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Irradiance (also known as flux density, radiant emittance or radiant exitance is defined by normal component of radiant intensity integrated over the entire hemispheric solid angle. are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. The term Irradiance is used when the electromagnetic radiation is incident on the surface whereas Radiant exitance or radiant emittance is used when the radiation is emerging from the surface. The SI unit for Irradiance is watts per square metre (W·m^-2) and the cgs unit is ergs per square centimeter per second (erg·cm^-2·s^-1. These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity or radiance, which has unit of W·m_-2·sr^-1.

When Irradiance defined for monochromatic radiation, it is called called spectral irradiance, and has SI units W·m^-3, or in common notation 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.

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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. Sometimes confusingly called "intensity".
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 Jλ	watt per square metre	W·m−2	emitted plus reflected power leaving a surface
Spectral radiance	Lλ or Lν	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	Eλ or Eν	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

[edit] See also

• Photometry (optics) Main Photometry/Radiometry article - explains technical terms
• Albedo
• Illuminance
• Fluence
• Insolation