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Solid angle is derived from the Greek stereos, and the symbol for solid angle is Ω (Greek letter omega). The steradian (symbol: sr) is the SI unit (International System of Units) of solid angle. The solid angle can be defined in terms of an area on the surface of a sphere which is centred at the vertex of the angle. The solid angle is a dimensionless measure (like the radian) of how big an object appears to an observer (a point).
Solid angle can be simply given by,
 [2671a]
Therefore, dΩ, as indicated in Figure 2671, is given by,
 [2671b]
Figure 2671. Schematic illustration of scattering of an electron by a single atom.
Everyone knows what a 90° angle looks like, and even referring to it as π/2 radians is relatively easy to understand. However, it is a different matter altogether with solid angles since it must be imagined in threedimensional (3D) space without an obvious reference. For instance, one steradian is equal to (180/π)^{2} square degrees. Table 2671 lists some typical values of solid angles of some specific shapes.
Table 2671. Typical values of solid angles of specific shapes.
Shapes 
Solid angle measured at the centre (sr) 
Complete sphere 
4π 
Face of a cube 
2π/3 
If the solid angle Ω in steradians is known, then the corresponding area of the surface of any sphere can be calculated by,
A_{S} = R^{2}·Ω  [2671c] where,
R  The radius of the sphere.
