Irradiance - Solar Energy

Solar Energy

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 of solar irradiance over a time period is solar irradiation or insolation. Irradiation is generally measured in J/m2 and is represented by the symbol H.

SI radiometry units
  • v
  • t
  • e
Quantity Symbol SI unit Symbol Dimension Notes
Radiant energy Qe joule J M⋅L2⋅T−2 energy
Radiant flux Φe watt W M⋅L2⋅T−3 radiant energy per unit time, also called radiant power.
Spectral power Φeλ watt per metre W⋅m−1 M⋅L⋅T−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 M⋅L2⋅T−3 power per unit solid angle.
Spectral intensity Ieλ watt per steradian per metre W⋅sr−1⋅m−1 M⋅L⋅T−3 radiant intensity per wavelength.
Radiance Le watt per steradian per square metre W⋅sr−1⋅m−2 M⋅T−3 power per unit solid angle per unit projected source area.

confusingly called "intensity" in some other fields of study.
Spectral radiance Leλ
or
Leν		 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		 M⋅L−1⋅T−3
or
M⋅T−2		 commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.
Irradiance Ee watt per square metre W⋅m−2 M⋅T−3 power incident on a surface, also called radiant flux density.

sometimes confusingly called "intensity" as well.
Spectral irradiance Eeλ
or
Eeν		 watt per metre3
or
watt per square metre per hertz		 W⋅m−3
or
W⋅m−2⋅Hz−1		 M⋅L−1⋅T−3
or
M⋅T−2		 commonly measured in W⋅m−2⋅nm−1
or 10−22W⋅m−2⋅Hz−1, known as solar flux unit.
Radiant exitance /
Radiant emittance		 Me watt per square metre W⋅m−2 M⋅T−3 power emitted from a surface.
Spectral radiant exitance /
Spectral radiant emittance		 Meλ
or
Meν		 watt per metre3
or

watt per square
metre per hertz

 W⋅m−3
or
W⋅m−2⋅Hz−1		 M⋅L−1⋅T−3
or
M⋅T−2		 power emitted from a surface per wavelength or frequency.
Radiosity Je or Jeλ watt per square metre W⋅m−2 M⋅T−3 emitted plus reflected power leaving a surface.
Radiant exposure He joule per square metre J⋅m−2 M⋅T−2
Radiant energy density ωe joule per metre3 J⋅m−3 M⋅L−1⋅T−2
See also: SI · Radiometry · Photometry

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