Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. In Optics, radiometry is the field that studies the Measurement of Electromagnetic radiation, including Visible light. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Area is a Quantity expressing the two- Dimensional size of a defined part of a Surface, typically a region bounded by a closed Curve. "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·m^-2), while the cgs units are ergs per square centimeter per second (erg·cm^-2·s^-1, often used in astronomy). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. M^2 redirects here For other uses see M². CM2 redirects here The centimetre-gram-second system ( CGS) is a system of physical units. M^2 redirects here For other uses see M². CM2 redirects here The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units. In Physics, intensity is a measure of the time-averaged Energy Flux. In Radiometry, radiant intensity is a measure of the

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·m^-3, 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.

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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. Radiant energy is the Energy of Electromagnetic waves The quantity of radiant energy may be calculated by integrating Radiant flux (or power The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In Radiometry, radiant intensity is a measure of the The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area and falls M^2 redirects here For other uses see M². CM2 redirects here Sometimes confusingly called "intensity".
Irradiance	E, I	watt per square metre	W·m−2	power incident on a surface. Sometimes confusingly called "intensity". In Physics, intensity is a measure of the time-averaged Energy Flux.
Radiant exitance / Radiant emittance	M	watt per square metre	W·m−2	power emitted from a surface. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit
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

See also

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

In the fields of Optics and Heat transfer, radiosity is the total emitted and reflected radiation leaving a surface Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area and falls The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit This article deals with the usage of this term in Optics and Lighting. The albedo of an object is the extent to which it diffusely reflects light from the sun In photometry, illuminance is the total Luminous flux incident on a surface per unit Area. In Physics, fluence or integrated Flux is defined as the number of particles that intersect a unit area. Insolation is a measure of Solar radiation energy received on a given surface area in a given time

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