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Physical size
Surname Irradiance
Formula symbol ,
Size and
unit system
unit dimension
SI W · m -2 M · T −3

The irradiance ( Engl. : Irradiance , radiant flux density , and the radiation flux density , outdated: radiation current density ) is the term for the entire power of the incoming electromagnetic energy impinging on a surface, based on the size of the area.

The photometric equivalent of the irradiance is the illuminance E v , which also includes the special properties of human perception. To distinguish this, the symbol E e is often used for the irradiance , whereby the index “e” means that the irradiance is purely energetic , i.e. H. is an objective measurand . In the field of electrical engineering, irradiance is often used synonymously with intensity , but the latter generally refers to waves.

Analogous to the irradiance, there is the specific radiation , which describes the radiant power emanating from an area per area. It should not be confused with irradiation (measured in J⋅m −2 ), which describes the accumulated energy per unit area as a time-integrated quantity.


The irradiance is defined as the radiant flux  through the irradiated area  :


  • = Radiance
  • = Angle of the solid angle element to the surface normal . The cosine factor takes into account that in the case of irradiation from any given direction, only the projection of the surface perpendicular to this direction occurs as the effective receiving surface.
  • = Solid angle element.

General definition in the field

The radiation distribution more generally, i. H. not necessarily collimated , radiation is given by a direction-dependent  radiance ( : spherical coordinates ). In this case, the irradiance in direction ( ) is defined as


  • Unit vectors
  • the relationship

The following are also defined:

  • the scalar irradiance (engl .: scalar irradiance ) that takes into account the beam density regardless of the direction:
  • the vectorial irradiance (engl .: vectorial irradiance ) having a net represents irradiance (with direction):
where the components and the irradiance mean in the x , y and z directions.

Gershun equation

The Gershun equation (according to Andre Aleksandrovich Gershun , 1903–1952) relates the scalar and vectorial irradiance to the absorption coefficient  :

Since the scattering coefficient does not appear in the relationship , the absorption coefficient can be determined  in any radiation distribution - regardless of the scattering - by determining the two irradiance levels:

Spectral irradiance

The spectral irradiance (unit: W m −2 Hz −1 ) indicates which radiation power hits the body at the frequency from the entire half-space per unit area and per unit frequency interval:

with the spectral radiance

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.


  • DIN-Taschenbuch 22. Units and terms for physical quantities . Beuth Verlag, 1999, ISBN 3-410-14463-3
  • Erich Helbig: Basics of light measurement technology . 2nd edition, Akademische Verlagsgesellschaft Geest & Portig K.-G., Leipzig, 1977, DNB 770197817
  • Gershun, A. (1936/1939): Svetovoe Pole (English: The Light Field ), Moscow 1936. Translated by P. Moon and G. Timoshenko (1939) in Journal of Mathematics and Physics, 18, 51-151