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A graphical representation o 1 steradian. The sphere haes radius r, an in this case the aurie o the patch on the surface is A = r2. The solid angle is θ = A/r2 so in this case θ = 1. The entire sphere haes a solid angle o 4π sr ≈ 12.56637 sr.

The steradian (seembol: sr) (or squared radian) is the SI unit o solid angle. It is uised in three-dimensional space, an functions analogously tae the manner in which the radian quantifies planar angles. The name is derived frae the Greek stereos for "solid" an the Laitin radius for "ray, beam".

The steradian, lik the radian, is dimensionless, essentially acause a solid angle is the ratio atween the aurie subtendit an the square o its distance frae the vertex: baith the numerator an denominator o this ratio hae dimension length2. It is uisefu, however, tae distinguish atween dimensionless quantities o different nature, so in practice the seembol "sr" is uised tae indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W·sr−1). The steradian wis umwhile an SI supplementary unit, but this category wis abolished frae the SI in 1995 an the steradian is nou considered an SI derived unit.