Irradiance
Physical size  

Surname  Irradiance  
Formula symbol  ,  

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 : Irradiance , radiant flux density , and the radiation flux density , outdated: radiation current density ) is the term for the entire power of the incoming electromagnetic energy impinging on a surface, based on the size of the area.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 timeintegrated quantity.
definition
The irradiance is defined as the radiant flux through the irradiated area :
With
 = 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 directiondependent radiance ( : spherical coordinates ). In this case, the irradiance in direction ( ) is defined as
With
 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 halfspace 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 
W ( watt ) 
Radiant energy through time 
Luminous flux luminous flux, luminous power 
lm ( lumens ) 

Radiant intensity irradiance, radiant intensity 
W / sr  Radiation flux through solid angles 
Luminous intensity luminous intensity 
cd = lm / sr ( candela ) 

Irradiance irradiance 
W / m ^{2}  Radiation flux through the receiver surface 
Illuminance 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, radiance 
W / m ^{2} sr  Radiant intensity through effective transmitter area 
Luminance luminance 
cd / m ^{2}  
Radiant energy amount of radiation, radiant energy 
J ( joules ) 
by radiation transmitted energy 
Amount of light luminous energy, quantity of light 
lm · s  
Irradiation 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 
literature
 DINTaschenbuch 22. Units and terms for physical quantities . Beuth Verlag, 1999, ISBN 3410144633
 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, 51151