Irradiance: Definition from Answers.com

Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power per unit area of electromagnetic radiation at a surface. "Irradiance" is used when the electromagnetic radiation is incident on the surface. "Radiant exitance" or "radiant emittance" is used when the radiation is emerging from the surface. The SI units for all of these quantities are watts per square metre (W/m²), while the cgs units are ergs per square centimeter per second (erg·cm⁻²·s⁻¹, often used in astronomy). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units.

All of these quantities characterize the total amount of radiation present, at all frequencies. It is also common to consider each frequency in the spectrum separately. When this is done for radiation incident on a surface, it is called spectral irradiance, and has SI units W/m³, or commonly W·m⁻²·nm⁻¹.

If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source.

1 Technical details
2 Solar energy
3 See also
4 References

Technical details

The irradiance of a light wave is given in terms of its electric field by

$I = \frac{c n \epsilon_0}{2} |E|^2$ ,

where E is the complex amplitude of the wave's electric field, n is the refractive index of the medium, $c$ is the speed of light in vacuum, and $ε 0$ is the vacuum permittivity.

Irradiance is also the time average of the component of the Poynting vector perpendicular to the surface.

Solar energy

Irradiance due to solar radiation is also called insolation. The global irradiance on a horizontal surface on Earth consists of the direct irradiance E_dir and diffuse irradiance E_dif. On a tilted plane, there is another irradiance component: E_ref, which is the component that is reflected from the ground. The average ground reflection is about 20% of the global irradiance. Hence, the irradiance E_tilt on a tilted plane consists of three components: E_tilt = E_dir + E_dif + E_ref.^[1]

The integral of solar irradiance over a time period is solar irradiation. Irradiation is measured in J/m² and is represented by the symbol H.^[1]

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SI radiometry units
Quantity	Symbol	SI unit	Abbr.	Notes
Radiant energy	Q	joule	J	energy
Radiant flux	Φ	watt	W	radiant energy per unit time, also called radiant power
Radiant intensity	I	watt per steradian	W·sr⁻¹	power per unit solid angle
Radiance	L	watt per steradian per square metre	W·sr⁻¹·m⁻²	power per unit solid angle per unit projected source area. called intensity in some other fields of study.
Irradiance	E, I	watt per square metre	W·m⁻²	power incident on a surface. sometimes confusingly called "intensity".
Radiant exitance / Radiant emittance	M	watt per square metre	W·m⁻²	power emitted from a surface.
Radiosity	J or J_λ	watt per square metre	W·m⁻²	emitted plus reflected power leaving a surface
Spectral radiance	L_λ or L_ν	watt per steradian per metre³ or watt per steradian per square metre per hertz	W·sr⁻¹·m⁻³ or W·sr⁻¹·m⁻²·Hz⁻¹	commonly measured in W·sr⁻¹·m⁻²·nm⁻¹
Spectral irradiance	E_λ or E_ν	watt per metre³ or watt per square metre per hertz	W·m⁻³ or W·m⁻²·Hz⁻¹	commonly measured in W·m⁻²·nm⁻¹

References

^ ^a ^b Quaschning, Volker (2003). "Technology fundamentals—The sun as an energy resource". Renewable Energy World 6 (5): 90–93. http://www.volker-quaschning.de/articles/fundamentals1/index_e.html.

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Irradiance

Contents

Technical details

Solar energy

See also

References