Radiant flux, intensity, irradiance, exitance and radiance
Optical power, directional power and surface densities are different quantities.
Key knowledge
Radiant flux Φ_e is radiant energy transferred per unit time. Radiant intensity I_e is emitted flux per solid angle for a point source. Irradiance E_e measures incident flux per receiving area; radiant exitance M_e measures outgoing flux per area. Radiance L_e resolves flux by both direction and projected area. Thus intensity in W/sr is not interchangeable with irradiance in W/m² or radiance in W/(m² sr).
Values and equations
Φ_e=dQ_e/dt; I_e=dΦ_e/dΩ; E_e=dΦ_e,in/dA; M_e=dΦ_e,out/dA; L_e=d²Φ_e/(dA cosθ dΩ)
- Φ_e,Φ_e,in,Φ_e,out
- Radiant flux and its incident/outgoing components W
- Q_e
- Radiant energy transferred J
- t
- Time s
- I_e
- Radiant intensity of the point-source model W sr⁻¹
- E_e,M_e
- Irradiance and radiant exitance W m⁻²
- L_e
- Radiance of the resolved beam W m⁻² sr⁻¹
- Ω
- Resolved solid angle sr
- A
- Actual receiving or emitting surface area m²
- θ
- Angle between the relevant outward or receiving normal and the resolved ray direction rad
Conditions and scope
- The derivatives describe local densities when they exist; finite quotients give averages over the stated area or solid angle.
- Radiant intensity strictly uses a point-source model; an extended source requires location-dependent radiance or a justified far-field approximation.
- For the radiance expression take dA cosθ as the positive projected area, cosθ>0, and resolve a directed beam element.
Sources · 8
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
International Commission on Illumination — CIE S 017:20205 sources
- International Lighting Vocabulary, 2nd edition: radiant fluxInternational Commission on Illumination — CIE S 017:2020Reference guide
- International Lighting Vocabulary, 2nd edition: radiant intensityInternational Commission on Illumination — CIE S 017:2020Reference guide
- International Lighting Vocabulary, 2nd edition: irradianceInternational Commission on Illumination — CIE S 017:2020Reference guide
- International Lighting Vocabulary, 2nd edition: radiant exitanceInternational Commission on Illumination — CIE S 017:2020Reference guide
- International Lighting Vocabulary, 2nd edition: radianceInternational Commission on Illumination — CIE S 017:2020Reference guide
- Photometric Calibrations, NIST Special Publication 250-95 (2018)Yuqin Zong, Maria E. Nadal, Benjamin K. Tsai and C. Cameron Miller — NISTReference guide · PDF
- Heat and Mass Transfer · Radiation modulePradip Dutta · IIScCourse notes · PDF
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileiraInmetro and Instituto Português da QualidadeReference guide · PDF
Evidence and review
Radiant energy, flux, intensity, irradiance and radiance have distinct time, area, direction and projected-area meanings and units.
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
Scope or convention differs
Directional emitted power per projected area and solid angle has radiance dimensions; total radiant power is measured in watts.
The heat-transfer notes call radiance intensity. The repository keeps radiant intensity (W/sr) distinct; this added source does not independently cover every local differential quantity.
Indian Institute of Science · NPTEL
- Heat and Mass Transfer · Radiation module IndiaEnglish
Projected area and intensity, PDF pp7–8
- Heat and Mass Transfer · Radiation module
Inmetro and Instituto Português da Qualidade
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira Brazil and PortugalPortugueseShared standard
2.3.4 Table4 watt and 2.3.4 examples of radiometric derived units
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira
Entry details
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- foundation-measurement-radiometric-quantities
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion