THE STERADIAN
 

The standard unit of solid angular measure is the steradian, symbolized sr. A solid angle of 1 sr is represented by a cone with its apex at the center of a sphere and intersecting the surface of the sphere in a circle such that, within the circle, the enclosed area on the sphere is equal to the square of the radius of the sphere. There are 4π, or approximately 12.56636, steradians in a complete sphere.