The Steradian:
The unit of solid angular measure is the steradian, represented by sr. A solid angle of 1 sr is symbolized by a cone with its apex at the center of a sphere and intersecting the surface of the sphere in a circle like, within the circle, the enclosed region on the sphere is equivalent to the square of the radius of the sphere. There are 4π, or around 12.56636, steradians in a complete sphere.