Irradiance

The irradiance (English: irradiance, radiant flux density ) is the term for the total power of the incoming electromagnetic energy impinging on a surface, based on the size of the area. It is measured in the SI unit watts per square meter with the unit symbol W / m².

The index e - which is omitted here - is at symbols in photometry for energy measurement size, that is as an objective measure, in the not incorporate the specific characteristics of human perception. In contrast, the index v stands for a visual measure, integrating the subjective characteristics of the human eye. The visual representation of the irradiance is the light intensity. In the field of electrical engineering, the irradiance is often used interchangeably with the intensity, but the latter generally refers to waves.

Analogous to the irradiance there is the emittance, which denotes the radiation emanating from a surface radiation power per unit area.

Definition

The irradiance is defined as the radiant flux per unit area dA d.phi:

With

• L = radiance
• = Angle of the solid angle element to the surface normal. The cosine factor takes into account that upon irradiation of an arbitrary, only the projection on this direction perpendicular to the surface occurs as a given direction as the effective receiving area.
• = Element of solid angle.

Terms defined in the field

The general radiation distribution, i.e. not necessarily collimated radiation fields is given by a direction dependent radiance L ( θ, φ ) ( θ, φ: spherical coordinates ). In this case, the radiation intensity in the direction ( θ0, φ0 ) is defined as

With

• Unit vectors
• The relationship

Also, are defined:

• The scalar irradiance (English: scalar irradiance ), which takes into account the beam density regardless of the direction:

• The vector irradiance (English: vectorial irradiance ), which represents a net irradiance ( with direction):

Gershun equation

The Gershun equation (after Andre Gershun Aleksandrovich, 1903-1952 ) sets the scalar and the vector irradiance in relation to the absorption coefficient a:

Since the relationship between the scattering coefficient does not appear, may be the absorption coefficient a in an arbitrary radiation distribution - regardless of the dispersion - be determined by the determination of the irradiance:

Spectral irradiance

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

With the spectral irradiance