"Spectral emittance" is an old term for this quantity. The latter is commonly measured in W⋅m −2⋅nm −1. Radiant exitance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "intensity". "Radiant emittance" is an old term for this quantity. This is the emitted component of radiosity. Radiant flux emitted by a surface per unit area. This is sometimes also confusingly called "spectral intensity". Radiosity of a surface per unit frequency or wavelength. Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. Non-SI units of spectral flux density include jansky ( 1 Jy = 10 −26 W⋅m −2⋅Hz −1) and solar flux unit ( 1 sfu = 10 −22 W⋅m −2⋅Hz −1 = 10 4 Jy). Irradiance of a surface per unit frequency or wavelength. Radiant flux received by a surface per unit area. Watt per steradian per square metre, per metre The latter is commonly measured in W⋅sr −1⋅m −2⋅nm −1. Radiance of a surface per unit frequency or wavelength. Watt per steradian per square metre per hertz Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. The latter is commonly measured in W⋅sr −1⋅nm −1. Radiant intensity per unit frequency or wavelength. Radiant flux emitted, reflected, transmitted or received, per unit solid angle. The latter is commonly measured in W⋅nm −1. Radiant flux per unit frequency or wavelength. This is sometimes also called "radiant power", and called luminosity in Astronomy. Radiant energy emitted, reflected, transmitted or received, per unit time. For instance, the irradiance of Alpha Centauri A (radiant flux: 1.5 L ☉, distance: 4.34 ly) is about 2.7 × 10 −8 W/m 2 on Earth. This is a good approximation because the distance from even a nearby star to the Earth is much larger than the star's diameter. In astronomy, stars are routinely treated as point sources even though they are much larger than the Earth.
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