Steradian

The steradian (symbol: sr) or square radian is an SI unit. It is like a radian, but for smooth 3D objects, for example, a sphere. A steradian is about one twelfth the surface area of a sphere, regardless of how big the sphere is.[1]

A steradian on a sphere

Sphere vs Steradian
  • The surface area of a sphere is
  • The surface area of a steradian is just

So a sphere measures 4π steradians, or about 12.57 steradians. Likewise a steradian is 1/12.57, or about 8% of a sphere. And because we measure an angle, it doesn't matter what size the sphere is, it will always measure 4π steradians.[2]

References
  1. Weisstein, Eric W. "Steradian". mathworld.wolfram.com. Retrieved 2020-03-27.
  2. "Steradian". www.mathsisfun.com. Retrieved 2020-03-27.
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