# Steradian

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## steradian

[stə′rād·ē·ən]
(mathematics)
The unit of measurement for solid angles; it is equal to the solid angle subtended at the center of a sphere by a portion of the surface of the sphere whose area equals the square of the sphere's radius. Abbreviated sr; sterad.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Steradian

a unit of measurement for solid angles. It is equal to the solid angle subtended at the center of a sphere by a portion of the sphere’s surface whose area is equal to the square of the sphere’s radius. The abbreviations used for the unit are sr and sterad. The solid angle of a sphere at its center is 4π steradians.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Fluence rate is wholly non-directional and includes UV radiation over 4[pi] steradians; that is, all of the possible directions from which UV rays might originate are included, and the influence of UV radiation received by the specified point is independent of direction.
Therefore the distribution of the first surface reflectance is determined by the distribution of the micro-facet normals which in turn is driven by the density function [equivalent to] ([THETA]), the relative density of micro-facet normals (per steradian) in vector direction [THETA].
where: F = flux density (W/[m.sup.2]-Hz) k = Boltzmann's constant (1.38 x [10.sup.-23] J/K) T = physical temperature of the blackbody (K) [lambda] = wavelength of the RF radiation (m) and [omega] = solid angle subtended by the blackbody source (steradians).
If the communication requirement is for the entire quadrant (hemispherical coverage), the function is normalized by [Pi]/2, the solid angle in steradians in a single quadrant.

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