solid angle
A surface formed by rays with a common origin, the vertex of the solid angle, and passing through a closed curve (a polyhedral angle is a special type of angle for which the curve is a polygon).

The unit for measuring solid angles is defined analogously to that for measuring the plane angles - radian. Since the area of a sphere is 4symbol PIr2, the total value of the solid angle at the center is 4symbol PI steradians. In other words, a full sphere represents a spherical angle of 4symbol PI steradians.

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