Steradian (sr): multiples and submultiples 
symbol: sr
The steradian or square radian is the unit of solid angle in the International System of Units (SI). It is used in threedimensional geometry, and is analogous to the radian, which quantifies planar angles. A steradian can be defined as the solid angle subtended at the centre of a unit sphere by a circular unit area on its surface. The solid angle is related to the area it cuts out of a sphere: Ω = (A / r^{2}) * sr Ω is the solid angle A is the surface area of the spherical cap r is the radius of the sphere 

solid angle  
measurement units  