In geometry, an **angle** can be defined as the figure formed by two rays meeting at a common endpoint (vertex).

## Solid angle

A **solid angle** is a 3D angle formed by a conical surface angular volume that is defined analogously to the definition of a plane angle in two dimensions.

Considering its vertex to lie at the center of a sphere, then a measure of its magnitude may be obtained from the ratio between the area (*L*^{2}) of the surface of the sphere cut off by the angle, and the square (*R*^{2}) of the sphere’s radius. Solid angles are measured in steradians (sr), an angle of one steradian subtending an angle of area *R*^{2} at a distance *R*.