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The units of W/m^{2 }and W/(m^{2 }nm) differ because the first measure does not specify the wavelength, meaning simply that a certain amount of radiative power per surface area was measured. When "nm" is included, the measurement is the "radiative power per surface area and per wavelength interval."

In a more formal sense, "irradiance," "spectral irradiance," and the associated units are defined as follows:

**Irradiance** *E,* is the radiant energy *dQ,* arriving per time interval *dt*, and per area dA, on a surface. The associated SI unit is W/m^{2}.

**Spectral irradiance** *E*_{λ}, is the radiant energy *dQ,* arriving per time interval *dt*, per area dA, and per wavelength interval *d*λ, on a surface. The associated unit is W/(m^{2 }nm).

Irradiance can be calculated from spectral irradiance by integrating over wavelength.

A full discussion of irradiance and its units can be found in BSI's Application Note "Light Quantities, Units, and Conversion Factors."