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Irradiance, radiant emittance, and radiant exitance are radiometry
Radiometry

In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. 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....
 terms for the power of electromagnetic radiation
Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
 at a surface, per unit area
Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
. "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
Si

Si, si, or SI may refer to :...
 units for all of these quantities are watt
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s per square metre (W·m-2), while the cgs units are ergs per square centimeter per second
Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
 (erg·cm-2·s-1, often used in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
).






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Irradiance, radiant emittance, and radiant exitance are radiometry
Radiometry

In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. 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....
 terms for the power of electromagnetic radiation
Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
 at a surface, per unit area
Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
. "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
Si

Si, si, or SI may refer to :...
 units for all of these quantities are watt
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s per square metre (W·m-2), while the cgs units are ergs per square centimeter per second
Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
 (erg·cm-2·s-1, often used in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
). These quantities are sometimes called intensity
Intensity (physics)

In physics, intensity is a Measurement of the time averaging energy flux. The word "intensity" here is not synonymous with "wikt:strength", "wikt:amplitude", or "wikt:level", as it sometimes is in colloquial speech....
, but this usage leads to confusion with radiant intensity
Radiant intensity

In radiometry, radiant intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. The SI unit of radiant intensity is watts per steradian ....
, 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·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.

Technical details

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

where E is the complex
Complex number

In mathematics, the complex numbers are an extension of the real numbers obtained by adjoining an imaginary unit, denoted i, which satisfies:...
 amplitude
Amplitude

Amplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation....
 of the wave's electric field
Electric field

In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field ....
, n is the refractive index
Refractive index

The refractive index of a medium is a measure for how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index of 1.5, which means that in glass, light travels at times the speed of light in a vacuum....
 of the medium, is the speed of light
Speed of light

The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second....
 in vacuum
Vacuum

A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty....
, and is the vacuum permittivity.

Irradiance is also the time average of the component of the Poynting vector
Poynting vector

In physics, the Poynting vector can be thought of as representing the energy flux of an electromagnetic field. It is named after its inventor John Henry Poynting....
 perpendicular to the surface.

Solar energy

Irradiance due to solar radiation is also called insolation
Insolation

Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day ....
. The global irradiance on a horizontal surface on Earth consists of the direct irradiance Edir and diffuse irradiance Edif. On a tilted plane, there is another irradiance component: Eref, which is the component that is reflected from the ground. The average ground reflection is about 20% of the global irradiance. Hence, the irradiance Etilt on a tilted plane consists of three components: Etilt = Edir + Edif + Eref.

The integral
Integral

Integration is an important concept in mathematics, specifically in the field of calculus and, more broadly, mathematical analysis. Given a function ƒ of a Real number variable x and an interval [a, b] of the real line, 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.

See also

  • Photometry (optics)
    Photometry (optics)

    Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
     Main Photometry/Radiometry article—explains technical terms
  • Albedo
    Albedo

    The albedo of an object is the extent to which it diffusely reflects light from the Sun. It is therefore a more specific form of the term reflectivity....
  • Illuminance
    Illuminance

    In photometry , illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception....
  • Fluence
    Fluence

    In physics, fluence or integrated flux is defined as the number of particles that intersect a unit area . Its units are m–2 . In particular, it is used to describe the strength of a radiation field, in which case the unit used is J/m2....
  • Insolation
    Insolation

    Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day ....
  • Light diffusion
  • Solar azimuth angle
    Solar azimuth angle

    The solar azimuth angle is the azimuth angle of the sun. It is most often defined as the angle between the line from the observer to the sun projected on the ground and the line from the observer due south....
  • Solar irradiation
  • Solar noon