The SI unit of solid angle is the steradian (sr), which is defined to be the solid angle that subtends an area equal to the square of the radius of a circle. By this definition, a full sphere subtends...The SI unit of solid angle is the steradian (sr), which is defined to be the solid angle that subtends an area equal to the square of the radius of a circle. By this definition, a full sphere subtends an area equal to the area of a sphere (4πr2) divided by the square of its radius (r2) - so a full sphere is 4π steradians, and a hemisphere is 2π steradians.
