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Showing 1681–1709 of 1709 reference entries

Measurement · PhysicsRadiant flux, intensity, irradiance, exitance and radiance

Optical power, directional power and surface densities are different quantities.

DefinitionMeasurement · PhysicsPhotometric weighting, lumen, candela, lux and luminance

Photometry weights radiation with a specified standard visual response.

DefinitionMeasurementWriting SI quantities and unit symbols

Symbol form, spacing and parentheses preserve the meaning of reported quantities.

DefinitionMeasurementGram prefixes and the kilogram exception

Mass prefixes attach to gram even though kilogram is the SI base unit.

DefinitionMeasurementRounding a result with its uncertainty

Report useful digits without treating extra digits as proof of accuracy.

MethodMeasurement · Physics · ChemistryCommon non-SI units and exact SI conversions

Common non-SI unit names require their correct SI scale factors.

DefinitionPhysics · ChemistryGas pressure as molecular momentum transfer

Molecular collisions link macroscopic pressure to random translational motion.

ModelChemistry · PhysicsOrdinary aggregate states and bulk behavior

Solid, liquid and gas describe bulk behavior rather than chemical identity.

ModelChemistryInferring empirical and molecular formulas from composition

Element mass ratios become atom ratios through molar masses, with uncertainty and sample identity retained.

MethodChemistryLewis structures and valence-electron accounting

A Lewis diagram accounts for bonding and nonbonding valence electrons in a specified connectivity.

MethodChemistryElectron-domain geometry and molecular shape

VSEPR distinguishes the arrangement of electron regions from the positions of atoms.

ModelChemistryElectronegativity, bond polarity and molecular polarity

Polar bonds can cancel in a molecule; molecular geometry matters.

ModelChemistrySolutions, solutes, solvents and solvation

Dissolution disperses specified molecular or ionic species in a solvent.

DefinitionChemistrySolubility, saturation and supersaturation

A saturation limit is an equilibrium property under specified conditions, not a synonym for concentration.

DefinitionChemistryFiltration separates retained solids from a fluid

Ordinary filtration separates undissolved particles; dissolved species can remain in the filtrate.

MethodChemistryDistillation and vapor–liquid composition

Distillation uses vapor–liquid partitioning and condensation, with a limit at an azeotrope.

MethodBiologyGene, allele, genotype and phenotype

Genomic sequence variants and observable traits describe different levels of biological information.

DefinitionBiology · ChemistryEnzyme substrates, active sites and induced fit

Substrate recognition and catalysis depend on a dynamic molecular environment.

ModelBiology · ChemistryMonomers, polymers and hydrolysis

Covalent subunit linkage and water-assisted cleavage explain biological polymer structure.

DefinitionBiology · ChemistrySugars, glycosidic linkages and polysaccharides

Sugar connectivity distinguishes energy-storage and structural polysaccharides.

DefinitionBiology · ChemistryFatty acids, storage lipids and membrane lipids

Lipids include several architectures rather than one repeating-monomer polymer class.

DefinitionBiology · ChemistryAmino-acid residues and peptide linkages

The peptide backbone and variable side chains are different parts of protein structure.

DefinitionBiology · ChemistryNucleotides and DNA/RNA backbone polarity

Bases carry sequence information on a chemically directional sugar–phosphate backbone.

DefinitionBiologyCell membrane, cell wall, cytosol and compartments

A cell's boundary, fluid interior and specialized structures have different roles.

DefinitionEarth & cosmos · MeasurementEarthquake location, source magnitude and local intensity

A source location or magnitude does not describe the shaking experienced at every site.

DefinitionEarth & cosmosAtmospheric composition and thermal layers

Composition and temperature structure describe different features of Earth's atmosphere.

ModelEarth & cosmosDrainage basins, outlets and water divides

Surface drainage boundaries depend on the chosen outlet and need not bound groundwater flow.

DefinitionEarth & cosmosThe celestial sphere and the observer's sky

A sky direction is an angular observation, not an object's three-dimensional distance.

ModelEarth & cosmos · MeasurementRight ascension, declination and reference epochs

Celestial angular coordinates require a declared reference system and date conventions.

Definition
DefinitionSource checked

Radiant flux, intensity, irradiance, exitance and radiance

MeasurementPhysics

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.

  1. International Commission on Illumination — CIE S 017:20205 sources
    1. International Lighting Vocabulary, 2nd edition: radiant fluxInternational Commission on Illumination — CIE S 017:2020Reference guidecie.co.atRead source
    2. International Lighting Vocabulary, 2nd edition: radiant intensityInternational Commission on Illumination — CIE S 017:2020Reference guidecie.co.atRead source
    3. International Lighting Vocabulary, 2nd edition: irradianceInternational Commission on Illumination — CIE S 017:2020Reference guidecie.co.atRead source
    4. International Lighting Vocabulary, 2nd edition: radiant exitanceInternational Commission on Illumination — CIE S 017:2020Reference guidecie.co.atRead source
    5. International Lighting Vocabulary, 2nd edition: radianceInternational Commission on Illumination — CIE S 017:2020Reference guidecie.co.atRead source
  2. Photometric Calibrations, NIST Special Publication 250-95 (2018)Yuqin Zong, Maria E. Nadal, Benjamin K. Tsai and C. Cameron Miller — NISTReference guide · PDFnvlpubs.nist.govRead source
  3. Heat and Mass Transfer · Radiation modulePradip Dutta · IIScCourse notes · PDFarchive.nptel.ac.inRead source
  4. O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileiraInmetro and Instituto Português da QualidadeReference guide · PDFgov.brRead source

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.

Related reference entries

Explore relationships within this area of study and its connections to other areas.

Showing 1–3 of 3 relationships

Scientific relationships · 3 / 3

The relationship type describes the connection. Its linked statements, sources and conditions define the scope.

  1. Uses definition
    Relationship sources · 6

    Linked statements

    • 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).

    Directional flux densities use the solid-angle definition.

  2. Related to
    Relationship sources · 6

    Linked statements

    • 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).

    The existing σT⁴ relation describes blackbody radiant exitance, rather than directional radiant intensity.

  3. Uses definition
    Relationship sources · 3

    Linked statements

    • 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.

    Photometric densities use corresponding optical geometry after applying the specified visual weighting.

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