Photometric weighting, lumen, candela, lux and luminance
Photometry weights radiation with a specified standard visual response.
Key knowledge
Photopic luminous flux weights spectral radiant flux by the standard V(λ) response. Luminous intensity is luminous flux per solid angle; illuminance is incident luminous flux per receiving area; luminance resolves flux per projected area and direction. Their units are lumen, candela, lux and cd/m² respectively. These quantities are not physical radiant power or a direct measurement of an individual observer’s subjective brightness.
Values and equations
Φ_v=K_m∫₀∞Φ_e,λ(λ)V(λ)dλ; I_v=dΦ_v/dΩ; E_v=dΦ_v,in/dA; L_v=d²Φ_v/(dA cosθ dΩ)
- Φ_v,Φ_v,in
- Luminous flux and incident luminous flux lm
- K_m
- Maximum luminous efficacy for the normalized standard spectral convention lm W⁻¹
- Φ_e,λ
- Spectral radiant flux per unit wavelength W m⁻¹
- λ
- Wavelength coordinate for the stated spectral convention m
- V(λ)
- Standard normalized photopic spectral response 1
- I_v
- Luminous intensity cd = lm sr⁻¹
- E_v
- Illuminance lx = lm m⁻²
- L_v
- Luminance cd m⁻²
- Ω
- Solid angle sr
- A
- Receiving or emitting area m²
- θ
- Angle defining positive projected area, cosθ>0 rad
Conditions and scope
- The weighting formula uses the specified standard photopic observer, a consistently defined wavelength coordinate and its matching V(λ) function; other visual adaptation regimes require other conventions.
- The directional derivatives and projected-area conditions follow the corresponding radiometric definitions; I_v strictly uses a point-source model.
- K_m is the maximum efficacy associated with the chosen normalized spectral convention; it is not to be identified without qualification with the defining K_cd at exactly 540 THz.
Sources · 5
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- Photometric Calibrations, NIST Special Publication 250-95 (2018)Yuqin Zong, Maria E. Nadal, Benjamin K. Tsai and C. Cameron Miller — NISTReference guide · PDF
International Commission on Illumination — CIE S 017:20202 sources
- Elementos de FotometriaGuido Stolfi · Escola Politécnica da USPCourse notes · PDF
- 国際単位系（SI）第9版 · Japanese translation, V3.01National Metrology Institute of Japan, AISTReference guide · PDF
Evidence and review
Standard photopic weighting converts spectral radiant quantities to photometric quantities; maximum luminous efficacy and the SI defining efficacy constant have distinct roles.
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
Standard-observer spectral weighting relates radiometric and photometric quantities, with luminous flux, intensity, illuminance and luminance having their distinct units.
The lecture simplifies the 683 factor; ARK distinguishes exact Kcd at the defining frequency from the spectral maximum and from subjective brightness.
Universidade de São Paulo · Escola Politécnica
- Elementos de Fotometria BrazilPortugueseShared standard
2.1–2.3 and 2.5, pp5–7
- Elementos de Fotometria
National Metrology Institute of Japan, AIST
- 国際単位系（SI）第9版 · Japanese translation, V3.01 JapanJapaneseShared standard
2.3.1 Candela, pp19–20
- 国際単位系（SI）第9版 · Japanese translation, V3.01
Entry details
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