Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. The other two terms are used interchangeably for radiation emerging from a surface. The SI units for all of these quantities are watts per square metre (W/m2). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, 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/m3, 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.

Quantity Symbol SI unit Abbr. Notes
Radiant energy Q joule J energy
Radiant intensity I watt per steradian W·sr−1 power per unit solid angle
Radiance L watt per steradian per square metre W·sr−1·m−2 power per unit solid angle per unit projected source area.

Sometimes confusingly called "intensity".

Irradiance E watt per square metre W·m−2 power incident on a surface.

Sometimes confusingly called "intensity".

Radiant exitance / Radiant emittance M watt per square metre W·m−2 power emitted from a surface.

Sometimes confusingly called "intensity".

or
Lν
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

commonly measured in W·sr−1·m−2·nm−1
or
Eν
watt per metre3 or
watt per square metre per hertz
W·m−3
or
W·m−2·Hz−1
commonly measured in W·m−2·nm−1