steradi

Steradi is not a recognized term in standard physics or metrology. In most English-language sources, the unit of solid angle is the steradian, with the plural steradians. The form “steradi” may appear as a misspelling or as a nonstandard plural in some non-English texts, but it is not used in formal documentation.

A steradian measures solid angle, the three-dimensional analogue of the radian for planar angles. It is defined as the ratio of a surface area on a sphere to the square of the sphere’s radius: Ω = A / r^2. For a unit sphere (radius = 1), the solid angle directly equals the subtended surface area. The total solid angle around a point is 4π steradians, which corresponds to the surface area of a unit sphere. In differential form, dΩ = sinθ dθ dφ, where θ is the polar angle and φ the azimuthal angle.

Steradians are used in radiometry and photometry to describe how much of the sky or a light source is being sampled or emitted. They appear in expressions for radiant intensity, luminous flux, and irradiance, often in conjunction with angular distributions. For example, the solid angle of a cone with apex angle 2θ is Ω = 2π(1 − cosθ). Because steradian is a dimensionless unit, care is taken to distinguish angular measures from plain ratios in calculations.

Etymology-wise, the term steradian combines a root meaning solid (stere-) with radian, the unit of plane angle. In standard usage, the correct term remains steradian; “steradi” is not an accepted plural in formal contexts.