Recall how a (plane) angle is defined in two-dimensions Let a small
transverse line element Dl be placed at a distance r from a point O Then
the angle subtended by Dl  at O can be approximated as Dq = Dl/r Likewise, in three-dimensions the solid angle* subtended by a small
perpendicular plane area DS, at a distance r, can be written as
DW = DS/r2