Quantity

Unit

Dimension

Notes

Name

Symbol[nb 1]

Name

Symbol

Symbol

Radiant energy

Qe[nb 2]

joule

J

M⋅L2⋅T−2

energy

Radiant flux

Φe[nb 2]

watt

W

M⋅L2⋅T−3

radiant energy per unit time, also called radiant power.

Spectral power

Φeλ[nb 2][nb 3]

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[nb 3]

watt per steradian per metre

W⋅sr−1⋅m−1

M⋅L⋅T−3

radiant intensity per wavelength.

Radiance

Le

watt per steradian persquare 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[nb 3]
or
Leν[nb 4]

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 eitherwavelength or frequency.
 

Irradiance

Ee[nb 2]

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[nb 3]
or
Eeν[nb 4]

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.[nb 5]
 

Radiant exitance /
Radiant emittance

Me[nb 2]

watt per square metre

W⋅m−2

M⋅T−3

power emitted from a surface.

Spectral radiant exitance /
Spectral radiant emittance

Meλ[nb 3]
or
Meν[nb 4]

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 unit wavelength or frequency.
 

Radiosity

Je

watt per square metre

W⋅m−2

M⋅T−3

emitted plus reflected power leaving a surface.