In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power. This article deals with the usage of this term in Optics and Lighting.

Radiometry is important in astronomy, especially radio astronomy, and is important for Earth remote sensing. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Radio astronomy is a subfield of Astronomy that studies celestial objects at radio frequencies. Earth remote sensing is data collection on the environment, Geology, Climate, and other characteristics of the Earth by means of Sensors The measurement techniques categorized as radiometry in optics are called photometry in some astronomical applications, contrary to the optics usage of the term. Photometry is a technique of Astronomy concerned with measuring the Flux, or intensity of an Astronomical object 's Electromagnetic

Spectroradiometry is the measurement of absolute radiometric quantities in narrow bands of wavelength. ^[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. 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. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit 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

• Reflectivity

References

1. ^ Leslie D. 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 In photometry and Heat transfer, reflectivity is the fraction of incident radiation reflected by a surface Stroebel and Richard D. Zakia (1993). Focal Encyclopedia of Photography, 3rd ed. , Focal Press, p. Focal Press is a publisher of media technology books and it is an imprint of Elsevier. 115. ISBN 0240514173.  

External links

• Radiometry and photometry FAQ Professor Jim Palmer's Radiometry FAQ page (University of Arizona).

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