The steradian (sr) is the SI unit of the solid angle (Greek letter Omega). According to calculator.org, one definition of a solid angle is a certain area (for example, a circle) on a sphere's surface centered at the angle's vertex. The solid angle for a sphere can be calculated by the formula Omega = (S/r squared), where "S" is the "surface area subtended by the intersection of the sphere and the cone."
Instructions
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1
Decide if you have an area that has already been calculated. A sphere measures 4pi steradians, so half a sphere would measure 4pi/2.
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2
Square the radius of the sphere.
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3
Divide the subtended area by the amount you calculated in Step 2.