GEOMETRY OF RADIATION Plane Angle Solid Angle Differential Solid Angle ω unit circle unit sphere 1 α ω α = arc length [ radian ] ω = area on a unit sphere α [ steradian], [ sr ] dA r α s ω A dω = = circle of 2π ⋅ 1 2π ⋅ r sphere of r 4π ⋅ 12 4π ⋅ r 2 radius r