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 \left(4 \pi r^{2}\right) divided by the square of its radius \left(r^{2}\right) - so a full sphere is 4 \pi steradians, and a hemisphere is 2 \pi steradians.
