The quantity solid angle (denoted by the Greek letter Ω) is a measure of
the field of view that a given object covers.
The coresponding unit is steradian (sr).
In the Figure, the solid angle subtended by the area A (in m2)
on the surface of the sphere with radius r (in m) is:
Ω = A × r -2
The solid angle subtended by the whole sphere measered from its interior is 4π sr.