Physical size
Formula symbol ${\ displaystyle I}$
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 : ${\ displaystyle I}$${\ displaystyle \ mathrm {d} \ Phi}$ ${\ displaystyle \ Phi}$${\ displaystyle \ mathrm {d} \ Omega}$

${\ displaystyle I \ mathrm {d} \ Omega = \ mathrm {d} \ Phi}$

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: ${\ displaystyle 4 \ pi \, \ mathrm {sr}}$

${\ displaystyle I = {\ frac {\ Phi} {4 \ pi \, \ mathrm {sr}}} \,}$.

## 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 . ${\ displaystyle I _ {\ mathrm {v}}}$

## 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 ${\ displaystyle \ Phi _ {\ mathrm {e}}}$ W ( watt ) Radiant energy through time Luminous flux luminous flux, luminous power ${\ displaystyle \ Phi _ {\ mathrm {v}}}$ lm ( lumens ) Radiant intensity irradiance, radiant intensity ${\ displaystyle I _ {\ mathrm {e}}}$ W / sr Radiation flux through solid angles Luminous intensity luminous intensity ${\ displaystyle I _ {\ mathrm {v}}}$ cd = lm / sr ( candela ) Irradiance irradiance ${\ displaystyle E _ {\ mathrm {e}}}$ W / m 2 Radiation flux through the receiver surface Illuminance illuminance ${\ displaystyle E _ {\ mathrm {v}}}$ lx = lm / m 2 ( lux ) Specific radiation emission current density, radiant exitance ${\ displaystyle M _ {\ mathrm {e}}}$ W / m 2 Radiation flux through the transmitter surface Specific light emission luminous exitance ${\ displaystyle M _ {\ mathrm {v}}}$ lm / m 2 Radiance radiance, radiance, radiance ${\ displaystyle L _ {\ mathrm {e}}}$ W / m 2 sr Radiant intensity through effective transmitter area Luminance luminance ${\ displaystyle L _ {\ mathrm {v}}}$ cd / m 2 Radiant energy amount of radiation, radiant energy ${\ displaystyle Q _ {\ mathrm {e}}}$ J ( joules ) by radiation transmitted energy Amount of light luminous energy, quantity of light ${\ displaystyle Q _ {\ mathrm {v}}}$ lm · s Irradiation irradiation, radiant exposure ${\ displaystyle H _ {\ mathrm {e}}}$ J / m 2 Radiant energy through the receiver surface Exposure luminous exposure ${\ displaystyle H _ {\ mathrm {v}}}$ lx s Radiation yield radiant efficiency ${\ displaystyle \ eta _ {\ mathrm {e}}}$ 1 Radiation flux through absorbed (mostly electrical) power Luminous efficiency (overall) luminous efficacy ${\ displaystyle \ eta _ {\ mathrm {v}}}$ 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".