Steradian (sr)
Unit used to express the dimensionless quantity of solid angleFraction of the surface of a sphere covered by a particular object, as seen by an observer at the sphere's center. For a small region with area, Δa, the numerical value of the solid angle is: where r is the radius of the sphere. Although the solid angle is a. A sphere subtends a solid angle of 4p (»12.5663706) for an observer at the centre of the sphere. This factor appears in the formula for the surface area, SA, of a sphere with radius, r:
