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Physical size
Surname Radiance
Formula symbol
Size and
unit system
unit dimension
SI W · sr -1 M · L 2 · T −3

Radiation intensity or radiation intensity , also called radiation intensity (but the term is also used in a different meaning - see intensity (physics) ), is a term from radiometry . The radiant intensity is the portion of the radiant power that is emitted by a radiation source (e.g. light source ) in a given spatial direction into the solid angle element of any small size :

The power is measured in watts (W), the solid angle in steradians (sr); the SI unit of the radiation intensity is therefore W / sr.

If the energy is radiated isotropically , i.e. within the entire solid angle of equally in each direction, the radiation intensity in each direction is:


Photometric equivalent

The corresponding quantity in photometry is the light intensity . It results from the radiant intensity by them with the photometric radiation equivalent , which includes the sensitivity curve of the human eye, weighted .

Relationship with other radiometric quantities

radiometric quantity Symbol a) SI unit description photometric equivalent b) symbol SI unit
Radiant flux
radiant power, radiant flux, radiant power
( watt )
Radiant energy through time Luminous flux
luminous flux, luminous power
( lumens )
Radiant intensity
irradiance, radiant intensity
W / sr Radiation flux through solid angles Luminous intensity
luminous intensity
cd = lm / sr
( candela )
W / m 2 Radiation flux through the receiver surface Illuminance
lx = lm / m 2
( lux )
Specific radiation
emission current density, radiant exitance
W / m 2 Radiation flux through the transmitter surface Specific light emission
luminous exitance
lm / m 2
radiance, radiance, radiance
W / m 2 sr Radiant intensity through effective transmitter area Luminance luminance
cd / m 2
Radiant energy
amount of radiation, radiant energy
( joules )
by radiation transmitted energy Amount of light
luminous energy, quantity of light
lm · s
irradiation, radiant exposure
J / m 2 Radiant energy through the receiver surface Exposure
luminous exposure
lx s
Radiation yield
radiant efficiency
1 Radiation flux through absorbed (mostly electrical) power Luminous efficiency
(overall) luminous efficacy
lm / W
a)The index "e" is used to distinguish it from the photometric quantities. It can be omitted.
b)The photometric quantities are the radiometric quantities, weighted with the photometric radiation equivalent K , which indicates the sensitivity of the human eye.

Individual evidence

  1. Ludwig Bergmann , Clemens Schaefer : Optics: Wave and particle optics . In: Bergmann-Schaefer textbook on experimental physics . tape 3 . Walter de Gruyter, 2004, ISBN 3-11-017081-7 , p. 637 ( limited preview in Google Book search).
  2. electropedia , International Electrotechnical Dictionary (IEV) of the International Electrotechnical Commission : Entries 731-01-23 (area "fiber optic communication") and 845-01-30 (area lighting) have the translation: radiant intensity = "radiation intensity"
  3. Robert Wichard Pohl : Introduction to Optics . 8th edition. Springer, Heidelberg 1948, ISBN 978-3-662-01855-2 , p.  321 ff . (Online: [1] )
  4. electropedia , International Electrotechnical Dictionary (IEV) of the International Electrotechnical Commission : Entry 705-02-04 (section "Communication by radio waves") has the translation radiation intensity = "Radiation intensity (in a given direction)"; the German edition of the dictionary , published by the German Commission for Electrical, Electronic and Information Technologies , adds the footnote: "In lighting technology, preferably referred to as radiation intensity".