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==== Solid Angle ====
**Definition:**\\
If an object of arbitrary shape subtends a region at the center of a sphere, and its projected area on a spherical surface of radius r is A, then the solid angle (denoted by the symbol $\Omega$) subtended by the object at the center of the sphere is:$$\Omega = \frac{A}{r^2}$$
**Unit:** steradian (sr)\\
{{ :yanding:成像基础知识:光学:辐射度学与光度学:立体角.png?300 |}}
(Image source: https://commons.wikimedia.org/wiki/File:Angle_solide_coordonnees.svg)