The steradian (symbol: sr) is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane. The name is derived from the Greek stereos for "solid" and the Latin radius for "ray, beam". The steradian, like the radian, is dimensionless because 1 sr = m2·m−2 = 1. It is useful, however, to distinguish between dimensionless quantities of different nature, so in practice th… (More on Steradian)