|steradian (stĭ-rā'dē-ən) Pronunciation Key
A unitless measure of solid angles. A solid angle projecting from the center of a sphere and cutting its surface has a measure of s/r2 steradians, where s is the surface area of the sphere cut out by the solid angle, and r is the radius of the sphere. See also radian.
unit of solid-angle measure in the International System of Units (SI), defined as the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere's radius. Since the complete surface area of a sphere is 4pi times the square of its radius, the total solid angle about a point is equal to 4pi steradians. Derived from the Greek for solid and the English word radian, a steradian is, in effect, a solid radian; the radian is an SI unit of plane-angle measurement defined as the angle of a circle subtended by an arc equal in length to the circle's radius
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