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In radiometry (measurement of electromagnetic radiation), irradiance is the radiant flux (power) received by a surface per unit area, and spectral irradiance is the irradiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of irradiance is the watt per square metre (W/m^{2}), while that of spectral irradiance is the watt per square metre per hertz (W·m^{−2}·Hz^{−1}) or the watt per square metre per metre (W·m^{−3})—commonly the watt per square metre per nanometre (W·m^{−2}·nm^{−1}). The CGS unit erg per square centimetre per second (erg·cm^{−2}·s^{−1}) is often used in astronomy. Irradiance is often called "intensity" in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.
Irradiance of a surface, denoted E_{e} ("e" for "energetic", to avoid confusion with photometric quantities), is defined as^{[1]}
where
If we want to talk about the radiant flux emitted by a surface, we speak of radiant exitance.
Spectral irradiance in frequency of a surface, denoted E_{e,ν}, is defined as^{[1]}
where ν is the frequency.
Spectral irradiance in wavelength of a surface, denoted E_{e,λ}, is defined as^{[1]}
where λ is the wavelength.
Irradiance of a surface is also, according to the definition of radiant flux, equal to the time-average of the component of the Poynting vector perpendicular to the surface:
For a propagating sinusoidal linearly polarized electromagnetic plane wave, the Poynting vector always points to the direction of propagation while oscillating in magnitude. The irradiance of a surface is then given by^{[2]}
This formula assumes that the magnetic susceptibility is negligible, i.e. that μ_{r} ≈ 1 where μ_{r} is the magnetic permeability of the propagation medium. This assumption is typically valid in transparent media in the optical frequency range.
The global irradiance on a horizontal surface on Earth consists of the direct irradiance E_{e,dir} and diffuse irradiance E_{e,diff}. On a tilted plane, there is another irradiance component, E_{e,refl}, 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_{e} on a tilted plane consists of three components:^{[3]}
The integral of solar irradiance over a time period is called "solar exposure" or "insolation".^{[3]}^{[4]}
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