a complete sphere, or to 57.2957795 degrees squared.

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