Solid angle and steradian
A solid angle measures a cone of directions rather than a plane rotation.
Key knowledge
The solid angle Ω subtended at a point is the area cut out on a sphere centred at that point divided by the sphere’s squared radius. Its coherent SI unit is the steradian (sr), with dimension one. A whole sphere subtends 4π sr. Keeping sr in optical units helps distinguish radiant intensity from flux even though the SI dimensions alone do not distinguish the quantities.
Values and equations
Ω=A_s/r²; Ω_whole_sphere=4π sr
- Ω,Ω_whole_sphere
- Solid angle and solid angle of the entire sphere sr
- A_s
- Area intercepted on the centred sphere m²
- r
- Positive radius of that sphere m
Conditions and scope
- Use the area on the centred sphere, not the unprojected area of an arbitrary target surface; r>0.
- Plane angle in radians and solid angle in steradians are different kinds of quantity.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- International Lighting Vocabulary, 2nd edition: solid angleInternational Commission on Illumination — CIE S 017:2020Reference guide
- The International System of Units, 9th edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guide
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileiraInmetro and Instituto Português da QualidadeReference guide · PDF
Evidence and review
Solid angle is centred spherical area divided by radius squared, with steradian units and full-sphere value 4π.
2026-10-10 · Independent Chemistry, Biology and Earth/Cosmos editor (AI editorial review). Source checks and independent AI editorial review of compact reference claims, equations, units, conditions and conceptual mappings. This is not human specialist certification, laboratory accreditation, whole-book verification, or a claim that all possible foundational knowledge is present.
Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
Steradians describe solid angle through spherical area divided by squared radius; a complete sphere subtends 4π sr.
A generic target area is not interchangeable with its corresponding spherical patch.
Inmetro and Instituto Português da Qualidade
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira Brazil and PortugalPortugueseShared standard
2.3.4 Table4 note(c), printed p14 / PDF p30
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira
Entry details
Permanent link to this entry ↗- Record identity
- foundation-measurement-solid-angle
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion