Steradian  

A graphical representation of 1 steradian.
The sphere has radius r, and in this case the area A of the highlighted surface patch is r^{2}. The solid angle Ω equals [A/r^{2}] sr which is 1 sr in this example. The entire sphere has a solid angle of 4πsr.


Unit information  
Unit system  SI derived unit 
Unit of  Solid angle 
Symbol  sr 
The steradian (symbol: sr) or square radian^{[1]}^{[2]} is the SI unit of solid angle. It is used in threedimensional geometry, and is analogous to the radian which quantifies planar angles. Whereas an angle in radians, projected onto a circle, gives a length on the circumference, a solid angle in steradians, projected onto a sphere, gives an area on the surface. The name is derived from the Greek στερεός stereos 'solid' + radian.
The steradian, like the radian, is a dimensionless unit, essentially because a solid angle is the ratio between the area subtended and the square of its distance from the center: both the numerator and denominator of this ratio have dimension length squared (i.e. L^{2}/L^{2} = 1, dimensionless). It is useful, however, to distinguish between dimensionless quantities of a different nature, so the symbol "sr" is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W⋅sr^{−1}). The steradian was formerly an SI supplementary unit, but this category was abolished in 1995 and the steradian is now considered an SI derived unit.
A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface. For a general sphere of radius r, any portion of its surface with area A = r^{2} subtends one steradian.^{[3]}
The solid angle is related to the area it cuts out of a sphere:
Because the surface area A of a sphere is 4πr^{2}, the definition implies that a sphere measures 4π (≈ 12.56637) steradians. By the same argument, the maximum solid angle that can be subtended at any point is 4π sr.
Since A = r^{2}, it corresponds to the area of a spherical cap (A = 2πrh) (where h stands for the "height" of the cap), and the relationship h/r = 1/2π holds. Therefore, one steradian corresponds to the plane (i.e. radian) angle of the crosssection of a simple cone subtending the plane angle 2θ, with θ given by:
This angle corresponds to the plane aperture angle of 2θ ≈ 1.144 rad or 65.54°.
A steradian is also equal to the spherical area of a polygon having an angle excess of 1 radian, to 1/4π of a complete sphere, or to (180/π)^{2} ≈ 3282.80635 square degrees.
The solid angle of a cone whose crosssection subtends the angle 2θ is:
Millisteradians (msr) and microsteradians (μsr) are occasionally used to describe light and particle beams.^{[4]}^{[5]} Other multiples are rarely used.
Solid angles over 4π—the solid angle of a full Euclidean sphere—are rarely encountered.
Look up steradian in Wiktionary, the free dictionary. 
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