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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 4
r2, the total value of the solid angle at the center is 4 steradians. In other words, a full sphere represents a spherical angle of 4 steradians.
